To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and ...To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and Yongchuan areas of the Sichuan Basin for porosity and permeability experiments and a triaxial compression and sound wave integration experiment at the maximum temperature and pressure of 120 ℃ and 70 MPa. The results show that the microscopic porosity and permeability change and the macroscopic rock deformation are mutually constrained, both showing the trend of steep and then gentle variation. At the maximum temperature and pressure, the porosity reduces by 34%–71%, and the permeability decreases by 85%–97%. With the rising temperature and pressure, deep shale undergoes plastic deformation in which organic pores and clay mineral pores are compressed and microfractures are closed, and elastic deformation in which brittle mineral pores and rock skeleton particles are compacted. Compared with previous experiments under high confining pressure and normal temperature,the experiment under high temperature and high pressure coupling reveals the effect of high temperature on stress sensitivity of porosity and permeability. High temperature can increase the plasticity of the rock, intensify the compression of pores due to high confining pressure, and induce thermal stress between the rock skeleton particles, allowing the reopening of shale bedding or the creation of new fractures along weak planes such as bedding, which inhibits the decrease of permeability with the increase of temperature and confining pressure. Compared with the triaxial mechanical experiment at normal temperature, the triaxial compression experiment at high temperature and high pressure demonstrates that the compressive strength and peak strain of deep shale increase significantly due to the coupling of temperature and pressure. The compressive strength is up to 435 MPa and the peak strain exceeds 2%, indicating that high temperature is not conducive to fracture initiation and expansion by increasing rock plasticity. Lithofacies and mineral composition have great impacts on the porosity, permeability and rock mechanics of deep shale. Shales with different lithologies are different in the difficulty and extent of brittle failure. The stress-strain characteristics of rocks under actual geological conditions are key support to the optimization of reservoir stimulation program.展开更多
Based on analysis of pore features and pore skeleton composition of shale,a“rigid elastic chimeric”pore skeleton model of shale gas reservoir was built.Pore deformation mechanisms leading to increase of shale porosi...Based on analysis of pore features and pore skeleton composition of shale,a“rigid elastic chimeric”pore skeleton model of shale gas reservoir was built.Pore deformation mechanisms leading to increase of shale porosity due to the pore skeleton deformation under overpressure were sorted out through analysis of stress on the shale pore and skeleton.After reviewing the difficulties and defects of existent porosity measurement methods,a dynamic deformed porosity measurement method was worked out and used to measure the porosity of overpressure Silurian Longmaxi Formation shale under real formation conditions in southern Sichuan Basin.The results show:(1)The shale reservoir is a mixture of inorganic rock particles and organic matter,which contains inorganic pores supported by rigid skeleton particles and organic pores supported by elastic-plastic particles,and thus has a special“rigid elastic chimeric”pore structure.(2)Under the action of formation overpressure,the inorganic pores have tiny changes that can be assumed that they don’t change in porosity,while the organic pores may have large deformation due to skeleton compression,leading to the increase of radius,connectivity and ultimately porosity of these pores.(3)The“dynamic”deformation porosity measurement method combining high injection pressure helium porosity measurement and kerosene porosity measurement method under ultra-high variable pressure can accurately measure porosity of unconnected micro-pores under normal pressure conditions,and also the porosity increment caused by plastic skeleton compression deformation.(4)The pore deformation mechanism of shale may result in the"abnormal"phenomenon that the shale under formation conditions has higher porosity than that under normal pressure,so the overpressure shale reservoir is not necessarily“ultra-low in porosity”,and can have porosity over 10%.Application of this method in Well L210 in southern Sichuan has confirmed its practicality and reliability.展开更多
As an important pilot target for shale gas exploration and development in China,the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen,which severely increases exploration ri...As an important pilot target for shale gas exploration and development in China,the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen,which severely increases exploration risk.Accordingly,this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions,isotope features,and geochemical characteristics of associated gases.The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir.Specifically,the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume.Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas.The normally high nitrogen content(18.05%-40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage.Specifically,the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale,accompanied by nitrogen generation.In comparison,the abnormally high nitrogen content(86.79%-98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults,which results in hydrocarbon loss and nitrogen intrusion,acting as the key factor for deconstruction of the primary shale gas reservoir.Results of this study not only enrich research on genetic mechanism of high-maturity N_@ shale gas reservoirs,but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.展开更多
Shale,as a kind of brittle rock,often exhibits different nonlinear stress-strain behavior,failure and timedependent behavior under different strain rates.To capture these features,this work conducted triaxial compress...Shale,as a kind of brittle rock,often exhibits different nonlinear stress-strain behavior,failure and timedependent behavior under different strain rates.To capture these features,this work conducted triaxial compression tests under axial strain rates ranging from 5×10-6 s-1 to 1×10-3 s-1.The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates.These strain rate-dependent mechanical behaviors of shale are originated from damage growth,which is described by a damage parameter.When axial strain is the same,the damage parameter is positively correlated with strain rate.When strain rate is the same,with an increase of axial strain,the damage parameter decreases firstly from an initial value(about 0.1 to 0.2),soon reaches its minimum(about 0.1),and then increases to an asymptotic value of 0.8.Based on the experimental results,taking yield stress as the cut-off point and considering damage variable evolution,a new measure of micro-mechanical strength is proposed.Based on the Lemaitre’s equivalent strain assumption and the new measure of micro-mechanical strength,a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established.Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.展开更多
Fracture prediction is a technical issue in the field of petroleum exploration and production worldwide.Although there are many approaches to predict the distribution of cracks underground,these approaches have some l...Fracture prediction is a technical issue in the field of petroleum exploration and production worldwide.Although there are many approaches to predict the distribution of cracks underground,these approaches have some limitations.To resolve these issues,we ascertained the relation between numerical simulations of tectonic stress and the predicted distribution of fractures from the perspective of geologic genesis,based on the characteristics of the shale reservoir in the Longmaxi Formation in Dingshan;the features of fracture development in this reservoir were considered.3 D finite element method(FEM)was applied in combination with rock mechanical parameters derived from the acoustic emissions.The paleotectonic stress field of the crack formation period was simulated for the Longmaxi Formation in the Dingshan area.The splitting factor in the study area was calculated based on the rock breaking criterion.The coefficient of fracture development was selected as the quantitative prediction classification criteria for the cracks.The results show that a higher coefficient of fracture development indicates a greater degree of fracture development.On the basis of the fracture development coefficient classification,a favorable area was identified for the development of fracture prediction in the study area.The prediction results indicate that the south of the Dingshan area and the DY3 well of the central region are favorable zones for fracture development.展开更多
Through graptolite identification in profiles,graptolite zone division,contour map compilation,and analysis of mineral composition,TOC content,lamina distribution features of shale samples,the biostratigraphic and res...Through graptolite identification in profiles,graptolite zone division,contour map compilation,and analysis of mineral composition,TOC content,lamina distribution features of shale samples,the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out.There are 4 graptolite zones(WF1 to WF4)in Wufeng Formation and 9(LM1 to LM9)in Longmaxi Formation,and the different graptolite zones can be calibrated by lithology and electrical property.The shale layers of these graptolite zones have two depocenters in the southwest and northeast,and differ in mineral composition,TOC,and lamina types.Among them,the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale,the upper part of WF1-WF2 and WF3 have horizontal bedding hybrid shale with organic matter,the LM1-LM4 mainly consist of organic-rich siliceous shale with horizontal bedding,and the LM5-LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding.The mineral composition,TOC and lamina types of shale depend on the paleo-climate,paleo-water oxidation-reduction conditions,and paleo-sedimentation rate during its deposition.Deposited in oxygen-rich warm water,the lower parts of WF1 and WF2 graptolite zones have massive bedding,low TOC and silicon content.Deposited in cooler and oxygen-rich water,the WF4 has massive bedding,high calcium content and low TOC.Deposited in anoxic water with low rate,the upper part of WF2,WF3,and LM1-LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae.Deposited in oxygen rich water at a high rate,the graptolite zones LM5-LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.展开更多
Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakth...Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakthroughs have been made in deeper shale gas resources at depth of 3500–4500 m.To promote the effective production of shale gas in this area,this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation.The results show that:(1)The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoirs.In the relatively deep water area in deep-water shelf,grade-I reservoirs with a larger continuous thickness develop.The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions.The research shows that the uranium to thorium mass ratio greater than 1.25 indicates relatively deep water in anoxic reduction environment,and the uranium to thorium mass ratio of 0.75–1.25 indicates semi-deep water in weak reduction and weak oxidation environment,and the uranium to thorium mass ratio less than 0.75 indicates relatively shallow water in strong oxidation environment.(2)The propped fractures in shale reservoirs subject to fracturing treatment are generally 10–12 m high,if grade-I reservoirs are more than 10 m in continuous thickness,then all the propped section would be high-quality reserves;in this case,the longer the continuous thickness of penetrated grade-I reservoirs,the higher the production will be.(3)The shale gas reservoirs at 3500–4500 m depth in southern Sichuan are characterized by high formation pressure,high pressure coefficient,well preserved pores,good pore structure and high proportion of free gas,making them the most favorable new field for shale gas exploration;and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production.(4)High production wells in the deep shale gas reservoirs are those in areas where Long11-Long13 sub-beds are more than 10 m thick,with 1500 m long horizontal section,grade-I reservoirs penetration rate of over 90%,and fractured by dense cutting+high intensity sand injection+large displacement+large liquid volume.(5)The relatively deep-water area in the deep-water shelf and the area at depth of 3500–4500 m well overlap in the southern Sichuan,and the overlapping area is the most favorable shale gas exploration and development zones in the southern Sichuan in the future.With advancement in theory and technology,annual shale gas production in the southern Sichuan is expected to reach 450×108 m3.展开更多
Due to the depletion of conventional hydrocarbon resources,both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources,especially those generated and accum...Due to the depletion of conventional hydrocarbon resources,both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources,especially those generated and accumulated within source rocks.In an attempt to further understand the mechanisms of these resources,detailed mineralogical,lithological,and geochemical studies were performed to compare source rock samples from i)the Longmaxi Formation of the Lower Silurian in the Sichuan Basin(China),and ii)the Semiluksk Formation of the Frasnian Stage of the Upper Devonian in the Volga-Ural region of the East European Platform(Russia).The results showed that the main mineral of the source rocks of both formations is chalcedony that formed during an outpouring of deep fluids onto the sea bed.Simultaneous thermal analyses of this mineral indicated similar thermophysical properties to those of the hydrothermal-sedimentary chalcedony from jasper.Moreover,a direct relationship between the chalcedony content and the total organic matter content in the samples from the two formations was established.The presence of biophilic chemical elements in the siliceous component of the source rock samples indicate that high total organic content values were likely due to the presence of biophilic chemical elements in deep fluids,which led to the large-scale development of biota and subsequent accumulation of organic matter during sedimentation.The findings also revealed that the organic matter in the source rocks of the two regions was at different stages of maturity.The organic matter in samples from the Volga-Ural region was less mature and only at the early stage of oil generation,whereas the organic matter in samples from the Sichuan Basin reached both oil and gas generation stages.The Longmaxi Formation is already in the shale gas exploration and development stage,and the Semiluksk Formation could also be regarded as an exploration target for shale oil reservoirs.展开更多
Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng For...Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces(i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.展开更多
The water adsorption by shale significantly affects shale gas content and its seepage capacity.However,the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore s...The water adsorption by shale significantly affects shale gas content and its seepage capacity.However,the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore structure.The relationship between the adsorbed water content at different relative humidities(RHs)and shale compositions,as well as shale pore structure and the spatial configuration relationship between organic matter(OM)and clay minerals,was investigated to clarify the controlling factors and mechanisms of water adsorption by Longmaxi Formation shale from the Southern Sichuan Basin in China.Consequently,the water adsorption process could be generally divided into three different stages from 0%RH to 99%RH.Furthermore,the Johnston’s clay mine ral interlayer pore structure model(JCM),the Freundlich model(FM)and the Dubinin-Astakhov model(DAM)were tested to fit the three water adsorption stages from low RH to high RH,respectively.The fitting results of the JCM and FM at lower RHs were far from good,while the fitting results of DAM at higher RHs were acceptable.Accordingly,two revised models(LRHM and MRHM)considering the spatial configuration relationship between OM and clay minerals were proposed for the two stages with lower RHs,and performed better fitting results indicating the pronounced effect of the spatial configuration relationship between OM and clay minerals on the water adsorption process of Longmaxi Formation shale.The outcomes of this study will contribute to clarifying the water distribution characteristics in the pore network of shale samples with variable water contents.展开更多
Based on feild outcrop survey,lithology observation and systematic sampling,and with the scanning electron microscope,rock pyrolysis,biomarker compound test and analysis,this paper discussed source rock characteristic...Based on feild outcrop survey,lithology observation and systematic sampling,and with the scanning electron microscope,rock pyrolysis,biomarker compound test and analysis,this paper discussed source rock characteristics and paleoenvironment in western Hubei.The lithology at the bottom of Lower Silurian Longmaxi Formationn in this study area is mainly dark carbonaceous shale,the mineral composition is mostly clay minerals,quartz and feldspar,the content of illite and kaolinite is high,but also contains a minor amount of strawberry pyrite and graptolite fossils.The test and analysishow that the total organic carbon content(TOC)of the source rock is 0.8-6.32%,the average value is 3.42%,and the vitrinite reflectance(Ro)is 1.57-2.95%.The type of organic matter is mainly typeⅡ2.The overall abundance of organic matter is high,and the degree of thermal evolution has reached the stage of high maturity and over maturity.All these have become the good material basis and conditions for the formation of shale gas.The paleoenvironment of the source rocks indicates that the organic matter is a mixed source of planktonic algae and bacteria material.The large amount of hopane also indicates that the bacterial activity is strong during the deposition of organic matter;The water of the ancient lake is almost freshwater and brackishwater in some parts;The redox property of the water is in a strong reduction environment and with a stable stratification,which is also conducive to the preservation of organic matter.The above research results will provide the application of shale gas exploration in western Hubei.展开更多
Deep shale layer in the Lower Silurian Longmaxi Formation,southern Sichuan Basin is the major replacement target of shale gas exploration in China.However,the prediction of"sweet-spots"in deep shale gas rese...Deep shale layer in the Lower Silurian Longmaxi Formation,southern Sichuan Basin is the major replacement target of shale gas exploration in China.However,the prediction of"sweet-spots"in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale.Based on petrological,acoustic and hardness measurements,variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated.The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type.Geochemical characteristics of organic-rich shale samples(TOC>2%)indicate that these shale samples have a framework of microcrystalline quartz grains;the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore.The lean-organic shale samples(TOC<2%),with quartz primarily coming from terrigenous debris,feature plastic clay mineral particles as the support frame in rock texture.Intergranular pores in these samples are between clay particles,and show features of soft pores in mechanical property.The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content,and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content.Also due to the difference in microtexture,the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson’s ratio as well as elasticity modulus versus Poisson’s ratio.The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness,while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content.The study results can provide a basis for well-logging interpretation and"sweet spot"prediction of Longmaxi Formation shale gas reservoirs.展开更多
The assessment of gas storage capacity is crucial to furthering shale gas exploration and development in the eastern Sichuan Basin,China.Eleven organic-rich shale samples were selected to carry out the high pressure m...The assessment of gas storage capacity is crucial to furthering shale gas exploration and development in the eastern Sichuan Basin,China.Eleven organic-rich shale samples were selected to carry out the high pressure methane sorption,low-pressure N_(2)/CO_(2) gas adsorption,and bulk and skeletal density measurements to investigate the methane storage capacity(MSC).Based on the relative content of clay,carbonates,quartz+feldspar,we grouped the 11 samples into three lithofacies:silica-rich argillaceous shale(CM-1),argillaceous/siliceous mixed shale(M-2),and clay-rich siliceous shale(S-3).The total porosity of the shale samples varies from 3.4% to 5.6%,and gas saturation ranges from 47% to 89%.The measured total gas amount ranges from 1.84 mg/g to 4.22 mg/g with the ratio of free gas to total gas amount ranging from 52.7% to 70.8%.Free gas with high content in the eastern Sichuan Basin may be the key factor controlling amount of shale gas production.The TOC content critically controls the MSC of shales,because micropore,mesopore volumes and the specific surface areas associated with organic matter provide the storage sites for the free and adsorbed gas.The methane sorption capacities of samples from different lithofacies are also affected by clay minerals and moisture content.Clay minerals can provide additional surface areas for methane sorption,and water can cause a 7.1%-42.8% loss of methane sorption capacity.The total porosity,gas-bearing porosity,water saturation,free gas and adsorbed gas number of samples from different lithofacies show subtle differences if the shale samples had similar TOC contents.Our results suggest that,in the eastern Sichuan Basin,clay-rich shale lithofacies is also prospecting targets for shale gas production.展开更多
Due to the existence of water content in shale reservoir,it is quite meaningful to clarify the effect of water content on the methane adsorption capacity(MAC)of shale.However,the role of spatial configuration relation...Due to the existence of water content in shale reservoir,it is quite meaningful to clarify the effect of water content on the methane adsorption capacity(MAC)of shale.However,the role of spatial configuration relationship between organic matter(OM)and clay minerals in the MAC reduction process is still unclear.The Silurian Longmaxi Formation shale samples from the Southern Sichuan Basin in China were prepared at five relative humidity(RH)conditions(0%,16%,41%,76%,99%)and the methane adsorption experiments were conducted on these water-bearing shale samples to clarify the MAC reduction process considering the spatial configuration relationship between clay minerals and OM and establish the empirical model to fit the stages.Total organic carbon(TOC)content and mineral compositions were analyzed and the pore structures of these shale samples were characterized by field-emission scanning electron microscopy(FE-SEM),N2 adsorption and high-pressure mercury intrusion porosimetry(HPMIP).The results showed that the MAC reduction of clay minerals in OM occurred at different RH conditions from that of clay minerals outside OM.Furthermore,the amount of MAC reduction of shale samples prepared at the same RH condition was negatively related with clay content,which indicated the protection role of clay minerals for the MAC of water-bearing shale samples.The MAC reduction process was generally divided into three stages for siliceous and clayey shale samples.And the MAC of OM started to decline during stage(1)for calcareous shale sample mainly because water could enter OM pores more smoothly through hydrophobic pathway provided by carbonate minerals than through hydrophilic clay mineral pores.Overall,this study will contribute to improving the evaluation method of shale gas reserve.展开更多
As a serious problem in drilling operation,wellbore instability restricts efficient development of shale gas.The interaction between the drilling fluid and shale with hydration swelling property would have impact on t...As a serious problem in drilling operation,wellbore instability restricts efficient development of shale gas.The interaction between the drilling fluid and shale with hydration swelling property would have impact on the generation and propagation mechanism of cracks in shale formation,leading to wellbore instability.In order to investigate the influence of the hydration swelling on the crack propagation,mineral components and physicochemical properties of shale from the Lower Silurian Longmaxi Formation(LF)were investigated by using the XRD analysis,cation exchange capabilities(CEC)analysis,and SEM observation,and we researched the hydration mechanism of LF shale.Results show that quartz and clay mineral are dominated in mineral composition,and illite content averaged 67%in clay mineral.Meanwhile,CEC of the LF shale are 94.4 mmol/kg.The process of water intruding inside shale along microcracks was able to be observed through high power microscope,meanwhile,the hydration swelling stress would concentrate at the crack tip.The microcracks would propagate,bifurcate and connect with each other,with increase of water immersing time,and it would ultimately develop into macro-fracture.Moreover,the macrocracks extend and coalesce along the bedding,resulting in the rock failure into blocks.Hydration swelling is one of the major causes that lead to wellbore instability of the LF shale,and therefore improving sealing capacity and inhibition of drilling fluid system is an effective measure to stabilize a borehole.展开更多
Through the field emission scanning electronic microscope(FESEM)and the nitrogen adsorption test,pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied.Result showe...Through the field emission scanning electronic microscope(FESEM)and the nitrogen adsorption test,pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied.Result showed that the pore type includes organic pore,intercrystalline pore,dissolution intracrystalline pore and interparticle pore,and the organic pore was one of major pore types;among the organic pore,the micropore had large pore volume and specific surface area,and was the main storage space of shale gas.Through study on effect of total organic carbon(TOC),organic matter maturity(Ro),diagenesis and tectonism on shale porosity,influence of TOC on porosity could be divided into four stages:the rapid increasing stage(TOC from 0 to 2%),the slow decreasing stage(TOC from 2 to 3%),the rapid increasing stage(TOC from 3 to 4%or 6%)and the rapid decreasing stage(TOC>4%or 6%);influence of the maturity on porosity of shale could be divided into three stages:the rapid decreasing stage(Ro from 1.5 to 2.2%),the rapid increasing stage(Ro from 2.2 to 2.7%)and the rapid decreasing stage(Ro>2.7%);during the high thermal evolution stage,the organic diagenesis was stronger than the inorganic diagenesis;the tectonism had a great impact on porosity,and the more intense the tectonism was,the smaller the porosity would be.The evolution of shale porosity of the Longmaxi Formation underwent five stages:the immature rapid compaction stage(Ro<0.7%),the mature hydrocarbon generation and dissolution stage(Ro from 0.7 to 1.3%),the high mature pore closed stage(Ro from 1.3 to 2.2%),the overmature secondary pyrolysis stage(Ro from 2.2 to 2.7%)and the overmature slow compaction stage(Ro>2.7%);among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages。展开更多
Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identif...Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identified in the Wufeng Formation to Longmaxi Formation,e.g.,siliceous shale,silty shale,argillaceous shale,calcareous shale,shell-bearing argillaceous limestone/calcareous mudstone,siltstonefine sandstone and bentonite.For shale in the Wufeng and Longmaxi Formations,the depositional mode was not only the suspension deposition under the low-energy hydrodynamic condition,but also the biogenic deposition,the storm deposition and the bottom current deposition indicated by large amount of siliceous biological fossils,biological detritus with size graded structure,silt laminations,lenticular bedding,ripple cross-bedding and other sedimentary structures.Trace element analysis suggested that the siliceous shale in theWufeng Formation and the lower part of Longmaxi Formation was developed in the oxygen-dysoxic/anoxic depositional environment,which was rich in organic matters;then due to influence of the sea level gradually decline and bottom current,the silty and argillaceous shales were formed,and the oxygen-dysoxic/anoxic depositional environment was destroyed,gradually leading to the worse preservation condition of organic matter.展开更多
In this paper,the pore structure characteristics of shale samples from the Lower Silurian Longmaxi Formation in South of Sichuan Basin of China were investigated by total organic carbon(TOC)content determination,X-ray...In this paper,the pore structure characteristics of shale samples from the Lower Silurian Longmaxi Formation in South of Sichuan Basin of China were investigated by total organic carbon(TOC)content determination,X-ray diffraction(XRD),scanning electron microscope(SEM),low pressure nitrogen adsorption(LPNA)and high pressure mercury injection(HPMI).The fractal dimension of shale samples was calculated based on Frenkel-Halsey-Hill(FHH)model and thermodynamic relation model.The results showed that the major mineral compositions of shales were quartz and clay content.Organic pores,intergranular pores,intragranular pores,microfractures were widely developed in the shale samples,of which organic pores were the most developed.The pore morphology was mainly ink bottle-shaped pores and slit-shaped pores;the pore size distribution of shale samples was complex with multiple distribution peaks,the pore size between 3 and 40 nm occupied the most of storage space.The fractal dimension Dn1 of pores between 2 nm and 10 nm was 2.7177e2.7933,while fractal dimension Dn2 of pores between 10 nm and 50 nm was 2.2439e2.5468.The fractal dimension Dr of macropores calculated by the thermodynamic model was 2.6401e2.7025.展开更多
The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs.In this study,the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation,Southe...The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs.In this study,the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation,Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc(CCNBD).Based on the experimental data,the relationships among the mode-I fracture toughness,the density,the acoustic time and the clay mineral content were analyzed.The shale samples for fracture toughness test from cores were commonly limited,the investigation of fracture toughness using well logs was necessary.Therefore,a prediction model was proposed by correlating the fracture toughness with well logs responses.The results indicate that the fracture toughness of shale samples is from 0.4744 MPa$m1/2 to 1.0607 MPa$m1/2 with an average of 0.7817 MPa$m1/2,indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale.The clay mineral content and the density have a positive effect on the fracture toughness,whereas the acoustic time plays a negative role on the fracture toughness.The clay mineral content has an important effect on the relationships among fracture toughness,acoustic time and density.The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation,and it has certain guiding significance in the exploration and development of oil and gas reservoirs.展开更多
For shale of Lower Silurian Longmaxi Formation in Chongqing,southeast Sichuan Basin,characteristics of micro-nano pores in marine shale reservoirs were well studies by means of Field-Emission Scanning Electron Microsc...For shale of Lower Silurian Longmaxi Formation in Chongqing,southeast Sichuan Basin,characteristics of micro-nano pores in marine shale reservoirs were well studies by means of Field-Emission Scanning Electron Microscope and Low-temperature Low-pressure Adsorption Experiment of CO_(2)and N_(2).Results showed that six types of pore were developed in the shale of Longmaxi Formation,i.e.,organic pores,intergranular pores,intragranular pores,intercrystalline pores,dissolution pores and microfractures,among which the organic pores and intragranular pores in interlayers of clay minerals were most developed,and a plenty of dissolution pores were also well developed because of high thermal evolution degree.BET specific surface area of the shale in Longmaxi Formation ranged from 3.5 to 18.1 m^(2)/g,BJH total pore volume was from 0.00234 to 0.01338 cm^(3)/g,DA specific surface area of micropores vaired from 1.3 to 7.3 m^(2)/g,and DA pore volume ranged from 0.00052 to 0.00273 cm^(3)/g.The specific surface area of micropores in the shale accounted for 23.1%-80.2%of total specific surface area with an average of 50.3%,and the pore volume of micropores accounted for 12.1%-48.5%of total pore volume with an average of 32.3%.Micropore was the main storage space in shale reservoir for methane adsorption,that because capacities of specific surface area provided by micropores were considerably greater than those provided by mesopores and macropores.Pore size distribution of the shale was complex,and multiple different peaks occurred in the pore size curves,showing two or three peaks in the range from 0 to 100 nm and four peaks occasionally.TOC had a good linear relationship with pore structure parameters of micropores,mesopores t macropores and total pores in the shale,indicating that TOC was the most important control factor for micron-to nano-pore structure in the shale.After normalization of pore structure parameters to TOC,the pore structure parameters of total pores and mesopores t macropores,had positive linear relationships with content of clay minerals but negative linear relationships with content of brittle minerals,indicating that clay minerals and brittle minerals mainly controlled development of mesopores and macropores in the shale.展开更多
基金Supported by the National Natural Science Foundation of China(41872124,42130803)Sinopec Key Science and Technology Project(P20046).
文摘To investigate the porosity, permeability and rock mechanics of deep shale under temperature-pressure coupling, we selected the core samples of deep shale from the Lower Silurian Longmaxi Formation in the Weirong and Yongchuan areas of the Sichuan Basin for porosity and permeability experiments and a triaxial compression and sound wave integration experiment at the maximum temperature and pressure of 120 ℃ and 70 MPa. The results show that the microscopic porosity and permeability change and the macroscopic rock deformation are mutually constrained, both showing the trend of steep and then gentle variation. At the maximum temperature and pressure, the porosity reduces by 34%–71%, and the permeability decreases by 85%–97%. With the rising temperature and pressure, deep shale undergoes plastic deformation in which organic pores and clay mineral pores are compressed and microfractures are closed, and elastic deformation in which brittle mineral pores and rock skeleton particles are compacted. Compared with previous experiments under high confining pressure and normal temperature,the experiment under high temperature and high pressure coupling reveals the effect of high temperature on stress sensitivity of porosity and permeability. High temperature can increase the plasticity of the rock, intensify the compression of pores due to high confining pressure, and induce thermal stress between the rock skeleton particles, allowing the reopening of shale bedding or the creation of new fractures along weak planes such as bedding, which inhibits the decrease of permeability with the increase of temperature and confining pressure. Compared with the triaxial mechanical experiment at normal temperature, the triaxial compression experiment at high temperature and high pressure demonstrates that the compressive strength and peak strain of deep shale increase significantly due to the coupling of temperature and pressure. The compressive strength is up to 435 MPa and the peak strain exceeds 2%, indicating that high temperature is not conducive to fracture initiation and expansion by increasing rock plasticity. Lithofacies and mineral composition have great impacts on the porosity, permeability and rock mechanics of deep shale. Shales with different lithologies are different in the difficulty and extent of brittle failure. The stress-strain characteristics of rocks under actual geological conditions are key support to the optimization of reservoir stimulation program.
基金Supported by the National Science and Technology Major Project of China(2017ZX05035).
文摘Based on analysis of pore features and pore skeleton composition of shale,a“rigid elastic chimeric”pore skeleton model of shale gas reservoir was built.Pore deformation mechanisms leading to increase of shale porosity due to the pore skeleton deformation under overpressure were sorted out through analysis of stress on the shale pore and skeleton.After reviewing the difficulties and defects of existent porosity measurement methods,a dynamic deformed porosity measurement method was worked out and used to measure the porosity of overpressure Silurian Longmaxi Formation shale under real formation conditions in southern Sichuan Basin.The results show:(1)The shale reservoir is a mixture of inorganic rock particles and organic matter,which contains inorganic pores supported by rigid skeleton particles and organic pores supported by elastic-plastic particles,and thus has a special“rigid elastic chimeric”pore structure.(2)Under the action of formation overpressure,the inorganic pores have tiny changes that can be assumed that they don’t change in porosity,while the organic pores may have large deformation due to skeleton compression,leading to the increase of radius,connectivity and ultimately porosity of these pores.(3)The“dynamic”deformation porosity measurement method combining high injection pressure helium porosity measurement and kerosene porosity measurement method under ultra-high variable pressure can accurately measure porosity of unconnected micro-pores under normal pressure conditions,and also the porosity increment caused by plastic skeleton compression deformation.(4)The pore deformation mechanism of shale may result in the"abnormal"phenomenon that the shale under formation conditions has higher porosity than that under normal pressure,so the overpressure shale reservoir is not necessarily“ultra-low in porosity”,and can have porosity over 10%.Application of this method in Well L210 in southern Sichuan has confirmed its practicality and reliability.
基金financially supported by the National Science and Technology Major Project (2017ZX05063002–009)National Natural Science Foundation of China (41772150)+1 种基金Sichuan Province’s Key Project of Research and Development (18ZDYF0884)Qian Ke He Platform Talents [2017]5789-16。
文摘As an important pilot target for shale gas exploration and development in China,the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen,which severely increases exploration risk.Accordingly,this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions,isotope features,and geochemical characteristics of associated gases.The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir.Specifically,the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume.Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas.The normally high nitrogen content(18.05%-40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage.Specifically,the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale,accompanied by nitrogen generation.In comparison,the abnormally high nitrogen content(86.79%-98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults,which results in hydrocarbon loss and nitrogen intrusion,acting as the key factor for deconstruction of the primary shale gas reservoir.Results of this study not only enrich research on genetic mechanism of high-maturity N_@ shale gas reservoirs,but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.
基金financially supported by the China Scholarship Council projectthe National Natural Science Foundation of China(grants No.51574218,41688103,51678171,51608139,U1704243 and 51709113)+4 种基金the Guangdong Science and Technology Department(grant No.2015B020238014)the Guangzhou Science Technology and Innovation Commission(grant No.201604016021)the High-level Talent Research Launch Project(grant No.950318066)the Shandong Provincial Natural Science Foundation,China(grants No.ZR2017PD001 and ZR2018BD013)the Science Foundation of Chinese Academy of Geological Sciences(grant No.JYYWF20181201)
文摘Shale,as a kind of brittle rock,often exhibits different nonlinear stress-strain behavior,failure and timedependent behavior under different strain rates.To capture these features,this work conducted triaxial compression tests under axial strain rates ranging from 5×10-6 s-1 to 1×10-3 s-1.The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates.These strain rate-dependent mechanical behaviors of shale are originated from damage growth,which is described by a damage parameter.When axial strain is the same,the damage parameter is positively correlated with strain rate.When strain rate is the same,with an increase of axial strain,the damage parameter decreases firstly from an initial value(about 0.1 to 0.2),soon reaches its minimum(about 0.1),and then increases to an asymptotic value of 0.8.Based on the experimental results,taking yield stress as the cut-off point and considering damage variable evolution,a new measure of micro-mechanical strength is proposed.Based on the Lemaitre’s equivalent strain assumption and the new measure of micro-mechanical strength,a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established.Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.
基金supported by the Open Fund (PLN 201718) of State Key Laboratory of Oil and Gas Reservoir Geology and ExploitationSouthwest Petroleum University and the Open Fund (SEC-2018-04) of Collaborative Innovation Center of Shale Gas Resources and EnvironmentSouthwest Petroleum University and the National Science and Technology Major Project of China (2017ZX05036003-003)
文摘Fracture prediction is a technical issue in the field of petroleum exploration and production worldwide.Although there are many approaches to predict the distribution of cracks underground,these approaches have some limitations.To resolve these issues,we ascertained the relation between numerical simulations of tectonic stress and the predicted distribution of fractures from the perspective of geologic genesis,based on the characteristics of the shale reservoir in the Longmaxi Formation in Dingshan;the features of fracture development in this reservoir were considered.3 D finite element method(FEM)was applied in combination with rock mechanical parameters derived from the acoustic emissions.The paleotectonic stress field of the crack formation period was simulated for the Longmaxi Formation in the Dingshan area.The splitting factor in the study area was calculated based on the rock breaking criterion.The coefficient of fracture development was selected as the quantitative prediction classification criteria for the cracks.The results show that a higher coefficient of fracture development indicates a greater degree of fracture development.On the basis of the fracture development coefficient classification,a favorable area was identified for the development of fracture prediction in the study area.The prediction results indicate that the south of the Dingshan area and the DY3 well of the central region are favorable zones for fracture development.
基金Supported by the China National Science and Technology Major Project(2017ZX05035-001)。
文摘Through graptolite identification in profiles,graptolite zone division,contour map compilation,and analysis of mineral composition,TOC content,lamina distribution features of shale samples,the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out.There are 4 graptolite zones(WF1 to WF4)in Wufeng Formation and 9(LM1 to LM9)in Longmaxi Formation,and the different graptolite zones can be calibrated by lithology and electrical property.The shale layers of these graptolite zones have two depocenters in the southwest and northeast,and differ in mineral composition,TOC,and lamina types.Among them,the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale,the upper part of WF1-WF2 and WF3 have horizontal bedding hybrid shale with organic matter,the LM1-LM4 mainly consist of organic-rich siliceous shale with horizontal bedding,and the LM5-LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding.The mineral composition,TOC and lamina types of shale depend on the paleo-climate,paleo-water oxidation-reduction conditions,and paleo-sedimentation rate during its deposition.Deposited in oxygen-rich warm water,the lower parts of WF1 and WF2 graptolite zones have massive bedding,low TOC and silicon content.Deposited in cooler and oxygen-rich water,the WF4 has massive bedding,high calcium content and low TOC.Deposited in anoxic water with low rate,the upper part of WF2,WF3,and LM1-LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae.Deposited in oxygen rich water at a high rate,the graptolite zones LM5-LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.
基金Supported by the China National Science and Technology Major Project(2016ZX05062)the PetroChina Science and Technology Major Project(2016E-0611)
文摘Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakthroughs have been made in deeper shale gas resources at depth of 3500–4500 m.To promote the effective production of shale gas in this area,this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation.The results show that:(1)The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoirs.In the relatively deep water area in deep-water shelf,grade-I reservoirs with a larger continuous thickness develop.The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions.The research shows that the uranium to thorium mass ratio greater than 1.25 indicates relatively deep water in anoxic reduction environment,and the uranium to thorium mass ratio of 0.75–1.25 indicates semi-deep water in weak reduction and weak oxidation environment,and the uranium to thorium mass ratio less than 0.75 indicates relatively shallow water in strong oxidation environment.(2)The propped fractures in shale reservoirs subject to fracturing treatment are generally 10–12 m high,if grade-I reservoirs are more than 10 m in continuous thickness,then all the propped section would be high-quality reserves;in this case,the longer the continuous thickness of penetrated grade-I reservoirs,the higher the production will be.(3)The shale gas reservoirs at 3500–4500 m depth in southern Sichuan are characterized by high formation pressure,high pressure coefficient,well preserved pores,good pore structure and high proportion of free gas,making them the most favorable new field for shale gas exploration;and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production.(4)High production wells in the deep shale gas reservoirs are those in areas where Long11-Long13 sub-beds are more than 10 m thick,with 1500 m long horizontal section,grade-I reservoirs penetration rate of over 90%,and fractured by dense cutting+high intensity sand injection+large displacement+large liquid volume.(5)The relatively deep-water area in the deep-water shelf and the area at depth of 3500–4500 m well overlap in the southern Sichuan,and the overlapping area is the most favorable shale gas exploration and development zones in the southern Sichuan in the future.With advancement in theory and technology,annual shale gas production in the southern Sichuan is expected to reach 450×108 m3.
基金supported by grants from the Russian Foundation for Basic Research(project No.18-35-00136)the Chinese NSFC(project No.41811530094)+1 种基金the Government of the Republic of Tatarstan(project No.18-45-160003)supported by subsidies allocated as part of the state’s support for the Kazan Federal University in order to increase its global competitiveness in leading research and educational centers.
文摘Due to the depletion of conventional hydrocarbon resources,both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources,especially those generated and accumulated within source rocks.In an attempt to further understand the mechanisms of these resources,detailed mineralogical,lithological,and geochemical studies were performed to compare source rock samples from i)the Longmaxi Formation of the Lower Silurian in the Sichuan Basin(China),and ii)the Semiluksk Formation of the Frasnian Stage of the Upper Devonian in the Volga-Ural region of the East European Platform(Russia).The results showed that the main mineral of the source rocks of both formations is chalcedony that formed during an outpouring of deep fluids onto the sea bed.Simultaneous thermal analyses of this mineral indicated similar thermophysical properties to those of the hydrothermal-sedimentary chalcedony from jasper.Moreover,a direct relationship between the chalcedony content and the total organic matter content in the samples from the two formations was established.The presence of biophilic chemical elements in the siliceous component of the source rock samples indicate that high total organic content values were likely due to the presence of biophilic chemical elements in deep fluids,which led to the large-scale development of biota and subsequent accumulation of organic matter during sedimentation.The findings also revealed that the organic matter in the source rocks of the two regions was at different stages of maturity.The organic matter in samples from the Volga-Ural region was less mature and only at the early stage of oil generation,whereas the organic matter in samples from the Sichuan Basin reached both oil and gas generation stages.The Longmaxi Formation is already in the shale gas exploration and development stage,and the Semiluksk Formation could also be regarded as an exploration target for shale oil reservoirs.
基金Supported by the PetroChina Science and Technology Project(2021DJ1904)PetroChina Exploration and Production Company Marine Shale Gas Selection Project(kt2018-01-06)。
文摘Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation-Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation-Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces(i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.
基金supported by the National Natural Science Foundation of China(No.41972145)National Science and Technology Major Project of China(No.2017ZX05035—002)+1 种基金the Foundation(No.PRP/indep-2-1904,PRP/indep-3-1707 and No.PRP/indep-3-1615)of State Key Laboratory of Petroleum Resources and Prospecting from China University of Petroleum in Beijingfundamental Research Funds for China University of Geosciences under Award Number 35832019035。
文摘The water adsorption by shale significantly affects shale gas content and its seepage capacity.However,the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore structure.The relationship between the adsorbed water content at different relative humidities(RHs)and shale compositions,as well as shale pore structure and the spatial configuration relationship between organic matter(OM)and clay minerals,was investigated to clarify the controlling factors and mechanisms of water adsorption by Longmaxi Formation shale from the Southern Sichuan Basin in China.Consequently,the water adsorption process could be generally divided into three different stages from 0%RH to 99%RH.Furthermore,the Johnston’s clay mine ral interlayer pore structure model(JCM),the Freundlich model(FM)and the Dubinin-Astakhov model(DAM)were tested to fit the three water adsorption stages from low RH to high RH,respectively.The fitting results of the JCM and FM at lower RHs were far from good,while the fitting results of DAM at higher RHs were acceptable.Accordingly,two revised models(LRHM and MRHM)considering the spatial configuration relationship between OM and clay minerals were proposed for the two stages with lower RHs,and performed better fitting results indicating the pronounced effect of the spatial configuration relationship between OM and clay minerals on the water adsorption process of Longmaxi Formation shale.The outcomes of this study will contribute to clarifying the water distribution characteristics in the pore network of shale samples with variable water contents.
基金Supported by project of National Natural Science Foundation of China(NSFC)(No.42072122).
文摘Based on feild outcrop survey,lithology observation and systematic sampling,and with the scanning electron microscope,rock pyrolysis,biomarker compound test and analysis,this paper discussed source rock characteristics and paleoenvironment in western Hubei.The lithology at the bottom of Lower Silurian Longmaxi Formationn in this study area is mainly dark carbonaceous shale,the mineral composition is mostly clay minerals,quartz and feldspar,the content of illite and kaolinite is high,but also contains a minor amount of strawberry pyrite and graptolite fossils.The test and analysishow that the total organic carbon content(TOC)of the source rock is 0.8-6.32%,the average value is 3.42%,and the vitrinite reflectance(Ro)is 1.57-2.95%.The type of organic matter is mainly typeⅡ2.The overall abundance of organic matter is high,and the degree of thermal evolution has reached the stage of high maturity and over maturity.All these have become the good material basis and conditions for the formation of shale gas.The paleoenvironment of the source rocks indicates that the organic matter is a mixed source of planktonic algae and bacteria material.The large amount of hopane also indicates that the bacterial activity is strong during the deposition of organic matter;The water of the ancient lake is almost freshwater and brackishwater in some parts;The redox property of the water is in a strong reduction environment and with a stable stratification,which is also conducive to the preservation of organic matter.The above research results will provide the application of shale gas exploration in western Hubei.
基金Supported by the National Natural Science Foundation of China(41774136)China National Science and Technology Major Project(2017ZX05035004)
文摘Deep shale layer in the Lower Silurian Longmaxi Formation,southern Sichuan Basin is the major replacement target of shale gas exploration in China.However,the prediction of"sweet-spots"in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale.Based on petrological,acoustic and hardness measurements,variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated.The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type.Geochemical characteristics of organic-rich shale samples(TOC>2%)indicate that these shale samples have a framework of microcrystalline quartz grains;the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore.The lean-organic shale samples(TOC<2%),with quartz primarily coming from terrigenous debris,feature plastic clay mineral particles as the support frame in rock texture.Intergranular pores in these samples are between clay particles,and show features of soft pores in mechanical property.The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content,and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content.Also due to the difference in microtexture,the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson’s ratio as well as elasticity modulus versus Poisson’s ratio.The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness,while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content.The study results can provide a basis for well-logging interpretation and"sweet spot"prediction of Longmaxi Formation shale gas reservoirs.
基金funding supports from the Major Programs of National Natural Science Foundation of China(Nos.41690134 and 41690131)the National Natural Science Foundation of China(No.41872139).
文摘The assessment of gas storage capacity is crucial to furthering shale gas exploration and development in the eastern Sichuan Basin,China.Eleven organic-rich shale samples were selected to carry out the high pressure methane sorption,low-pressure N_(2)/CO_(2) gas adsorption,and bulk and skeletal density measurements to investigate the methane storage capacity(MSC).Based on the relative content of clay,carbonates,quartz+feldspar,we grouped the 11 samples into three lithofacies:silica-rich argillaceous shale(CM-1),argillaceous/siliceous mixed shale(M-2),and clay-rich siliceous shale(S-3).The total porosity of the shale samples varies from 3.4% to 5.6%,and gas saturation ranges from 47% to 89%.The measured total gas amount ranges from 1.84 mg/g to 4.22 mg/g with the ratio of free gas to total gas amount ranging from 52.7% to 70.8%.Free gas with high content in the eastern Sichuan Basin may be the key factor controlling amount of shale gas production.The TOC content critically controls the MSC of shales,because micropore,mesopore volumes and the specific surface areas associated with organic matter provide the storage sites for the free and adsorbed gas.The methane sorption capacities of samples from different lithofacies are also affected by clay minerals and moisture content.Clay minerals can provide additional surface areas for methane sorption,and water can cause a 7.1%-42.8% loss of methane sorption capacity.The total porosity,gas-bearing porosity,water saturation,free gas and adsorbed gas number of samples from different lithofacies show subtle differences if the shale samples had similar TOC contents.Our results suggest that,in the eastern Sichuan Basin,clay-rich shale lithofacies is also prospecting targets for shale gas production.
基金supported by the National Science and Technology Major Project of China(No.2017ZX05035-002)the National Natural Science Foundation of China(No.41972145)the Foundation of State Key Laboratory of Petroleum Resources and Prospecting from China University of Petroleum in Beijing(Nos.PRP/indep-3-1707,PRP/indep-3-1615)。
文摘Due to the existence of water content in shale reservoir,it is quite meaningful to clarify the effect of water content on the methane adsorption capacity(MAC)of shale.However,the role of spatial configuration relationship between organic matter(OM)and clay minerals in the MAC reduction process is still unclear.The Silurian Longmaxi Formation shale samples from the Southern Sichuan Basin in China were prepared at five relative humidity(RH)conditions(0%,16%,41%,76%,99%)and the methane adsorption experiments were conducted on these water-bearing shale samples to clarify the MAC reduction process considering the spatial configuration relationship between clay minerals and OM and establish the empirical model to fit the stages.Total organic carbon(TOC)content and mineral compositions were analyzed and the pore structures of these shale samples were characterized by field-emission scanning electron microscopy(FE-SEM),N2 adsorption and high-pressure mercury intrusion porosimetry(HPMIP).The results showed that the MAC reduction of clay minerals in OM occurred at different RH conditions from that of clay minerals outside OM.Furthermore,the amount of MAC reduction of shale samples prepared at the same RH condition was negatively related with clay content,which indicated the protection role of clay minerals for the MAC of water-bearing shale samples.The MAC reduction process was generally divided into three stages for siliceous and clayey shale samples.And the MAC of OM started to decline during stage(1)for calcareous shale sample mainly because water could enter OM pores more smoothly through hydrophobic pathway provided by carbonate minerals than through hydrophilic clay mineral pores.Overall,this study will contribute to improving the evaluation method of shale gas reserve.
基金This research was supported by the United Fund Project of National Natural Science Foundation of China(Grant No.U1262209)the National Natural Science Foundation of China(NSFC)(Grant No.51274172).
文摘As a serious problem in drilling operation,wellbore instability restricts efficient development of shale gas.The interaction between the drilling fluid and shale with hydration swelling property would have impact on the generation and propagation mechanism of cracks in shale formation,leading to wellbore instability.In order to investigate the influence of the hydration swelling on the crack propagation,mineral components and physicochemical properties of shale from the Lower Silurian Longmaxi Formation(LF)were investigated by using the XRD analysis,cation exchange capabilities(CEC)analysis,and SEM observation,and we researched the hydration mechanism of LF shale.Results show that quartz and clay mineral are dominated in mineral composition,and illite content averaged 67%in clay mineral.Meanwhile,CEC of the LF shale are 94.4 mmol/kg.The process of water intruding inside shale along microcracks was able to be observed through high power microscope,meanwhile,the hydration swelling stress would concentrate at the crack tip.The microcracks would propagate,bifurcate and connect with each other,with increase of water immersing time,and it would ultimately develop into macro-fracture.Moreover,the macrocracks extend and coalesce along the bedding,resulting in the rock failure into blocks.Hydration swelling is one of the major causes that lead to wellbore instability of the LF shale,and therefore improving sealing capacity and inhibition of drilling fluid system is an effective measure to stabilize a borehole.
基金supported by the National Natural Science Foundation of China(No.41502150)Major science and technology special projects of PetroChina Co.Ltd(No.2016E-0611)Stra-tegic Priority Research Programof the Chinese Academy of Sciences(No.XDB10010504).
文摘Through the field emission scanning electronic microscope(FESEM)and the nitrogen adsorption test,pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied.Result showed that the pore type includes organic pore,intercrystalline pore,dissolution intracrystalline pore and interparticle pore,and the organic pore was one of major pore types;among the organic pore,the micropore had large pore volume and specific surface area,and was the main storage space of shale gas.Through study on effect of total organic carbon(TOC),organic matter maturity(Ro),diagenesis and tectonism on shale porosity,influence of TOC on porosity could be divided into four stages:the rapid increasing stage(TOC from 0 to 2%),the slow decreasing stage(TOC from 2 to 3%),the rapid increasing stage(TOC from 3 to 4%or 6%)and the rapid decreasing stage(TOC>4%or 6%);influence of the maturity on porosity of shale could be divided into three stages:the rapid decreasing stage(Ro from 1.5 to 2.2%),the rapid increasing stage(Ro from 2.2 to 2.7%)and the rapid decreasing stage(Ro>2.7%);during the high thermal evolution stage,the organic diagenesis was stronger than the inorganic diagenesis;the tectonism had a great impact on porosity,and the more intense the tectonism was,the smaller the porosity would be.The evolution of shale porosity of the Longmaxi Formation underwent five stages:the immature rapid compaction stage(Ro<0.7%),the mature hydrocarbon generation and dissolution stage(Ro from 0.7 to 1.3%),the high mature pore closed stage(Ro from 1.3 to 2.2%),the overmature secondary pyrolysis stage(Ro from 2.2 to 2.7%)and the overmature slow compaction stage(Ro>2.7%);among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages。
基金The work was supported by the China Postdoctoral Science Foundation(No.2016M591350)the National Natural Science Foundation of China(No.41202103)Sinopec Scientific and Technological Project(G5800-14-ZS-KJB158).
文摘Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identified in the Wufeng Formation to Longmaxi Formation,e.g.,siliceous shale,silty shale,argillaceous shale,calcareous shale,shell-bearing argillaceous limestone/calcareous mudstone,siltstonefine sandstone and bentonite.For shale in the Wufeng and Longmaxi Formations,the depositional mode was not only the suspension deposition under the low-energy hydrodynamic condition,but also the biogenic deposition,the storm deposition and the bottom current deposition indicated by large amount of siliceous biological fossils,biological detritus with size graded structure,silt laminations,lenticular bedding,ripple cross-bedding and other sedimentary structures.Trace element analysis suggested that the siliceous shale in theWufeng Formation and the lower part of Longmaxi Formation was developed in the oxygen-dysoxic/anoxic depositional environment,which was rich in organic matters;then due to influence of the sea level gradually decline and bottom current,the silty and argillaceous shales were formed,and the oxygen-dysoxic/anoxic depositional environment was destroyed,gradually leading to the worse preservation condition of organic matter.
文摘In this paper,the pore structure characteristics of shale samples from the Lower Silurian Longmaxi Formation in South of Sichuan Basin of China were investigated by total organic carbon(TOC)content determination,X-ray diffraction(XRD),scanning electron microscope(SEM),low pressure nitrogen adsorption(LPNA)and high pressure mercury injection(HPMI).The fractal dimension of shale samples was calculated based on Frenkel-Halsey-Hill(FHH)model and thermodynamic relation model.The results showed that the major mineral compositions of shales were quartz and clay content.Organic pores,intergranular pores,intragranular pores,microfractures were widely developed in the shale samples,of which organic pores were the most developed.The pore morphology was mainly ink bottle-shaped pores and slit-shaped pores;the pore size distribution of shale samples was complex with multiple distribution peaks,the pore size between 3 and 40 nm occupied the most of storage space.The fractal dimension Dn1 of pores between 2 nm and 10 nm was 2.7177e2.7933,while fractal dimension Dn2 of pores between 10 nm and 50 nm was 2.2439e2.5468.The fractal dimension Dr of macropores calculated by the thermodynamic model was 2.6401e2.7025.
基金by the National Science and Technology Major Project(Grant No.2019A-3307)the National Natural Science Foundation of China(Grant No.41872167)。
文摘The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs.In this study,the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation,Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc(CCNBD).Based on the experimental data,the relationships among the mode-I fracture toughness,the density,the acoustic time and the clay mineral content were analyzed.The shale samples for fracture toughness test from cores were commonly limited,the investigation of fracture toughness using well logs was necessary.Therefore,a prediction model was proposed by correlating the fracture toughness with well logs responses.The results indicate that the fracture toughness of shale samples is from 0.4744 MPa$m1/2 to 1.0607 MPa$m1/2 with an average of 0.7817 MPa$m1/2,indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale.The clay mineral content and the density have a positive effect on the fracture toughness,whereas the acoustic time plays a negative role on the fracture toughness.The clay mineral content has an important effect on the relationships among fracture toughness,acoustic time and density.The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation,and it has certain guiding significance in the exploration and development of oil and gas reservoirs.
基金This work was supported by National Natural Science Foundation of China(No.41690134)National Science and Technology Major Project of China(No.2017ZX05035002-006).
文摘For shale of Lower Silurian Longmaxi Formation in Chongqing,southeast Sichuan Basin,characteristics of micro-nano pores in marine shale reservoirs were well studies by means of Field-Emission Scanning Electron Microscope and Low-temperature Low-pressure Adsorption Experiment of CO_(2)and N_(2).Results showed that six types of pore were developed in the shale of Longmaxi Formation,i.e.,organic pores,intergranular pores,intragranular pores,intercrystalline pores,dissolution pores and microfractures,among which the organic pores and intragranular pores in interlayers of clay minerals were most developed,and a plenty of dissolution pores were also well developed because of high thermal evolution degree.BET specific surface area of the shale in Longmaxi Formation ranged from 3.5 to 18.1 m^(2)/g,BJH total pore volume was from 0.00234 to 0.01338 cm^(3)/g,DA specific surface area of micropores vaired from 1.3 to 7.3 m^(2)/g,and DA pore volume ranged from 0.00052 to 0.00273 cm^(3)/g.The specific surface area of micropores in the shale accounted for 23.1%-80.2%of total specific surface area with an average of 50.3%,and the pore volume of micropores accounted for 12.1%-48.5%of total pore volume with an average of 32.3%.Micropore was the main storage space in shale reservoir for methane adsorption,that because capacities of specific surface area provided by micropores were considerably greater than those provided by mesopores and macropores.Pore size distribution of the shale was complex,and multiple different peaks occurred in the pore size curves,showing two or three peaks in the range from 0 to 100 nm and four peaks occasionally.TOC had a good linear relationship with pore structure parameters of micropores,mesopores t macropores and total pores in the shale,indicating that TOC was the most important control factor for micron-to nano-pore structure in the shale.After normalization of pore structure parameters to TOC,the pore structure parameters of total pores and mesopores t macropores,had positive linear relationships with content of clay minerals but negative linear relationships with content of brittle minerals,indicating that clay minerals and brittle minerals mainly controlled development of mesopores and macropores in the shale.
