Deep in-situ rock mechanics considers the influence of the in-situ environment on mechanical properties,differentiating it from traditional rock mechanics.To investigate the effect of in-situ stress,pore pressure pres...Deep in-situ rock mechanics considers the influence of the in-situ environment on mechanical properties,differentiating it from traditional rock mechanics.To investigate the effect of in-situ stress,pore pressure preserved environment on the mechanical difference of sandstone,four tests are numerically modeled by COMSOL:conventional triaxial test,conventional pore pressure test,in-situ stress restoration and reconstruction test,and in-situ pore pressure-preserved test(not yet realized in the laboratory).The in-situ stress restoration parameter is introduced to characterize the recovery effect of in-situ stress on elastic modulus and heterogeneous distribution of sandstone at different depths.A random function and nonuniform pore pressure coefficient are employed to describe the non-uniform distribution of pore pressure in the in-situ environment.Numerical results are compared with existing experimental data to validate the models and calibrate the numerical parameters.By extracting mechanical parameters from numerical cores,the stress-strain curves of the four tests under different depths,in-situ stress and pore pressure are compared.The influence of non-uniform pore pressure coefficient and depth on the peak strength of sandstone is analyzed.The results show a strong linear relationship between the in-situ stress restoration parameter and depth,effectively characterizing the enhanced effect of stress restoration and reconstruction methods on the elastic modulus of conventional cores at different depths.The in-situ pore pressurepreserved test exhibits lower peak stress and peak strain compared to the other three tests,and sandstone subjected to non-uniform pore pressure is more prone to plastic damage and failure.Moreover,the influence of non-uniform pore pressure on peak strength gradually diminished with increasing depth.展开更多
In-situ stress is a critical factor influencing the permeability of coal reservoirs and the production capacity of coalbed methane(CBM)wells.Accurate prediction of in-situ stress and investigation of its influence on ...In-situ stress is a critical factor influencing the permeability of coal reservoirs and the production capacity of coalbed methane(CBM)wells.Accurate prediction of in-situ stress and investigation of its influence on coal reservoir permeability and production capacity are significant for CBM development.This study investigated the CBM development zone in the Zhengzhuang area of the Qinshui Basin.According to the low mechanical strength of coal reservoirs,this study derived a calculation model of the in-situ stress of coal reservoirs based on the multi-loop hydraulic fracturing method and analyzed the impacts of initial fractures on the calculated results.Moreover,by combining the data such as the in-situ stress,permeability,and drainage and recovery data of CBM wells,this study revealed the spatial distribution patterns of the current in-situ stress of the coal reservoirs and discussed the impacts of the insitu stress on the permeability and production capacity.The results are as follows.(1)Under given fracturing pressure,longer initial fractures are associated with higher calculated maximum horizontal principal stress values.Therefore,ignoring the effects of the initial fractures will cause the calculated values of the in-situ stress to be less than the actual values.(2)As the burial depth increases,the fracturing pressure,closure pressure,and the maximum and minimum horizontal principal stress of the coal reservoirs in the Zhengzhuang area constantly increase.The average gradients of the maximum and minimum horizontal principal stress are 3.17 MPa/100 m and 2.05 MPa/100 m,respectively.(3)Coal reservoir permeability is significantly controlled by the magnitude and state of the current in-situ stress.The coal reservoir permeability decreases exponentially with an increase in the effective principal stress.Moreover,a low lateral pressure coefficient(less than 1)is associated with minor horizontal compressive effects and high coal reservoir permeability.(4)Under similar conditions,such as resource endowments,CBM well capacity is higher in primary structural coal regions with moderate paleotectonic stress modification,low current in-situ stress,and lateral pressure coefficient of less than 1.展开更多
High geostress will become a normality in the deep because in-situ stress rises linearly with depth.The geological structure grows immensely intricate as depth increases.Faults,small fractures,and joint fissures are w...High geostress will become a normality in the deep because in-situ stress rises linearly with depth.The geological structure grows immensely intricate as depth increases.Faults,small fractures,and joint fissures are widely developed.The objective of this paper is to identify geostress anomalies at a variety of locations near faults and to demonstrate their accumulation mechanism.Hydrofracturing tests were conducted in seven deep boreholes.We conducted a test at a drilling depth of over one thousand meters to reveal and quantify the influence of faults on in-situ stresses at the hanging wall,footwall,between faults,end of faults,junction of faults,and far-field of faults.The effect of fault sites and characteristics on the direction and magnitude of stresses has been investigated and compared to test boreholes.The accumulation heterogeneity of stresses near faults was illustrated by a three-dimensional numerical simulation,which is utilized to explain the effect of faults on the accumulation and differentiation of in-situ stress.Due to regional tectonics and faulting,the magnitude,direction,and stress regime are all extremely different.The concentration degree of geostress and direction change will vary with the location of faults near faults,but the magnitude and direction of in-situ stress conform to regional tectonic stress at a distance from the faults.The focal mechanism solution has been verified using historical seismic ground motion vectors.The results demonstrate that the degree of stress differentiation varies according to the fault attribute and its position.Changes in stress differentiation and its ratio from strong to weak occur between faults,intersection,footwall,end of faults,and hanging wall;along with the sequence of orientation is the footwall,between faults,the end of faults,intersection,and hanging wall.This work sheds new light on the fault-induced stress accumulation and orientation shift mechanisms across the entire cycle.展开更多
With the rapid increase of energy demand and the increasingly highlighted environmental problems, clean, safe and widely distributed geothermal resources have become a hot spot for renewable resources development. The...With the rapid increase of energy demand and the increasingly highlighted environmental problems, clean, safe and widely distributed geothermal resources have become a hot spot for renewable resources development. The state of in-situ stress is a major control parameter for multiple links including well location, fracture inspiration and reservoir assessment, so how to determine the accurate state of in-situ stress in the deep thermal reservoir becomes a core problem drawing widely attention and urgent to be solved. Based on features of hot dry rock reservoir in terms of temperature and pressure and the comparison analysis, this article proposes the method of Anelastic Strain Recovery(ASR) as an effective method for determining the state of in-situ stress in the area with HDR resources distributed and explains the availability of ASR method by application examples.展开更多
The tight sandstone in the Tarim Basin has the characteristics of large burial depth and development of nature fractures due to concentrated in-situ stress. Identifying the present-day in-situ stress orientation is im...The tight sandstone in the Tarim Basin has the characteristics of large burial depth and development of nature fractures due to concentrated in-situ stress. Identifying the present-day in-situ stress orientation is important in hydrocarbon exploration and development, but also a key scientific question in understanding naturally fractured reservoirs. This paper presents a case study where we integrate various methods using wireline and image-log data, to identify present-day in-situ stress direction of ultra-deep fractured tight sandstone reservoirs, in the Kuqa depression. We discuss the formation mechanism of the elliptical borehole, compares the advantages and applicable conditions of the double caliper method,resistivity image logs and array sonic logs method. The well borehole diameter is measured orthogonally,then the ellipse is fitted, and the in-situ stress orientation is identified by the azimuth of the short-axis borehole, but it fails in the borehole expansion section, the fracture development section and the borehole collapse section. The micro-resistivity image logs method reveals the borehole breakouts azimuth, and also the strike of induced fractures, which are used to determine the orientation of in-situ stress. However, under water-based mud conditions, it’s hard to distinguish natural fractures from induced fractures by image logs. Under oil-based mud conditions, the induced fractures are difficult to identify due to the compromised image quality. As for the sonic log, shear waves will split when passing through an anisotropic formation, shear waves will split during propagation, and the azimuth of fast shear waves is consistent with the orientation of in-situ stress. However, it is usually affected by the anisotropy caused by the excessively fast rotation of the well log tools, so that the azimuth of fast shear wave cannot effectively reflect the orientation of the in-situ stress. Based on comprehensive assessment and comparison, in this paper we propose a method integrating various logging data to identify the orientation of in-situ stress. Among various types of logging data, the breakouts azimuth identified by image logs is proved to be the most credible in identifying the orientation of in-situ stress, while using the direction of induced fractures under water-based mud conditions is also viable. However, the azimuth of the fast shear wave is consistent with the orientation of maximum in-situ stress only when the rotation speed of the logging tool is low. The caliper method can be used as a reference for verifying the other two methods. Using this integrated method to study the orientation of in-situ stress in the Keshen8 trap, the results show that faults are an important factor affecting the direction of in-situ stress, while multi-level faults will produce superimposed effects that cause the current direction of in-situ stress to change.展开更多
The in-situ stress field is a key factor controlling the successful construction of a large number of underground structures in mountainous areas,and is intensively affected by the mountainvalley topography.The effect...The in-situ stress field is a key factor controlling the successful construction of a large number of underground structures in mountainous areas,and is intensively affected by the mountainvalley topography.The effects of mountain-valley morphology(the width of the mountain top platform,mountain height,slope angle,and width of the valley bottom)on the distribution of the in-situ stress field were analyzed and interpreted using numerical modeling techniques,where the spatial distribution and maximum values of the horizontal and vertical stresses were analyzed.The results showed that there existed a critical value of the topographic influence depth,where the in-situ stress distribution varied significantly as mountain-valley morphology,after which the influence diminishes.Tectonic action has a more remarkable influence on the in-situ stress distribution than gravitational action under the same mountain-valley morphology.Moreover,the relationships between the magnitudes of these stress components and the morphology variables are described using empirical formulas,which can be directly applied to different topographies to rapidly achieve a rational estimation.The findings of this study can be very useful for quickly understanding the in-situ stress distribution and as stress measurement guidelines.展开更多
Knowledge of the present-day in-situ stress distribution is greatly import-ant for better understanding of conventional and unconventional hydro-carbon reservoirs in many aspects,e.g,reservoir management,wellbore stab...Knowledge of the present-day in-situ stress distribution is greatly import-ant for better understanding of conventional and unconventional hydro-carbon reservoirs in many aspects,e.g,reservoir management,wellbore stability asssment,etc.In tectonically stable regions,the present-day in-situ stress field in terms of stress distribution is 1argely controlled by lithological changes,which can be predicted through|a numerical simulation method incorporating specific mechanical properties of the subsurface reservoir.In this study,a workflow was presented to predict the present-day in-situ stress field based on the finite element method(FEM).Sequentially,it consists of:i)building a three-dimensional(3D)geometric framework,i)creating a 3D petrophysical parameter field,11)integrating the geometric framework with petrophysical parameters,iv)setting up a 3D heterogeneous geomechanical model,and finally,v)calculating the present-day in-situ stress distribution and calibrating the prediction with measured stress data,e.g.,results from the extended leak-off tests(XLOTs).The approach was sucessfully applied to the Block W in Ordos Basin of central China.The results indicated that the workflow and models presented in this study could be used as an effective tool to provide insights into stress perturbations in subsurface reservoirs and geological references for subsequent analysis.展开更多
Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at v...Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at varying orientations were applied. The results indicate that stress concentrations, roadway deformation and failure increase in magnitude and extent as the excavation angle with respect to the maximum horizontal stress increases. In addition, the stress adjacent to the coal-rock interface sharply varies in space and evolves with time; coal is much more vulnerable to deformation and failure than rock.The results provide insights into the layout of roadways excavated through a coal seam. Roadways should be designed parallel or at a narrow angle to the maximum horizontal stress. The concentrated stress at the top corner of the face-end should be reduced in advance, and the coal seam should be reinforced immediately after excavation.展开更多
In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is ...In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is based on the distribution of acoustic velocity in different directions around rock cores. The heterogeneity of core samples, such as fractures and gravel contained, can also lead to wave velocity anisotropy. Therefore, the corresponding reliability evaluation method is established to exclude some other anisotropy factors caused by non-tectonic stresses. In this paper, the reliability of testing results is evaluated from three aspects, i.e. phase difference, anisotropy index and waveform, to remove the factors caused by non-tectonic stresses.展开更多
The differences of rock mechanical properties were analyzed based on triaxial compression test in low permeability reservoirs of the Bonan Oilfield. Through the analysis of reservoir mechanics, the influence mechanism...The differences of rock mechanical properties were analyzed based on triaxial compression test in low permeability reservoirs of the Bonan Oilfield. Through the analysis of reservoir mechanics, the influence mechanisms of different mechanical properties of rocks on reservoir in-situ stress were studied. By means of stress ellipse and finite element simulation, the influence rules of different mechanical properties of rocks on in-situ stress field were discussed. For the low permeability reservoirs of the Bonan Oilfield, the coarser rock has a larger Young’s modulus value and a lower Poisson’s ratio. The rock mechanical parameters and stress-strain relationship of sandstone facies and mudstone facies are different. Different rocks have different mechanical properties, which cause extra stress at the lithological contact interface, and the existence of extra stress affects the reservoir in-situ stress. Without considering the influence of structural features on the in-situ stress field, the reservoir in-situ stress is controlled by the magnitude of extra stress and the angle between lithological contact surface and boundary stress.展开更多
The relative change of in-situ stress is an inevitable outcome of differential movement among the crust plates. Conversely, changes of in-situ stress can also lead to deformation and instability of crustal rock mass, ...The relative change of in-situ stress is an inevitable outcome of differential movement among the crust plates. Conversely, changes of in-situ stress can also lead to deformation and instability of crustal rock mass, trigger activity of faults, and induce earthquakes. Hence, monitoring real-time change of in-situ stress is of great significance. Piezomagnetic in-situ stress monitoring has good and longtime applications in large engineering constructions and geoscience study fields in China. In this paper, the new piezomagnetic in-situ stress monitoring system is introduced and it not only has overall improvements in measuring cell's structure and property, stressing and orienting way, but also enhances integration and intelligence of control and data transmission system, in general, which greatly promotes installing efficiency of measuring probe and quality of monitoring data. This paper also discusses the responses of new piezomagnetic system in large earthquake events of in-situ stress monitoring station at Qiaoqi of Baoxing and Wenxian of Gansu. The monitoring data reflect adjustments and changes of tectonic stress field at the southwestern segment of and the northern area near the Longmenshan fault zone, which shows that the new system has a good performance and application prospect in the geoscience field. Data of the Qiaoqi stress-monitoring station manifest that the Lushan Earthquake did not release stress of the southwestern segment of the Longmenshan fault zone adequately and there still probably exists seismic risk in this region in the future. Combined with absolute in-situ stress measurement, carrying out long-term in-situ stress monitoring in typical tectonic position of important regions is of great importance for researchers to assess and study regional crust stability.展开更多
A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage potential w...A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage potential windblast and periodic caving hazards associated with these conglomerate strata,the in-situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. Overcore stress measurements have indicated that the vertical stress is the lowest principal stress so that hydraulic fractures placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in-situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden strata overhanging the goaf indicated by subsidence monitoring.展开更多
Measuring in-situ stress by using the Kaiser effect in rocks has such advantages as time- effi ciency, low cost and little limitation, but the precision of the method is dependent on rock properties and delay time of ...Measuring in-situ stress by using the Kaiser effect in rocks has such advantages as time- effi ciency, low cost and little limitation, but the precision of the method is dependent on rock properties and delay time of the measurement. In this paper, experiments on the Kaiser effect in limestones were performed, and it was found that the limestones had good ability to retain a memory of their recent stress history and high time-sensitivity. The longer the experiment was delayed from the extraction of the stone, the larger the Felicity ratio was. As the Felicity ratio approached 1, signifi cant Kaiser effect was observed. In-situ stress should be determined by the limestone measurements when the delay time was 40-120 days. Finally, the in-situ stress in a limestone formation could be successfully measured in practice.展开更多
To solve the technical cruxes of the conventional system in deep rock mass, an automatic testing system for hydraulic frac- turing that includes a single tube for hydraulic loop, a pressure-relief valve, central-tubel...To solve the technical cruxes of the conventional system in deep rock mass, an automatic testing system for hydraulic frac- turing that includes a single tube for hydraulic loop, a pressure-relief valve, central-tubeless packers, and a multichannel real-time data acquisition system was used for in-situ stresses measurement at great depths (over 1000 m) in a coalfield in Juye of Northern China. The values and orientations of horizontal principal stresses were determined by the new system. The virgin stress field and its dis- tributing law were decided by the linear regression from the logged 37 points in seven boreholes. Besides, the typical boreholes ar- ranged in both the adjacent zone and far away zone of the faults were analyzed, respectively. The results show that a stress concentra- tion phenomenon and a deflection in the orientation of the maximal horizontal stress exist in the adjacent zone of the faults, which further provides theoretical basis for design and optimization of mining.展开更多
In-situ stress is an essential parameter for design and construction of most engineering projects that involve excavation in rocks.Progress in in-situ stress measurement from the 1950s in China is briefly introduced.S...In-situ stress is an essential parameter for design and construction of most engineering projects that involve excavation in rocks.Progress in in-situ stress measurement from the 1950s in China is briefly introduced.Stress relief by overcoring technique and hydraulic fracturing technique are the two main techniques for in-situ stress measurement in China at present.To make them suitable for application at great depth and to increase their measuring reliability and accuracy,a series of techniques have been developed.Applications and achievements of in-situ stress measurement in Chinese rock engineering,including mining,geotechnical and hydropower engineering,and earthquake prediction,are introduced.Suggestions for further development of in-situ stress measurement are also proposed.展开更多
A multivariable regression analysis of the in-situ stress field, which considers the non-linear deformation behavior of faults in practical projects, is presented based on a newly developed three-dimensional displacem...A multivariable regression analysis of the in-situ stress field, which considers the non-linear deformation behavior of faults in practical projects, is presented based on a newly developed three-dimensional displacement discontinuity method (DDM) program. The Bar-ton-Bandis model and the Kulhaway model are adopted as the normal and the tangential deformation model of faults, respectively, where the Mohr-Coulomb failure criterion is satisfied. In practical projects, the values of the mechanical parameters of rock and faults are restricted in a bounded range for in-situ test, and the optimal mechanical parameters are obtained from this range by a loop. Comparing with the traditional finite element method (FEM), the DDM regression results are more accurate.展开更多
Reliable information of in-situ stress state is necessary for the design and construction of most important rock projects. As most rock projects are getting deeper and deeper, traditional techniques of in-situ stress ...Reliable information of in-situ stress state is necessary for the design and construction of most important rock projects. As most rock projects are getting deeper and deeper, traditional techniques of in-situ stress measurement are not very suitable. The current techniques of in-situ stress measurement and their insufficiency for use at great depth are analyzed. Some basic ideas of the development of new techniques and the improvement of current techniques for use at great depth are provided.展开更多
In-situ stress is a key reservoir parameter to evaluate reservoir permeability,hydraulic fractures,coal seam deformation and coalbed methane(CBM)recovery.With limited CBM test wells in the Zhengzhuang field,southeast ...In-situ stress is a key reservoir parameter to evaluate reservoir permeability,hydraulic fractures,coal seam deformation and coalbed methane(CBM)recovery.With limited CBM test wells in the Zhengzhuang field,southeast of the Qinshui basin,North China,the in-situ stress data is inadequate for CBM exploration and development.It is necessary to find a method to predicate in-situ stress through other exploration data such as geophysical well loggings.In this study,we provide a new well-logging databased model to predicate the in-situ stress based on 17 sets of well test data and comprehensive logging data.As a distinguished characteristic of this model,different structural compartmentalization of CBM reservoirs was considered.A regional adaptive residual strain index was introduced to the model.Based on the model,the in-situ stress distribution in the Zhengzhuang field were evaluated systematically,and the influences of in-situ stress on permeability and the propagation of hydro-fractures were discussed.Results indicate that the magnitude of the maximum(S_(Hmax),14.19e45.40 MPa)and minimum horizontal stresses(Shmin,10.62e28.38 MPa)and the gravitational stress(Sv,9.58e30.82 MPa)all show positive correlations with burial depths.The in-situ stress fields in the study area are characterized by 1)SHmax>Sv>Shmin in shallow layers(<700 m),indicating a dominant strike-slip faulting stress regime;2)Sv z S_(Hmax)>S_(hmin)and S_(Hmax)>Sv>S_(hmin)in the depth of 700e1050 m,suggesting a transformed regime;and 3)S_(Hmax)>Sv>S_(hmin)in deep layers(>1050 m),indicating a strike-slip faulting stress regime.The S_(Hmax)in the study area is orientated by NE-SW,with a trend of 40e49.Resulted from the change of the in-situ stress regimes from shallow(500 m)to deep layers(~1000 m),the reservoir permeability variation shows a typical increase followed by decrease.The presence of natural fractures significantly affect the propagation pattern of hydraulic fractures,and the length difference between the major and branch fractures increases with increasing stress anisotropy in the Zhengzhuang field.展开更多
Four months after the Wenchuan M_s 8 earthquake in western Sichuan,China,in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake haz...Four months after the Wenchuan M_s 8 earthquake in western Sichuan,China,in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment.In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths.The maximum horizontal stress in shallow depths(～20 m) is about 4.3 MPa,oriented N19°E,in the epicenter area at Yingxiu Town,about 9.7 MPa,oriented N51°W,at Baoxing County in the southwestern Longmenshan range,and about 2.6 MPa,oriented N39°E,near Kangding in the southernmost zone of the Longmenshan range.Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth.A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range,and is still moderate in the epicenter zone.These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.展开更多
基金supported by the National Natural Science Foundation of China(Nos.51827901 and 52121003)the 111 Project(No.B14006)+1 种基金the Yueqi Outstanding Scholar Program of CUMTB(No.2017A03)the Fundamental Research Funds for the Central Universities(No.2022YJSNY13).
文摘Deep in-situ rock mechanics considers the influence of the in-situ environment on mechanical properties,differentiating it from traditional rock mechanics.To investigate the effect of in-situ stress,pore pressure preserved environment on the mechanical difference of sandstone,four tests are numerically modeled by COMSOL:conventional triaxial test,conventional pore pressure test,in-situ stress restoration and reconstruction test,and in-situ pore pressure-preserved test(not yet realized in the laboratory).The in-situ stress restoration parameter is introduced to characterize the recovery effect of in-situ stress on elastic modulus and heterogeneous distribution of sandstone at different depths.A random function and nonuniform pore pressure coefficient are employed to describe the non-uniform distribution of pore pressure in the in-situ environment.Numerical results are compared with existing experimental data to validate the models and calibrate the numerical parameters.By extracting mechanical parameters from numerical cores,the stress-strain curves of the four tests under different depths,in-situ stress and pore pressure are compared.The influence of non-uniform pore pressure coefficient and depth on the peak strength of sandstone is analyzed.The results show a strong linear relationship between the in-situ stress restoration parameter and depth,effectively characterizing the enhanced effect of stress restoration and reconstruction methods on the elastic modulus of conventional cores at different depths.The in-situ pore pressurepreserved test exhibits lower peak stress and peak strain compared to the other three tests,and sandstone subjected to non-uniform pore pressure is more prone to plastic damage and failure.Moreover,the influence of non-uniform pore pressure on peak strength gradually diminished with increasing depth.
基金sponsored by the National Natural Science Foundation of China(42002181)projecta public bidding project of 2020 Shanxi Provincial Science and Technology Program(20201101002-03).
文摘In-situ stress is a critical factor influencing the permeability of coal reservoirs and the production capacity of coalbed methane(CBM)wells.Accurate prediction of in-situ stress and investigation of its influence on coal reservoir permeability and production capacity are significant for CBM development.This study investigated the CBM development zone in the Zhengzhuang area of the Qinshui Basin.According to the low mechanical strength of coal reservoirs,this study derived a calculation model of the in-situ stress of coal reservoirs based on the multi-loop hydraulic fracturing method and analyzed the impacts of initial fractures on the calculated results.Moreover,by combining the data such as the in-situ stress,permeability,and drainage and recovery data of CBM wells,this study revealed the spatial distribution patterns of the current in-situ stress of the coal reservoirs and discussed the impacts of the insitu stress on the permeability and production capacity.The results are as follows.(1)Under given fracturing pressure,longer initial fractures are associated with higher calculated maximum horizontal principal stress values.Therefore,ignoring the effects of the initial fractures will cause the calculated values of the in-situ stress to be less than the actual values.(2)As the burial depth increases,the fracturing pressure,closure pressure,and the maximum and minimum horizontal principal stress of the coal reservoirs in the Zhengzhuang area constantly increase.The average gradients of the maximum and minimum horizontal principal stress are 3.17 MPa/100 m and 2.05 MPa/100 m,respectively.(3)Coal reservoir permeability is significantly controlled by the magnitude and state of the current in-situ stress.The coal reservoir permeability decreases exponentially with an increase in the effective principal stress.Moreover,a low lateral pressure coefficient(less than 1)is associated with minor horizontal compressive effects and high coal reservoir permeability.(4)Under similar conditions,such as resource endowments,CBM well capacity is higher in primary structural coal regions with moderate paleotectonic stress modification,low current in-situ stress,and lateral pressure coefficient of less than 1.
基金financially supported by the National Natural Science Foundation of China(Nos.51574015 and 51934001)the National Key Science and Technology Programs,China(No.2018YFC0808004)。
文摘High geostress will become a normality in the deep because in-situ stress rises linearly with depth.The geological structure grows immensely intricate as depth increases.Faults,small fractures,and joint fissures are widely developed.The objective of this paper is to identify geostress anomalies at a variety of locations near faults and to demonstrate their accumulation mechanism.Hydrofracturing tests were conducted in seven deep boreholes.We conducted a test at a drilling depth of over one thousand meters to reveal and quantify the influence of faults on in-situ stresses at the hanging wall,footwall,between faults,end of faults,junction of faults,and far-field of faults.The effect of fault sites and characteristics on the direction and magnitude of stresses has been investigated and compared to test boreholes.The accumulation heterogeneity of stresses near faults was illustrated by a three-dimensional numerical simulation,which is utilized to explain the effect of faults on the accumulation and differentiation of in-situ stress.Due to regional tectonics and faulting,the magnitude,direction,and stress regime are all extremely different.The concentration degree of geostress and direction change will vary with the location of faults near faults,but the magnitude and direction of in-situ stress conform to regional tectonic stress at a distance from the faults.The focal mechanism solution has been verified using historical seismic ground motion vectors.The results demonstrate that the degree of stress differentiation varies according to the fault attribute and its position.Changes in stress differentiation and its ratio from strong to weak occur between faults,intersection,footwall,end of faults,and hanging wall;along with the sequence of orientation is the footwall,between faults,the end of faults,intersection,and hanging wall.This work sheds new light on the fault-induced stress accumulation and orientation shift mechanisms across the entire cycle.
基金founded by Project of National Natural Science Foundation of China “Study on the Anelastic Strain Recovery Compliance in the In-situ Stress Measurement by ASR Method”, No 41404080the Project of Geological Survey “Survey on the In-situ Stress Field in Southern China”
文摘With the rapid increase of energy demand and the increasingly highlighted environmental problems, clean, safe and widely distributed geothermal resources have become a hot spot for renewable resources development. The state of in-situ stress is a major control parameter for multiple links including well location, fracture inspiration and reservoir assessment, so how to determine the accurate state of in-situ stress in the deep thermal reservoir becomes a core problem drawing widely attention and urgent to be solved. Based on features of hot dry rock reservoir in terms of temperature and pressure and the comparison analysis, this article proposes the method of Anelastic Strain Recovery(ASR) as an effective method for determining the state of in-situ stress in the area with HDR resources distributed and explains the availability of ASR method by application examples.
基金financial support for this work comes from the Science Foundation of CUPB(No.2462017YJRC023)supported by the Exploration and Development Research Institute of Petro China Tarim Oilfield Branch Company
文摘The tight sandstone in the Tarim Basin has the characteristics of large burial depth and development of nature fractures due to concentrated in-situ stress. Identifying the present-day in-situ stress orientation is important in hydrocarbon exploration and development, but also a key scientific question in understanding naturally fractured reservoirs. This paper presents a case study where we integrate various methods using wireline and image-log data, to identify present-day in-situ stress direction of ultra-deep fractured tight sandstone reservoirs, in the Kuqa depression. We discuss the formation mechanism of the elliptical borehole, compares the advantages and applicable conditions of the double caliper method,resistivity image logs and array sonic logs method. The well borehole diameter is measured orthogonally,then the ellipse is fitted, and the in-situ stress orientation is identified by the azimuth of the short-axis borehole, but it fails in the borehole expansion section, the fracture development section and the borehole collapse section. The micro-resistivity image logs method reveals the borehole breakouts azimuth, and also the strike of induced fractures, which are used to determine the orientation of in-situ stress. However, under water-based mud conditions, it’s hard to distinguish natural fractures from induced fractures by image logs. Under oil-based mud conditions, the induced fractures are difficult to identify due to the compromised image quality. As for the sonic log, shear waves will split when passing through an anisotropic formation, shear waves will split during propagation, and the azimuth of fast shear waves is consistent with the orientation of in-situ stress. However, it is usually affected by the anisotropy caused by the excessively fast rotation of the well log tools, so that the azimuth of fast shear wave cannot effectively reflect the orientation of the in-situ stress. Based on comprehensive assessment and comparison, in this paper we propose a method integrating various logging data to identify the orientation of in-situ stress. Among various types of logging data, the breakouts azimuth identified by image logs is proved to be the most credible in identifying the orientation of in-situ stress, while using the direction of induced fractures under water-based mud conditions is also viable. However, the azimuth of the fast shear wave is consistent with the orientation of maximum in-situ stress only when the rotation speed of the logging tool is low. The caliper method can be used as a reference for verifying the other two methods. Using this integrated method to study the orientation of in-situ stress in the Keshen8 trap, the results show that faults are an important factor affecting the direction of in-situ stress, while multi-level faults will produce superimposed effects that cause the current direction of in-situ stress to change.
基金supported by the Key Projects of the Yalong River Joint Fund of the National Natural Science Foundation of China(U1865203)the National Natural Science Foundation of China(51279201)Partial support from the Youth Innovation Promotion Association CAS。
文摘The in-situ stress field is a key factor controlling the successful construction of a large number of underground structures in mountainous areas,and is intensively affected by the mountainvalley topography.The effects of mountain-valley morphology(the width of the mountain top platform,mountain height,slope angle,and width of the valley bottom)on the distribution of the in-situ stress field were analyzed and interpreted using numerical modeling techniques,where the spatial distribution and maximum values of the horizontal and vertical stresses were analyzed.The results showed that there existed a critical value of the topographic influence depth,where the in-situ stress distribution varied significantly as mountain-valley morphology,after which the influence diminishes.Tectonic action has a more remarkable influence on the in-situ stress distribution than gravitational action under the same mountain-valley morphology.Moreover,the relationships between the magnitudes of these stress components and the morphology variables are described using empirical formulas,which can be directly applied to different topographies to rapidly achieve a rational estimation.The findings of this study can be very useful for quickly understanding the in-situ stress distribution and as stress measurement guidelines.
文摘Knowledge of the present-day in-situ stress distribution is greatly import-ant for better understanding of conventional and unconventional hydro-carbon reservoirs in many aspects,e.g,reservoir management,wellbore stability asssment,etc.In tectonically stable regions,the present-day in-situ stress field in terms of stress distribution is 1argely controlled by lithological changes,which can be predicted through|a numerical simulation method incorporating specific mechanical properties of the subsurface reservoir.In this study,a workflow was presented to predict the present-day in-situ stress field based on the finite element method(FEM).Sequentially,it consists of:i)building a three-dimensional(3D)geometric framework,i)creating a 3D petrophysical parameter field,11)integrating the geometric framework with petrophysical parameters,iv)setting up a 3D heterogeneous geomechanical model,and finally,v)calculating the present-day in-situ stress distribution and calibrating the prediction with measured stress data,e.g.,results from the extended leak-off tests(XLOTs).The approach was sucessfully applied to the Block W in Ordos Basin of central China.The results indicated that the workflow and models presented in this study could be used as an effective tool to provide insights into stress perturbations in subsurface reservoirs and geological references for subsequent analysis.
基金provided by the Fundamental Research Funds for the Central Universities(No.2014QNA02)the Program for Changjiang Scholars and Innovative Research Team in University(No.IRT13098)+3 种基金the National Basic Research Program of China(No.2011CB201205)the National Natural Science Foundation of China(No.51404261)the Natural Science Foundation of Jiangsu Province(No.BK20140196)China PostdoctoralScience Foundation funded project(No.2014M551057)
文摘Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at varying orientations were applied. The results indicate that stress concentrations, roadway deformation and failure increase in magnitude and extent as the excavation angle with respect to the maximum horizontal stress increases. In addition, the stress adjacent to the coal-rock interface sharply varies in space and evolves with time; coal is much more vulnerable to deformation and failure than rock.The results provide insights into the layout of roadways excavated through a coal seam. Roadways should be designed parallel or at a narrow angle to the maximum horizontal stress. The concentrated stress at the top corner of the face-end should be reduced in advance, and the coal seam should be reinforced immediately after excavation.
基金Supported by the PetroChina Company Limited (112002Kt0090001)
文摘In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is based on the distribution of acoustic velocity in different directions around rock cores. The heterogeneity of core samples, such as fractures and gravel contained, can also lead to wave velocity anisotropy. Therefore, the corresponding reliability evaluation method is established to exclude some other anisotropy factors caused by non-tectonic stresses. In this paper, the reliability of testing results is evaluated from three aspects, i.e. phase difference, anisotropy index and waveform, to remove the factors caused by non-tectonic stresses.
基金Supported by the China National Major Demonstration Project(2017ZX05072)
文摘The differences of rock mechanical properties were analyzed based on triaxial compression test in low permeability reservoirs of the Bonan Oilfield. Through the analysis of reservoir mechanics, the influence mechanisms of different mechanical properties of rocks on reservoir in-situ stress were studied. By means of stress ellipse and finite element simulation, the influence rules of different mechanical properties of rocks on in-situ stress field were discussed. For the low permeability reservoirs of the Bonan Oilfield, the coarser rock has a larger Young’s modulus value and a lower Poisson’s ratio. The rock mechanical parameters and stress-strain relationship of sandstone facies and mudstone facies are different. Different rocks have different mechanical properties, which cause extra stress at the lithological contact interface, and the existence of extra stress affects the reservoir in-situ stress. Without considering the influence of structural features on the in-situ stress field, the reservoir in-situ stress is controlled by the magnitude of extra stress and the angle between lithological contact surface and boundary stress.
基金finically supported by the Sino Probe-06-01,Special Fund Research in the Public Interest (Grant No. 201211076)National Key Basic Project (973) (Grant No. 2008CB425702)
文摘The relative change of in-situ stress is an inevitable outcome of differential movement among the crust plates. Conversely, changes of in-situ stress can also lead to deformation and instability of crustal rock mass, trigger activity of faults, and induce earthquakes. Hence, monitoring real-time change of in-situ stress is of great significance. Piezomagnetic in-situ stress monitoring has good and longtime applications in large engineering constructions and geoscience study fields in China. In this paper, the new piezomagnetic in-situ stress monitoring system is introduced and it not only has overall improvements in measuring cell's structure and property, stressing and orienting way, but also enhances integration and intelligence of control and data transmission system, in general, which greatly promotes installing efficiency of measuring probe and quality of monitoring data. This paper also discusses the responses of new piezomagnetic system in large earthquake events of in-situ stress monitoring station at Qiaoqi of Baoxing and Wenxian of Gansu. The monitoring data reflect adjustments and changes of tectonic stress field at the southwestern segment of and the northern area near the Longmenshan fault zone, which shows that the new system has a good performance and application prospect in the geoscience field. Data of the Qiaoqi stress-monitoring station manifest that the Lushan Earthquake did not release stress of the southwestern segment of the Longmenshan fault zone adequately and there still probably exists seismic risk in this region in the future. Combined with absolute in-situ stress measurement, carrying out long-term in-situ stress monitoring in typical tectonic position of important regions is of great importance for researchers to assess and study regional crust stability.
文摘A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage potential windblast and periodic caving hazards associated with these conglomerate strata,the in-situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. Overcore stress measurements have indicated that the vertical stress is the lowest principal stress so that hydraulic fractures placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in-situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden strata overhanging the goaf indicated by subsidence monitoring.
文摘Measuring in-situ stress by using the Kaiser effect in rocks has such advantages as time- effi ciency, low cost and little limitation, but the precision of the method is dependent on rock properties and delay time of the measurement. In this paper, experiments on the Kaiser effect in limestones were performed, and it was found that the limestones had good ability to retain a memory of their recent stress history and high time-sensitivity. The longer the experiment was delayed from the extraction of the stone, the larger the Felicity ratio was. As the Felicity ratio approached 1, signifi cant Kaiser effect was observed. In-situ stress should be determined by the limestone measurements when the delay time was 40-120 days. Finally, the in-situ stress in a limestone formation could be successfully measured in practice.
基金This work was financially supported by the National Natural Science Foundation of China (No. 50490271)
文摘To solve the technical cruxes of the conventional system in deep rock mass, an automatic testing system for hydraulic frac- turing that includes a single tube for hydraulic loop, a pressure-relief valve, central-tubeless packers, and a multichannel real-time data acquisition system was used for in-situ stresses measurement at great depths (over 1000 m) in a coalfield in Juye of Northern China. The values and orientations of horizontal principal stresses were determined by the new system. The virgin stress field and its dis- tributing law were decided by the linear regression from the logged 37 points in seven boreholes. Besides, the typical boreholes ar- ranged in both the adjacent zone and far away zone of the faults were analyzed, respectively. The results show that a stress concentra- tion phenomenon and a deflection in the orientation of the maximal horizontal stress exist in the adjacent zone of the faults, which further provides theoretical basis for design and optimization of mining.
文摘In-situ stress is an essential parameter for design and construction of most engineering projects that involve excavation in rocks.Progress in in-situ stress measurement from the 1950s in China is briefly introduced.Stress relief by overcoring technique and hydraulic fracturing technique are the two main techniques for in-situ stress measurement in China at present.To make them suitable for application at great depth and to increase their measuring reliability and accuracy,a series of techniques have been developed.Applications and achievements of in-situ stress measurement in Chinese rock engineering,including mining,geotechnical and hydropower engineering,and earthquake prediction,are introduced.Suggestions for further development of in-situ stress measurement are also proposed.
基金financially supported by the Western Transport Technical Project of the Ministry of Transport, China (No. 2009318000046)
文摘A multivariable regression analysis of the in-situ stress field, which considers the non-linear deformation behavior of faults in practical projects, is presented based on a newly developed three-dimensional displacement discontinuity method (DDM) program. The Bar-ton-Bandis model and the Kulhaway model are adopted as the normal and the tangential deformation model of faults, respectively, where the Mohr-Coulomb failure criterion is satisfied. In practical projects, the values of the mechanical parameters of rock and faults are restricted in a bounded range for in-situ test, and the optimal mechanical parameters are obtained from this range by a loop. Comparing with the traditional finite element method (FEM), the DDM regression results are more accurate.
文摘Reliable information of in-situ stress state is necessary for the design and construction of most important rock projects. As most rock projects are getting deeper and deeper, traditional techniques of in-situ stress measurement are not very suitable. The current techniques of in-situ stress measurement and their insufficiency for use at great depth are analyzed. Some basic ideas of the development of new techniques and the improvement of current techniques for use at great depth are provided.
基金We acknowledge financial support from the National Natural Science Foundation of China(4183042741872123)the National Major Research Programfor Science and Technology in China(2016ZX05043-001).
文摘In-situ stress is a key reservoir parameter to evaluate reservoir permeability,hydraulic fractures,coal seam deformation and coalbed methane(CBM)recovery.With limited CBM test wells in the Zhengzhuang field,southeast of the Qinshui basin,North China,the in-situ stress data is inadequate for CBM exploration and development.It is necessary to find a method to predicate in-situ stress through other exploration data such as geophysical well loggings.In this study,we provide a new well-logging databased model to predicate the in-situ stress based on 17 sets of well test data and comprehensive logging data.As a distinguished characteristic of this model,different structural compartmentalization of CBM reservoirs was considered.A regional adaptive residual strain index was introduced to the model.Based on the model,the in-situ stress distribution in the Zhengzhuang field were evaluated systematically,and the influences of in-situ stress on permeability and the propagation of hydro-fractures were discussed.Results indicate that the magnitude of the maximum(S_(Hmax),14.19e45.40 MPa)and minimum horizontal stresses(Shmin,10.62e28.38 MPa)and the gravitational stress(Sv,9.58e30.82 MPa)all show positive correlations with burial depths.The in-situ stress fields in the study area are characterized by 1)SHmax>Sv>Shmin in shallow layers(<700 m),indicating a dominant strike-slip faulting stress regime;2)Sv z S_(Hmax)>S_(hmin)and S_(Hmax)>Sv>S_(hmin)in the depth of 700e1050 m,suggesting a transformed regime;and 3)S_(Hmax)>Sv>S_(hmin)in deep layers(>1050 m),indicating a strike-slip faulting stress regime.The S_(Hmax)in the study area is orientated by NE-SW,with a trend of 40e49.Resulted from the change of the in-situ stress regimes from shallow(500 m)to deep layers(~1000 m),the reservoir permeability variation shows a typical increase followed by decrease.The presence of natural fractures significantly affect the propagation pattern of hydraulic fractures,and the length difference between the major and branch fractures increases with increasing stress anisotropy in the Zhengzhuang field.
基金the auspice of National Key Basic Project(973)(granted No.2008CB425702)National Science and Technology Project(granted No.SinoProbe-06)
文摘Four months after the Wenchuan M_s 8 earthquake in western Sichuan,China,in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment.In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths.The maximum horizontal stress in shallow depths(～20 m) is about 4.3 MPa,oriented N19°E,in the epicenter area at Yingxiu Town,about 9.7 MPa,oriented N51°W,at Baoxing County in the southwestern Longmenshan range,and about 2.6 MPa,oriented N39°E,near Kangding in the southernmost zone of the Longmenshan range.Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth.A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range,and is still moderate in the epicenter zone.These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.
