摘要
为揭示煤矿基本顶细砂岩定向水力压裂裂缝起裂、扩展规律,在煤矿井下原位获取并加工成300 mm×300 mm×300 mm大尺寸细砂岩,在试样正中布置直径26 mm的压裂孔,采用专用切槽钻头垂直钻孔轴向预制长12 mm的三维楔形横槽,开展大型真三轴定向水力压裂试验与高能工业CT扫描,研究了原生层理方向与水平应力差大小对水力裂缝起裂压力、扩展形态、水压-时间曲线、压裂体积的影响规律,并引入定向偏转距概念(预制切槽处裂缝沿其方向定向扩展不发生偏转的距离)来表征定向压裂效果。试验结果表明:预制横向切槽可驱使其附近裂缝沿着切槽定向起裂、扩展,裂缝形态分为单一横切型和复杂“H”型;水压-时间曲线根据裂纹扩展阶段的不同,分为平缓式波动型和断崖式跌落型。水平应力差对切槽处的裂缝定向偏转距影响程度大于层理方向。高水平应力差作用下切槽尖端应力集中程度更高,穿越层理面能力更强,裂缝从切槽尖端起裂后与层理交汇后不发生偏转,切槽定向效果较好;而低水平应力差作用下裂缝扩展时遇到层理易发生转向,切槽定向效果差。高水平应力差作用下裂缝定向偏转距为低水平应力差作用下的10倍,前者切槽可定向裂缝扩展至试样边界,后者切槽仅可控制其附近裂缝扩展方向,之后逐渐偏转至与最大水平主应力方向平行。层理平行切槽时,裂缝平均起裂压力、压裂体积是垂直切槽时的1.7倍;高水平应力差作用下裂缝平均起裂压力、压裂体积是低水平应力差作用下的1.3倍。层理效应在低水平应力差作用下明显,当切槽与层理方向一致时,切槽附近层理最易被激活并沿切槽定向扩展,裂缝宽度与形态复杂多样,反之,较难被激活,裂缝形态单一;而高水平应力差作用下不同方向的层理均能被激活,裂缝扩展充分,形成形态复杂多样的缝网。
To reveal the initiation and propagation law of directional hydraulic fractures of main roof fine sandstone,the true tri-axial directional hydraulic fracturing and high-energy CT scanning tests were carried out on the 300 mm×300 mm×300 mm large-scale sandstone specimens obtained in the underground mine,a 26 mm diameter fracturing hole is drilled in the center of each sample,and a special grooved drill bit is used to prefabricate a 12 mm three-dimensional wedge-shaped transverse groove in the axial direction.The effects of bedding direction and horizontal stress differences on the hydraulic crack initiation pressure,extension pattern,hydraulic pressure-time curve and fracturing volume are studied,and the concept of directional deflection distance,over which the crack propagates in the direction of the groove without deflection,is introduced to characterize the effect of directional fracturing.The test results show that the premade transverse grooves can drive the crack to initiate and extend along the groove.The crack patterns of specimens with transverse grooves are divided into two types:“一”and“H”.The hydraulic pressure-time curve can be divided into gentle wave type and cliff drop type according to the crack propagation stage.The horizontal stress difference affects the directional deflection distance more than the bedding direction.In the case of high horizontal stress difference,the stress concentration at the tip of the groove is high,which promotes the ability of cutting through the bedding planes.After the crack initiation at the tip of the groove,the crack does not deflect when intersecting with the bedding planes,and extends to the boundary of the specimen.In the case of low horizontal stress difference,the crack is prone to deflect when it comes across the bedding planes,resulting in the poor directional propagation performance of the groove.The average directional deflection distance for the high horizontal stress difference is 10 times of that for the low horizontal stress difference.The former can extend the directional crack to the specimen boundary,and the latter can only control the local crack propagation,then the crack will gradually deflect to the direction of the largest principal stress.The average initiation pressure and fracturing volume for specimens with bedding planes parallel to the grooves is 1.7 times of that perpendicular to the grooves,and the average crack initiation pressure and fracturing volume for specimens with high horizontal stress difference is 1.3 times of that with low horizontal stress difference.The bedding effect is obvious at the low horizontal stress difference,especially when the groove is parallel to the bedding plane,and the bedding planes near the groove are most likely activated and extended along the groove direction.Otherwise,it is hard to be activated.However,the bedding planes can be activated under horizontal stress difference,which complex cracks net are formed.
作者
吴拥政
杨建威
WU Yongzheng;YANG Jianwei(Coal Mining and Design Department,Tiandi Science&Technology Co.,Ltd.,Beijing 100013,China;Coal Mining Branch,China Coal Research Institute,Beijing 100013,China;State Key Laboratory of Coal Mining and Clean Utilization,Beijing 100013,China)
出处
《煤炭学报》
EI
CAS
CSCD
北大核心
2020年第3期927-935,共9页
Journal of China Coal Society
基金
国家重点研发计划资助项目(2017YFC0603003)
国家自然科学基金资助项目(51774186)。
关键词
细砂岩
真三轴
横向切槽
定向水力压裂
裂缝扩展
定向偏转距
fine sandstone
true tri-axial
directional premade crack
directional hydraulic fracturing
fracture propagation
directional deflection distance
