页岩气储层层理方向对水力压裂裂纹扩展的影响

页岩气储层具有不同于常规储层的层理结构，使得其水力压裂规律也与常规水力压裂有所不同。为研究天然层理方向对水力压裂过程中裂纹扩展的影响，利用三轴水力压裂实验系统进行了页岩水力压裂实验，并基于扩展有限元法开发了水力压裂起裂判据，建立了三维页岩气储层水力压裂计算模型，研究了层理方向对页岩储层水力压裂裂纹扩展的影响。结果表明：①页岩气储层水力压裂裂纹扩展规律由原地应力状态和层理面结构及强度共同决定，层理方向是水力压裂裂纹扩展方向的主控因素，若压裂后层理面法向拉应力先达到层理面抗拉强度，裂纹沿层理方向扩展，反之，裂纹则垂直于最小地应力方向扩展；②裂纹沿层理面扩展时，层理法向与最小地应力方向夹角增加，起裂和扩展压力增大，裂纹面积减小；③裂纹整体呈椭球非平面扩展，随着压裂液的注入，裂纹面积增加，地层总滤失率增加，裂纹扩展速度减小。压裂实验与模型计算所得的压裂裂纹扩展规律相吻合，从而验证了页岩气储层水力压裂模型的有效性。

Impacts of bedding directions of shale gas reservoirs on hydraulically induced crack propagation

Shale gas reservoirs are different from conventional ones in terms of their bedding architectures, so their hydraulic fracturingrules are somewhat different. In this paper, shale hydraulic fracturing tests were carried out by using the triaxial hydraulic fracturingtest system to identify the effects of natural bedding directions on the crack propagation in the process of hydraulic fracturing. Then, thefracture initiation criterion of hydraulic fracturing was prepared using the extended finite element method, on this basis, a 3D hydraulicfracturing computation model was established for shale gas reservoirs. And finally, a series of studies were performed about the effectsof bedding directions on the crack propagation created by hydraulic fracturing in shale reservoirs. It is shown that the propagation rulesof hydraulically induced cracks in shale gas reservoirs are jointly controlled by the in-situ stress and the bedding plane architecture andstrength, with the bedding direction as the main factor controlling the crack propagation directions. If the normal tensile stress of beddingsurface reaches its tensile strength after the fracturing, cracks will propagate along the bedding direction, and otherwise vertical to theminimum in-situ stress direction. With the propagating of cracks along bedding surfaces, the included angle between the bedding normaldirection and the minimum in-situ stress direction increases, the fracture initiation and propagation pressures increase and the crack areasdecrease. Generally, cracks propagate in the form of nonplane ellipsoids. With the injection of fracturing fluids, crack areas and totalformation filtration increase and crack propagation velocity decreases. The test results agree well with the calculated crack propagationrules, which demonstrate the validity of the above-mentioned model.Shale gas; Hydraulic fracturing; Laboratory test; Bedding direction; Damage; Fracture initiation; Crack; Filtration; Extendedfinite element method