水力裂缝在地层界面的扩展行为

采用岩石断裂力学的方法,分析了水力裂缝与地层界面相交时水力裂缝沿其高度方向可能发生的3种扩展行为:1裂缝停止扩展;2裂缝沿地层界面转向扩展;3裂缝直接穿过地层界面进入隔层内,并给出各种行为发生的判断准则。该模型中考虑了分层地应力、分层岩石力学参数、地层界面效应、储层厚度及施工参数的影响。研究表明,对于水力裂缝与地层界面相交后停止扩展的情况,裂缝存在一个临界长度。裂缝超过临界长度后,将沿地层界面转向扩展或穿过地层界面进入隔层内。得出临界裂缝长度的计算方法。对于水力裂缝与地层界面相交后穿过地层界面进入隔层内的情况,分段计算应力强度因子并将其叠加,给出了裂缝在隔层中延伸距离的计算方法及验证实例。

Extending behavior of hydraulic fracture on formation interface

There are three types of possible behavior of hydraulic fractures during extending along the height, including the stopping extension, extending along formation interface and directly penetrating into separating formation, when the hydraulic fractures meet the formation interface. The three types of extending behavior of hydraulic fractures were analyzed by using rock fracture mechanics. The judging criterion for the extending behavior was presented. The layered earth stress, layered rock mechanics parameters, formation interface effect, reservoir thickness and operating parameters were taken into consideration in the model. The research results indicated that there existed a critical fracture length for the case of stopping extension. The hydraulic fractures will extend along the formation interface or penetrate into the separating formation when the length of fracture is longer than the critical length. The method was introduced to calculate the critical fracture length. For the case of directly penetrating into the separating formation, the vertical growth dimension of fracture was estimated by the sectional calculation of stress intensity factors and their superposition. An application was given for validating the research results.