页岩储层中水力裂缝穿过天然裂缝的判定准则

Criteria for judging whether hydraulic fractures cross natural fractures in shale reservoirs

为了准确预测页岩储层水力压裂过程中水力裂缝是否会穿过天然裂缝及其穿出方向,基于最大周向应力理论,建立了沿任意方位延伸的水力裂缝穿过天然裂缝的判定准则,给出了水力裂缝穿过天然裂缝时穿出角度的计算方法。将该准则与实验结果进行对比,吻合度较高。现有的判定准则只能判断沿水平最大主应力方向延伸的水力裂缝能否穿过天然裂缝,新的判定准则在现有准则的基础上对其拓展,使之更加适用于页岩储层。研究结果表明:在高水平主应力差、高水平主应力比值、高逼近角和高天然裂缝壁面摩擦系数的条件下,水力裂缝更易穿过天然裂缝;水平主应力差是主控因素且有一个临界值,大于该值时,水力裂缝才可能穿过天然裂缝,反之在任何条件下水力裂缝均无法穿过天然裂缝。同时还认识到,在低逼近角、高水平主应力差情况下,水力裂缝也有可能穿过天然裂缝。

In this paper, new criterion is set up on the basis of maximum circumferential stress theory to predict accurately if hydrau- lic fractures extending in any orientation cross natural fractures during hydraulic fracturing of shale reservoirs and a method is provided to calculate breakthrough angles. Based on experimental results, the new criterion is higher in coincidence. The new criterion is developed from the old one which can be only used for the hydraulic fractures extending in the direction of maximum horizontal prin- cipal stress, and it is more suitable for shale reservoirs. It is indicated by the analysis results that hydraulic fractures tend to run through natural fractures more easily in the condition of high horizontal principal stress difference, high horizontal principal stress ra- tio, high approaching angle and high friction coefficient of natural fracture wall surface. Horizontal principal stress difference is the main controlling factor with a critical value, and hydraulic fractures can not run through natural fractures until the critical value is reached, otherwise the hydraulic fractures can never run through natural fractures in any case. It is also shown that hydraulic frac- tures may run through natural fractures in the condition of low approaching angle and high horizontal principal stress difference.