页岩Ⅰ型断裂韧性的分形计算方法与应用

Fractal calculation method of model Ⅰ fracture toughness of shale rock and its application

水平井分段体积压裂是页岩气商业规模开发的重要工艺措施,如何评价页岩储层可压性是该工艺成功的关键。页岩断裂韧性是可压性评价的重要支撑参数,从I型断裂裂纹(裂缝)微观形态入手,结合断裂力学理论和分形理论,建立了页岩I型断裂韧性分形计算方法;借助晶体劈裂功法计算页岩表面能,采用密度、声波时差2种参数获取计算结果,对比分析新的分形方法计算数据、传统方法预测数据与试验测试数据,新的分形方法计算平均误差为3.63%,传统预测方法平均误差为?10.53%,验证了方法的准确性;参考实例水平井测井解释数据,计算了水平段I型断裂韧性指数的全井筒连续性剖面,结合可压性级别与较大储层改造体积概率关系,优选可压性级别为III级及II级改造。建立的页岩I型断裂韧性分形计算方法对定量评价页岩储层可压性具有重要意义。

Shale gas development in commercial scale benefits from the key technology- staged fracturing of horizontal wells, and the evaluation of shale fracability is also the key for the technology. Shale fracture toughness is an important factor for the evaluation of fracability. Based on the microstructure of Model Ⅰ fracture, combining the fracture mechanics with the fractal theory, this paper established a new calculation method of shale fracture toughness. The surface energy of shale can be calculated by means of crystal splitting, and the calculation results can be obtained easily via rock density and acoustic time. This paper presents a comparison between the fractal calculation results and the experimental test results and traditional predicting results. The comparison shows that the mean error of fractal calculation is 3.63% and the mean error of traditional predicting is -10.53%, validating the accuracy of the new method. Referring to the horizontal well logging data of horizontal wells, this method can be used to establish the wellbore model-Ⅰ fracture toughness index continuity. Considering the relationship between the fracability level and the probability of stimulated reservoir volume, the class Ⅲ and class Ⅱ fracability level is preferred. The fractal calculation method of modelⅠfracture toughness of shale is important in the perspective of quantitative evaluation of shale gas reservoir fracability.