扰动荷载下页岩体积压裂缝网演化的断裂力学机制

The shale gas extraction process in multi-well or multistage fracturing will produce disturbed stress field which will inevitable cause the stress shadowing effect and then influence the in-situ shale rock fracturing. Thus, it is of important significance to study the basic fracture mechanics characteristics of shale under disturbance load and then to investigate the evolution mechanism of fracture network during the reservoir fracturing process. It can be found that there are rare research on fracture characteristics of shale and few reports have been published involving the stress sensitivity of reservoir fracture network using the rock fracture mechanics theory. Hence, this project will conduct fracture mechanics tests on shale of Lungmachi Formation under static and dynamic disturbance respectively. Using the theory of plastic revision factor and statistical method, the nonlinear fracture toughness of shale and its variation can be obtained and then the shale fracture criterion considering effect of additional stress fields can be built. Based on the above research，the volume fracturing tests under disturbed load can be carried out on the shale reservoir and the features of network distributions can be monitored by acoustic emission. Combined the fracture mechanics criterion and physical experiment model, the numerical model can be established to study the influence of stress shadowing effect on fracture network initiation and propagation and establish the expression of initiation pressure as well as evaluate the reservoir reconstruction. All these above results can provide theoretical basis for the multi-well and multistage fracturing of the exploration and development of shale gas.

页岩气开采过程中的多井或多级压裂会产生扰动应力场，其引起的应力阴影效应影响着原位页岩储层压裂改造效果，所以探明扰动荷载作用下页岩的断裂力学特征，并基于此研究其体积压裂过程中缝网演化机制具有重要意义。目前对页岩断裂特征方面的研究尚显不足，而且很少有借助岩石断裂力学理论研究压裂效果的应力敏感性。为此，本项目拟选取龙马溪组页岩开展室内静态及动态扰动下的断裂力学实验，采用断裂力学中塑性修正因子理论和统计学方法，计算页岩断裂的非线性断裂韧度值并分析其变化规律，构建考虑附加扰动应力场影响下的页岩断裂力学判据。在此基础上进行页岩储层在扰动荷载下的体积压裂实验并声发射监测其缝网展布特征，结合断裂力学判据和物理实验模型建立数值计算模型，研究应力阴影效应对压裂缝网起裂与扩展机制的影响，建立起裂压力表达式并评价储层改造效果，从而为页岩气勘探开发中的多井、多级压裂提供理论依据。

页岩储层的大规模体积压裂作为影响产能的核心科学技术，越来越多的得到重视。页岩气开采过程中的多井或多级压裂会产生扰动应力场，其引起的应力阴影效应影响着原位页岩储层压裂改造效果，所以探明扰动荷载作用下页岩的断裂力学特征，并基于此研究其体积压裂过程中缝网演化机制具有重要意义。目前对页岩断裂特征方面的研究尚显不足，而且很少有借助岩石断裂力学理论研究压裂效果的应力敏感性。.本项目拟围绕扰动荷载下页岩体积压裂缝网演化（起裂、扩展与转向）机制这一关键问题，重点开展以下工作：①扰动荷载下页岩破裂特征及断裂判据研究：依据国际岩石力学学会（ISRM）建议的方法分别测试页岩Ⅰ型、Ⅱ型断裂韧度值，分析试件破裂特征并计算Ⅰ型、Ⅱ型静态断裂韧度值。②扰动应力场影响下的压裂缝网演化及应力阴影效应研究：基于上述页岩断裂力学参量及可压裂性测试结果，结合页岩储层所处的地应力环境，设计实验室尺度下真三轴状态页岩体积压裂试验。③建立体积压裂数值计算模型及应力场敏感性分析：利用①②获取的岩体断裂力学参数并结合压裂工艺建立体积压裂地质力学模型，模拟体积压裂试验，并与声发射定位获取的缝网演化过程进行对比分析，以验证数值计算模型的科学性和有效性，达到直观压裂缝演化的目的。.研究结果表明：三向恒定荷载越大，页岩起裂压力相应增大；不管是模拟垂向井还是水平井，主破裂面都趋向于平行最大主应力面；沿井筒方向的地应力一样的情况下，侧向压力越大，试件起裂压力越大；侧向压力差越小，试件压裂缝越复杂；侧向扰动荷载作用下页岩起裂压力比恒定荷载状态下的起裂压力低10MPa左右，说明外力扰动真三轴状态下试件更易被压裂；模拟水平井的压裂过程中，降压扰动页岩试件更易被压裂。本项目研究成果为页岩气开采中的多级、多井压裂施工提供重要的理论基础和技术支撑。