纵向无限级多薄层储层裂缝穿层扩展规律

Fracture through-layer propagation law in longitudinal infinite multiple thin layer reservoirs

煤系页岩纵向多岩性交替叠置发育,层数多、单层厚度小,呈多薄层特征,多层合采是有效提高此类储层纵向动用程度的关键技术,迫切需要一套适用于多薄层储层的裂缝穿层扩展快速预测模型。文中研究基于尖端应力强度因子计算式求解裂缝纵向穿层扩展,根据建库寻解方法避免超越方程的计算,缝长和缝高方向扩展通过连续的压力分布进行耦合求解,建立了一套考虑纵向无限级层数地层的裂缝穿层扩展模型。研究结果表明:各小层间力学性质和物性的差异,导致裂缝纵向穿层扩展出现突变现象,高度剖面轮廓变得不平滑。裂缝纵向穿层扩展主要取决于层间应力差和滤失系数差,泵注排量对裂缝高度方向的调控能力随层间应力差的增大逐渐减弱,随层间滤失系数差的增大逐渐增强。文中模型能够快速进行大量数值模拟计算,开展影响因素分析,对煤系页岩压裂现场施工参数优化具有指导意义。

Coal-bearing shale exhibits multi-lithology alternation in the longitudinal direction,with multiple layers and small single-layer thickness,showing a characteristic of multiple thin layers.Multi-layer co-production is a key technology to effectively improve the longitudinal stimulation degree of such reservoirs,and a rapid prediction model of fracture through-layer propagation applicable to multiple thin layer reservoirs is urgently needed.In this study,the longitudinal through-layer propagation of fracture was solved based on the calculation formula of tip stress intensity factor.According to the method of establishing database first and then finding solution,the calculation of transcendental equation was avoided.Then,the propagation of fracture length and height was coupled and solved by continuous pressure distribution,and a model of fracture through-layer propagation suitable for longitudinal infinite layers of strata is established.The results show that due to the differences in mechanical properties and physical properties between the small layers,an abrupt change in fracture longitudinal propagation occurs,and the contour of height profile becomes unsmooth.The longitudinal through-layer propagation of fractures primarily depends on the stress difference and filtration coefficient difference between layers.The control ability of injection rate on the height direction of fractures gradually weakens with the increase of stress difference between layers,and strengthens with the increase of filtration coefficient difference between layers.The established model can quickly carry out a large number of numerical simulation calculations and analyze the influencing factors,which is of guiding significance for optimizing the field construction parameters of coal-bearing shale fracturing.