层理发育的页岩气储集层压裂裂缝扩展模拟

Numerical simulation of hydraulic fracture propagation in laminated shale reservoirs

以涪陵页岩气田焦石坝背斜主体区为研究对象,通过室内岩石力学实验和直剪实验明确了层理发育的岩石力学各向异性特征及其参数;结合室内实验评价结果,基于离散元方法建立了考虑力学各向异性、层理弱面和纵向应力差异的目标储集层三维压裂裂缝扩展模型,分析了不同层理弱面发育密度、层理强度以及压裂工程参数(射孔簇数、排量和压裂液黏度)条件下的水力裂缝展布规律。研究表明:综合考虑层理弱面和纵向应力差异的影响,研究区3~4 MPa的隔层应力差异将缝高控制在应力遮挡层内,缝高低于40 m;若不考虑层理弱面影响,缝高扩展预测结果明显较高。高密度层理缝的开启增加了水力裂缝复杂性,但显著限制了水力裂缝缝高的延伸。通过降低簇数、提高排量、增加前置液阶段高黏度压裂液用量及比例,可以减少层理弱面与纵向应力差异对缝高扩展的限制,促使裂缝纵向延伸。利用裂缝扩展模型对涪陵页岩气田焦页A井压裂施工段进行模拟,模拟结果与微地震裂缝监测结果相符。

The main area of the Jiaoshiba anticline of the Fuling shale gas field was taken as the research object, laboratory rock mechanical experiments and direct shear experiments were conducted to clarify the mechanical anisotropy characteristics and parameters of rock samples with rich beddings. Based on the experimental results, a 3 D fracture propagation model of the target reservoir taking mechanical anisotropy, weak bedding plane and vertical stress difference into account was established by the discrete element method to analyze distribution patterns of hydraulic fractures under different bedding densities, mechanical properties, and fracturing engineering parameters(including perforation clusters, injection rates and fracturing fluid viscosity). The research results show that considering the influence of the weak bedding plane and longitudinal stress difference, the interlayer stress difference 3–4 MPa in the study area can control the fracture height within the zone of stress barrier, and the fracture height is less than 40 m. If the influence of the weak bedding plane is not considered, the simulation result of fracture height is obviously higher. Although the opening of high-density bedding fractures increases the complexity of hydraulic fractures, it significantly limited the propagation of fracture height. By reducing the number of clusters, increasing the injection rate, and increasing the volume and proportion of high-viscosity fracturing fluid in the pad stage, the restriction on fracture height due to the bedding plane and vertical stress difference can be reduced, and the longitudinal propagation of fractures can be promoted. The fracture propagation model was used to simulate one stage of Well A in Fuling shale gas field, and the simulation results were consistent with the micro-seismic monitoring results.