砾石影响下裂缝扩展规律数值模拟

Numerical Simulation of Fracture Propagation Pattern in the Presence of Gravel

大规模水力压裂是致密砂砾岩储层经济高效开发的必备技术,砾石参数是影响水力裂缝形态以及整体改造效果的关键因素。砾石的存在使压裂改造施工难度增大,裂缝形态复杂、迂曲度高,支撑缝长较短,难以达到设计的裂缝导流能力。基于连续-非连续单元法(CDEM),建立了含砾石二维流-固全耦合裂缝扩展模型,系统研究了应力差、砾石含量和施工排量对裂缝扩展形态的影响。数值模拟结果表明:基于CDEM方法的多相复合介质水力压裂模型能够准确模拟砾石影响下裂缝扩展形态;裂缝遇到高强度砾石时易偏转绕流,产生迂曲裂缝;高应力下,水力裂缝先绕砾偏转,后转向至水平最大主应力方向扩展;砾石影响下,裂缝扩展会憋压形成“高压区”,促进产生局部微裂缝,提高储层改造体积。研究为砂砾岩储层压裂设计优化奠定了理论基础。

Large-scale hydraulic fracturing is the essential technology for cost-effective and efficient development of tight sandy conglomerate reservoirs. Gravel parameters are considered the key factors that influence the hydraulic fracture geometry. The existence of gravels can make it even harder to complete fracturing stimulation. The complex fracture geometry,high tortuosity,and shorter supporting fracture length make it difficult to achieve the designed fracture conductivity. In this research,the continuous-discontinuous element method(CDEM)was applied to establish a 2D full fluid-solid coupling fracture propagation model to explore the influence of stress difference,gravel content,and flow rate on the fracture propagation geometry. The numerical simulation results show that:the multi-phase media hydraulic fracturing model based on the CDEM method can accurately simulate the overall fracture propagation geometry under the influence of gravel;the fractures will divert and generate a tortuous fracture when meeting high strength gravel;in reservoirs with high stress,hydraulic fractures tend to pass around the gravel and then divert to the direction of the maximum horizontal principal stress;in the presence of gravel,the fracture propagation will shape a high-pressure zone and generate more micro-fractures,thus increasing the stimulated reservoir volume. This study lays a theoretical basis for optimization of the fracturing design of conglomerate reservoirs.