非稳态渗流下砂砾岩水力裂缝扩展数值模拟

Numerical simulation on hydraulic fracture propagation in glutenite under the effect of unsteady seepage

由于砾石的存在,砂砾岩储层水力压裂裂缝扩展机理不明确,裂缝形态难以控制。将砂砾岩储层中砾石表征为基质—交界面—砾石的复合结构,结合断裂力学与损伤力学的方法,建立了应力场与非稳定渗流场共同作用下砂砾岩储层水力裂缝扩展的数学模型并进行了求解,分析了主应力差、砾石粒径、砾石含量、排量等因素对裂缝扩展形态和岩体破裂压力的影响。研究结果表明:砂砾岩储层水力裂缝形态复杂,次生羽状裂缝发育;水力裂缝遇砾发生绕砾、穿砾与止裂现象,并以绕砾扩展为主;在研究范围内,水力裂缝长度随主应力差、砾石粒径、排量的增大而增大,随砾石含量的增大而减小;水力压裂过程中岩体的破裂压力与主应力差、砾石含量、排量成正比,与砾石粒径成反比。

Due to the existence of gravels, the propagation mechanisms of hydraulic fractures in glutenite reservoirs are not defned and fracture morphology can be hardly controlled. In this paper, the gravel in glutenite reservoirs was characterized as a three-phase com-plex structure of matrix-interface-gravel. Then, the mathematical model for the propagation of hydraulic fractures in glutenite reservoirs under the joint effect of stress feld and unsteady seepage feld was established by means of fracturing mechanics and damage mechanics comprehensively, and it was solved. Finally, the effects of principal stress difference, gravel particle diameter, gravel content and fow rate on fracture propagation morphology and rock fracturing pressure were analyzed. The following research results were obtained. First, the hydraulic fractures in glutenite reservoirs are morphologically complex with developed secondary pinnate fractures. When hydraulic fractures meet gravels, gravel bypassing, gravel penetrating and crack arresting occur, and gravel bypassing propagation is dominant. Second, the length of hydraulic fractures in the study area increases with the increase of principal stress difference, gravel particle di-ameter and fow rate and with the decrease of gravel content. Third, the rock fracturing pressure in the process of hydraulic fracturing is proportional to principal stress difference, gravel content and fow rate and is in inverse proportion to gravel particle diameter.