基于流固耦合的水力压裂过程断层失稳规律研究

STUDY ON FAULTS INSTABILITY LAW IN HYDRAULIC FRACTURING BASED ON FLUID-SOLID COUPLING METHOD

水力压裂技术通过高压注入大量压裂液破碎致密页岩,该过程对周围环境产生的潜在影响(如水力压裂激活断层),受到社会的广泛关注。为了更加安全、合理地进行页岩气开采,优化水力压裂施工设计及减小其对周围环境的影响,有必要开展水力压裂注水过程储层地应力演变及断层稳定性研究。本文针对水力压裂缝网与天然裂缝系统沟通后储层导流能力抬升情况,基于流固耦合模型,分析了水力压裂过程中孔隙压力动态扩散、断层破裂带体积应变及断层稳定性变化规律。模拟中开采井的注入压力考虑西南地区页岩气藏水力压裂实际施工压力,计算结果表明:随着注水过程的进行,孔隙压力扩散明显,邻近断层区域有效应力减小,断层稳定性降低;断层核两侧破裂带体积应变(挤压、拉伸)呈现相反变化趋势,邻近注水井一侧的断层破裂带及断层核内孔隙压力场抬升明显(抬升约1.1倍),造成邻近断层局部失稳,但断层整体未出现失稳。

Hydraulic fracturing breaks tight shale by injecting a large amount of fracturing fluid under high pressure.The potential impact of this process on the surrounding environment,such as hydraulic fracturing-induced reactivation of faults,has received extensive attention from society.In order to develop shale gas more safely and reasonably,optimize hydraulic fracturing construction design,and reduce its impact on the surrounding environment,it is necessary to carry out research on the evolution of reservoir in-situ stress and fault stability during the water injection process of hydraulic fracturing.In this work,the properties of pore pressure diffusion,volume strain variation in fault fracturing zone,and fault stability were analyzed based on the fluid-solid coupling method,focusing on the enhancement of reservoir conductivity after the hydraulic fracture network connects with the natural fractures.The injection pressure refers to the hydraulic fracturing pressure in Southwestern shale gas reservoirs in China.Results show that with the progress of the water injection process,the pore pressure diffusive zone increases,and the effective stress near the fault zone decreases,which induces a higher risk of faults reactivation.The volumetric strain of the fracturing zones on both sides of the fault presents an opposite trend,either in compression or extension.The pore pressure field in the fault fracturing zone and fault core adjacent to the injection well increases significantly(the increment ratio is around 1.1),resulting in the local instability of the adjacent fault,but the entire fault zone is stable.