Effect of hydraulic fracture deformation hysteresis on CO_2 huff-n-puff performance in shale gas reservoirs

Abstract

目的 在页岩气藏CO_2吞吐过程中, 水力裂缝处于循环载荷作用下时, 极易发生不可逆变形(变形滞后), 影响吞吐效果。本文旨在建立考虑水力裂缝变形滞后的页岩气藏CO_2吞吐流固耦合模型, 形成相应的高效求解方法, 并开展流固耦合数值模拟研究, 以揭示变形滞后对CO_2吞吐的影响规律。 创新点 1. 建立考虑水力裂缝变形滞后、复杂裂缝系统和特殊流动机理的页岩气藏多组分流固耦合模型, 并形成相应的三维高效数值模拟技术; 2. 揭示水力裂缝变形滞后对页岩气藏CO_2吞吐的影响规律。 方法 1. 建立考虑水力裂缝变形滞后、复杂裂缝系统和特殊流动机理的页岩气藏多组分流固耦合模型; 2. 基于结构化网格构造高效稳定的多组分流固耦合模型数值求解算法; 3. 通过流固耦合数值模拟, 揭示水力裂缝变形滞后对页岩气藏CO_2吞吐的影响规律。 结论 1. 水力裂缝变形滞后会阻碍 CO_2注入期间裂缝渗透率的恢复, 对CO_2吞吐有负面影响; 2. 较低的初始水力裂缝导流能力和生产压力、较晚的吞吐启动时间、较高的注入压力和较多的循环次数均会增强变形滞后的负面影响; 3. CO_2吞吐效果与初始水力裂缝导流能力、吞吐启动时间、注入压力和循环次数呈正相关, 与生产压力呈负相关。 As a promising enhanced gas recovery technique, CO_2 huff-n-puff has attracted great attention recently. However, hydraulic fracture deformation hysteresis is rarely considered, and its effect on CO_2 huff-n-puff performance is not well understood. In this study, we present a fully coupled multi-component flow and geomechanics model for simulating CO_2 huff-n-puff in shale gas reservoirs considering hydraulic fracture deformation hysteresis. Specifically, a shale gas reservoir after hydraulic fracturing is modeled using an efficient hybrid model incorporating an embedded discrete fracture model (EDFM), multiple porosity model, and single porosity model. In flow equations, Fick’s law, extended Langmuir isotherms, and the Peng-Robinson equation of state are used to describe the molecular diffusion, multi-component adsorption, and gas properties, respectively. In relation to geomechanics, a path-dependent constitutive law is applied for the hydraulic fracture deformation hysteresis. The finite volume method (FVM) and the stabilized extended finite element method (XFEM) are applied to discretize the flow and geomechanics equations, respectively. We then solve the coupled model using the fixed-stress split iterative method. Finally, we verify the presented method using several numerical examples, and apply it to investigate the effect of hydraulic fracture deformation hysteresis on CO_2 huff-n-puff performance in a 3D shale gas reservoir. Numerical results show that hydraulic fracture deformation hysteresis has some negative effects on CO_2 huff-n-puff performance. The effects are sensitive to the initial conductivity of hydraulic fracture, production pressure, starting time of huff-n-puff, injection pressure, and huff-n-puff cycle number.