流-固-热耦合的CO2压裂数值模拟方法研究

The core of CO2-fracturing treatments in shale gas reservoirs is complex-fracture propagation under thermo-hydro-mechanical coupling conditions. Based on the principle of stress superimposition via adding the changed stresses due to temperature and pressure perturbation to fracture-induced stresses, we build a thermo-hydro-mechanical CO2-fracturing model, where the boundary element method (BEM) is used to solve the displacements and stresses related to embedded fractures, and the mixed finite volume and finite element method is used to calculate the pressure and temperature of matrix and the stresses within matrix. The embedded discrete fracture method (EDFM) is adopted to estimate the leaking fluid from artificial fractures to intact matrix, micro-fractured matrix, and stabilized natural fractures, which combined with criteria for fracture initiation and propagation, activation of natural fractures, and fracture interaction between artificial and natural fractures enables us to model complex-fracture propagation dominated by toughness, viscosity, and/or leak-off. Using true triaxial fracturing experimental system, we will perform CO2-fracturing experiments under various stress conditions, different dip angles of fracture, and different temperatures to validate the proposed crossing criterion, as well as the fracturing model. The goal of this project is to establish an effective CO2-fracturing model so as to provide a fundamental basis for high-efficiency fracturing treatments and great enhancement of productivity associated with shale gas reservoirs.

页岩气储层CO2压裂的核心问题是流动场、温度场和应力场控制下的裂缝扩展。项目利用应力叠加原理将流体压力、温度扰动引起的应力变化与人工裂缝产生的扰动应力变化进行加合，采用边界元方法计算嵌入式离散裂缝的形变与扰动应力，采用有限体积-有限元方法计算基质和裂缝中的压力和温度以及基质中应力的变化，建立流-固-热多场耦合的CO2压裂模型；利用嵌入式离散裂缝方法计算CO2从人工裂缝到完整基质、含微裂缝的基质及未激活的天然裂缝的滤失量，并结合裂缝起裂与扩展、天然裂缝激活和裂缝交叉准则，实现对天然裂缝性页岩气储层内断裂韧性、粘性和滤失共同主导的复杂裂缝扩展过程的模拟；采用真三轴压裂实验系统，开展不同应力组合、不同裂缝倾角、不同温度的CO2压裂实验，对所建立的三维裂缝交叉准则和裂缝扩展模拟方法实施验证。形成有效的CO2压裂复杂裂缝扩展模拟方法，为实现页岩气储层的高效压裂与产能提升提供理论基础。

评估页岩气储层CO2压裂效果的关键是准确预测流动场、温度场和应力场控制下的裂缝（缝网）扩展，然而目前尚缺乏液态/超临界CO2驱动下天然裂缝性储层中的三维裂缝（缝网）扩展流-固-热耦合模型。本项目采用解析和数值相结合的方法解决边界单元影响系数求解时面临的奇异和超奇异点积分难题，研发了高精度混合三维DDM方法，该混合方法比纯数值方法计算效率提高32%；利用应力叠加原理将流体压力和温度扰动应力与人工裂缝诱导应力进行加合，采用边界元法求解裂缝变形与扩展，利用有限体积-有限元法求解岩体中流体流动、热流动和基质变形，进而建立流-固-热多场耦合的CO2压裂模型；采用固定应力-固定应变两步解耦方法求解流体流动、热流动、岩石变形与裂缝扩展；利用扩展嵌入式离散裂缝方法计算CO2从非平面人工裂缝到完整基质、含微裂缝的基质及未激活的天然裂缝的非均匀滤失量；采用真三轴压裂实验系统开展CO2压裂实验，验证所建立的三维裂缝交叉准则和裂缝扩展模拟方法的准确性，形成了有效的CO2压裂三维非平面裂缝扩展数值模拟方法；探明了CO2压裂过程中由于流体的高流动性、高压缩性及热应力等有利于提高人工裂缝网络复杂度的重要机理；初步掌握了三维裂缝之间的相互力学作用行为及三维裂缝协同竞争扩展规律，为实现页岩气储层的高效压裂与产能提升提供理论基础。