基于黏聚型裂纹本构方程的煤岩水力压裂多场耦合模型及数值算法研究

Hydraulic fracturing is a widely used technology for coalbed methane exploitation. Coals exhibit evident ductile failure characteristics, which have significant effects on crack propagation laws and crack network patterns. The main source of ductile failure of coals is the damage of fracture process zones ahead of crack tips, so the multiphysics models of hydraulic fracturing in coals should take into account the influence of fracture process zones. Combining laboratory experiments and theoretical analysis, the project will utilize the interface damage mechanics to construct the cohesive crack constitutive models for the failure laws of fracture process zones of coals, revealing the laws and mechanisms of ductile fracture of coals. Secondly, through theoretical analysis the multiphysics model of hydraulic fracturing in coals based on the cohesive crack constitutive model will be established, which is able to clarify the interaction between fracture process zones and solid skeleton deformation, seepage flow in pores/cracks, leakage, etc. Thirdly, through the improvement of phase field methods in computational fracture mechanics, a numerical algorithm will be developed and implemented to simulate crack initiation, propagation, branching and merging, clarifying the laws of the complex crack network in hydraulic fracturing in coals. Finally, laboratory experiments of hydraulic fracturing in coals will be conducted using large size coal samples subjected to tri-axial loading, in order to verify or modify the theoretical models. The project can provide the theoretical basis for hydraulic fracturing in coals, and promote the efficiency and safety of coalbed methane exploitation.

水力压裂是开采煤层气的重要技术手段。煤岩呈现明显的韧性破坏特征，对裂纹的起裂扩展规律和缝网形态有关键影响。煤岩韧性破坏的主要原因是裂纹尖端局部区域(断裂过程区)的渐进损伤，因此煤岩水力压裂的多场耦合模型需要充分考虑断裂过程区的作用。本项目拟通过实验研究和理论分析，采用界面损伤力学，针对煤岩断裂过程区的破坏规律建立黏聚型裂纹本构方程，揭示煤岩韧性断裂的规律及机理；通过理论分析，构建基于黏聚型裂纹本构关系的煤岩水力压裂多场耦合模型，阐明断裂过程区的损伤与固体骨架变形、孔裂隙渗流、压裂液滤失等物理机制的相互作用；改进计算断裂力学中的相场法，发展数值算法，编制程序，对裂纹的起裂、扩展、分叉、融合进行数值模拟，揭示煤岩水力压裂复杂缝网形成的规律；进行煤岩的大尺寸真三轴水力压裂实验，验证和修正理论模型。本项目可以为煤岩水力压裂提供理论基础，促进煤层气的高效安全绿色开采。

水力压裂是开采煤层气的重要技术手段。煤岩断裂过程区的渐进式损伤导致了煤岩的韧性破坏，对水力裂缝的起裂扩展规律和缝网形态有重要影响。本项目采用不同煤阶煤岩进行了紧凑圆盘张拉实验和贯通剪切实验，基于PPR模型对煤岩断裂过程区建立了I／II复合型断裂的黏聚型裂纹本构；将黏聚型裂纹模型代入断裂力学控制方程组，进行煤岩三点弯曲梁的数值模拟，并通过对比数值模拟和物理实验结果，验证了黏聚型本构模型；将煤岩制备成半圆盘试件，并分别采用N2和CO2施加围压，通过三点弯曲实验研究了不同围压下的断裂韧度变化规律；耦合黏聚型裂纹本构、裂隙流方程以及岩石骨架变形方程等，构建了煤岩水力压裂多场耦合方程组；通过自编程序和有限元软件二次开发实现了等几何边界元法、相场法等数值算法，对煤岩水力压裂规律进行了模拟和分析；进行了三轴压裂物理实验，研究了水平应力差、注液速率、压裂液黏度、界面力学特性等因素对裂缝形态的影响，本项目的研究成果有助于深入理解煤岩韧性破坏规律，为煤岩水力压裂提供理论支持。