各向异性煤岩水力压裂缝网形成机理及数值仿真研究

Hydraulic fracturing is one of the most widely used techniques in the exploitation of coalbed methane (CBM) and the commercial exploitation of CBM strongly depends on the fracturing network. Due to the complex structure (e.g., anisotropy, cleats, natural fracture, et al.), the hydraulic fracturing mechanism of the coals is complicate and it needs gain a better understanding. In order to further study the mechanisms of fracturing network of the anisotropic coals containing numerous cleats of the anisotropic coals during hydraulic fracturing, the mechanical properties (i.e., damage and crack) of the anisotropic coals will be identified and quantified using macro&microscopic experimental measurements and the mechanisms of the fractures of the anisotropic coals will be identified through true three axial testing system combining with acoustic emission measurements. The hydraulic fracturing of the anisotropic coals will be proposed and implanted in the extended finite element method (EFEM) program. We will try to find out a new algorithm to solve the problem of fracturing interaction. The fracturing network of the anisotropic coals will be simulated using the proposed EFEM and the influenced of the fracturing network on the exploitation of CBM will be evaluated. This study will be helpful to understand the fracturing network of natural reservoirs and provide key technique support for the development of unconventional energies.

水力压裂是煤层气开采应用最为广泛的技术，而水力压裂缝网形成及其效果直接决定了煤层气开采成功与否。煤岩具有显著各向异性、割理发育、并存在天然裂缝，导致水力压裂致缝过程极为复杂，缝网形成机制仍然不清。本项目拟采用室内宏细观力学测试技术，研究各向异性煤岩裂纹扩展机理，确定煤岩力学参数及断裂韧度；利用真三轴水力压裂设备，开展不同应力、水压力和层理方向的模型试验，结合声发射损伤定位跟踪技术研究各向异性煤岩水力压裂的裂纹形成机理，建立各向异性煤岩水力压裂模型，确定其临界条件；利用扩展有限元网格无需适应煤岩内部复杂结构面的演化优势，研究适应各向异性煤岩的水力压裂扩展有限元数值模拟技术，探究煤岩缝网形成机制，提出各向异性煤岩水力压裂缝网效果评估方法，为煤层气开采提供科学的评估依据。本研究将为我国煤层气及其他非常规能源开采中面临的水力压裂缝网形成机理及科学评估提供可靠的试验数据，理论支撑和借鉴。

水力压裂是煤层气开采应用最为广泛的技术，而水力压裂缝网形成及其效果直接决定了煤层气开采成功与否。煤岩具有显著各向异性、割理发育、并存在天然裂缝，导致水力压裂过程极为复杂，三维缝网形成机制仍然不清。本项目采用室内宏细观力学测试技术，开展了不同层理方向的煤岩力学特性试验，确定了各向异性煤岩的基本力学参数；利用真三轴水力压裂设备，开展了不同井筒方位、泵注参数和射孔参数的压裂物模试验，结合声发射损伤定位跟踪技术研究了各向异性煤岩水力压裂裂缝扩展机理，建立了各向异性煤岩水力压裂模型；基于水力压裂实际物理过程，提出了描述裂缝存在对渗流场影响的新型弱不连续加强函数，建立了基于扩展有限元的各向异性煤岩水力压裂求解方法，研究了水力裂缝与单一界面的相作用机理。基于裂尖应力局部性原理，提出了一种裂缝网络形成快速模拟方法，在此基础上分别研究了含天然裂缝各向异性煤岩与层状各向异性煤岩在不同力学参数与泵注参数条件下的裂缝形态特征，揭示了各向异性煤岩水力压裂缝网形成机理。研究成果为我国煤层气及其他非常规能源开采中面临的水力压裂缝网形成机理及科学评估提供了可靠的试验数据、理论支撑和借鉴。