Computational Fracking: 3D Numerical framework for multiphysics modelling of propagating fractures in rock

近年来，地下勘探技术的进步使非常规油气矿产开采的地下岩石深度开发成为可能。例如，水力压裂（HF）技术的最新进展使得从以前认为不经济的地下深层页岩地层中开采石油/天然气成为可能。然而，在非常规油气开采中使用HF引起了争议，因此一些国家暂停了其在非常规碳氢化合物开采中的使用。反对HF的人声称，使用HF会带来严重的环境风险，比如污染地下水资源，耗尽淡水供应并引发地震活动。为了更好地理解高频过程，申请人建议开发、实施、验证和验证一个3D计算多物理场框架。HF模型应考虑流体在多孔介质和离散裂纹中的流动，以演化出复杂的三维断裂模式，如裂纹分支和裂纹合并。它应该适用于至少两个阶段的建模。将采用扩展有限元法（XFEM）来模拟压裂网络中的流体流动。较小的裂缝将由多孔介质模型来解释，该模型将渗透率与孔隙度联系起来，而孔隙度又取决于各向异性损伤张量。将考虑跨不连续面的孔隙压力增加、流体滞后、支撑剂颗粒造成的裂缝张开以及裂缝引起的渗透率增加等现象。提出的计算框架将通过与实验和收集的现场数据进行比较来验证。将进行参数研究，以回答HF中一些最紧迫的问题，例如，不同阶段裂缝网络之间的相互作用，裂缝网络侵入相邻岩层的可能性或裂缝与现有天然断层相交的相互作用，等等。

Advances in underground exploration technology made in recent years have allowed for developments at great depth of subsurface rock for unconventional gas/oil mineral exploitation. For example, recent advances in hydraulic fracturing (HF) have allowed for commercially viable extraction of oil/gas from deep underground shale formations previously deemed uneconomical to exploit. However, the use of HF in unconventional oil/gas extraction has generated controversy, so that several countries have imposed moratorium on its use for unconventional hydrocarbon extraction. Opponents of HF claim that its use poses severe environmental risks such as contamination of groundwater resources, that it depletes freshwater supply and induces seismicity. To gain a better understanding of the HF-process, the applicant proposes to develop, implement, verify and validate a 3D computational multiphysics framework. The HF model should allow for fluid flow through porous media and discrete cracks for evolving complex three-dimensional fracture patterns such as crack branching and crack coalescence. It should be applicable to model at least two stages. The extended finite element method (XFEM) will be employed and devised to model the fluid flow through the fracturing network. Smaller fractures will be accounted for by a porous media model that links the permeability to the porosity which in turn depends on an anisotropic damage tensor. Phenomena including the increase of pore pressure across the discontinuity, the fluid lag, the fracture opening by the proppant particles and the crack induced permeability increase will be taken into account. The proposed computational framework will be validated by comparison to experiments and site data to be collected. Parameter studies will be performed in order to answer some of the most pressing issues in HF, e.g. the interaction between fracture networks at different stages, the possibility of the fracture network encroaching into adjacent layers of rock or the interaction of fractures with existing natural faults that intersect the shale seam, to name a few.