A fully coupled XFEM model for hydraulic fracturing in multi-phase porous media

页岩气彻底改变了美国的油气行业，在包括英国在内的许多国家都是一种潜在的资源。产量取决于水力压裂增产。在渗透率极低的页岩中，压裂增产取决于天然裂缝网络（张开的或填充的）的存在，该网络具有较大的表面积可用于连接井。页岩气有可能彻底改变能源供应，并对英国乃至全世界的经济发展、能源安全和多样性产生重大影响。这取决于通过改进当前对所涉过程的理解来提高效率。由于环境原因，页岩气的开发遇到了相当大的阻力（例如，目前在法国被禁止）。与增产过程有关的主要问题是：(i)压裂作业（场地准备等）对地面扰动的强度；（二）压裂用水、储存和处置大量返排液；诱发地震活动的可能性；（四）压裂液泄漏到地表含水层的可能性。尽管水力压裂技术已经应用了好几年，但人们仍然缺乏对水力压裂过程的全面了解。为了提高效率，满足政治、环境和公众的关注，有必要提高我们对压裂增产过程中发生的地质力学过程的理解，这是本提案的重点。在这项研究中，我们将利用我们在多孔介质中裂缝扩展建模方面的丰富经验，为具有现有间断层的多孔介质的水力压裂开发一个完全耦合的水力-力学模型。将考虑两种流体：一种代表压裂液，另一种代表宿主流体。通过裂缝的流动将根据润滑假设来定义，而通过基体的流动将表示为达西流动。采用扩展有限元法（XFEM）捕获力学模型中的断裂不连续，通过内聚断裂模型定义断裂扩展准则。开发的模型将提供一个有价值的工具，可用于(i)评估水力压裂过程和裂缝扩展，（ii）预测诱发地震活动，以及（iii）评估系统的潜在泄漏和地下环境中污染物的去向。使用这种新颖的方法，可以进一步了解所涉及的过程及其影响。

Gas produced from shale has revolutionised the oil and gas industry in the US and is a potential resource in many countries including the UK. Production is dependent on hydraulic fracture stimulation. In shales, which are extremely low permeability, fracture stimulation depends on the presence of a natural fracture network (open or filled) having a large surface area available for connection to the well. Shale gas has the potential to revolutionise the energy supply and have a significant impact on the economic development and energy security and diversity in the UK and throughout the world. This is contingent on increasing the efficiency by improving the current understanding of the processes involved. Shale gas development has met with considerable resistance (e.g. currently banned in France) for environmental reasons. The main concerns relating to the stimulation process are: (i) The intensity of surface disturbance by fracturing operations (site preparation etc); (ii) Fracturing water usage, storage and disposal of large volumes of flow back fluid; (iii) The potential for induced seismicity; (iv) The potential for leakage of fracturing fluids to surface aquifers.Although hydraulic fracturing has been used for several years, a thorough understanding of fracking processes is still lacking. To increase efficiency and meet political, environmental and public concerns it is essential to improve our understanding of the geomechanical processes which occur during fracture stimulation which is the focus of this proposal. In this study, we will build on our extensive experience in modelling of fracture propagation in porous media to develop a fully coupled hydro-mechanical model for hydro-fracturing of porous media with existing discontinues. Two fluids will be considered: one representing the fracturing fluid and the other the host fluid. Flow through fracture will be defined based on lubrication assumption, while flow through the matrix will be expressed as Darcy flow. The fracture discontinuity in the mechanical model will be captured using eXtended Finite Element Method (XFEM) while the fracture propagation criterion will be defined through cohesive fracture model. The developed model will provide a valuable tool that can be used for (i) evaluating the hydraulic fracturing process and fracture propagation, (ii) predicting induced seismicity, and (iii) assessing potential leakage from the system and fate of contaminants in subsurface environment. Using this novel approach further understanding of the processes involved and their impacts can be achieved.