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

从页岩中提炼的天然气给美国的油气行业带来了革命性的变化，在包括英国在内的许多国家都是一种潜在的资源。生产依赖于水力压裂增产。在渗透率极低的页岩中，裂缝增产取决于天然裂缝网络(开放或填充)的存在，该网络具有可用于连接油井的大表面积。页岩气有可能彻底改变能源供应，并对英国和世界各地的经济发展、能源安全和多样性产生重大影响。这取决于通过改善目前对所涉过程的理解来提高效率。由于环境原因，页岩气开发遇到了相当大的阻力(例如，法国目前禁止开发页岩气)。与刺激过程有关的主要问题是：(I)压裂作业(场地准备等)对地表的干扰强度；(Ii)压裂水的使用、储存和大量返排液的处置；(Iii)诱发地震的可能性；(Iv)压裂液泄漏到地表含水层的可能性。尽管水力压裂已经使用了几年，但对压裂过程仍然缺乏透彻的了解。为了提高效率并满足政治、环境和公众关注的问题，必须提高我们对骨折刺激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，我们将以我们在模拟多孔介质中裂缝扩展方面的丰富经验为基础，开发一个完全耦合的流体力学模型，用于考虑现有不连续面的多孔介质的水力压裂。将考虑两种流体：一种代表压裂液，另一种代表主体流体。通过裂缝的流动将基于润滑假设来定义，而通过基质的流动将被表示为达西流动。用扩展有限元方法捕捉力学模型中的裂隙不连续性，用粘结断裂模型定义裂隙扩展准则。所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污染物在地下环境中的去向。使用这一新的方法，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.