CO2-Enhanced Gas Recovery (CO2-EGR): Multi-Scale Simulation of Rarefied Gas Flows in Porous Media

北美的页岩气革命在价格、消费方面改变了能源行业，并有助于减少二氧化碳排放。在英国，非常规天然气可以取代迅速枯竭的北海储量，并有助于建立一个更强大、更有竞争力的经济。然而，尽管许多国家/地区都想复制这一成功，但由于页岩气开采历史较短（始于本世纪）和非常规油气藏生产跨度较长（通常大于20年），进展有限。由于工程经验有限，页岩气开采过程目前仍处于试错阶段。为了使非常规储层中的页岩气开采和碳封存经济安全，我们需要量化非常规储层中常见的超致密多孔介质中的气体输运。了解气体流动有助于确定页岩地层的泄油面积和寿命，从而优化生产工艺。例如，我们可以更好地确定威尔斯井之间的距离，以实现相同的生产目标，但大大减少了钻井威尔斯的数量和相应的环境影响。我们还可以确定在CO2提高气体回收（CO2-EGR）阶段应该注入多少CO2以及井应该密封多长时间。最后，需要新的气体传输结果来评估可以储存多少二氧化碳，从而设计长期的碳储存策略。超致密多孔介质气体渗透率的实验测量是非常困难的。基于页岩样品数字图像的数值模拟成为理解非平凡气体输运的关键。这是支持最近的进展，获得高分辨率图像的页岩使用聚焦离子束/扫描电子和氦离子显微镜。随着这项技术的进步，该研究项目将开发新的动力学模型和高性能的计算机代码，以研究超致密多孔介质中的CO2-EGR，其中传统的Navier-Stokes方程失败，分子动力学模拟过于昂贵。基于页岩的数字图像，我们将研究可以优化生产过程的因素，以最大限度地从页岩中回收甲烷。这项基础研究将使我们能够对页岩气生产率做出明智的预测，特别是有助于评估CO2-EGR和后续长期CO2封存的经济和环境价值。

The shale gas revolution in North America has transformed the energy sector in terms of prices, consumption, and helped to reduce CO2 emission. In the UK, unconventional gas could replace rapidly depleting North Sea reserves and help to build a stronger and more competitive economy. However, although many countries/regions want to copy this success, limited progress has been made, due to the short history of shale gas extraction (started from this century) and long production span (usually larger than 20 years) of unconventional reservoirs. The shale gas extraction process is currently trial-and-error, as limited engineering experience has been gained.To make shale gas extraction and carbon sequestration in unconventional reservoirs economical and safe, we need to quantify the gas transport in the ultra-tight porous media typically found in unconventional reservoirs. Understanding of the gas flow helps to determine the drainage area and life span of the shale formations, which leads to optimized production process. For example, we can better determine the distance between the wells to achieve the same production goal but with much reduced numbers of drilling wells and the corresponding environmental impact. We can also determine how much CO2 should be injected and how long the well should be sealed in CO2-enhanced gas recovery (CO2-EGR) stage. Finally, new gas transport results are needed to assess how much CO2 can be stored, and hence to design long-term carbon storage strategies. Experimental measurement of gas permeability is extremely difficult for ultra-tight porous media. Numerical simulation, based on digital images of shale samples, becomes key to understanding the non-trivial gas transport. This is supported by recent advances in obtaining high-resolution images of shale rocks by using Focused-Ion-Beam/Scanning Electron and Helium-Ion Microscopes. With this technology advancement, this research project will develop new kinetic models and high-performance computer codes to investigate CO2-EGR in ultra-tight porous media, where the conventional Navier-Stokes equations fail and Molecular Dynamics simulations are too expensive. Based on the digital image of shale rocks, we will investigate factors that could optimise the production process for maximum recovery of methane from shale. This fundamental research will enable us to make well-informed predictions of shale gas production rates, and, in particular, help to assess the economic and environmental value of CO2-EGR and subsequent long-term CO2 sequestration.