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

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.

(1) Knowledge Impact
The proposed fundamental research has the potential to make reliable prediction of shale gas production, and in particular, helps to assess the economic and environmental values of CO2-enhanced gas recovery and the subsequent long-term CO2 storage in depleted shale reservoirs. The research outcome will be published in leading high-impact journals in the field.

To ensure maximum dissemination and increase the academic benefits, the applicant, PRDA, and PhD student will attend a range of international and national conferences and workshops, including the Annual Meeting of the APS Division of Fluid Dynamics, International Conference on Porous Media, International Symposium on Rarefied Gas Dynamics, Scottish Fluid Mechanics Meeting, and UK InterPore Conference on Porous Media.

To maximise impact in academia in UK, the applicant will organise a Christmas workshop on Multi-Scale Characterisation and Simulation of Shale Gas Flows in Strathclyde (Dec 2020). We will highlight our research findings and software capability to around 50 participants from the UK universities working in closely related fields.

Software is a key component in delivering high-quality engineering science and will be one of the major vehicles for the impact of this project. Key codes will be published in Computer Physics Communications and released open-source. To engage industrial beneficiaries especially in the oil & gas sector, we will make the best use of the industrial links at Strathclyde Oil/Gas Institute (https://www.strath.ac.uk/research/oilgasinstitute/) to present our work and promote our software. Also, the collaboration with the King Fahd University of Petroleum and Minerals in Saudi Arabia, which has strong connections with the oil & gas industry worldwide, will help to disseminate our research outputs and software to a wide range of organizations/companies working in shale gas extraction and CO2 storage.

(2) People Impact
This project will contribute to the training and education of UK engineers with the necessary multidisciplinary knowledge and skills to develop future flow technologies for unconventional gas recovery and carbon storage. One PDRA and one PhD student will be trained through this project, who will be able to continue to work to sustain UK effort in this field in either industry or academia.

In order to inform the next generation of engineers, our research findings will be incorporated into the undergraduate and postgraduate courses at University of Strathclyde. For instance, the applicant has successfully provided the easy-to-run Matlab code for the 4th year students to gain basic understanding of non-equilibrium flows in their individual projects, where they can simulate gas flows in arbitrary two-dimensional porous media. The applicant will also integrate the research findings to the 'Advanced Topics in Fluid Systems Engineering' course for 5th-year MEng students which he is teaching, so the future mechanical engineers will be equipped with fundamental knowledge of non-equilibrium gas flows especially in porous media.

(3) Societal and Economic Impact
This research aims to develop efficient simulation software to make reliable prediction of shale gas production, and hence has the potential to assess the economic and environmental values of CO2-EGR, as well as the non-aqueous fracking using supercritical CO2 instead of poisonous chemicals. If the research proves that the CO2-EGR and non-aqueous fracking can greatly enhance the shale gas production, it will ease the public's concerns on environmental impact of shale gas extraction including water contamination. Meanwhile, the carbon sequestration by shale reservoirs will decrease the atmospheric green house gas level and hence reduce the impact of climate change. And this research could help to assess the CO2 storage capacity in depleted shale reservoirs.