Evaluation of impact of surface diffusion on methane recovery via carbon dioxide injection in shale reservoirs

Dimitris Spanakos,Sean P Rigby

doi:10.1016/j.fuel.2021.121928

Abstract

Injection of carbon dioxide into shale reservoirs is a promising technology for enhancing natural gas recovery and reducing greenhouse gas emissions. Nanoscale phenomena contribute to a significant difference in mass transfer processes within shale-gas reservoirs compared to conventional gas reservoirs. Previous investigations have shown the significance of surface diffusion to gas transfer mechanisms. Surface diffusion was added to an established apparent permeability model, which was then applied for the first time to numerical reservoir simulations to model CO2 injection techniques. Most publications to date have used a theoretical model to predict surface diffusion coefficient in a low-pressure condition, whereas, in this paper, it has been estimated from gravimetric experiments. Shale reservoirs, with different reservoir and petrophysical properties, were generated to investigate the efficiency of transport of CO2 via surface diffusion. A recently proposed fractal model for surface diffusion was used to investigate the impact of rock surface roughness on CH4 production. The results show that surface diffusion plays a significant role in increasing CH4 recovery by up to 3.2% when the average pore radius is less than 2 nm. In particular, a high surface fractal dimension can potentially enhance CH4 production by up to 1.5% and should not be neglected when the average pore radius is less than 1 nm. In areas with high surface capacity, adsorption of CO2 and desorption of CH4 molecules may increase by up to 2.74% and 2.3%, respectively, when compared to models with no surface diffusion. In all the reservoirs examined, geostatistical reservoir simulations showed that reservoir heterogeneity is not favourable to methane recovery via CO2 injection techniques, except for the Barnett shale reservoir. To the best of our knowledge, this work is the first to implement an apparent model within a reservoir simulator to investigate the impact of surface diffusion on methane recovery via CO2 injection techniques at various shale reservoirs with different properties.

Highlights

The world today is faced with a scarcity of conventional energy sources due to population growth and technological advancement [1,2]
The characteristic half time of the adsorption process was obtained by fitting a single exponential Linear Driving Force (LDF) model to the experimental data
In the areas of Bedford, Canoga, and Burlington of the Marcellus shale reservoir, there is an increase of CH4 production by CO2 injection compared to no injection scenario due to high fracture conductivity

Summary

Introduction

The world today is faced with a scarcity of conventional energy sources due to population growth and technological advancement [1,2]. The global demand of natural gas is predicted to increase by 45% by 2040, and 30–50% of its supply is expected to come from shale gas [3]. The production potential of shale gas reservoirs, which are amenable to hydraulic fracturing and horizontal drilling techniques, has been scru tinized in order to overcome the depletion of conventional reservoirs and to supply the world with greater quantities of clean-burning energy [4]. There has been a paradigm shift in thinking towards unconventional gas that is to a large extent, changing the world energy landscape, leading to a rapid expansion of shale gas production, especially over the past decade. There has been an increase in supply from the shale resources in North America [5]. CO2 injection is used for enhanced oil recovery [7]

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