Injection of Flue Gas Improves CO2 Permeability and Storage Capacity in Coal: A Promising Technology

Swelling of coal thus reducing permeability is the main detrimental for any carbon dioxide (CO2) capture and storage (CCS) projects. Additionally, CO2 capture from flue gas or direct air is an expensive process. The current commercial simulators are impaired of combining various effects such as fluid segregation, adsorption, Darcy's flow, and permeability change in coal. The objective of this study is to develop a numerical model to simulate flue gas injection in coal. The study is motivated by encouraging preliminary results from lab-scale experiments of injection of flue gas (ideally a mixture of Nitrogen and CO2) in coal. Bench-scale experiments demonstrated the swelling reduction caused by the selective flow of surrogate flue gas N2-CO2 mixture, based on the fluid stratification at sub-critical conditions where the density of pure components in a vertical container causes stratification as predicted from the Grashof number and the thermodynamic properties of fluids. The numerical simulation aims to reproduce and upscale the results from bench-scale Darcy experiments performed in subbituminous coal flowing pure species and mixture of CO2 and N2 at in-situ conditions. A thorough review of the material balance equations coupled with geomechanical stresses and adsorption mechanisms is observed and implemented in a Newton-Raphson model. The stratified fluid in the vertical column is injected in batches onto the coal sample at reservoir conditions producing a cyclic flow of CO2-rich mixture, followed by a N2-rich phase. The repetitive cycles of batch pumping of the stratified CO2-N2 mixture allow the periodic adsorption and desorption interactions, maintaining a high permeability compared to the reduced flow of pure CO2 and the CO2 adsorption in the coal matrix regulated by its partial pressure. Pure CO2 flow in coal resulted in a permeability reduction from 3 to 0.1 mD. The novel optimized CO2-N2 mixture flow ensures an average permeability of 2 mD, while preserving 70% of the maximum CO2 storage capacity. Carbon dioxide storage (CCS) in a suitable geologic setting such as unmineable coal seams are getting research attention for fighting global warming. The model provides important guidelines for the optimization of CO2 capture storage (CCS) in coalbed based on novel strategy of flowing a surrogate flue gas N2-CO2, minimizing the coal swelling due to the adsorption mechanisms, and consequently maintaining a high permeability, while ensuring adsorption and consequently permanent storage of CO2. The proposed methodology offers not only to improve permeability of coal, but also considers the possibility of injecting flue gas mixtures from combustion processes, reducing considerably the cost of surface facilities for CO2 treatment prior to injection. The successful implementation of this technology could potentially solve the problem of global warming at a low-cost process of injection and storage of CO2.