Modeling CH₄ Displacement by CO₂ in Deformed Coalbeds during Enhanced Coalbed Methane Recovery

Abstract

Gas adsorption and desorption and displacement has a significant effect on coal deformation and permeability evolution during the primary recovery of coalbed methane (CBM) and enhanced coalbed methane recovery (ECBM). The objectives are to (1) quantify the coal deformation and permeability change caused by methane (CH 4 ) displacement with carbon dioxide (CO 2 ) and ( 2) model the transportation of CH 4 and CO 2 in deformed coalbed. In this study, the gas adsorption and desorption and displacement, coal deformation, and permeability evolution during CBM and ECBM recovery were described by an internally consistent adsorption-strain-permeability model, of which the simplified local density (SLD) adsorption theory, a theoretical strain model, and a matchstick-based permeability model were rigorous coupled. The coupled model was then verified with all of the CH 4 and CO 2 measured gas adsorption and desorption and coal strain data published in the past 60 years. Next, sensitivity analysis was further conducted on the coupled model to highlight and calibrate its performance. Finally, the coupled model was integrated into the Transport of Unsaturated Groundwater and Heat Simulator (TOUGH2) to simulate the ECBM process. The results show that the coupled model can simultaneously describe gas adsorption and desorption and displacement, coal deformation, and permeability evolution during ECBM recovery with only six parameters, including slit width, solid−solid interaction potential energy parameter, surface areas of CH 4 and CO 2 , adsorption expansion modulus, and initial porosity. The coupled model can predict both CH 4 and CO 2 adsorption and the induced coal deformation fairly accurately at a pressure up to 20 MPa, and the average relative errors are within 9.76% and 9.14%, respectively. The results also suggest that the adsorption capacity of CO 2 is 2−5 times as large as that of CH 4 , and the volumetric strain induced by CO 2 adsorption is 2−8 times as large as that caused by CH 4 adsorption. While the stronger adsorption capacity of CO 2 on coal offers an option for CO 2 -ECBM, matrix swelling due to CH 4 displacement with CO 2 may narrow down or even close the cleat, significantly reducing the permeability and thus impacting the injection efficiency. Last but not least, the original TOUGH2 simulator predicts similar results with several other CBM simulators. However, it is impossible that 90% of CH 4 can be displaced within 90 days. Considering the coal deformation and permeability change due to CH 4 displacement with CO 2 , the modified TOUGH2 simulator shows that only 24% of CH 4 is displaced in the first 90 days, and it takes about 1800 days to displace 90% or more. Advances in the understanding of CH 4 displacement by CO 2 and their transportation mechanisms in coal seams suggests that the success of CO 2 -ECBM depends on the optimal management of matrix swelling.

Molecular simulation of CO2/CH4/H2O competitive adsorption and diffusion in brown coal

“…19 In addition to experimental work, much simulation work has also recently been carried out. Zeng et al 20 used a coupled adsorption-strain model and Transport of Unsaturated Groundwater and Heat Simulator (TOUGH2) to simulate the enhanced coalbed methane recovery process. Their results suggested that it takes about 1800 days to displace 90% of methane considering the coal deformation and permeability change.…”

Swelling Characteristics and Interaction Mechanism of High-Rank Coal during CO2 Injection: A Molecular Simulation Study

“…The pore volume obtained by the NLDFT model is slightly lower than the analysis result of the simulation, which might be caused by the effect of ash in the coal matrix. 54 This indicates that the molecular simulation method is accurate for analyzing the pore size distribution of micropores in coal.…”

A dynamic evolution model of coal permeability during enhanced coalbed methane recovery by N2 injection: experimental observations and numerical simulation

“…Result indicated that super-critical CO 2 on CH 4 recovery was independent of coal rank or maturity, whereas high CO 2 pressure may lead to a negative effect on CH 4 production. Zeng et al 22 proposed an internally-consistent adsorption-strain-permeability model, including gas adsorption/desorption, coal deformation, and permeability evolution, and described the ECBM process of CH 4 displacement by CO 2 . Moreover, Yang et al 23 conducted molecular simulation to reveal the competitive adsorption mechanism of CH 4 , CO 2 , and N 2 in coal.…”