(398d) Modeling the Sequestration and Transportation of CO2 in Deformed Coalbed during Enhanced Coalbed Methane Recovery

Gas adsorption/desorption 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 due to methane displacement with CO2, (2) model the sequestration and transportation of CO2 in deformed coal.

In this study, the adsorption-induced coal deformation model was first developed on the basis of surface energy theory, followed by its validation with all the coal strain data published in the past 60 years. Next, the adsorption-strain-permeability coupled model was developed by rigorously coupling simplified local density (SLD) adsorption theory, adsorption strain model, and cleat permeability model. The coupled model was verified with well test data in San Juan Basin followed. Last but not least, the coupled model was integrated into the Transport of Unsaturated Groundwater and Heat Simulator (TOUGH2) to simulate an ECBM case widely used.

The results show that the strain model can predict the coal deformation induced by methane or CO2 adsorption at pressure up to 80 MPa, and the average relative errors between the measured and predicted results are all within 15.51%. The relation between gas adsorption and the induced strain is not always linear, the gas adsorption induced coal deformation is more remarkable at a higher adsorption amount. The results also show that the coupled model can describe the permeability evolution effectively by adjusting only five parameters, including slit length, solid-solid interaction potential energy parameter, surface area, adsorption expansion modulus, and cleat parameter. The improved Palmer and Mansoori permeability model has the best performance in fitting well test permeability data. The original TOUGH2 simulator predicts similar results with several other CBM simulators. However, it’s impossible that all the methane can be displaced within 90 days. Considering the coal deformation and permeability change due to methane displacement with CO2, the modified TOUGH2 shows that only 24% methane is displaced in 90 days, and it takes about 1800 days to displace all the methane. Advances in the understanding of sequestration and transportation of CO2 help to improve the ECBM technology.