Title: Simulation of coal permeability under non-isothermal CO₂ injection

Authors: Hongyan Qu; Jishan Liu; Zhejun Pan; Yan Peng; Fujian Zhou

Addresses: State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, 18 Fuxue Road, Changping, Beijing, 102249, China; State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Haidian, Beijing, 100083, China; Unconventional Natural Gas Institute, China University of Petroleum, 18 Fuxue Road, Changping, Beijing, China ' School of Mechanical and Chemical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia ' Earth Science and Resource Engineering, Ian Wark Laboratory, CSIRO, Bayview Ave, Clayton VIC 3168, Australia ' School of Petroleum Engineering, China University of Petroleum, 18 Fuxue Road, Changping, Beijing, 102249, China; School of Mechanical and Chemical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia ' Unconventional Natural Gas Institute, China University of Petroleum, 18 Fuxue Road, Changping, Beijing, 102249, China

Abstract: CO₂ injection into coal seams is a non-isothermal process, which has significant impact on coal permeability but has not been well studied. In this paper, a non-isothermal model coupled with nonlinear gas flow and matrix deformation was developed. The effects of temperature change on each term of the effective strain during the CO₂ injection scenarios, as well as the variations of fluid properties over a range of sub- and supercritical-thermodynamic conditions were investigated. This model involves the balance of thermal energy and the law of heat transfer. Two non-isothermal cases of CO₂ injection were studied and compared with the isothermal case. The results show that CO₂ injection into coal seams reduces coal permeability for all three cases. The coal matrix expands with temperature increase due to the thermal expansion and shrinks due to the decrease in adsorption amount. However, the final permeability with low-temperature CO₂ injection remains lower than that with high-temperature gas injection since the effect of sorption-induced strain on permeability outweighs that of the thermal deformation. The increase in temperature leads to the reduction in coal swelling (with the decrease of CO₂ adsorption capacity), resulting in larger cleat aperture and higher coal permeability for the cases studied in this work. [Received: November 29, 2015; Accepted: June 22, 2016]

Keywords: coal permeability; non-isothermal; ECBM; thermal effect; CO₂ adsorption.

DOI: 10.1504/IJOGCT.2017.084317

International Journal of Oil, Gas and Coal Technology, 2017 Vol.15 No.2, pp.190 - 215

Received: 08 Dec 2015
Accepted: 22 Jun 2016
Published online: 05 Jun 2017 *

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