6653 r2010 American Chemical Society pubs.acs.org/EF Energy Fuels 2010, 24, 6653–6661 : DOI:10.1021/ef100165w Published on Web 11/08/2010 Experimental Study on Fluid Transport Processes in the Cleat and Matrix Systems of Coal Fengshuang Han, †,‡,§ Andreas Busch, ) Bernhard M. Krooss, § Zhenyu Liu, ^ Niels van Wageningen, ) and Jianli Yang* ,† † State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P.R. China, ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China, § Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, D-52056 Aachen, Germany, ) Shell International Exploration and Production B.V., 2288 GS Rijswijk-ZH, The Netherlands, and ^ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China Received February 10, 2010. Revised Manuscript Received September 29, 2010 The fluid transport phenomena were studied on a cleat-plug (with cleats) and a matrix-plug (cleat-free) of a Chinese coal under controlled confining stresses (10-40 MPa). The single-phase fluid flow tests with argon (Ar) and water were performed under the steady-state. The gas breakthrough tests (two- phase fluid flow tests) with helium (He), argon (Ar), methane (CH 4 ), and carbon dioxide (CO 2 ) were conducted on the water-wetted plugs under the nonsteady state by monitoring the gas pressure changes with time in two closed compartments separated by the plug. It was found that the permeability of both the matrix- and cleat-plugs for Ar and water are measurable under the test confining stresses and the steady-state. The cleats in the cleat-plug are not easily closed by the high confining stress, evidenced by the fact that the permeability of the cleat-plug under the 40 MPa confining stress is still higher than that of the matrix-plug under the 20 MPa confining stress. The permeability coefficient of a confined coal with respect to gas varies mainly with the mean gas pressure and the effective stress. A mathematical equation combining these two factors is proposed to model the observed permeability data with respect to Ar for the cleat-plug under the confining stresses of 10-40 MPa. The associated coefficients of determination (R 2 ) with the regressions are in the range of 0.88-0.98. It is encouraging that the prediction made by this model may be extended to the other conditions. During the gas breakthrough tests, a residual pressure difference between the up- and downstream pressures is observed for the water- wetted cleat-plug, which is indicative of the capillary forces in the gas/water/coal system. However, the up- and downstream pressure profiles for the water-wetted matrix-plug vary continuously over time and no capillary breakthrough is observed. It represents a mixed gas transport process including diffusion and imbibition. The different flow patterns for gas passing through the water-wetted cleat- and matrix-plugs are due to the differences in their capillary threshold pressures, which in turn depend on the throats radii of the largest interconnected flow paths of the plugs and the types of the fluid. 1. Introduction Carbon capture and storage (CCS) is attracting people’s attention as a measure for mitigating global climate change. Several types of geological formations, including depleted oil and gas reservoirs, deep saline formations, and unminable coal seams, are considered as potential options for under- ground CO 2 storage. Among them, unminable coal seams represent a promising opportunity, because the injected CO 2 may enhance coal bed methane recovery (CO 2 -ECBM), which could partly offset the costs of CCS. Technology development and application for the CO 2 - ECBM process is still at a nascent stage. There is a lack of knowledge on the mechanism of the CO 2 -ECBM process due to the complexity of the coal seam structure and the mixed fluid transport processes. A detailed understanding of the transport mechanisms of fluids in the coal seams is important for revealing the mechanism of the CO 2 -ECBM process and evaluating the efficiencies for CO 2 trapping and CH 4 recovery. The physical structure of the coal seams is generally characterized by a dual porosity configuration: a naturally occurring network of fractures called the cleat system, and a highly heterogeneous porous structure surrounded by the cleats called the matrix system. 1,2 It is understood that during CBM production the transport process of CH 4 in the cleats can be described as laminar flow and obeys Darcy’s law while the flow of CH 4 in the matrix may be considered as diffusive flow. 1,2 The fluid flow in the cleat system is relatively fast and well understood while the fluid flow in the matrix system is slow and lacks of detailed understanding. 3,4 The characteristics of fluid transport in the matrix system are controlled by both the properties of the coal matrix (e.g., noncovalent bonds in the coal structure and porosity of the coal matrix) and the properties of the fluid (e.g., viscosity, *To whom correspondence should be addressed. Telephone/Fax: þ86-351-4048571. E-mail: jyang@sxicc.ac.cn. (1) Harpalani, S.; Chen, G. L. Geotech. Geol. Eng. 1997, 15, 303–325. (2) Shi, J. Q.; Durucan, S. Fuel 2003, 82, 1219–1229. (3) Van Wageningen, W.; Maas, J. Methane Symposium, Tuscaloosa, AL, May 21-25, 2007. (4) Van Bergen, F. Ph.D. Dissertation, Utrecht University, Utrecht, The Netherlands, 2009.