Gas Sorption and Transport in Poroelastic Coals

Field: Environmental Science (Physical Sciences)

Gas Sorption and Transport in Poroelastic Coals Jun Yi; Jun Yi Chongqing University of Science and Technology Search for other works by this author on: This Site Google Scholar I. Yucel Akkutlu; I. Yucel Akkutlu U. of Alberta Search for other works by this author on: This Site Google Scholar Clayton Vernon Deutsch Clayton Vernon Deutsch U. of Alberta Search for other works by this author on: This Site Google Scholar Paper presented at the Rocky Mountain Oil & Gas Technology Symposium, Denver, Colorado, U.S.A., April 2007. Paper Number: SPE-103180-MS https://doi.org/10.2118/103180-MS Published: April 16 2007 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Yi, Jun, Akkutlu, I. Yucel, and Clayton Vernon Deutsch. "Gas Sorption and Transport in Poroelastic Coals." Paper presented at the Rocky Mountain Oil & Gas Technology Symposium, Denver, Colorado, U.S.A., April 2007. doi: https://doi.org/10.2118/103180-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Rocky Mountain Petroleum Technology Conference / Low Permeability Reservoirs Symposium Search Advanced Search AbstractIn this paper, natural gas sorption and transient diffusion processes are described within coals exhibiting bimodal (macro- and micro-) pore structure on space-time continuum. Single-component gas is distributed in the microporous solid at adsorbed and dissolved states and in the macropores as free gas. The coal matrix is poroelastic, namely, its solid material manifests swelling and shrinkage effects due to the sorption phenomena under effective overburden stress. Gas transport is Fickian in nature and described by molecular and surface diffusion processes simultaneously taking place in the macro and micropores, respectively. A free gas concentration dependent apparent diffusion coefficient is explicitly derived. Initial/boundary value problems are constructed considering the cases of gas uptake by and release from the coal. Consequently, influences of sorption phenomena on solid/macropore volumes and on the overall gas transport are numerically investigated using a finite difference approach. It is found that transport is primarily hindered by equilibrium sorption and takes place at a rate significantly less (typically 1–10 per cent) than that in the macropores only. Macroporosity variations are non-uniform in space and time. These mainly relate to availability of the sorbed gas in microporous solid; thus, the swelling and shrinkage effects are closely associated with the affinity of solid material to the gas component. Only in gas-coal systems with large sorption capacity the solid material is observed to shrink during the gas release and swell as the gas is sorbed by the coal. The estimated macropore volume changes could be as high as ±10 percent, although their effect on the overall gas transport is negligible.1. IntroductionAs an unconventional natural gas resource, coalbed methane receives worldwide attention. Deep coal seams that are not accessible for mining are suitable for in-situ gas production using conventional drilling, well completion and gas recovery technologies. Hence, a vast amount of natural gas is globally available.Unlike conventional resources, however, gas storage, flow and transport processes are quite complex mainly due to an intricate nature of coalbeds. Coals exhibit multi-scale heterogeneity often characterized by a distinctive matrix structure involving bimodal pore size distribution. It could be viewed as a microporous solid material penetrated by a network of interconnected macropores and uniformly distributed fractures, i.e., cleats. Solid material has large internal surface area and preferential affinity for natural gas components such as methane and carbon dioxide. At high coalbed pressures, therefore, a significant amount of natural gas resides in the microporous voids at an adsorbed state (Thimons and Kissel, 1973), and is stored in intra-molecular voids within complex macromolecular structure of the solid material at an absorbed/dissolved state (Karacan, 2003, Larsen, 2004, Romanov et al., 2006 and Medek et al., 2006).Influence of gas adsorption and dissolution on the gascoal system is twofold. Presence of the sorbed gas may have swelling/shrinkage effects on the microporous solid material, which consequently may change the specific surface areas and total pore volume of the coal matrix (Romanov et al., 2006). Secondly, when its migration is promoted, the sorbed gas may cause a considerable retardation effect on flow and transport in the matrix (Yi et al., 2006). Hence, during primary and enhanced coalbed methane productions, depending on the coal material properties and the original coalbed conditions, availability of the sorbed gas is anticipated to bring about a complex and dynamic nature to the pressure drawdown and maintenance operations.Based on the premise that injection and production operations are controlled by flow through the interconnected macropores and fractures, a series of experimental (Harpalani and Scraufnagel, 1990, Seidle et al., 1991) and theoretical (Palmer and Mansoori, 1998, Cui and Bustin, 2005, Shi and Durucan 2005, Mavor and Gunter, 2006) works appeared in literature, focusing on overall influences of the adsorbed gas and effective stress on the transient behavior of volume averaged flow permeability. These analyses ignored the presence of a fraction of dissolved gas in the coal material. Commonly, it is argued that gas pressure variations in the macropores and fractures promote de/sorption in coal, which is accompanied by shrinkage/swelling of the solid material. Keywords: coal bed methane, Upstream Oil & Gas, macroporosity, coefficient, uptake, coal seam gas, gas concentration, poroelastic coal, macropore, gas sorption Subjects: Unconventional and Complex Reservoirs, Coal seam gas This content is only available via PDF. 2007. Society of Petroleum Engineers You can access this article if you purchase or spend a download.