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Sorbed gas capacities in shale gas reservoirs – Influence of pore structure

Daniel Ross

Department of Earth and Ocean Sciences, University of British Columbia

Vancouver, Canada, V6T 1Z4

E mail: [email protected]

As suggested by previous coal research, pore structure (primarily the microporosity) influences the ability of a coal to adsorb gas. With greater amounts of micropores (with larger surface areas), more adsorption sites are available for gas to sorb onto. It is apparent from shale research that the understanding of gas adsorption onto organic-rich material is heavily biased towards coals and there have been few systematic studies investigating pore structure of marine-type organics.

It is evident that the relationship between pore structure and marine organic matter in shales is influenced by thermal maturation. Immature shales (enriched in matrix bituminite) show moderate correlation between organic content and gas sorbed but is unrelated to the micropore structure. The increased sorption of methane with organic content, independent of micropore volumes, implies gas is stored through solution in the bituminite. Thermally mature shales have greater micropore volumes which allow more gas to be adsorbed per wt% organic carbon. Structural transformation of the bituminite during maturation-induced diagenesis appears to play a key role in the creation of these adsorption sites.

Inorganics comprise the majority of the bulk composition yet little is understood about their adsorptive properties. Quartz and calcite have low internal surface area and are unlikely to contribute to the adsorption capacity. Clays have notable surface areas and may adsorb methane to their internal structure on a dry basis. The clay-gas adsorption process is inhibited under moisture equilibrated conditions due to the hydrophilic nature of clays allowing water to occupy potential adsorption sites.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid