The Effects of TOC and Clay Mineralogy on Gas Storage Potential in Shale Gas Reservoirs

In shale gas reservoirs, gas adsorption is an important mechanism of gas storage. In a recent study, methane adsorption behaviour for samples obtained from two different regional shale gas reservoirs (Shale Gas Reservoir 1 and Shale Gas Reservoir 2) has been modelled using the Langmuir Isotherm which assumes that gas adsorption occurs as a monomolecular layer adsorbed onto microporous surfaces. The influence of shale properties on the Langmuir Volume (VL) and Langmuir Pressure (PL) has been investigated in order to determine which properties have the greatest influence on the nature and variability of the Langmuir Isotherms. This investigation has shown that total organic carbon (TOC) and clay mineralogy may have significant controls on adsorption behaviour.

A positive correlation exists between TOC and VL. An increase in micropore volume associated with increases in TOC, as reported in previous studies, is used to explain this relationship. Shale Gas Reservoir 2 samples show a consistent decrease in PL with increasing TOC. This implies that more methane can be adsorbed at lower pressures and hence, typically, shallower depths.

A local positive correlation between total clay mineral abundance and VL implies that clay minerals can contribute considerably to methane adsorption capacity, likely due to their structure providing gross regions of microporosity, and hence can potentially contribute significantly to the total reservoir gas-in-place. In contrast, local negative correlations suggest that clay minerals can also locally inhibit methane adsorption, which may be explained by clay bound water occupying potential adsorption sites. The majority of Shale Gas Reservoir 1 samples show that decreasing PL is correlated with increases in total clay content. This implies that clay minerals allow more methane to be adsorbed at lower pressures. Some samples indicate that clay mineral abundance has greater control on isotherm behaviour than TOC. Correlation of clay mineral type with VL for both Shale Gas Reservoir groups indicates that illite clay mixtures may have a significant control on methane adsorption capacity. Therefore, the abundance and morphology of the clay minerals and their resultant effect on microporous surface area is important as they likely provide adsorption sites allowing for significant gas storage.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California