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Abstract
The presence of kerogen in source rocks gives rise to a plethora of potential gas storage mechanisms. Proper estimation of the gas reserve requires knowledge of the quantities of free and adsorbed gas in rock pores and kerogen. Traditional methods of reserve estimation such as the volumetric and material balance approaches are insufficient because they do not consider both the free and adsorbed gas compartments present in kerogens. Modified versions of these equations are based on adding terms to account for hydrocarbons stored in kerogen. None of the existing models considered the effect of kerogen maturing on methane gas adsorption. In this work, a molecular modeling was employed to explore how thermal maturity impacts gas adsorption in kerogen. Four different macromolecules of kerogen were included to mimic kerogens of different maturity levels; these were folded to more closely resemble the nanoporous kerogen structures of source rocks. These structures form the basis of the modeling necessary to assess the adsorption capacity as a function of the structure. The number of double bonds plus the number and type of heteroatoms (O, S, and N) were found to influence the final configuration of the kerogen structures, and hence their capacity to host methane molecules. The degree of aromaticity increased with the maturity level within the same kerogen type. The fraction of aromaticity gives rise to the polarity. We present an empirical mathematical relationship that makes possible the estimation of the adsorption capacity of kerogen based on the degree of polarity. Variations in kerogen adsorption capacity have significant implications on the reservoir scale. The general trend obtained from the molecular modeling was found to be consistent with experimental measurements done on actual kerogen samples. Shale samples with different kerogen content and with different maturity showed that shales with immature kerogen have small methane adsorption capacity compared to shales with mature kerogen. In this study, it is shown for the first time that the key factor to control natural gas adsorption is the kerogen maturity not the kerogen content.
Highlights
Source rocks have become an important resource for satisfying worldwide energy demands
Arabia used to experimentally temperature, adsorbate and adsorbent. The latter, in our case,were is the kerogen structure at some investigate the relationship between thermal maturity and methane adsorption capacity described predefined maturity levels
Three samples collected from an actual shale basin in Saudi Arabia were used to experimentally investigate the relationship between thermal maturity and methane adsorption capacity described in the previous section
Summary
Source rocks have become an important resource for satisfying worldwide energy demands. Desorption in the smaller pores becomes augmented by the induced pressure gradient During their transition from the rock matrix to the wellbore, natural gas molecules are subjected to transport and storage mechanisms that deviate from those applicable to classical reservoirs. These intricacies have implications for the methods used for reserve assessments. Reserve estimation methods for source rocks were conceptualized on dividing reservoir units into inorganic and organic constituents The former is modelled similar to the classical hydrocarbons traps while the latter is described with some adsorption parameters that are approximated empirically. Understanding the adsorption behavior is of vital importance to properly select and use a given reserve estimation method
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