Effect of Kerogen Thermal Maturity on Methane Adsorption Capacity: A Molecular Modeling Approach

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.

Diffusion Behavior of Methane in 3D Kerogen Models

“…In addition, the CH 4 adsorption capacity increases with thermal maturity in Type II kerogen series. These results match those observed in earlier modeling studies [43][44][45]49,53 and experimental measurements. 82−86 The CH 4 adsorption capacity was found to be the highest in Type II-D kerogens compared to the other kerogen types.…”

“…In both studies, a positive correlation has been shown between methane adsorption and kerogen maturity. This is further supported by a later study by Alafnan et al Tesson and Firoozabadi reported methane adsorption in both rigid and flexible Type II-A kerogens and concluded that kerogen flexibility has a limited effect on the self-diffusion of methane. It is worth pointing out that while ensuring flexibility of the kerogen matrix, their MD simulations were conducted in the NVT ensemble, where deformation was not accounted for.…”

The Impact of Pore Structure on Kerogen Geomechanics

“…The convergence in the density is consistent with estimated mechanical moduli as shown in Figure 8. Similar observations of the number of units needed to form a realistic kerogen structure are reported in the literature [79][80][81][82][83][84].…”

Reserves Estimation for Coalbed Methane Reservoirs: A Review

“…Moreover, the storage capacity is highly influenced by the origin and the maturation of organic matter. The adsorption capacity positively correlates with maturation as reported by different studies [20][21][22].…”