Molecular-Scale Quantitative Evaluation of the Competitive Sorption of Methane and Carbon Dioxide on the Different Constituents of Organic-Rich Mudrocks
Authors:   The University of Texas at Austin, Gomaa Ibrahim , Heidari Zoya ,   The University of Texas at Austin, Espinoza Nicolas ,   The University of Texas at Austin
Published: November 2024

Carbon dioxide (CO2) and methane (CH4) adsorption in organic-rich mudrocks can significantly be affected by clay minerals and kerogen as dominant components of the rock. Previous publications have explored the adsorption of CH4 and CO2 on kerogen and clay structures separately and overlooked their competitive adsorption behavior on the molecular scale. The objectives of this paper are to (a) evaluate the CH4 and CO2 adsorption capacity of different kerogen types and thermal maturity levels under reservoir pressure and temperature, (b) evaluate the CH4 and CO2 adsorption capacity of illite and kaolinite with different pore structure, and finally (c) improve the conventional Langmuir adsorption models to account for the competitive adsorption of CH4 and CO2 within the different components of organic-rich mudrocks. We used realistic kerogen molecular models that were condensed and optimized to mimic the actual kerogen structures. Kerogen molecules of different types (e.g., type I, II, and III) and different thermal maturity levels were transformed into dense porous structures through an annealing process. Meanwhile, illite and kaolinite samples were modeled, honoring their chemical composition, surface charges, and pore size. We then performed Grand Canonical Monte Carlo (GCMC) simulations to evaluate the CH4 and CO2 adsorption isotherms for kerogen and clay structures. To investigate the effect of reservoir temperature on the adsorption capacity, adsorption isotherms were constructed for a pressure range of 1 to 20 MPa under temperatures of 300, 330, and 360 K. The change of gas adsorbate density along with atomic radial distribution function (RDF) was evaluated to quantify the interfacial interactions between the gases and the adsorbent surface. Moreover, the diffusion coefficients of CH4 and CO2 were calculated for the simulated kerogen and clay structures. Results showed that the available pore volume for kerogen type III was found to be twice as much as kerogen type I. This is a result of the increase in the kerogen aromaticity from 29% for kerogen type I to about 57% for kerogen type III. In addition, increasing thermal maturity from kerogen type IIA through type IID led to an increase in the available pore volume and kerogen aromaticity by 50% and 92.6%, respectively. These changes in the kerogen geochemistry and pore structure led to variations in the adsorption capacity for both CH4 and CO2. Meanwhile, The CH4 and CO2 adsorption capacity values of illite and kaolinite were much lower than that of kerogen molecules. Pore size and surface area in clay minerals were proven to be the main controlling factor in determining the clay adsorption capacity for the tested gases. However, the negatively charged illite samples showed more adsorption affinity to the polar CO2 molecules than the nonpolar CH4 molecules. The proposed methods improve the conventional Langmuir adsorption models of organic-rich mudrocks that used kerogen as the only adsorbent component. Moreover, the improved adsorption models can be used as guidelines for enhanced gas recovery or CO2 sequestration applications in organic-rich mudrock. This also can be extended to different formations where both kerogen and clay particles may coexist.