Speaker
Description
The term "kerogen" is defined as the organic matter (OM) that produces oil during the geological process of thermal maturation, in which the OM is progressively exposed to higher temperatures and pressures. The kerogen maturity indicates whether it is in a state of oil generation (immature), gas generation (mature), or above its hydrocarbon production stage (overmature). In the so-called van Krevelen diagram, the maturity of kerogen is characterised through a two-dimensional diagram showing the evolution of the atomic H/C and O/C ratios. Kerogen has gained a lot of attention due to the emergence of shale gas, as it is the key phase impacting hydrocarbon recovery or carbon sequestration processes, with fluid molecules mainly trapped within its amorphous microporosity (pore size < 20 Å), which is very close to that of biochars. Owing to the inherent complexity, heterogeneity, and diversity of such carbon microstructures, predicting their thermodynamic properties remains challenging.
In recent years, a strategy based on the replica exchange molecular dynamics (REMD) method has been developed to obtain models of kerogens directly related to their organic precursor (see presentation by Jean-Marc Leysale). The main advantage of this method is that the pore space is not prescribed for the microstructures but is the emergent result of the decomposition process as simulated.
Here we use 11 models built by this technique from a fatty acid precursor (type I) at various maturities (H/C ratio from 1.3 to 0.3), allowing the study of the transition from very immature to overmature microstructures. Notably, their mechanical properties shift from soft viscoelastic immature matrices to hard elastic mature ones. Given this diversity in mechanical behaviour, it is important to account for the poromechanical coupling between the adsorbed fluid and the kerogen structure, as some can be significantly prone to adsorption-induced swelling. This is achieved by alternating between molecular simulations in the grand-canonical (µVT) and the isobaric-isothermal (NPT) ensembles for a large number of cycles until both the volume V and the number of adsorbed molecules N fluctuate around equilibrium values, thus giving access to the adsorption isotherm and the volumetric swelling. The imposed chemical potential of the fluid corresponds to a bulk fluid at the same mechanical pressure P that is imposed on the system (unjacketed or drained condition, as in most adsorption experiments). We indeed show for adsorption of both pure CH₄ and CO₂ at 318 K that the immature kerogens (H/C > 0.7) can exhibit large swelling above 10 bars, as opposed to the mature ones (H/C ≲ 0.7), where swelling remains below 5% even at 500 bars. In this study, the applicability of the conventional Tòth adsorption model to describe the evolution of adsorption properties with the H/C ratio is examined, along with the relevance of results obtained by neglecting poromechanical coupling (i.e., the rigid matrix assumption). The impact of adsorption-induced swelling on diffusion in such conditions will be anticipated in light of the findings of previous studies.
| Country | France |
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