Impact of micro-scale characteristics of shale reservoirs on gas depletion behavior: A microscale discrete model

Shale gas has become increasingly significant in the global energy supply. Mineral heterogeneity in shales importantly impacts gas transport within the shale matrix and therefore the depletion history curve. A microscale discrete coupling model is introduced to clarify mass transfer and mechanical interactions, as well as their impact on gas transport properties, ranging from individual mineral through ensemble field scale. The model uses a mineral morphology thin-section obtained through tescan integrated mineral analyzer with the mechanical parameters, controlling both elastic and viscosity behavior of each mineral, achieved through nanoindentation. A coupled model for poromechanical evolution is proposed and solved using COMSOL. The applicability of the model results are validated against field data using a dimensionless approach. This confirms that in the early stages of gas depletion, gas is primarily liberated from inorganic minerals, whereas in later stages, it is predominantly sourced from adsorbed gas from the organic matter. Over time, the permeability of the inorganic minerals decreases, and a higher Young’s modulus of the minerals results in a greater ultimate permeability ratio. Evolution of the effective diffusion coefficient for the organic matter is controlled by multiple components. A negative correlation exists between mineral grain size and the creep effects, indicating that larger grain sizes result in smaller creep magnitudes during gas production. The Young’s modulus of inorganic matter is inversely correlated with the diffusion coefficient, while an increase in the Young’s modulus in the organic matter corresponds to a higher diffusion coefficient. The proposed model complements the traditional continuum dual-medium method and provides a clearer understanding of the interactions between minerals during gas depletion behavior.