Molecular simulation calculation method for shale gas adsorption in realistic shale model

Abstract

An accurate evaluation of the adsorption capacity of shale formations relies on a quantitative determination of the capacity for shale gas adsorption. However, the molecular simulations at the microscopic scale performed by most research institutes do not provide an accurate reflection of the results of macroscopic experiments. In addition, due to the multi-scale pore structure and non-uniform rock composition, the adsorption micro-mechanisms of shale reservoirs are complex and diverse, and related theories still need to be studied and perfected. This research used X-ray diffraction and total organic carbon experimental data to propose a shale molecular model based on data from actual shale formations. The proposed model considers mineral and organic contents, mineral cell structures, lattice parameters, mineral arrangements, and pore diameters, and it corresponds well to actual shale formations. Under temperature and pressure conditions of the shale formation, the established shale molecular model corresponds more closely to actual formation conditions in terms of shale gas adsorption capacity and specific surface area than previous single-mineral models. At the same time, the variation between the adsorption simulation result obtained from the shale molecular model and the result from the adsorption experiment was less than 5%. This further confirms the authenticity and reliability of the constructed shale molecular model. On the basis of shale molecular model, deep shale gas adsorption data at pressures above 30 MPa were predicted. It's show that the adsorption capacity of shale gas approaches the saturated adsorption capacity when the shale formation pressure reaches 75 MPa.