Study of gas sorption, stress effects and analysis of effective porosity and permeability for shale gas reservoirs

Abstract

The gas sorption capacity, effective porosity, and effective permeability are important to shale gas reservoir characterization, gas-in-place estimation, and production behavior prediction. Previous studies have ignored the absorption gas in gas sorption measurements; however, the absorption gas is dissolved into kerogen and cannot be underestimated. In this study, experiments are performed on shale samples to quantify the gas sorption capacity with new sights and to analyze the effective porosity and permeability in adsorbed and absorbed gas considering gas sorption and stress impacts. Then, data are also obtained for quantitative analysis. This study revealed that the proposed gas sorption model (sum of adsorption and absorption of gases) is as accurate as the Langmuir model. Based on this model, converted excessive to absolute gas sorption and the contribution of the absorption gas to the total gas sorption are estimated. The porosity and permeability in adsorbed/absorbed and sorption gas increases with increasing pore pressure, whereas the effective porosity and permeability decreases with increasing pore pressure. The effective porosity is affected more at low pressure, whereas it is almost the same or changes slightly at higher pressure; however, the effective permeability is dependent on the adsorbed porosity. The amount of absorbed gas increases linearly with increasing pore pressure, and that it followed Henry's law, whereas the effective absorbed porosity decreases linearly with increasing pore pressure. The effective absorbed permeability increases at a lower rate at low pressure, while at a high pressure, the rate of increase is higher. It is also observed that the adsorbed/absorbed porosity, such as gas sorption, is a function of the total organic carbon and specific surface area. However, both parameters vary with kerogen quality. The combined impact of gas sorption and stress may change the controlling factors of shale gas properties and enable the accurate measurement of the effective porosity and permeability.