Study on the Coupled Effect of Effective Stress and Methane Gas Adsorption on the Porosity and Permeability of Shale Gas Reservoirs

Abstract

Shale gas is considered very important unconventional hydrocarbon resources. Due to the technological advances of hydraulic fracturing, the development of shale gas has become the main focus in recent years. Porosity and permeability are the most important petrophysical parameters during the production of shale gas. A considerable amount of research work has been carried out on stress law of porosity and permeability. However, nearly none of them considered the effects of methane adsorption. This paper utilizes the simplified local-density (SLD) theory to study adsorption of supercritical gas in shale gas reservoirs. On the basis of the basic features of high pressure supercritical adsorption of shale gas, Peng—Robinson equation is used to describe adsorbed fluid. The interaction between the gas molecules and porewalls of shale is considered using Lennard-Jones potential. Finally, we establish the SLD model to do regression analysis for the adsorption experiments data. The density of adsorbed phase and free phase density could be obtained applying SLD model and then the amount of gas adsorption can be determined. The Gibbs adsorption amount calculated using the SLD model is used to establish matrix strain model. Finally, the strain model is incorporated into widely used analytical porosity and permeability models to develop the coupled model with consideration of the coupled effect of gas adsorption and stress on the porosity and permeability of shale gas reservoirs. And the trend of variation of porosity and permeability of shale rocks taking account of the effects of stress and gas adsorption can be obtained. Lab experiments of gas adsorption of methane gas are made on three shale samples. The developed SLD model is applied to describe gas adsorption data. The outcome indicates that SLD model can properly analyze and fit the experimental data. From the results calculated by the new developed model of porosity and permeability, we can conclude that porosity ratios and permeability ratios of gas shales decrease with the increase of pore pressure, which is contrary to the tendency of changes in porosity and permeability only taking account of the effects of effective stress. This result demonstrates that gas adsorption has very large impact on pore volume, therefore the deformation of matrix induced by the adsorption of methane gas cannot be neglected. The proposed model could further be used for the accurate evaluation of storage capacity of shale gas reservoirs and gas production of wells.