Variations of Gas Flow Regimes and Petro-Physical Properties During Gas Production Considering Volume Consumed by Adsorbed Gas and Stress Dependence Effect in Shale Gas Reservoirs

摘要

Gas in shale gas reservoirs is stored by two mechanisms: free gas and adsorbed gas. The volume occupied by adsorbed gas is overlooked by current industry standards and numerical modeling. Also, stress dependence effect of matrix does not draw as much attention as hydraulic fractures. These two effects significantly impact Original Gas in Place (OGIP), gas flow regimes, and petro-physical properties during gas production. Therefore, development of an improved model and simulator with consideration of the volume consumed by adsorbed gas and the stress dependence effect of matrix is critical. In this paper, we develop a coupled model system for shale gas reservoir simulation. The equations considering the impact of gas adsorption/desorption upon effective porosity and permeability are derived and incorporated into the simulator. In particular, both Langmuir and BET (Brunauer, Emmett, and Teller) isotherms are incorporated using a unified formula. The stress dependence effect of matrix is included as well, which also affects effective porosity and permeability by decreasing the pore size. Subsequently, we demonstrate the numerical implementation of the mathematical model followed by the validation of our model using both experimental data and production data of shale gas fields. Finally, we discuss the dynamic changes of gas flow regimes, apparent permeability, and effective porosity during gas production using the new simulator. The results show that the model can predict the OGIP more accurately than the other models in the literature. In addition, we observe the dynamic changes of effective porosity, apparent permeability and gas flow regimes during gas production. Since gas desorption and stress dependence effect change the effective flow radius, they affect the absolute permeability directly, and further affect apparent permeability indirectly through increasing/decreasing the Knudsen number. The impacts of gas desorption and stress dependence effect on absolute permeability and Knudsen number are usually converse. Commonly, the gas flow regime belongs to slip and transition flows in shale gas reservoirs. In some cases, molecular-free regime (Knudsen diffusion) may appear. This work improves the simulation of gas production in shale gas reservoirs and makes a significant progress of numerical simulation of shale gas reservoirs.