Effects of Multiple Transport Mechanisms on Shale Gas Production Behavior

Abstract

Abstract Gas transport in extremely low-permeability shale formations is a multi-mechanisms-coupling process that includes gas adsorption-desorption, Knudsen diffusion, and slip flow. Incorporating these mechanisms into a unified mathematical model is crucial to obtain a more accurate shale gas production behavior. The objective of this paper is to investigate the effect of each mechanism on shale gas production behavior. We establish shale gas dual-porosity dual-permeability model that incorporates gas adsorption-desorption, Knudsen diffusion, and slip flow as well as the thermodynamics calculations. Peng-Robinson equation of state (PR-EOS) was used to calculate the gas density and compressibility factor by solving the cubic equation. In our model, gas adsorption-desorption obeys the Langmuir's isotherm as a function of reservoir pressure. Furthermore, gas viscosity changes with Knudsen number during the pressure depletion; and pore radius will increase when the adsorption gas desorbs from the pore wall. In the numerical method implementation, we combine the finite volume method with the experimenting pressure field approach to obtain the pressure distribution in the matrix and fracture systems. In this study, we investigate the effects of adsorption, Knudsen diffusion, and gas-slippage (Klinkenberg effect) on shale production behavior. We switch each mechanism on and off in the model to observe its effect on the production performance. It is observed from the results that among these mechanisms, adsorption has a great influence on gas production. Ignoring adsorbed gas would lead to a lower cumulative production. Meanwhile, the effects of Knudsen diffusion and slip flow are not as significant as adsorption. In particular for the example presented in this paper, neglecting adsorption will decrease the production nearly 80% compared to 15%-18% when Knudsen diffusion and slip flow are ignored. Knudsen diffusion and slip flow have less impact on the production because both mechanisms occurs in the matrix and contribute only to the apparent permeability, while the production is mainly determined by the fracture permeability, which is critical in determining fluid flow from the fracture system to the wellbore.