Abstract
Nanopores are extremely developed and randomly distributed in shale gas reservoirs. Due to the rarefied conditions in shale strata, multiple gas transport mechanisms coexist and need further understanding. The commonly used slip models are mostly based on Maxwell slip boundary condition, which assumes elastic collisions between gas molecules and solid surfaces. However, gas molecules do not rebound from solid surfaces elastically, but rather are adsorbed on them and then re-emitted after some time lag. A Langmuir slip permeability model was established by introducing Langmuir slip BC. Knudsen diffusion of bulk phase gas and surface diffusion of adsorbed gas were also coupled into our nanopore transport model. Considering the effects of real gas, stress dependence, thermodynamic phase changes due to pore confinement, surface roughness, gas molecular volume, and pore enlargement due to gas desorption during depressurization, a unified gas transport model in organic shale nanopores was established, which was then upscaled by coupling effective porosity and tortuosity to describe practical SGR properties. The bulk phase transport model, single capillary model, and upscaled porous media model were validated by data from experimental data, lattice Boltzmann method or model comparisons. Based on the new gas transport model, the equivalent permeability of different flow mechanisms as well as the flux proportion of each mechanism to total flow rate was investigated in different pore radius and pressure conditions. The study in this paper revealed special gas transport characteristics in shale nonopores and provided a robust foundation for accurate simulation of shale gas production.
Highlights
In recent decades, shale gas has drawn great attention globally, especially with its successful development in North America [1,2,3]
This is because inelastic gas–solid collisions, thermodynamic phase changes due to pore confinement, the dynamic hindering effect of adsorbed gas, real gas effects, and geomechanical effects are simultaneously considered in our model
In contrast with previous studies, for gas flow in pores with radii less than 0.7 nm, with pressure decreasing, the non-Darcy flow effect is stronger at first, and weakens abruptly. This is because surface diffusion dominates the flow in small pores when the pressure is larger than 10 MPa, but plays only a slight role when the pressure is less than 10 MPa (Figure 14)
Summary
Shale gas has drawn great attention globally, especially with its successful development in North America [1,2,3]. Established an AP model for gas transport in shale nanopores considering multiple effects, such as non-Darcy flow and surface diffusion. Note: σv is tangential momentum accommodation coefficient; Kn is Knudsen number; b is slip coefficient in B–K model [31]; γ is a parameter in Zhang’s model to describe slip surface location In those slip velocity models, the content of molecules diffusively reflected from solid walls is artificially assumed to be σv , whereas (1 − σv ) is the ratio of molecules specularly reflected to incident molecules. Based our work on the LSP model, we propose a new gas transport model coupling viscous flow, slippage effect, Knudsen diffusion, surface diffusion, and adsorption/desorption. To make our gas transport model more applicable to practical situations, we re-expressed it in an AP form and upscaled it from a single capillary model to a porous media form, which coupled porosity and tortuosity into AP model and considered the effects of adsorbed gas volume on effective porosity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
