The complex gas dynamics in tight and shale reservoirs have become an important research topic in the oil and gas industry. This study proposes a steady-state flow test using adsorbing and non-adsorbing gases of single and binary gas components through tight adsorbing and non-adsorbing cores to investigate the true permeability value of its diffusion and slip counterparts.
A steady-state flow permeability test was chosen to capture the complex gas dynamics in nanopore throats and the presence of organic matter. 1-D experiments in adsorbing (shale) and non-adsorbing (sandstone) cores are conducted under high overburden pressure at room temperature. The pressure difference and gas flow rates across the cores are measured. Helium (base case) is flowed, followed by adsorbing gases (N2 and CH4). This is followed by flowing gas mixtures to verify whether the single component values can be used in multicomponent systems. The results are compared to existing theoretical and analytical models.
The apparent gas permeability for shale and sandstone decreases as the gas changes from non-adsorbing to adsorbing. This observation is not in line with the proposed hypothesis of the current models, where the flow mechanisms in tight and shale formations are treated like parallel resistors, where the total permeability is the addition of each component. The adsorbing gas significantly influences gas permeability when comparing the Klinkenberg plots for single and binary gas. The binary gas permeabilities skewed heavily to the gas with higher adsorbing capacity. Besides that, the adsorbing gas permanently changes the shale pore throat morphology by decreasing the pore radius, which significantly affects the flow mechanisms in shale.
The study centered on the dissection of the flow mechanisms (viscous flow, surface diffusion, and Knudsen diffusion) contributing to the permeability calculations. Viscous flow dominates the more permeable porous media, while Knudsen diffusion is in the shale. Besides that, the binary gas mixture in a standard steady-state flow test in permeability estimation is introduced. The binary gas mixture in permeability measurement introduces the effect of gas flooding on the measured permeability. The more adsorbing gas actively displaces the less adsorbing gas and contributes to the surface diffusion permeability.