The role of fluid-rock interactions in permeability behavior of shale with different pore fluids

The permeability of shale is a controlling factor in fluid flow, solute transport, and overpressure development in a sedimentary basin. However, shale permeabilities measured with different fluids can be very different. To investigate and characterize the effects of fluid-rock interactions on shale permeability, a series of flow experiments on three samples collected from the Carboniferous formation in the eastern Qaidam Basin, China, with flow directions perpendicular to the bedding plane, were conducted using deionized water, liquid CO2 and 1 mol/L NaCl solution as permeating fluids, respectively. A water permeability model under the influence of fluid-rock interactions was newly derived. The experimental results indicate that for the same sample, the liquid CO2 permeability is the highest permeability measured, followed by that of deionized water, and that the 1 mol/L NaCl solution permeability is the lowest permeability measured. The liquid CO2 flow obeys Darcy's law, showing a constant permeability, with values of 6.90 × 10−19 m2, 3.80 × 10−20 m2 and 1.59 × 10−18 m2. The movement of the deionized water and NaCl solution in these samples deviates from Darcy's law. The relationship between water permeability and pressure gradient follows a power function, with exponents ranging from 0.96 to 3.41 for deionized water and 0.34–3.30 for NaCl solution. The permeabilities measured with these two fluids exhibit nearly identical ranges (10−20–10−21 m2), but the sample permeated with NaCl solution generally shows a lower permeability under the same pressure gradient. The permeability reduction magnitude between the three liquid permeabilities and the helium absolute permeability was used to represent the fluid-rock interactions strength (FRIS). The FRIS is 0.25–0.96 for liquid CO2, 1.44–2.32 for deionized water and 1.89–3.09 for NaCl solution. The relations between the FRIS and mineral compositions and pore structure of shale samples indicate that the FRIS has a better correlation with the specific surface area and porosity.