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

Abstract

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 CO 2 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 CO 2 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 CO 2 flow obeys Darcy's law , showing a constant permeability, with values of 6.90 × 10 −19 m 2 , 3.80 × 10 −20 m 2 and 1.59 × 10 −18 m 2 . 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 m 2 ), 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 CO 2 , 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.