Shugang Yang

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The permeability of shale is a controlling factor in fluid migration, solute transport, and overpressure development in a sedimentary basin. However, shale permeabilities measured with different fluids can be very different. To investigate the effects of fluid type on shale liquid permeability, a series of flow experiments on three samples were conducted using deionized water, liquid CO2 and 1 mol/L NaCl solution as permeating fluids. The results indicate that the liquid CO2 flow obeys Darcy’s law, showing a constant permeability. The liquid CO2 permeabilities of samples C01, C02 and C03 are 6.90×10-19 m2, 3.80×10-20 m2 and 1.59×10-18 m2, respectively. The transport of the deionized water and NaCl solution in these samples deviates from Darcy’s law, and threshold pressure gradient is observed. The permeabilities measured with these two fluids exhibit nearly identical ranges (10-20~10-21 m2). The sample permeated with NaCl solution generally shows a lower permeability (under the same pressure gradient) but a higher threshold pressure gradient. The relationship between water permeability and pressure gradient follows a power function, with exponents ranging from 0.96~3.41 for deionized water and 0.34~3.30 for NaCl solution. The permeability reduction magnitude (ω) was defined to describe the difference between the three liquid permeabilities and the helium absolute permeability. The range of ω is 0.25~0.96 for liquid CO2, 1.44~2.32 for deionized water and 1.89~3.09 for NaCl solution. The dependence of permeability on fluid type results from the differences in the fluid properties (viscosity and polarity) and fluid-mineral interactions.