Comparative Investigation On Flow Characteristics Of Water,NaCl Solution And Liquid CO₂ In Shale

Shale plays a key role in the hydrological cycle,solute transport,CO₂ geological storage,nuclear waste disposal and oil/gas exploitation in a sedimentary basin.However,flow characteristics and permeabilities in shale measured with different fluids can be very different.To comprehensively investigate the flow characteristics of different fluids in shale nanopores and the influence of fluid type on the measurement results of shale permeability,a series of“steady-state and step-by-step depressurization”experiments on three samples（named as C01,C02 and C03）,obtained from the Carboniferous formation in the eastern Qaidam Basin,China,were conducted using deionized water,liquid CO₂ and 1 mol/L NaCl solution as permeating fluids,respectively.The helium absolute permeabilities of shale samples C01,C02,and C03 were 2.52×10^-18m²,3.48×10^-19m²,and 2.82×10^-18m²,respectively.The helium absolute permeability of shale is the highest permeability measured,followed by that of the liquid CO₂ and then the deionized water,and the 1 mol/L NaCl solution permeability is the lowest permeability measured.The liquid CO₂flow in shale obeys Darcy’s law,showing a constant permeability.The liquid CO₂permeabilities of samples C01,C02 and C03 are 6.90×10^-19m²,3.80×10^-20m² 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（TPG）is observed.When the pressure gradient is higher than the TPG,the deionized water and NaCl solution start to flow.When the NaCl solution is used as the permeate,the TPG values of samples C01,C02 and C03 are 0.468 MPa/cm,0.533 MPa/cm and 0.156 MPa/cm,respectively,which are slightly greater than the TPGs measured with deionized water（with of 0.136 MPa/cm,0.358 MPa/cm and 0.111 MPa/cm,respectively）.The permeabilities measured with these two fluids exhibit nearly identical ranges(10^-20～10^-21 m²).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 ofωis 0.25～0.96 for liquid CO2,1.44～2.32 for deionized water and 1.89～3.09 for NaCl solution.Based on the interfacial forces between shale and water,the water in the shale was quantitatively classified into strongly bound water,inner layer of weakly bound water,outer layer of weakly bound water and free water according to its movability.Two water-related mechanisms,“shear fluidity”and“electro-viscous effect”,are primarily responsible for the difference in movability between the free water and bound water.On the basis of the cylindrical pore assumption,the permeability model considering the simultaneous flow of free water and bound water is derived,and the dependence of pressure gradient and TPG on the deionized water permeability was demonstrated.The comparative analysis on the three liquid permeability model and variation characteristics shows that the moving ratio of deionized water increases with the increase of pressure gradient,resulting in the increase of permeability.The interactions between liquid CO₂and the shale surface are much smaller due to the weak polarity of liquid CO₂.Apart from the adsorption layer close to the mineral surfaces,most of the liquid CO₂ in shale pores is weakly affected by the solid surface and still exhibits the properties of bulk fluid,thus resulting in liquid CO₂ flow according with Darcy’s law and a basically constant permeability.The power function relationship between the NaCl solution permeability and the pressure gradient is attributed to the change of the pore structure with the effective stress,which results from the compression of the electric double layer and the decrease or even disappearance of the electric double layer repulsive force.These results also indicate that the movement of NaCl solution in geological media is a process of fluid-solid-stress coupling.