Diffusional and Electrical Tortuosity in Unconventional Shale Reservoirs

Abstract

Abstract Tortuosity is a porous medium property which measures the interconnectedness and sinuosity of the pore space. It is important in order to understand advective, diffusive transport and electric current flow in low porosity low permeability reservoirs such as unconventional shale reservoirs. However, experimental values of diffusional and electrical tortuosity of shales are scarce in the literature. To improve the understanding of diffusive transport and electrical flow in unconventional shale reservoirs, we have conducted the independent measurements of diffusional and electrical tortuosities of 12 shale samples from Utica, Bakken, Wolfcamp and Eagle Ford formations and 3 sandstones as control samples. For the purpose of this study, the samples were characterized by measurements of total organic carbon (TOC), mineralogy, and total porosity. Diffusional tortuosity was measured by immersing brine saturated samples into D2O to allow the diffusion of D2O into the samples. The rates at which D2O diffused into the samples were measured with 12 MHz NMR measurements. The combination of the effective diffusion coefficient of D2O and the bulk diffusion coefficient of D2O were used to compute the diffusional tortuosity. To measure electrical tortuosity, the samples were resaturated with 2.5% KCl brine prior to immersion into 6% KCl brine. The measurements of the samples’ resistivities as function of time after the immersion into the 6% KCl brine were used to compute ionic diffusion coefficients. Electrical tortuosity was calculated using the bulk ionic diffusion coefficient and the ionic diffusion coefficients of the samples. The results show that diffusional and electrical tortuosity have similar values. These tortuosities vary between 1.7 and 5.8 for shale samples while they have values between 1.4 and 3.9 for sandstones. We also observed that TOC exerts a primary control on the tortuosity of shales.