Pore-scale simulation of transport properties of carbonate rocks using FIB-SEM 3D microstructure: Implications for field scale solute transport simulations

Pore-scale simulations using a nanoscale FIB-SEM three dimensional microstructure have been used to estimate the absolute permeability and diffusive tortuosity of a representative limestone sample collected from the Arab Formation (Jurassic) carbonate gas reservoir of Abu Dhabi, United Arab Emirates. Estimated absolute permeability (k) values are in excellent agreement with reported experimental permeability values. However, estimated diffusive tortuosity (τ_d) values required to calculate effective diffusivity (D_eff) values of multiple species involved in thermochemical sulfate reduction (TSR) are unrealistically larger than hydraulic tortuosity (τ_h) values. Widely employed empirical correlations (e.g. τ_d = φ D_m/D_eff) which include only porosity as the factor affecting τ_d of natural porous media yield unrealistically smaller values than 1.0. Realistic τ_d values were only obtained if a constrictivity factor (δ) was included in the estimation of τ_d. Given the difficulty in estimating both δ and τ_d values simultaneously, it is proposed that D_eff values for reactive transport simulations of TSR, which are commonly calculated as function of φ and τ_d (e.g. D_eff = (φ/τ_d)D_m), should be instead estimated as function of a formation factor (F) (D_eff = FD_m). In the proposed expression F accounts for the combined effect of porosity, tortuosity and constrictivity (F = δφ/τ_d). Our pore-scale simulation results shows that F can be accurately estimated from pore-scale simulations using FIB-SEM 3D images.