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

Abstract

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 (τd) values required to calculate effective diffusivity (Deff) values of multiple species involved in thermochemical sulfate reduction (TSR) are unrealistically larger than hydraulic tortuosity (τh) values. Widely employed empirical correlations (e.g. τd = φ Dm/Deff) which include only porosity as the factor affecting τd of natural porous media yield unrealistically smaller values than 1.0. Realistic τd values were only obtained if a constrictivity factor (δ) was included in the estimation of τd. Given the difficulty in estimating both δ and τd values simultaneously, it is proposed that Deff values for reactive transport simulations of TSR, which are commonly calculated as function of φ and τd (e.g. Deff = (φ/τd)Dm), should be instead estimated as function of a formation factor (F) (Deff = FDm). In the proposed expression F accounts for the combined effect of porosity, tortuosity and constrictivity (F = δφ/τd). Our pore-scale simulation results shows that F can be accurately estimated from pore-scale simulations using FIB-SEM 3D images.