Post-CO2 injection alteration of the pore network and intrinsic permeability tensor for a Permo-Triassic sandstone

The aim of this study was to determine the process–structure–property relationships between the pre- and post- CO 2 injection pore network geometry and the intrinsic permeability tensor for samples of core from low-perme- ability Lower Triassic Sherwood Sandstone, UK. Samples were characterised using SEM-EDS, XRD, MIP, XRCT and a triaxial permeability cell both before and after a three-month continuous-flow experiment using acidic CO 2 -rich saline fluid. The change in flow properties was compared to those predicted by pore-scale numerical modelling using an implicit finite volume solution to the Navier–Stokes equations. Mass loss and increased secondary porosity appeared to occur primarily due to dissolution of intergranular cements and K-feldspar grains, with some associ- ated loss of clay, carbonate and mudstone clasts. This resulted in a bulk porosity increase from 18 to 25% and caused a reduction in mean diameter of mineral grains with an increase in apparent pore wall roughness, where the fractal dimension, D f , increased from 1.68 to 1.84. All significant dissolution mass loss occurred in pores above c. 100 lm mean diameter. Relative dilation of post-treatment pore area appeared to increase in relation to initial pore area, suggesting that the rate of dissolution mass loss had a positive relationship with fluid flow velocity; that is, critical flow pathways are preferentially widened. Variation in packing density within sedimentary planes (occur- ring at cm-scale along the - z plane) caused the intrinsic permeability tensor to vary by more than a factor of ten. The bulk permeability tensor is anisotropic having almost equal value in - z and - y planes but with a 68% higher value in the - x plane (parallel to sedimentary bedding planes) for the pretreated sample, reducing to only 30% higher for the post-treated sample. The intrinsic permeability of the post-treatment sample increased by one order of magnitude and showed very close agreement between the modelled and experimental results