Experimental and numerical study of chemo-hydro-mechanical effects of CO2 injection on permeable limestone

Abstract

Carbon capture and storage (CCS) in saline aquifers is a proven technology aimed at reducing atmospheric CO2 emissions and mitigating the climate change crisis. CO2 injection acidifies the formation water inducing mineral dissolution and alterations in the pore structure and hydromechanical properties of the rock, particularly in carbonate reservoirs with large contents of fast-reacting minerals. Improving the current understanding of the form, extent, and governing mechanisms of such interactions is central to optimizing and securing the implementation of CCS and serves as the primary goal of this study. To achieve this goal, this study combines (i) percolation experiments with CO2-saturated water and HCl solutions on cm-scale cores of highly permeable Pont du Gard Limestone and (ii) 3D Darcy-scale reactive transport simulations of the performed experiments. Effluent chemistry analyses, X-ray Micro Computed Tomography (XMCT) imaging, and measurements of the hydromechanical properties of intact and altered specimens are employed to quantify acid-induced changes in the two acid-rock systems. Further, a digital rock approach is developed to construct heterogeneous permeability maps of the intact specimens from CT images that feed as inputs into 3D Darcy-scale reactive transport models. Experimental results show that the acid type and pore space heterogeneity have primary control on dissolution patterns formed in limestone specimens and the resulting alterations in their hydromechanical properties. Under the flow conditions of these experiments, the complete dissociation of HCl as a strong acid leads to rapid limestone dissolution and the formation of compact dissolution patterns that only affect the hydromechanical properties at the core inlet. In contrast, partial dissociation of H2CO3 as a weak acid extends the dissolution reaction along the core and induces wormhole formation that markedly enhances the rock permeability. Altered cores render significant attenuation in both mechanical rock properties and ultrasonic velocities. Chemically-driven alterations in rock stiffness are reproduced using a Differential Effective Medium (DEM) homogenization approach. Numerical simulations using the 3D Darcy-scale reactive transport model satisfactorily reproduce the experimentally measured changes in effluent chemistry, porosity, permeability, and the observed dissolution features in CT images of reacted limestone samples. Simulation results indicate that the pore space heterogeneity controls calcite dissolution from the very beginning of acidic fluid injections while the acid type becomes progressively important as the reaction front further penetrates into the rock. The compact dissolution pattern formed in the HCl-limestone system can be numerically ‎captured using the classical Kozeny-Carman porosity-permeability relationship with a ‎power-law exponent of 3 applied to the grid blocks of the numerical domain. In the case of CO2 injection, however, formation of wormhole by continuous acid renewal exerts strong feedback between the fluid flow and the dissolution reaction. This dissolution pattern can only be reproduced using an exponent as large as 15 that increases to ≈ 27 for the bulk behavior of the core containing a wormhole. This demonstrates that acid-induced permeability evolution in carbonate rocks is highly scale-dependent. The percolation experiments performed using CO2-saturated water represent a severe scenario of CO2-brine-rock interactions in carbonate reservoirs that needs to be considered in predictions and monitoring of CO2 storage. This study highlights (1) the importance of small-scale heterogeneities in controlling flow properties and localization of flow and chemical reactions in limestones and (2) the need for developing rigorous upscaling approaches to account for them in field-scale simulations.