Dissolved CO₂Increases Breakthrough Porosity in Natural Porous Materials

Abstract

When reactive fluids flow through a dissolving porous medium, conductive channels form, leading to fluid breakthrough. This phenomenon is caused by the reactive infiltration instability and is important in geologic carbon storage where the dissolution of CO in flowing water increases fluid acidity. Using numerical simulations with high resolution digital models of North Sea chalk, we show that the breakthrough porosity is an important indicator of dissolution pattern. Dissolution patterns reflect the balance between the demand and supply of cumulative surface. The demand is determined by the reactive fluid composition while the supply relies on the flow field and the rock's microstructure. We tested three model scenarios and found that aqueous CO dissolves porous media homogeneously, leading to large breakthrough porosity. In contrast, solutions without CO develop elongated convective channels known as wormholes, with low breakthrough porosity. These different patterns are explained by the different apparent solubility of calcite in free drift systems. Our results indicate that CO increases the reactive subvolume of porous media and reduces the amount of solid residual before reactive fluid can be fully channelized. Consequently, dissolved CO may enhance contaminant mobilization near injection wellbores, undermine the mechanical sustainability of formation rocks and increase the likelihood of buoyance driven leakage through carbonate rich caprocks.

Patterns of entropy production in dissolving natural porous media with flowing fluid

“…We used a conventional CFD approach (finite volume method to solve the Stokes equation on binarised tomogram [ 48 ]) to show that the influence is rock type specific and is predominately reflected in the interconnectivity of pore spaces that require discretisation. We have also applied the reactor network model on tomographic data at three different resolutions [ 34 ]. The results suggested that both the effect of the voxel size and that of the initial microstructure are secondary compared to the apparent solubility of the solid in the flowing fluid.…”

“…We adopted the model framework of Yang et al . [ 32 – 34 , 37 ]. The mathematical scheme is designed to treat the assemblage of greyscale voxels as a network of ideal chemical reactors.…”

Effect of Cumulative Surface on Pore Development in Chalk

“…In geologic carbon sequestration (DePaolo & Cole, ; Fitts & Peters, ; Zhang & Liu, ) and in acid stimulation in oil fields (Hoefner & Fogler, ), injecting erosive fluids generates complex flow pathways. Managing the development of such networks, that is, controlling the rate of network expansion while regulating the size distribution of the growing channels, is essential for properly distributing CO 2 into porous formations (Abdoulghafour et al, ; Deng et al, , ; Ellis et al, ; Luquot & Gouze, ; Noiriel, ; Yang et al, ) and for effectively recovering hydrocarbons from reservoirs (Fredd & Fogler, ; Hoefner & Fogler, ; Rege & Fogler, ).…”

“…In this study, we define cumulative surface (CS) as the amount of surface area to which fluid has access within the residence time. This concept was first coined to quantify the reactive subvolume of porous media (Yang et al, ; Yang, Bruns, Rogowska, et al, ). Here we further argue that CS, by integrating the effects of residence time, reaction rate, and initial pore heterogeneity into one quantity, also helps identify the trajectory of localized pore development.…”

Direct Observation of Coupled Geochemical and Geomechanical Impacts on Chalk Microstructure Evolution under Elevated CO2Pressure

“…This disequilibrium is the driving force for geochemical reactions which modify rock properties, such as mineral dissolution and precipitation. This modification, in turn, provides feedback on the migration of fluid that carries the reactants and products for these reactions (Yang et al, 2016a). This coupled evolution of flow field and microstructure through chemistry is far from being understood because of the difficulties in direct experimental observation (Noiriel, 2015) and in numerically handling mathematical models based on free boundary problems of partial differential equations (Chadam et al, 1986;Ortoleva et al, 1987;Szymczak and Ladd, 2012;Yang et al, 2016b).…”