Toward a Spatiotemporal Understanding of Dolomite Dissolution in Sandstone by CO₂-Enriched Brine Circulation

Abstract

In this study, we introduce a stochastic method to delineate the mineral effective surface area (ESA) evolution during a recycling reactive flow-through transport experiment on a sandstone under geologic reservoir conditions, with a focus on the dissolution of its dolomite cement, Ca 1.05 Mg 0.75 Fe 0.2 (CO 3 ) 2 . CO 2 -enriched brine was circulated through this sandstone specimen for 137 cycles (∼270 h) to examine the evolution of in situ hydraulic properties and CO 2enriched brine−dolomite geochemical reactions. The bulk permeability of the sandstone specimen decreased from 356 mD before the reaction to 139 mD after the reaction, while porosity increased from 21.9 to 23.2% due to a solid volume loss of 0.25 mL. Chemical analyses on experimental effluents during the first cycle yielded a dolomite reactivity of ∼2.45 mmol m −3 s −1 , a corresponding sampleaveraged ESA of ∼8.86 × 10 −4 m 2 /g, and an ESA coefficient of 1.36 × 10 −2 , indicating limited participation of the physically exposed mineral surface area. As the dissolution reaction progressed, the ESA is observed to first increase and then decrease. This change in ESA can be qualitatively reproduced employing scanning electron microscopy-image-based stochastic analyses on dolomite dissolution. These results provide a new approach to analyze and upscale the ESA during geochemical reactions, which are involved in a wide range of geoengineering operations.

The Effect of Mineral Dissolution on the Effective Stress Law for Permeability in a Tight Sandstone

“…The entire flow‐through experiment consists of three continuous stages: prereaction stage, reaction stage, and postreaction stage, employing the reactive transport system described in Ma et al. (2019) and Lima et al. (2019).…”

“…Effluent fluid samples are collected both at the inlet and the outlet of the reaction cell to infer fluid‐rock reactions during the experiment and to analyze the fluid samples, employing inductively coupled plasma optical emission spectrometry (ICP‐OES) in the Laboratory of Inorganic Chemistry at ETH Zurich (details are provided in Text S2). Using the measured ion concentration, the dolomite reactivity can be calculated as , where ΔC is the concentration difference of ion i between the fluid inlet and the fluid outlet, x i is the stoichiometric coefficient of ion i in dolomite, and V is the bulk volume of the sandstone specimen (details are provided in Text S3) (Ma et al., 2019). Furthermore, the theoretical maximum Ca concentration at the experiment conditions is calculated, employing an equilibrium model using PFLOTRAN (Lichtner et al., 2019).…”

“…The entire flow-through experiment consists of three continuous stages: prereaction stage, reaction stage, and postreaction stage, employing the reactive transport system described in Ma et al (2019) and Lima et al (2019). Throughout the three stages, the sandstone specimen stays inside the system and is saturated with the circulating fluids.…”

Thermally driven fracture aperture variation in naturally fractured granites

“…Insulation around the cell inlet and the cell body assured that the injected fluid temperature is maintained at that of the specimen. A complete description of the flow-through system is provided in Ma et al (2019). A set of preliminary tests showed that our system can reach a maximum temperature of ∼ 140 °C, given the pressure that was applied in this study.…”

Contributions of visible and invisible pores to reactive transport in dolomite

“…those occurring at the scale of individual pores in a rock, in fact govern the bulk (i.e. continuum-scale) geochemical interactions between rocks and fluids (e.g., Li et al, 2006;Molins et al, 2012;Plümper et al, 2017;Ma et al, 2019). Indeed, some have argued that the study of pore-scale geochemical processes should be considered a discipline in its own right (Steefel et al, 2015).…”