Estimation of Effective Surface Area: A Study on Dolomite Cement Dissolution in Sandstones

Abstract

Fluid-rock reaction plays a critical role in many natural and/or engineered hydrogeological processes. It is well-known that the reaction rate highly depends on the effective surface areas (ESA) of minerals that participate in the reactions. However, quantifying the ESA has been reported to be challenging. In this study, we analyzed the ESA of dolomite cement (Ca1.05Mg0.75Fe0.2(CO3)2) in a multi-mineral sandstone from a geothermal formation. We first combined BET gas absorption measurements with high resolution SEM-EDS (1.2 μm) image processing, to obtain the physically accessible surface area (ASA) of the dolomite cement, which was 0.065 m2/g. Then, we delineated the ESA evolution during the dissolution of the dolomite cement, by performing a reactive flow-through transport experiment under reservoir conditions. We circulated CO2-enriched brine through the specimen for 137 cycles (~270 hours) to examine the evolution of in-situ hydraulic properties and CO2-based geochemical fluid-rock reactions, with the focus on the dissolution of dolomite cement. Water chemistry analyses on the effluent samples yielded a sample-averaged ESA of 8.86 × 10-4 m2/g and an effective coefficient of the reactive surface area for dolomite of 1.36 × 10-2, indicating a limited participation of the physical surface area. As the dissolution reaction progressed, the ESA was observed to first increase, then decrease. This change in ESA can be qualitatively reproduced by employing an SEM image-based stochastic approach on dolomite dissolution. Our results provide insights into CO2-based geochemical reactions that may occur, for example, during carbon capture, utilization, and storage operations.