Caprock Assessment for a CCS Project in Abu Dhabi, Part 1: The Critical Role of Geochemistry in Carbonate Saline Aquifers

Abstract Geologic carbon storage is an important process to achieve net zero. The success of carbon capture and storage (CCS) projects requires a thorough evaluation of sites to assess storage capacity and mechanisms, and to define associated risks (e.g. containment). Used models are often threatened by the lack of proper data acquisition. In this study, we perform specialized laboratory tests, involving exposure of caprock samples to CCS conditions to generate key understanding of fluid-rock interactions that may impact integrity. Mineralogy and elemental composition of caprock samples of different lithologies at multiple depths, cored from the investigated CCS site, are characterized pre and post treatment with CO2-saturated brine, using X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) and thin sections. The injection CO2 is analyzed for impurities, and formation brine ionic composition is accurately measured, as one of the key aspects to investigate fluid-rock chemical reactions. The CO2 solubility in the formation brine is measured and rock samples are subsequently exposed to the CO2-saturated brine at reservoir conditions for set periods of time. The investigated CCS site consists of a carbonate saline aquifer. Three different caprock lithotypes were evaluated: anhydrite, argillaceous carbonate, and shale/silty shale. Compositional variations within each lithology were defined to create benchmarks for the area of interest. The anhydrite facies show no porosity and little compositional variations, with occasional evidence of the presence of halite, clay minerals and dolomite impurities. The argillaceous carbonate consists of a dolomitized fabric, with poor to moderate porosity and varying presence of halite and clay minerals. The shale samples are composed of clay-rich matrices with minor quartz presence, and microporosity is expected. The results of brine ionic composition, and mineralogy and elemental composition of rock samples post-exposure to CO2-saturated brine at given P-T, indicate: 1) Little chemical reactions during the anhydrite samples treatment, mostly dissolution of carbonate impurities, 2) Dissolution of halite and alteration of clay minerals, when present in small proportions, during the argillaceous carbonate samples treatment, 3) Alteration of clay minerals (potassium leaching), authigenic K-feldspar formation, and carbonate dissolution during the exposure of shale samples. No evident impact on porosity is observed (under further investigation). The impact of these changes in rock mechanics and seal integrity is currently under investigation. An innovative laboratory program was designed to produce critical data for CCS site evaluation, such as CO2 solubility in formation brine at reservoir conditions, and chemical reactions leading to dissolution/precipitation for the given brine ionic composition and rock type. The generated data set for the various caprock lithotypes creates benchmarks for more realistic modeling inputs.