Integration of well logs, mineralogy and geochemical analysis using cuttings of a natural analogue for CO2 injection in the Barra Velha Formation

One of the techniques for carbon capture and storage (CCS) is the injection of CO₂ into depleted reservoirs. After the injection, the CO₂ interacts with the host rock, causing dissolution and precipitation of minerals. For complex systems, such as carbonate rocks, it is difficult to predict the outcome of those reactions along the time, reason why natural analogues are valuable assets for understanding those changes, but they are scarce. In Brazil, the intense industrial activities in Pre-salt rocks provided many wells for potential CCS purposes. Based on that, the present work aims to characterize a natural analogue for CO₂ injection in the Barra Velha Formation (BVE), reservoir rocks of Santos Basin, Brazil. BVE is composed of lacustrine limestones, presenting shrubs and/or spherulites calcite growth structures, commonly dolomitized or silicified, and magnesian clay matrix. In the present work, 91 cutting samples from the exploratory well 1-BRSA-1363-RJS (Peroba) were used for lithological description and X-ray Fluorescence (XRF), and 31 cuttings for X-ray Diffractometry (XRD) analysis. The results were interpreted alongside well logs data acquired during wireline, such as Gamma Ray, Density, Neutron, Resistivity, Sonic, Photoelectric Factor, and Spectral Gamma Ray. The lithological description, mineralogy and chemistry showed different proportions of dolomite and quartz, which can be associated with changes in the well logs to delimit the well into six zones. Different types of dolomites were described, presenting distinct habits, colors and distributions along the zones, which could be related to the result of different minerals interacting with CO₂. The basal interval presents a characteristic increase in MgO and SiO₂, usually related to magnesian clays. However, XRD results clarified that clay minerals are not found at the studied interval. This chemical trend is related to intense dolomitization and silicification processes that provoked the entire dissolution of the clayey matrix, propitiating new phases of dolomite and the precipitation of dawsonite. In this way, the presence of CO₂ affected the host rocks causing mineral transformation along the geological time.