Mineralogical and Geochemical Studies of a Potential Co2 Storage Site's Core Samples Via Experimental and Digital Core Analyses – A case Study
Authors: Sazali W. M. L. , Yong W. P. , Mohd Ali M. T. , Md Shah S. S. , Abu Bakar Z. A. , Amir Rashidi M. R. 
Published: February 2025

Abstract
To achieve the aspiration of net zero carbon emissions by 2050, PETRONAS is looking into several depleted hydrocarbon fields as candidates for CO2 storage sites. The criteria for a good CO2 storage site include the storage capacity, location from the source of CO2, and the low impact of the CO2-rock-fluid reaction during CO2 injection and storage. One of the candidates is F Field, a depleted carbonate gas field located in Central Luconia, offshore Sarawak, Malaysia. The preliminary studies confirmed the suitability of F Field as a potential storage site based on the reservoir volume and proximity to a high-CO2 gas field. Mineralogical and geochemical studies were conducted to identify the risks associated with the chemical reactions between the formation brine, injected CO2, and limestone reservoir. As a depleted field, F Field's appraisal well was drilled in 1977. This proved to be challenging because of the age of the caprock samples. After selecting the samples based on well log data, we carefully cleaned and dried the samples using the continuous immersion Soxhlet (CIS) method and humidity drying, respectively. The mineralogical and geochemical studies comprised static ageing experiment on the selected samples, where we observed the changes in the samples’ mineralogy and porosity after contact with supercritical CO2 and brine at reservoir pressure and temperature. The changes in the samples’ mineralogy were perceived in the labs via ICP and XRD, while the changes in samples’ porosity were determined from conventional porosimeter. The changes were also examined digitally by scanning the pre- and post-ageing samples using QEMSCAN and HeliScan microCT. Two samples were selected from two potential injection zones. Interestingly, one sample was dominated by dolomite, whereas the other was dominated by calcite. The calcite-dominated sample is in the deeper zone, and the XRD readings are consistent with the mineral mapping from QEMSCAN. After ageing, there are not many changes are observed in the mineral weight percentage and ICP's total dissolved solids. Minimal changes are also observed in the sample's porosity. However, the small changes are due to the combination of dissolution and precipitation of the samples’ particles. Hence, despite a noticeable geochemical reaction during static ageing, F Field's samples can maintain their integrity because the net changes are still within the safety limit. Mineralogical and geochemical studies are parts of the experimental feasibility study in determining the best candidate of a CO2 storage site. To complete the study, injectivity and geomechanics experimental studies are currently ongoing using the samples from F Field. Later, all data from the laboratories would be used as inputs for the CO2 sequestration modelling that combines simulations from reservoir engineering, geochemistry, geomechanics perspectives. When everything is in order, the findings from F Field will be compared with other storage site candidates, as part of the selection process.