New approach in analysing rock property changes in vuggy carbonates

Abstract

X Field is a HPHT carbonate gas field containing about 70% of CO2, and is located in offshore East Malaysia. In order for this field to be economical and environmental friendly, PETRONAS plans to reinject produced CO2 back into the reservoir. With this objective, PETRONAS Research Sdn Bhd is conducting a feasibility study on changes of reservoir rock properties due to this CO2 sequestration activity. As the available vuggy-carbonate core-plugs have a higher risk of damage, the team employed digital core analysis (DCA) technology for this study. The study consists of three phases. In the first phase, the selected core plugs were analysed using micro CT scan and image processing software. To mimic the gas reservoir, the core plugs were then saturated with synthethic brine, before undergoing centrifuge until Sw-irr from Pc data was reached. The second phase consisted of aging the core-plugs by immersion in supercritical CO2 at pressure and temperature of the reservoir for 45 days. In the third phase the core plugs were analysed via DCA again to determine changes caused by CO2 reaction in rocks. Due to the fragility of the selected samples, they the post aging DCA analysis revealed both mechanical and geochemical changes. However, the central part of the core-plugs was not mechanically damaged and could be used to analyse the changes in porosity mainly due to geochemical reaction. Our results indicate that there will be small impact of porosity changes due to CO2 storage activity in X Field. DCA has proven to be a very useful method in determining porosity changes in reservoir rocks. Not only can DCA determine porosity of broken core plugs, but it also can extract the unbroken parts of the rock for more accurate analysis.