Comparison of Porosity Change Due to Geochemical Reaction between Samples from High CO2 Field and Depleted Field

Abstract

When developing a high CO2 field, oil and gas companies must consider the best and most economical carbon capture and storage (CCS) plan. After considering the distance of the storage site and storage capacity, PETRONAS has identified 2 carbonate fields, known as X Field and N Field in East Malaysia as the potential CO2 storage site. Interestingly, both fields are different, as X field is a high CO2 green field, while N field is a depleted gas field. The research team’s initial hypothesis is that N Field would have more severe geochemical reaction between CO2, brine and carbonates compared to X Field, since X field is already saturated with CO2. In order to test the hypothesis, samples from these two fields were selected to undergo static batch reaction analysis, and changes in porosity were determined using Digital Core Analysis (DCA). Both X and N fields are carbonate gas fields, with aquifer zone located below gas zones. The aquifer zones are the preferable CO2 injection zone because the deeper the zone, the longer it will take for the plume migration to happen. For static batch reaction analysis, samples each field were selected from the aquifer zone. After Routine Core Analysis (RCA) and Quality Control (QC), the samples were scanned under the high resolution microCT scan, before they were saturated into the respective synthetic brine. After saturation is completed, both brine and samples were placed inside a batch reactor, where the reactor’s pressure and temperature are set according to the field’s pressure and temperature. Once stabilized, the supercritical CO2 is injected into the brine, and was left for 45 days with constant observation. After aging with supercritical CO2, the samples were then scanned under microCT scan once again, using the same resolution, before being analysed via image processing software. Using registration algorithm software, both pre and post CO2 aging images were overlapped and subtracted digitally. The difference images were analyzed to determine the change in porosity. Samples from X Field has around 1% p.u. increase in porosity, while samples from N field shows increment of 2% p.u. porosity. While N field (depleted field) has higher reaction compared to X field (high CO2) field as per hypothesis, the difference is very minimal, which is much less than expected. The usage of DCA in the analysis enabled the team to determine minute changes that were happening during CO2 batch reaction. Without DCA, the 1% changes usually regarded as equipment’s error margin. The next step would be modelling, where the lab results will be upscaling into field scale, for modelled longer period of time. Hence, although the porosity changes between X and N field are very small under laboratory condition, it can have greater impact in field scale.