Characterisation of porosity and permeability in reservoir seals using an experimental and upscaled modelling approach

Abstract

Carbon capture, utilisation, and storage (CCUS) provide a safe option to achieve net zero carbon emission in 2050. Captured CO2 is usually stored in a deep geological reservoir formation, overlain by a sealing formation known as caprock. Understanding caprock integrity is important in ensuring safe and long-term CO2 containment and storage. Therefore, this research aims to reduce the uncertainties/risk due to the caprock integrity via comprehensive characterisation analysis. However, since caprock is comprised of micropores and nanopores with very low permeability (less than one microDarcy or 10E-18 m2 ), conventional porosimeter and permeameter are not suitable to determine its porosity and permeability. In addition, coring operation in the caprock section is very difficult and expensive, leading to the limited or unavailability of preserved core samples for laboratory analyses. Hence, data from drill cuttings and well logs are used as alternatives when core samples are limited or unavailable. In this research, caprock core samples, drill cuttings, and well log data were selected from S Field, located in offshore East Malaysia, because it is one of the candidates for a CO2 storage site, with a total of 4 wells, including an appraisal well drilled in 2015. This study is comprised of experimental rock characterisation, analyses of well-log data integrated with lab data, and numerical modelling of advective CO2 transport. The rock characterisation analyses include x-ray diffraction (XRD), x-ray fluorescence (XRF), particle size analysis (PSA), thin section petrography, scanning electron microscopy with energy dispersive x-ray (SEM-EDX), low-pressure N2 (LP-N2) sorption analysis, mercury intrusion capillary pressure (MICP), nuclear magnetic resonance (NMR), unsteady state (USS) pulse decay, and helium pycnometry (HP). In addition, broad ion beam (BIB) and focused ion beam scanning electron microscopy (BIB-SEM) were used for digital core analysis (DCA) of caprock samples. Next, well log data from S Field was analysed and integrated with lab data to generate porosity and permeability trends of the caprock of S Field. The data was also used to calculate capillary entry pressure and CO2 column height as part of the caprock integrity assessment. Finally, we studied advective transport of CO2 in S Field using Peng-Robinson (PR) equations of state (EOS) and multiphase fluid flow method. The caprock of S Field has been identified as siltstones since it is dominated by quartz and silts from mineralogical analyses. The caprock is split into two facies, Seal A and Seal B, with differing percentages of clay minerals (20% and 40%, respectively). Seal A is shallower and lies between 800 and 1400 meters below the seafloor. Seal B, on the other hand, is situated between Seal A and the carbonate reservoir and has a burial depth of around 1400 to 1900 meters. The permeability and porosity values determined in the lab, however, do not differ substantially between the two facies. This could be because Seal B is considerably over-pressured compared to Seal A. This excessive pressure could lead to the preservation of porosity during compaction, consequently resulting in enhanced permeability. This finding is consistent with the time-to-depth conversion from seismic data, which identifies Seal B as being less compacted than Seal A. Based on the data integration of the calculated porosity, permeability, capillary entry pressure, and column height, it can be summarised that the seal layers of S Field can contain injected CO2 as long as the reservoir capacity is not exceeded. This finding is supported by the numerical flow models, which show no leakage across the seal in 10,000 years and contained leaks in 1 million years.