Reducing Uncertainty in CCS Caprock Seal Performance Using NMR: Insights from Offshore Peninsular Malaysia

Secure subsurface storage is a cornerstone of future low-carbon energy systems, with Carbon Capture and Storage (CCS) playing a central role in long-term greenhouse gas mitigation. The effectiveness of CCS depends on the ability of caprocks to act as durable sealing barriers that prevent buoyant CO₂ migration over geological timescales (>10⁴ years). Despite their importance, significant uncertainty remains in quantifying the sealing capacity of shale and mudstone caprocks, largely due to challenges in characterising their low porosity and nanoDarcy-scale permeability.

Mineralogical analyses show that the studied caprocks are dominated by clay minerals, primarily illite–smectite and kaolinite, with subordinate quartz. This mineralogical composition results in complex pore systems dominated by micro- to nano-scale pores and a high proportion of bound fluids, posing challenges for conventional petrophysical characterisation. Accurate assessment of porosity and permeability is therefore critical for evaluating seal integrity. Conventional laboratory techniques, including Helium Pycnometry, Mercury Intrusion Porosimetry (MIP), and Brunauer–Emmett–Teller (BET) analysis, provide valuable quantitative data and serve as calibration and validation references. However, these methods are often limited by sample preparation effects, incomplete representation of pore connectivity, and measurements conducted under non-representative stress and fluid conditions, highlighting the need for complementary non-destructive approaches.

Nuclear Magnetic Resonance (NMR) has been widely applied in reservoir rock characterisation due to its ability to resolve pore size distribution, porosity, and fluid behaviour. Its application to caprocks, however, remains limited because standard NMR workflows developed for sandstones and carbonates often yield inconsistent results in clay-rich, low-porosity formations. This study evaluates the applicability of NMR for caprock characterisation and develops caprock-specific workflows suitable for CCS seal assessment.

Sample selection focused on primary shale seals overlying reservoirs identified as potential CO₂ storage targets in the Malay Basin, offshore Peninsular Malaysia. The main target intervals comprise Groups B, D, and E from four fields hosting major developed reservoirs. Low-field NMR measurements were conducted using tailored protocols that account for low porosity, complex pore geometry, and clay-related effects. Mineral oil saturation was evaluated as a non-reactive alternative to brine and was found to provide more stable and repeatable porosity measurements by minimising clay swelling and chemical alteration.

NMR-derived porosity shows good agreement with conventional laboratory measurements. Analysis of T₂ relaxation distributions indicates that pore systems are dominated by bound fluid components, consistent with limited pore connectivity and strong capillary sealing behaviour. By integrating NMR with conventional petrophysical data, this work builds a high-resolution database of shale properties, reduces uncertainty in caprock seal performance, and supports safe and reliable CCS storage design. The outcomes are directly relevant to offshore Peninsular Malaysia and contribute to national energy transition and climate objectives.