Caprock Porosity and Permeability Characterisation for CO₂ Storage Seal Integrity Assessment Using Preserved Core and Drill Cuttings

Carbon capture and storage (CCS) requires sealing caprocks to ensure the stored CO₂ is contained in the reservoir and not leaking towards the surface. Many caprocks are composed of mudrocks, a siliciclastic sedimentary rock with a high clay content. We here analyse such a caprock from the Field S which is a potential CO₂ storage site in the Sarawak Basin, East Malaysia. The determination of important caprock properties, like porosity and permeability, is often difficult and requires fresh core material that is adequately stored for lab testing. When such core material is not available, drill cuttings might be used as alternatives to predict formation properties if suitable empirical relationships are available which is the objective of this study. Therefore, we performed a petrophysical characterisation of core and cutting samples using mercury intrusion porosimetry, unsteady state pulse decay permeameter, nuclear magnetic resonance and helium pycnometer measurements. Only MIP and helium pycnometer are suitable to determine porosity and permeability on drill cuttings, while plug samples can be used for permeability tests under subsurface stress conditions. The lab data has then been correlated with well log data to further understand the porosity and permeability trend for prediction purposes. The caprock in Field S is divided into two facies (Seal A and Seal B) with varying clay contents of ~20 and ~40% respectively. However, laboratory derived porosity and permeability values do not vary significantly between the two facies. This might be because Seal B (higher clay content) is significantly over-pressured compared to Seal A, potentially resulting in porosity preservation during compaction, resulting in increased permeability. This is consistent with the time-to-depth conversion from seismic data, where Seal B is identified as being undercompacted in comparison to Seal A. Here, we will present our workflow in predicting porosity and permeability of caprocks based on a petrophysical and mineralogical database developed for Seal A and B. We will discuss issues with this approach and its potential and highlight the difficulties in determining permeability from confined plug or unconfined cutting samples.