Old core, new tricks: a comparative study of old and new mudstone cores for applications in the energy transition

Abstract

Repurposing depleted oil and gas reservoirs for secondary storage may play an important role in the transition to low-carbon energy. The integrity of the cap rocks overlying the reservoirs is an important factor for gas storage and needs to be understood prior to repurposing. In some cases, old cap-rock cores collected during exploration and development of oil and gas fields may be available for characterization using modern techniques but after being stored for decades these cores are likely to have experienced many changes in moisture, which can lead to physical changes. A comparative study of samples taken from old, unpreserved mudstone core and samples from a recently acquired and preserved core taken from the same formation shows that the mineralogy, porosity and permeability results are relatively similar between the two cores. The differences in the porosity measurements between the old and new core samples are primarily due to natural variations in grain size, rather than the preservation status of the cores. Geomechanical data, however, show significant and non-systematic differences between the old samples and the new samples, suggesting that old core samples are not suitable for geomechanical characterization. In the absence of new, well-preserved core, old core samples may provide suitable porosity, permeability and mineralogical data, whereas the old, unpreserved core is unlikely to provide reliable geomechanical data. Supplementary material: Individual BIB-SEM porosity measurements for each sample are available at https://doi.org/10.6084/m9.figshare.c.6725765 Thematic collection: This article is part of the Fault and top seals 2022 collection available at: https://www.lyellcollection.org/topic/collections/fault-and-top-seals-2022