Comparative Evaluation of Threshold Pressure Measurement Techniques in Faulted and Shale Formations

Akpewe Messiah Eroh , Paul W.J Glover, Samuel Allshorn

Reliable estimation of capillary threshold pressure is essential for evaluating the sealing capacity of subsurface flow barriers, which has important applications in fluid storage. This study compares the results of capillary threshold pressure measurements obtained from four experimental techniques: (i) gas breakthrough, (ii) mercury injection capillary pressure (MICP), (iii) Porosimetry under Confining Stress (PUCS), and (iv) Field’s metal injection with SEM imaging, applied to faulted sandstone from the Vale of Eden and mudstone from the Sidmouth Formation of the Mercia Mudstone Group (MMG). Gas breakthrough tests demonstrated strong internal consistency and repeatability; however, the technique is inherently slow and has very low throughput, often requiring several months to complete a single test on a typical mudstone sample. The MICP method exhibited considerable variability among the analysed samples and was unable to access the matrix porosity of mudstone due to compressibility and pore structure collapse. Mercury instead, preferentially occupied fractures and micro-cracks, as confirmed by SEM imaging of Field’s metal injection, an artefact that is often misinterpreted as mercury filling pore volume. Field’s metal injection yielded higher threshold pressures and provided valuable pore-scale insights from SEM imaging, but the method lacks standardized conversion parameters, making comparison with other methods difficult. These findings underscore the limitations of conventional methods in characterizing tight formations and highlight the need for caution when interpreting MICP results, particularly for shale and other low-permeability rocks. Furthermore, they emphasize the importance of developing a robust experimental framework that addresses these shortcomings, especially for tight gas and shale formations where accurate measurement of flow properties is critical for subsurface storage applications, including CO₂ sequestration and the disposal of nuclear waste in deep geological repositories.