Quantification of Elastic and Strength Properties Degradation in High Porosity Sandstones Due to CO2 Interaction

ABSTRACT: A study has been conducted on CO2 interaction degradation effects on the mechanical properties of reservoir formations for two carbon capture and storage projects in offshore Peninsular Malaysia. This study focuses on two new potential CO2 fields storage (B-field and J-field), with each presenting higher porosities compared to other CO2 degradation studies reported in the region. In this study, difficulties arose when testing the high porosity sandstone samples using multi-stage triaxial testing method. These high porosity samples tend to fail prematurely before achieving at least 3 confining cycles, particularly the post-CO2 ageing samples. Subsequent adjustments were made to the unloading criteria, which were controlled based on the volumetric strain over axial strain ratio, to prevent any further premature failure to the subsequent samples. From the results of the B-field and J-field pre- and post-CO2 ageing multi-stage triaxial tests, a reduction varying from 4.4-23.9% were observed for unconfined compressive strength, whilst for friction angle and cohesion, a reduction of 8.5-25.9% and 13.4-20.5% were generally observed. On the other hand, for Young's modulus of the samples, the results showed a reduction ranging from 5.7-42.9% whereas Poisson's ratio has changes ranging between reduction of 7.5-21.4% to an increase of 5.6%. This study emphasises the considerable degradation of rock mechanical properties observed in sandstones due to their interaction with CO2. It reveals that exposure to CO2 can significantly alter the structural integrity of these rocks, leading to a decrease in strength and stiffness. This degradation is critical to understand, as it directly impacts the viability of potential CO2 storage sites. In addition, it reveals the property degradation necessitates careful adjustments to the unloading criteria when conducting multi-stage triaxial testing on high porosity samples that have undergone CO2 ageing as they are susceptible to premature failure, largely due to their amplified friability. This underscores the importance of adapting testing protocols to account for the degraded mechanical properties of these materials, ensuring more reliable assessments of their performance in CO2 storage applications.