Lateral Variations in Geomechanical Properties of Mudstones: An Outcrop Analogues of Caprocks for Geological Carbon Storage

ABSTRACT: A better understanding of geomechanical properties of mudstone caprocks over a targeted storage area is crucial for effective geological carbon storage. However, it is difficult to evaluate lateral variations in geomechanical properties in detail because primary subsurface information obtained through appraisal wells is limited in most cases. Utilization of onshore outcrops as an analogue of subsurface geology enables us to overcome limitation of primary subsurface data. The present study investigated lateral variations in geomechanical properties of semiconsolidated mudstones collected from outcrops with precise lateral correlation. Triaxial compression tests of the mudstone samples revealed that both deformation and strength-related parameters do not show remarkable lateral variation in a spatial range in which pressure disturbance is induced by CO2 injection. It is also inferred from the present study that outcrop samples can be used to evaluate lateral variations in geomechanical properties with precise lateral correlation and the triaxial compression tests under the same confining pressure condition. 1. INTRODUCTION For effective geological carbon storage (GCS), a better understanding of geomechanical properties of caprocks over a targeted storage area is crucial. Considering spatial range of pressure disturbance induced by CO2 injection, lateral variation of geomechanical properties in a few to tens of km need to be assessed in detail, whereas primary subsurface information obtained through appraisal wells is limited in most cases. Variations in geomechanical properties, such as Young's modulus and Poisson's ratio, have been evaluated in some case studies for the characterization of shale gas reservoirs that targeted tight mudstones (e.g., Rickman et al., 2008). On the other hand, regarding GCS, a few case studies have only been conducted. Especially, it is pointed out that studies targeted the Neogene–Quaternary soft sedimentary rocks regarding GCS are very limited (Fujii et al., 2024), although GCS projects targeting these rocks are extensively planed in regions such as Southeast Asia and Japan.