Shear behavior of sandstone fractures after exposure to CO₂ environment

Abstract

CCS (Carbon Capture and Storage) is a promising technique for greenhouse effect mitigation by geological sequestration of CO₂. Deep saline aquifers are considered the major storage reservoirs, mainly due to their abundance on the earth’s crust. Even though rock joint sets (fractures) are critical characteristics of rockmass (both for their mechanical and hydrological behavior), they are not simulated in detail in reservoir scale models for CO₂ storage. That's due to significant demands for computational power and model design complexity. Advances in discrete medium models and discrete fracture network representation for models/reservoirs of considerable size will allow the usage of jointed rock as a discrete medium, for assessing reservoir hydromechanically. Hence, assessments of the performance under CO₂ exposure of rock fractures inside CO₂ environment will be necessary. In this study, we tested planar and rough, induced fractured sandstone specimens for shear mechanical behavior, under reservoir conditions. Results revealed a friction angle reduction (−23%) of exposed rough fractures, compared to unexposed ones. The moderate fracture roughness, even though still present, did not contribute as much to shear strength under effective normal stresses up to 1.5 MPa, with rough fractures behaving likewise to planar ones. Gouge production on fracture surfaces contributes to shear strength loss, calcite dissolution is considered the major reason for such behavior. We also observed inadequacy for shear resistance on planar exposed specimens when normal stress exceeds a critical value.

Keywords:

• CO₂
• shear stress
• fracture
• sandstone
• carbon sequestration

Acknowledgements

We thank AUTH geology department staff (associate professors Papadopoulou L. and Kantiranis N and Ph.D.–cand. Kipouros N.), for RXD analysis and thin section preparation.

Disclosure statement

No potential conflict of interest was reported by the authors.