Conceptual injection of CO2 into backfilled stope

Abstract

The focus of conventional carbon capture and storage (CCS) is primarily in depleted oil and gas reservoirs, deep saline aquifers, and basaltic formations. This study explores novel conceptual CCS approach involving the storage of supercritical CO2 in mined-out underground stope. In this approach, the stope is backfilled while intentionally leaving an unfilled void space at the top, which serves as localized CO2 storage zone. The key objective is to evaluate whether backfilled stope opening can maintain sufficient structural integrity and stability for localized CO2 storage, while potentially making use of existing mining infrastructure in an economically efficient manner. A combined numerical modelling approach was adopted using ANSYS or RS3. ANSYS was used to simulate thermal response of the storage chamber and resulting thermally induced stresses in the cemented paste backfill (CPB). Subsequently, three-dimensional geomechanical modelling was carried out in RS3 to assess mechanical behavior of the surrounding rock mass and the backfill system. The RS3 model considered 4 sequential stages: virgin stress state, excavated stope, backfilled stope with and intentionally created void at the top, and CO2 injection stage. As gases cannot be directly modelled in RS3, the mechanical effect of stored CO2 were represented by equivalent loads applied to chamber boundaries. Under the assumed supercritical storage conditions, the chamber temperature reaches approximately 405K and induced thermal stress at the top of the backfill was about 3MPa. The modelling results indicate that excavation produces the greatest disturbance to the initial in siu stress regime, generating stress concentrations at crown and corners. In the virgin state, major principal stress ranged from 23 to 29MPa, whereas after the excavation maximum major principal stress increased to approximately 94MPa near the stope crown. Full stope excavation produced maximum total displacement of 0.042m, concentrated near roof and sidewalls. After backfilling maximum major principal stress decreased to 86MPa, indicating partial stress redistribution and improved support. After the CO2 injection, the mechanical response remained localized around the storage chamber. The displacement directly beneath the chamber reached a maximum of 0.013m, with deformation decreasing toward the stope walls because of confinement from surrounding rock mass. Overall, the study indicates that CO2 storage in the void space above the backfilled stope may be feasible at a conceptual level, although chamber boundary and upper backfill interface remain the most critical zones for further investigation.