Capillary heterogeneity trapping of CO₂ in a sandstone rock at reservoir conditions

TLDR

CO₂ storage in deep subsurface porous rocks is being developed worldwide to mitigate emissions, but preventing buoyant CO₂ migration to the surface remains a key concern. This study investigates how natural capillary heterogeneity in a sandstone core affects CO₂ saturation buildup and trapping through high‑pressure, high‑temperature core‑flooding experiments. Experiments were conducted at 9 MPa and 50 °C on a Mt. Simon sandstone core comprising a high‑permeability upstream region and a low‑permeability downstream barrier with high entry pressure, during which CO₂ accumulated upstream of the barrier during drainage.

Abstract

The storage of CO 2 in deep subsurface porous rocks is being developed worldwide for the mitigation of emission from large industrial sources such as power plants and steel manufacturing. A main concern of this technology is in ensuring that the upwardly buoyant CO 2 does not migrate to the surface. Simulation studies suggest that substantial amounts of CO 2 can be trapped within permeable sections of a reservoir by capillary forces and intra‐reservoir heterogenities, but there is little experimental observation of these phenomena. We report the results of CO 2 core flooding experiments at high pressure and temperature performed to investigate the impact of natural capillary heterogeneity in a sandstone rock on CO 2 saturation buildup and trapping. CO 2 and water were injected through a Mt. Simon sandstone core at 9 MPa pore pressure and 50°C. The core had two regions of distinct capillarity: An upstream 10 cm long region of the core consisted of a relatively high permeability and homogenous sand. A downstream 3 cm long region of the core consisted of a low permeability region characterized by significant cross‐bedding and a high capillary entry pressure for CO 2 . During a drainage process of CO 2 displacing water, CO 2 builds up upstream of the capillary barrier. Once in place, CO 2 on the upstream side of the barrier cannot be displaced during 100% water flooding leading to trapped saturations that are a factor 2–5 times higher than what would be expected from residual trapping alone.