Geomechanical Evaluation of a Depleted Chalk Reservoir for CO2 Storage

Abstract

Carbon capture and storage (CCS) is the process of capturing carbon dioxide (CO2) before it enters the atmosphere and storing it for centuries. Depleted hydrocarbon fields represent an opportunity for CO2 storage because of already installed facilities and equipment and available subsurface data recoding during the lifetime of the reservoir. With starting CO2 injection in depleted chalk formations that have experienced elastic and visco-plastic deformation during production, two main processes can occur simultaneously under visco-plastic deformation condition. First, reservoir expansion is often expected as reservoir pressure builds up and effective stresses drops. The second is the reservoir compaction that continues due to viscous deformation even after the cessation of hydrocarbon production. As a result, the reservoir top tends to go up because of build-up pressure which may be limited at the same time with viscous deformation that had induced from the production phase. Taking advantage of these two effects, this study investigates the safest scenarios for CO2 injection in depleted chalk formation to minimize geo-mechanical deformation. For this purpose, a rate dependent constitutive model is utilized to calculate deformation in 1D simulation during depletion and injection phases. The constitutive model is calibrated for pelagic chalk from the North Sea and takes into account the water weakening effect. The simulation uses experimental and well log data obtained along two wells in Harald East field. We evaluate the impact of various operational conditions for CO2 injection in depleted reservoirs on long-term deformation of the storage sites.

Keywords: depleted reservoirs, CO2 injection,creep, geomechanic, chalk