Wellbore integrity in carbon sequestration environments: 1. Experimental study of Cement–Sandstone/Shale–Brine–CO2

Abstract It is important to determine the geochemical reactions between common cements used in wellbore construction, formation mineralogy, and supercritical CO 2 stored in deep saline reservoirs, because abandoned and completed wells provide a pathway for release of the stored CO 2 back to overlying aquifers and to the atmosphere. Although it is known that alkaline cements readily react with acidic CO 2 -rich waters, the influence of the formation and cement mineralogy on the bonding of the wellbore cement to the caprock at pressure and temperature conditions associated with saline CO 2 storage reservoirs is uncertain. We reacted end member components of the heterolithic sandstone and shale unit that forms the upper section of the carbon storage reservoir at the Krechba Field, In Salah, Algeria with supercritical CO 2 and class G cement in a representative brine at 95 °C and 10 MPa in gold bag autoclaves to identify geochemical reactions that occur in the wellbore environment. The experimental system allows reaction progress of complex geochemical environments to be tracked by sampling the aqueous phase periodically over the two-month long experiments. Analysis of the solution chemistry over time and the solid products show that the wellbore environment is dominated by reactions between cement, carbonate, and clay minerals when exposed to CO 2 -rich fluid. Reaction of the hydrated cement with synthetic brine equilibrated with supercritical CO 2 rapidly forms amorphous silica, calcite, and aragonite. Similar reaction products were observed when cement reacted with sandstone and CO 2 (reservoir). However in the vicinity of the shale (caprock) cement minerals altered to calcium carbonate minerals and smectite (clay).