Geochemistry of Wellbore Integrity in CO2 Sequestration: Portland Cement-Steel-Brine-CO2 Interactions (Invited)
Abstract
Effective geologic sequestration of CO2 requires long-term storage with very low leak rates. Numerous studies have identified wells as one of the key risk factors for CO2 leakage including purpose-built injection and monitoring wells in addition to older wells in and above the storage reservoir. All wells have the potential to leak due to faulty construction or other defects. However, geochemical reactions induced by CO2 could result in damage to Portland cement and steel that are used in the well to isolate reservoir fluids from underground drinking water sources and the surface. This concern is based on the thermodynamic incompatibility of CO2-saturated aqueous fluids with Portland cement and steel, which leads to relatively rapidly reactions that form, principally, calcium carbonate and iron carbonate. Despite this thermodynamic fate, wellbore materials perform and maintain zonal isolation in field and experimental observations. This is understood as a consequence of coupled behavior between flow of reactants (CO2-water) and the rate of dissolution and precipitation of cement or corrosion of steel. In the restricted flow environments found in wellbore systems, cements are carbonated but do not suffer significant deterioration of hydrologic or mechanical properties. In fact, cement carbonation often results in reduced permeability and enhanced mechanical strength. While steel is susceptible to corrosion, wellbore environments allow development of protective iron carbonate scale. In addition, the presence of Portland cement, even carbonated cement, provides protection against significant rates of corrosion. The impact of geochemical reactions in the wellbore environment cannot be separated from coupled flow, thermal and mechanical processes. CO2-induced chemical reactions migrating upward from a storage reservoir will not result in the creation of defects or the wholesale dissolution of cement or steel. Defects must exist that allow CO2×brine to flow and to come in contact with well materials. These defects may originate in faulty construction or arise from thermal and mechanical stresses that crack cement or separate cement-casing and cement-formation interfaces. Once flow along defects occurs, CO2-induced reactions may aggravate or ameliorate the condition through dissolution or precipitation. Several experimental and field studies show that cement and steel have substantial self-healing tendencies. These arise from the precipitation of calcium and iron carbonates, but also appear to originate from swelling of residual phases and/or migration and reprecipitation of cement phases. The mechanical behavior of the cement system is of particular importance as plastic deformation of reacted and unreacted cement appears to provide limits to the aperture of defects and thus limits to the effective permeability of wellbore leaks.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V31D..05C
- Keywords:
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- 1012 GEOCHEMISTRY Reactions and phase equilibria;
- 1832 HYDROLOGY Groundwater transport;
- 1822 HYDROLOGY Geomechanics;
- 3630 MINERALOGY AND PETROLOGY Experimental mineralogy and petrology