- © 2013 Mineralogical Society of America
Geochemistry and wellbore integrity in CO2 sequestration
Effective geologic sequestration of CO2 requires long-term storage with very low leak rates. Injection wells are an obvious leakage pathway for CO2 because they perforate the confining caprock. In addition, sequestration sites are likely to use monitoring wells to assess performance and, in the case of depleted oil and gas fields, may contain 10s to 1000s of older operating and abandoned wells.
All wells may have leakage pathways due to faulty construction or other defects. However, it is the subject of this chapter to consider whether geochemical reactions induced by CO2 could result in damage to wells and the development of leaks. This concern is based on the thermodynamic incompatibility of CO2-saturated fluids with the Portland cement and steel used to prevent fluid migration to the surface. Portland cement is an alkaline substance with pH > 12.5 and is not in equilibrium with CO2-bearing fluids (pH < 6). Low-carbon steel used as well casing is subject to aggressive corrosion by carbonic acid. As a result, well integrity has been a central issue in risk analysis of sequestration sites (Gasda et al. 2004; IPCC 2005; Viswanathan et al. 2008; Nordbotten et al. 2009).
At the outset, it is important to bear in mind that geochemical reactions alone do not yield insight into wellbore integrity, which is governed primarily by the effective permeability of the Portland cement seal and the mechanical integrity of the system. Thus the significance of chemical reactions must be considered with respect to their impact to changes in permeability or in strength. There is an unfortunate tendency in the literature to not distinguish between reaction and impact. Often CO2 reactions are described as “degradation of cement” or “corrosion of cement” without adequately defining what these terms …