HR: 1340h
AN: U43C-1398    [Abstracts]
TI: Impact of Wellbore Cement Degradation on CO₂ Storage Integrity
AU: * Kutchko, B
EM: Barbara.Kutchko@netl.doe.gov
AF: US Department of Energy, National Energy Technology Laboratory, PO Box 10940, Pittsburgh, PA 15236, United States
AU: * Kutchko, B
EM: Barbara.Kutchko@netl.doe.gov
AF: Carnegie Mellon University, Department of Civil and Environmental Engineering 5000 Forbes Ave, Pittsburgh, PA 15213, United States
AU: Strazisar, B
EM: Brian.Strazisar@netl.doe.gov
AF: US Department of Energy, National Energy Technology Laboratory, PO Box 10940, Pittsburgh, PA 15236, United States
AU: Lowry, G
EM: glowry@cmu.edu
AF: Carnegie Mellon University, Department of Civil and Environmental Engineering 5000 Forbes Ave, Pittsburgh, PA 15213, United States
AU: Dzombak, D
EM: dzombak@cmu.edu
AF: Carnegie Mellon University, Department of Civil and Environmental Engineering 5000 Forbes Ave, Pittsburgh, PA 15213, United States
AU: Thaulow, N
EM: nthaulow@rjlg.com
AF: RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, PA 15146, United States
AB: The sequestration of CO₂ in underground geologic formations requires a thorough evaluation of potential leakage of the sequestered CO₂ through the numerous existing wellbores which penetrate them. Leakage rates of less than 1% per 100 years have been deemed necessary for geologic sequestration to be viable. Well bores are of particular interest because the cement used to line and/or plug the well, may be vulnerable to acid attack. Injected CO₂ will dissolve, becoming carbonic acid, which can readily react with calcium hydroxide and calcium silicate hydrate, key components in hardened cement. Laboratory experiments have been performed in order to determine the physical and chemical changes, as well as the rate of degradation of the cement under simulated sequestration reservoir conditions, including both aqueous and supercritical CO₂. Upon exposure to aqueous CO₂, hardened cement formed well-defined reaction zones by a 2-step process. The first step is the dissolution of Ca(OH) _{2 (s)} and subsequent precipitation of CaCO_{3 (s)}. The formation of CaCO_{3 (s)} has been reported to decrease cement permeability and increase its compressive strength. The second step is the dissolution of CaCO_{3 (s)} resulting in a leaching of calcium from the cement matrix. The resulting cement paste has a significant increase in porosity, is primarily composed of amorphous silica gel, and lacks structural integrity. Although it is clear that cement is degraded, the results of this study suggest that the reactions involved are slow. In fact, long term experiments show that the rate of degradation decreases over time, likely due to the precipitation of CaCO_{3 (s)} within the pore space of the cement. This phenomenon should limit the negative impact that chemical degradation will have on well bores. Supercritical CO₂ exposure (saturated with water vapor) led to a very different process by which CaCO_{3 (s)} was deposited throughout the matrix and on the surface, rather than within an isolated reaction zone. Over the one-year time period of the experiments, this condition led to a smaller amount of total degradation than in the aqueous phase. However, in this case, there was no deceleration of the reaction observed. It is unlikely that the diffusion controlled degradation process observed in these experiments would lead to well failure in well completions that are well cemented with neat Portland cement (without additives). Further investigation is required to evaluate the effect of cement additives, fractures or channels in the cement, and geomechanical stress.
DE: 1600 GLOBAL CHANGE
DE: 1834 Human impacts
SC: Union [U]
MN: 2007 Fall Meeting