Carbon dioxide (CO2) is injected into subsurface rocks either to improve hydrocarbon recovery or for permanent storage in geologic formations. At storage sites, CO2 injection wells are drilled and completed with multiple-string casings, which are cemented to the host rock. Cement is the primary means of protecting these casings from corrosive fluids and isolating storage zones from overlying fresh water aquifers by effectively sealing the casing-rock annulus. Over the life of the well, dissolved CO2 interacts with the casing-cement-rock system thereby degrading the hardened cement and resulting in loss of zonal isolation and structural integrity required to support the casing and to ensure long-term integrity of the well. Cement performance is often assessed through laboratory measurement of compressive strength, porosity and permeability. These parameters are indicators of mechanical integrity. Although compressive strength in the range of 0.7 to 5 MPa is generally sufficient to continue drilling after the casing is cemented, further hydration and chemical degradation occur throughout the operational life of the well. In CO2 injection wells, environmental conditions (i.e. CO2 concentration, pressure and temperature) around the cement result in further alteration in mechanical properties. This hostile environment causes severe mechanical damage and ultimate failure of cement sheath, potentially leading to micro-channelling and formation of micro-annuli. In this study, experiments were conducted to investigate the effect of CO2 concentration on mechanical integrity of well cements (Classes G and H) after exposure to carbonic acid environment. Cement cores were prepared and aged for 14 days in autoclave filled with 2% NaCl solution saturated with methane gas containing carbon dioxide. Aging tests were performed at 38°C and 42 MPa, varying CO2 concentration. To assess the level of degradation and describe the process of acid attack, compressive strengths of unexposed and exposed specimens were measured and compared. In addition, other properties of cement cores including porosity, permeability, stiffness/elasticity and Fourier Transform Infrared Spectroscopy (FTIR) measurements were obtained and used in the assessment. After 14 days of exposure, increase in CO2 concentration leads to insignificant changes in compressive strength due to combined effect of carbonation of cement hydrates (calcium silicate hydrates and calcium hydroxide) and subsequent leaching, with each process compensating for the other. Porosity, permeability and FTIR measurements are consistent with this observation. Under the experimental conditions adopted in this study, durability of Classes G and H cement are comparable. However, Class H cement becomes more elastic (i.e. less stiff) than Class G cement as CO2 concentration increases. FTIR mineralogy provides evidence that the mechanical behaviour of well cement after exposure to CO2-saturated brine is a direct consequence of interconnected chemical processes. Under carbon sequestration conditions, these processes counterbalance one another to ensure long-term integrity of the well for CO2 storage and containment.

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