Long-Term Study of Effects of CO2 Exposure on Cement Integrity Under Downhole Conditions

The industry is exploring for oil and gas in reservoirs that contain in-situ corrosive fluids, such as CO2 and H2S. In addition, there has been a push to sequester the CO2 produced from various industrial sources. To this effect, geological carbon dioxide storage is an economically viable method to help reduce greenhouse emissions and CO2 injection is an effective enhanced oil recovery (EOR) method. Ensuring long-term safety and efficacy is one of the primary technical challenges in this context attributed to potential degradation of the cement sheath, creating pathways for leakage. Design of CO2-resistant slurries is an important mitigation approach. This paper discusses an experimental evaluation of the chemical effects of CO2 on key properties of cement sheaths. Twelve slurries were developed based on the two primary approaches to mitigate CO2 degradation: (a) reduction of reactive hydration products and (b) reduction of cement matrix permeability. This paper reports results for four selected slurries in a carbonic acid environment. Seven samples of each slurry were cured and then subjected to an aqueous CO2 environment (carbonic acid) for 15, 30, 90, 180 and 365 days. Downhole conditions were simulated by running the carbonation tests at 165°F and constant CO2 partial pressure of 2, 000 psi. At the specified time points, samples were removed to perform chemical tests (inductive coupled plasma [ICP], water analysis, X-ray diffraction [XRD], thermogravimetric analysis [TGA], and dye penetration) and physical tests (ultimate compressive strength [UCS], Brinell hardness, permeability, and porosity). The general observation was improvement in terms of physical properties up to 90 days, followed by a decrease that continued throughout the experiment period. Formulations mixed with NaCl brine were additionally degraded by salt leaching. The best resistance to degradation was obtained using formulations with reduced portland content and elastomers to reduce permeability. Higher compressive strength was correlated with low permeability. Most slurries had a permeability lower than 0.1 millidarcy (mD) after one year of exposure. Chemical testing supports the interpretation that the early improvement in physical properties can be attributed to carbonate precipitation, while later degradation is attributed to the formation and leaching of calcium bicarbonate. Some slurries contained sodium chloride in mix water at saturation levels. In which case, sodium chloride leached out of the cement sheath and was confirmed by water analysis. In the carbonic acid (wet) environment, a dense layer of calcium carbonate was observed that corresponded to an increase in Brinell hardness This study demonstrates the efficacy of a testing protocol that can be used to evaluate various cement blends for long-term integrity under downhole conditions and different forms of CO2 attack. The results were used to design cement systems more resistant to CO2 degradation than conventional slurries.