CO2 attack on API Class-G cement and the role of CaO swelling cements admixtures

Abstract

API Class-G cement is Portland cement that commonly applied as zonal isolation material in oil and/or gas well. Corrosion against the zonal isolation could lead to potential leakage of formation fluid such as oil, gas, or water. It is well known that ordinary Portland cement is thermodynamically unstable in the CO2 rich environment, which can be found in the CO2 geological sequestration. The CO2 attack on API Class-G cement had been observed through experiments that simulate the downhole condition of Geological Sequestration of CO2. Some of the experiment revealed the detrimental effect of CO2 such as increase of porosity and permeability of the cement sheath corroded by the CO2 attack. The degradation of API Class-G cement by CO2 involves formation and crystallization of CaCO3 from Ca(OH)2 which is formed during hydration of cement. In this experiment, we examine the role of the addition of CaO swelling cement by weight fraction to mitigate the corrosion caused by CO2 against downhole hardened API Class-G cement thus inhibits the occurrence of leakage path of CO2. The characterization and analysis by XRD, optical microscope and cement paste uniaxial volumetric expansion were presented in this paper. The conclusion will provide insights into the design of carbonation resistant cement slurries using swelling cement admixtures.