Study of carbonation in a class G Portland cement matrix at supercritical and saturated environments

Abstract

The carbon dioxide (CO2) is a chemical compound that can be present at subsurface, inside the porous of reservoir rocks containing oil or natural gas, dissolved or free. Being its occurrence natural or artificial, when injected into the reservoir at high pressure in order to improve the oil recovery factor. Regardless the source, it has potential to cause serious problems to the cement used during certain operations in oil wells. When in contact with hydration products of the Portland cement, the CO2 reacts due to a specific phenomenon denominated carbonation. This transformation significantly affects the cementitious composite array, causing changes in the microstructure, chemical compounds and harmful consequences for physical properties, such as porosity/permeability and mechanical resistance. The present study investigated the effects of CO2 under two conditions, supercritical state and dissolved in water, in a class G Portland cement matrix that is used in the oil industry. Samples containing the same formulation were placed at the same time in an autoclave, exposed to mediums with supercritical CO2 or saturated in water and removed after 30 days. In order to corroborate in the discussions and conclusions, the following analyses were conducted: pH indicator for measuring carbonation depth, image analysis to quantify the percentage of the affected area, X-ray diffraction using the Rietveld method for phases identification and quantification, and thermogravimetric analysis in order to confirm the presence of certain compounds. The analysis showed very distinct results, being the samples that were exposed to saturated medium suffering a greater attack. Based on the images analysis, formation of carbonates identified by XRD and quantified by the Rietveld method, the attack on samples subject to the medium saturated with CO2 was 35 percentage points higher when compared with the supercritical medium.