Emprego de aditivos em pastas de cimento para poços de hidrocarbonetos e injeção de CO2: influência na resistência à degradação por ataque ácido

Abstract

The issue of greenhouse gases particularly the emission of carbon dioxide into the atmosphere, has concerned the world opinion about climate changing. Therefore new technologies to diminish these gases have been developed in order to restrain global warming. Geologic carbon sequestration is been appointed as one effective way to achieve this goal for a short period of time. However, researches show that when injecting carbon dioxide into sedimentary geological formations, deep saline aquifers, coal seams and depleted or abandoned fields of oil exploration the environment becomes more aggressive and the material used in oil well construction is subjected to successive chemical attacks, which may compromise its integrity, allowing gas leakage and occasional contamination. It has been observed that class G cement paste may suffer some degradation in a shorter period of time than expected oil well service life due to a chemical attack in presence of CO2. The cement paste plays an important role in isolating other well components from the production areas. Thence, admixtures, which modify cement compound properties, may be used in order to increase the slurry performance. The purpose of this paper is to determine the changes caused by adding these substances into properties of cement paste, both fresh and hardened. Also, it is necessary to study the influence of water reducing anti-foam and silica fume admixtures on the resistance to carbonation in cement pastes whether they are used individually whether together. Different amount of admixtures pastes have been subjected to carbonation tests in acid environment in two degradation environments wet superficial CO2 and water saturated with CO2 at temperature and pressure conditions of 70º C and 15MPa respectively during a 7 day time period. Differential thermal analysis (TGA) and the use of optical microscopy scanning electron (SEM) were used to determine the matrix changes of cement paste induced by the incorporation of admixtures. Changes in the strength were also evaluated. Results showed that all mechanical properties were improved with the use of admixtures tested, while the resistance to carbonation showed positive results in low w/c pastes, anti-foam and all the admixtures together. However, silica fume incorporation, individually and water reducing admixture provided a decrease of resistance to degradation in CO2 presence.