Sealing Quality Of Cementing Systems To Be Used on Carbon Capture & Storage CCS Wells

Abstract

Carbon capture and storage (CCS) is considered to be one of the most effective ways to dispose of CO2 generated in coal or gas fired power plants. Cementing such CCS wells presents a technical challenge since ordinary Portland cement (OPC) is known for its corrosion potential with CO2. Such corrosion involves formation and crystallization of CaCO3 from Ca(OH)2 which is formed during the hydration of cement. Furthermore, CaCO3 can be leached as Ca(HCO3)2. This process can create channels in the cementitious matrix for CO2 migration to the surface. Here, three Portland cement based systems were formulated with different mineral and chemical admixtures and compared with a common API Class G oil well cement with respect to CO2 stability. The systems investigated were based on various concepts. For one cementing system, the aim was to fill the pore space in the cementitious matrix with inorganic, CO2 inert particles while the second was treated with a number of chemical admixtures, particularly latex polymer, to achieve the same goal. The third system was based on slag cement (CEM III) formulated with fly ash. Hardened specimens (30×50 mm) of the four cementing systems were prepared and stored under supercritical CO2 at 90 °C and 400 bar for one and six months, respectively. The autoclave was half filled with a synthetic reservoir fluid, thus allowing storage in a CO2 saturated salt brine and in wet supercritical CO2 atmosphere. Ingress of CO2 into the hardened cement specimens was probed via phenolphthalein test. From all systems, only the slag cement (CEM III) blended with a reactive filler (fly ash) exhibited good CO2 resistance. This cement contains a specific ratio of SiO2:CaO which produces only a minor amount of portlandite which can carbonate. For the other cement samples, severe corrosion was observed. The cement system containing chemical admixtures and the neat API Class G system even exhibited severe cracking as a result of high crystallization pressure during CaCO3 growth. Still, carbonation rates were relatively low (~ 0.5 m/year). The major finding of this study is that common API oil well cement sufficiently withholds CO2 attack if formulated in such way that only minor amounts of CaCO3 are formed, and if the w/c ratio is high enough to provide sufficient pore space for expansion of CaCO3 crystals.