Evaluation of a low carbon cement containing calcined clay for oil well cementing

Abstract

Well cements play a key role in zonal isolation of wellbores and preventing fluid or gas migration on oil, gas and geothermal wells. Creating an environmentally safe cement with excellent placement and sealing properties presents a major goal of the petroleum and geothermal industry. Unfortunately, common API (American Petroleum Institute) Class oil well cement exhibits a relatively high CO₂ footprint of ∼800 kg CO₂/ton cement. To reduce this significant CO₂ emission in the production of oil well cement, calcined clay (CC) presents an option as a clinker substitute. To this end, in this study, a 50:50 wt/wt. blend of API Class G cement and a calcined clay was investigated with respect to its rheological and thickening behavior and its response to common oil well cement additives (dispersant, retarder, fluid loss additives (FLA)). Experiments were carried out at water-to-cement blend ratios of 0.44 and 0.50, respectively and at low to medium temperatures (27 ^°C, 50 ^°C and 80 ^°C). It was found that calcined clay increases water demand and plastic viscosity while yield point is decreased. Moreover, owed to the lower reactivity of calcined clay, it prolongs thickening time (pumping time) and requires less retarder. The results signify that within the temperature range tested here, this cement blend could achieve excellent pumpability, adjustable thickening times using a lignosulfonate retarder, and low fluid loss rates when a common FLA was applied. Furthermore, a climate-neutral cement exhibiting a CO₂ footprint of ∼450 kg CO₂/ton binder can be achieved, yet it requires a clinker substitution rate of 70 % and increased dispersant dosage. It is demonstrated that calcined clay presents a technically feasible and environmentally preferable alternative to OPC (Ordinary Portland Cement) clinker in oil well cements, and that it can significantly reduce the CO₂ footprint of well cements.