Evaluation of calcined Saudi calcium bentonite as cement replacement in low-density oil-well cement system

Fly ash is most commonly used as a partial replacement of Portland cement in lightweight oil-well cement systems. The fly ash is a waste material generated during coal combustion to generate energy. The material is facing an imminent phase-out due to global warming issues associated with coal combustion. This study, for the first time, investigates the potential of calcined calcium bentonite as an alternative pozzolanic material in oil-well cementing. The bentonite was calcined at 830 °C for 3 h. Lightweight cement slurries were formulated at 12.5 ppg using the calcined clay in the ratio of 10%–30% cement replacement and cured at a BHST of 50 °C and a BHP of 1000 psi. The optimum concentration was found to be 20%. A comparative study of the calcined calcium bentonite (CCB) and fly ash-based systems designed at 20% cement replacement was performed. The XRD and thermogravimetry analysis showed that CCB and fly ash exhibit similar pozzolanic reactivity and the cement systems designed with these materials show equivalent characteristics in terms of rheology, strength, pore size distribution, porosity, and microstructural properties. The addition of 5%–10% silica fume can be used to further improve the early-age compressive strength of low-density cement systems designed with calcined calcium bentonite.