In oil and gas wells construction, the cement sheath in the annulus between casing and formation plays a critical role throughout the producing life of the oil and gas well. Under elevated conditions of temperature and pressure, set cement changes its physical, chemical and mechanical properties. Sometimes these changes can lead to strength retrogression of cement. Recently, it was found that calcined clay (CC) and nanoclay (NC) significantly improve the hardened cement matrix properties as both materials have pozzolanic properties. Both additives have characteristics such as acid and sulfate resistance, low heat of hydration, low density, low permeability and the formed phases with the higher thermal endurance.

In this study, cement mixes with different percentages (1% & 2%) by weight of cement (BWOC) of calcined clay and nanoclay were investigated and compared with Class G cement slurry. Rheology, fluid loss and compressive strength properties of all cement slurries were measured.Rheological properties were measured at 50°C temperature and atmospheric pressure. The fluid loss test was conducted at 50°C and 1000 psi pressure. The uniaxial compressive strength (UCS) test was conducted for cement samples cured at 121°C temperature and 3000 psi pressure conditions for 24 hours and 5 days in HPHT curing chamber. Microstructural investigations were conducted using SEM analysis of cured samples.

It was observed that addition of nanoclay and calcined clay in Class G cement enhanced the rheological properties of cement slurries. The plastic viscosity increased with an increase in percentages of both additives. Higher values of yield point and plastic viscosity were obtained for calcined clay. In case fluid loss, a reduction in fluid loss was noticed for nanoclay. There was no appreciable change in fluid loss with the addition of calcined clay in mixes. Calcined clay-based cement slurries resulted in robust structure and sustained high compressive strength after exposure to high temperature high-pressure conditions. The nanoclay-based mixesgave higher strength compared to Class G cement slurry but lower than calcined clay. The compositional analysis showed that calcined clay gave low calcium/silica ratio and strong bonded structure.

This study will pave a way for researchers to study further the application of calcined clay in oil and gas cementing and its long-term durability.

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