The hydrated products of Portland cement drastically change after exposure to high-temperatures, compromising the cement physical properties, especially, its compressive and tensile strengths, this phenomenon is known as strength retrogression.

Previous studies showed that the use of silica flour (SF) enhances Class G oil wells cement (OWC) resistance to the strength retrogression due to the formation of long silica chains. In this work, the influence of adding modified montmorillonite nanoclay (NC) particles, which are nanoparticles of layered mineral silicates, on Class G cement strength retrogression resistance under the high-temperature condition of 300°C was evaluated.

Six cement slurries were considered in this study, the base sample which has no silica or nanoclay particles, one sample contains 35% BWOC of SF particles only, and 4 samples incorporating 1.0, 2.0, 3.0, and 4.0% BWOC of NC and 35% BWOC of SF were prepared and tested under conditions of low (38°C) and high (300°C) temperature after 7 days of curing. The 300°C was selected to represent one thermal cycle condition when steam is injected into the oil well to increase the oil production for the purpose of enhanced oil recovery (EOR). After preparation, the samples were poured into different molds with specific dimensions based on the targeted test, then cured at the low-temperature condition of 38°C using a water bath, the samples were cured for 7 days. Some of the samples cured at the low temperature for the whole period while others removed in the last three days and cured at a high temperature of 300°C to mimic one steam injection cycle condition.

In order to evaluate the effect of the NC particles on mitigating the cement strength loss at high-temperature, the unconfined compressive strength (UCS) and tensile strength tests were performed. The change in the permeability of the samples as a function of NC content and temperature were evaluated. The percentage loss in the water absorbed by NC particles after exposing the cement samples to the high-temperature condition (300°C) was measured.

The results revealed that the use of NC (up to 3.0% BWOC) can prevent the cement deterioration under extremely high-temperature conditions of 300°C. This is attributed to two facts, first of all, the NC particles reduced the initial permeability of the samples by filling the nanoscale porous these expected to dominate the control samples (i.e. sample with 0% nanoclay), secondly acceleration of the hydration reaction which results in formation of more stable forms of calcium silicate hydrates (CSH) which leads to enhancement in the cement matrix resistance to the expected forces. At high-temperature environment, the original permeability of the NC-based cement matrix increased mainly due to evaporation of the water absorbed by NC particles when their concentration is maintained below 3.0% BWOC, the use of NC content beyond that concentration (i.e. >3.0%) severely damaged the cement matric microstructure due to agglomeration of nanoparticles.

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