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In this paper, we report the results of the investigations on the effects of sodium hydroxide (NaOH) concentration and different ratio of silicate to hydroxide on the density, rheology and compressive strength of geopolymer cement system used in oil well. Different ratios of Class F fly ash is mixed with different ratios of sodium silicate to sodium hydroxide ratio (2.5, 1, 0.5 and 0.25) to produce different geopolymer slurry densities and dispersant is added to control the rheological properties of the system. The NaOH solution was prepared by diluting NaOH pellets with distilled water according to the specified molarity (8, 10, 12 and 14 M). The solution was then mixed with sodium silicate to form the alkaline solution. Class F fly ash were added to the reactive to form homogeneous mixture, which was tested for its density and rheological properties at surface temperature. The mixture was placed in a 50 mm mould and cured at 930C and 3000 psia for 24 hours and the cubes were tested for destructive compressive strength. The results showed that as the concentration of sodium hydroxide increases, the density of the geopolymer cement increases. There was no significant variation on the density of the geopolymer cement. Also, as the ratio of silicate/hydroxide increases, the viscosity of the slurry increases and the workability of the geopolymer cement become poorer. Furthermore, the compressive strength increases as the NaOH molarity increases however when it reaches 14M, the adverse effect to the strength development was observed.

The oil-based mud is preferred to drill highly technical and challenging formations due to its superior performance. However, the inadequate chemical and thermal stability of conventional additives have greatly influenced the performance of oil-based mud at high-temperature conditions. Therefore, it is critical to design an oil-based mud with additives that withstand and improve its performance at high-temperature conditions. The nanoparticles have emerged as an alternative to the conventional additives that can significantly enhance the rheological and filtration characteristics of oil-based mud at high-temperature conditions. In this research study, a novel formulation of OBM enhanced with GNP is formulated, and its performance at high-temperature conditions is investigated. An extensive experimental study has been performed to study the effect of graphene nanoplatelets on the rheological and filtration properties along with flow behaviour, viscoelastic properties, electrical stability and barite sagging of oil-based mud at high temperatures. The graphene nanoplatelets are characterised to ascertain their purity and morphology. The result shows that the graphene nanoplatelets exhibited efficient performance and improved the rheological and filtration properties of oil-based mud. The plastic viscosity and yield point are improved by 11% and 42%, with a concentration of 0.3 ppb. Similarly, the gel strength and barite sagging tendency are enhanced by 14% and 2%, respectively. The filtration loss is also significantly decreased by up to 62% and 46%, with 0.5 ppb concentration at 100 and 120 °C. The addition of GNP results in the formation of a thin mud cake compared to the base mud sample. The rheological modelling recommends the shear-thinning behaviour of oil-based mud (n < 1), which is correlated with the Herschel–Bulkley model. An Artificial Neural Network model is developed to predict the viscosity of OBM based on the four input parameters (concentration of nanoparticles, temperature, shear rate and shear stress). The results demonstrate that graphene nanoplatelets have a favourable impact on the performance of oil-based mud. The addition of graphene nanoplatelets, even at small concatenation, has significantly improved the properties of oil-based mud at high-temperature. Graphical abstract

Ensuring oil-well-integrity is one of the challenging tasks when cementing is designed. It has been well established that cement tends to degrade when exposed to a corrosive environment and at elevated temperatures. This paper presents the results of the uniaxial compressive strength of the qualified mixes of geopolymer cement containing fly ash as the precursor. Geopolymer cement samples were cured in the potable water heated at 60 °C and 90 °C for 24 h before testing for uniaxial compressive strength. Uniaxial confined compressive strength test was performed for samples cured at a 60 °C, and results of the samples bearing density of 13, 15, and 17 ppg were obtained as 4.12, 9.21 and 17.68 kPa, respectively. For 90 °C, the compressive strengths were 4.43, 15.34 and 78.14 kPa, respectively, for the samples bearing the same density. Samples cured at 90 °C showed the higher value of UCS as compared to the samples cured at 60 °C, and it was because heat is required to stimulate the polymeric reaction.

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