Effect of Al<inf>2</inf>O<inf>3</inf> nanoparticle on viscoelastic and filtration properties of a salt-polymer-based drilling fluid

Conventional bentonite-based drilling fluids have associated problems while drilling troublesome or high-temperature zone due to degradation of rheological properties and excessive fluid loss. Clay particles present in the drilling fluid tend to block the pore throats and cause blockage to hydrocarbon flow. Similarly, polymers along with the filtrate loss, in the invaded zone, alter the wettability due to which there is a reduction in relative permeability to oil. All of these factors collectively cause a fall in production rates and demand stimulation jobs. The use of nondamaging drilling fluids, a particle-free salt-polymer-based drilling fluid, has addressed these problems by yielding a substantially less filtrate loss. However, at moderately high depths with high temperatures, this drilling fluid fails to maintain rheological properties and hence sagging, carrying, and fluid loss problems remerge. Sagging increases the equivalent circulation density which eventually decreases rate of penetration and induces formation damages. Hence, a need to further improve its properties is much required. This study has investigated the effect of aluminum oxide nanoparticles (Al2O3 NP) on these salt-polymer-based drilling fluids. A significant change was observed in rheological and fluid loss properties of drilling fluid (DF) when doped with different concentrations (0.5%, 0.8%, and 1%) of Al2O3 NP compared to the Base DF. Steady shear (rotational) tests were conducted to study the degree of shear thinning behavior of all the drilling fluids. It was observed that shear thinning behavior of the DF was increased with the addition of Al2O3 NP. Rotational measurements at 30°C showed a decrease in viscosity and shear stress of Al2O3 NP DF with an increase in concentration. However, at elevated temperatures of 60°C and 80°C, the reduction in viscosity of Al2O3 NP DF was substantially less than Base DF. One percent Al2O3 NP DF showed higher viscosity as compared to 0.5%, 0.8% Al2O3 NP, and Base DF at these conditions. To investigate the viscoelastic properties, oscillation tests were performed. Amplitude sweep tests showed an increase in storage modulus (G’) with an increase in Al2O3 NP concentration. While Base and 0.5% Al2O3 NP DF showed viscoelastic liquid in nature, 0.8% and 1% Al2O3 NP DF showed dominant storage properties indicating its viscoelastic solid nature at 30°C, 60°C, and 80°C. From the frequency sweep tests at higher temperatures, 0.8% and 1% Al2O3 NP DF, unlike Base and 0.5% Al2O3 NP DF, showed increase in complex viscosity at lower frequencies indicating structural build-up. Both rotational and thixotropic tests showed less structural regain time for 1% Al2O3 NP DF compared to other drilling fluid. Lastly, fluid loss control property of all the drilling fluids was investigated by measuring the filtrate loss volume in an API filter press apparatus. One percent Al2O3 NP DF showed a greater reduction in filtrate volume with Base DF showing the highest.