A successful drilling operation requires an effective drilling fluid system. Due to the variety of downhole conditions across the globe, the fluid system should be designed to meet complex challenges such as High-Pressure/High-Temperature (HPHT) environments, while promoting better productivity with a minimum interference for completion operations. This study aims to improve the rheological and fluid loss properties of water-bentonite suspensions by using both commercial (C-NP) and custom-made (CM-NP) iron oxide (Fe3O4) nanoparticles (NP) as drilling fluid additives.

Superparamagnetic Fe3O4 NP were synthesized by the co-precipitation method. Both types of nanoparticles were characterized by a High Resolution Transmission Electron Microscope (TEM) and X-ray Diffraction (XRD). Base fluid (BF), made of deionized water and bentonite at 7wt%, was prepared according to American Petroleum Institute (API) procedures and nanoparticles were added at 0.5wt%. A Couette-type viscometer was used to analyze the rheological characteristics of these fluids at different shear rates and various temperatures (up to 158°F). The rheological parameters were obtained from analysis of viscometric data using non-linear regression. The API Low-Pressure/Low-Temperature (LPLT) and HPHT fluid filtrate volumes were measured, using a standard API LPLT static filter press (100 psi, 77°F) and an API HPHT filter press (300 psi, 250°F). Observation of the porous matrix morphology of the produced filter cakes was done with Scanning Electron Microscope (SEM).

TEM showed that the mean diameter of the CM-NP was 7–8 nm, with measured surface areas between 100–250 m2/g. The C-NP had an average diameter of <50 nm, as per manufacturer specifications. The XRD of the CM-NP revealed peaks corresponding to pure crystallites of magnetite (Fe3O4) with no impurities. Rheological analysis showed very good fitting by the Herschel-Bulkley model with coefficient of determination (R2) greater than 0.99. Rheological properties of all samples were affected by higher temperatures, with increase in yield stress, decrease in flow consistency index (K) and slight increase in flow behavior index (n). Fluid filtration results indicated a decrease in the LPLT fluid loss and an increase in the filter cake thickness compared to the BF upon addition of higher concentrations of C-NP, because of a decrease in filter cake permeability. At HPHT conditions, samples with 0.5wt% C-NP had a smaller fluid loss by 34.3%, compared to 11.9% at LPLT conditions. CM-NP exhibited even higher reduction in the fluid loss at HPHT conditions of 40%.

Such drilling fluids can solve difficult drilling problems and aid in achieving the reservoir’s highest potential by eliminating the use of aggressive, potentially damaging chemicals. Exploitation of the synergistic interaction of the utilized components can produce a water-based system with excellent fluid loss characteristics while maintaining optimal rheological properties.

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