An Improved Model for Accurate Description of Drilling Fluid Rheological Behaviour in Enhanced-Water and Low-Toxicity Oil-Based Muds

Abstract

In this study, a new rheological model was developed by the introduction of dimensionless plastic viscosity and shear rate correction factors (k and n) respectively to the plastic viscosity stress-term of the Bingham plastic rheological model with a view to correcting the under-estimation errors that are associated with the model at high shear rate condition in the pipe. Similarly, an effective yield stress τeff was also incorporated into the model to correctly describe the resistance to flow at low shear rate condition in the annulus by taking into account the effect of lowest shear rate stress on the yield-stress term of the model. The drilling fluid matrix consists of a low toxicity oil-based mud (LTOBM) and a high-performance, nanoparticle-enhanced water-based mud (EWBM). The rheological characteristics of these muds were evaluated at temperatures of 30°C, 60°C and 120°C and pressures of 14.7psi, 2500psi and 5000psi respectively. The experiments were conducted under API recommended standard equipment and procedures. Also, statistical tools were employed to quantify the degree of deviation of the model from experimental values. Empirical results of the new model comparative rheological performance analysis with different existing models that are commonly used in the oil industries showed that the developed model accurately predicts fluid rheology better than the Bingham plastic model at both high and low shear rates conditions in the LTOBM and EWBM at all tested temperatures and pressures. The new model performed better than the Bingham plastic and the power law models at 30°C and 14.7psi for the EWBM with lowest standard error deviation of 1.5096 as against 4.4392 and 2.2573 for BPRM and PLRM respectively. Similarly, the average absolute error of the new model for EWBM at 30°C and 14.7psia is 1.6874 unlike BPRM and PLRM with (EAA) values of 5.3249 and 2.8704 respectively. The new model is not significantly temperature and pressure dependent at high shear rate.