The design of drilling fluids is very important for the drilling operation success. The rheological properties play a key role in the performance of the drilling fluid. Therefore, studying the mud rheological properties of the water-based drilling fluid based on bentonite is essential. The main objectives of this study are to address the effect of pH changes on the rheological and filtration properties of the water-based drilling fluid based on bentonite and to provide a recommended pH range for this drilling fluid for a safe and high-performance drilling operation. Different samples of the water-based drilling fluid based on bentonite with different pH values were prepared, and the rheological properties such as plastic viscosity, yield point, and gel strength were measured. After that, the filtration test was performed under 300 psi differential pressure and 200 °F. The pH for the water-based drilling fluid based on bentonite significantly affects the mud rheology. The shear stress and shear rate relation were varying with the change in the pH. Increasing the pH from 8 to 12 resulted in decreasing the plastic viscosity by 53% and the yield point by 82%, respectively. The ratio of yield point / plastic viscosity was 1.4 for pH of 8 while it decreased to 0.5 for a pH of 11 and 12. There was a significant decrease in the gel strength readings by increasing the pH. The filtrate volume and filter cake thickness increased by increasing pH. The filtration volume increased from 9.5 cm³ to 12.6 cm³ by increasing the pH from 9 to 12. The filter cake thickness was 2 mm at 9 pH, while it was increased to 3.6 mm for 12 pH. It is recommended from the results to keep the pH of water-based drilling fluid based on bentonite in the range of 9 to 10 as it provides the optimum mud rheological and filtration properties. The findings of this study illustrated that keeping the pH in the range of 9 to 10 will reduce the plastic viscosity that will help in increasing the rate of penetration and reducing the required pump pressure to circulate the mud to the surface which will help to sustain the drilling operation. In addition, reducing the filtrate volume will produce a thin filter cake which will help in avoiding the pipe sticking and protect the environment. In general, optimizing the pH of the water-based drilling fluid based on bentonite in the range of 9 to 10 will improve the drilling operation and minimize the total cost.

Weighting agents such as barite, micromax, ilmenite, and hematite are commonly added to drilling fluids to produce high-density fluids that could be used to drill deep oil and gas wells. Increasing the drilling fluid density leads to highly conspicuous fluctuation in the drilling fluid characteristics. In this study, the variation in the drilling fluid’s rheological and filtration properties induced by adding different weighting agents was evaluated. For this purpose, several water-based drilling fluid samples were prepared and weighted up using the same concentration of various weighting materials including barite, micromax, ilmenite, and hematite. The characteristics of the used weighting agents’ (particle size distribution and mineralogy) were measured. Subsequently, the rheological properties of the drilling fluid were obtained using a Fann viscometer at 80 °F. The filtration test was carried out at 200 °F and 300 psi differential pressure to form a filter cake over the sandstone core samples. The properties of the formed filter cake layer such as thickness, porosity, and permeability were determined. Furthermore, the typical properties of core samples including porosity and permeability were assessed before and after the filtration test. The displayed results confirmed that the plastic viscosity (PV), yield point (YP), and filter cake sealing properties were all significantly influenced by the ratio of the large to fine particle size (D₉₀/D₁₀) of the weighting agents irrespective of the weighting material type. Among the examined weighting agents, barite showed novel potency to control both rheological and filter cake properties for 14 ppg drilling fluid. The results showed that D₉₀/D₁₀ is a key factor for the PV and YP properties as increasing the D₉₀/D₁₀ ratio caused PV increase and YP decrease, which indicated that the interaction among the loaded weighting materials in the drilling fluid dominated its viscosity.

Tracking the rheological properties of the drilling fluid is a key factor for the success of the drilling operation. The main objective of this paper is to relate the most frequent mud measurements (every 15 to 20 min) as mud weight (MWT) and Marsh funnel viscosity (MFV) to the less frequent mud rheological measurements (twice a day) as plastic viscosity (PV), yield point (YP), behavior index (n), and apparent viscosity (AV) for fully automating the process of retrieving rheological properties. The adaptive neuro-fuzzy inference system (ANFIS) was used to develop new models to determine the mud rheological properties using real field measurements of 741 data points. The data were collected from 99 different wells during drilling operations of 12 ¼ inches section. The ANFIS clustering technique was optimized by using training to a testing ratio of 80% to 20% as 591 data points for training and 150 points, cluster radius value of 0.1, and 200 epochs. The results of the prediction models showed a correlation coefficient (R) that exceeded 0.9 between the actual and predicted values with an average absolute percentage error (AAPE) below 5.7% for the training and testing data sets. ANFIS models will help to track in real-time the rheological properties for invert emulsion mud that allows better control for the drilling operation problems.