Characterization and performance study of OBM at various oil-water ratios

Abstract

Drilling fluids are an essential part of the drilling operation. One of the main functions of a properly maintained drilling fluid is to suspend cuttings, as well as weighting materials, in static conditions. Insufficient gel strength or low-shear viscosity can enhance settling of weight material, known as barite sag. Some drilling fluids display elastic and viscous characteristics at lowshear rates. This means that the fluid possess solid-like and liquid-like qualities. This is known as viscoelasticity. It is thought that viscoelastic behavior can give an advantageous knowledge about dynamic settlement. This involves studies of drilling fluids viscoelastic behavior at deformation rates far below the conventional viscometer range. This study assess the influence of increasing water fraction in oil based drilling fluids with the use of viscoelastic measurements, computing of the Unified hydraulics model and cuttings transport simulation. Four different oil based drilling fluids were used in this study. The density was equal for all samples at 1750 kg/m3 with an oil-water ratio increasing from 60:40 towards 90:10 and at the same time keeping the viscosifying-clay concentration constant. The experiments conducted in this thesis included dynamic and static sag measurements to asses which fluid was most susceptible for barite sag. In addition to conventional rheological methodology, a total of five different types of viscoelastic measurements was included in the search for a qualitative characterization. Viscosity profiles at shear rates far beyond conventional viscometer was obtained from a shear rate ramp. In the evaluation of yield stress in drilling fluids, a controlled stress ramp was performed. A performance evaluation has been performed with the use of hydraulic wellbore simulation and cuttings transport simulation. This was performed by computing the Unified hydraulics model and by using Landmark’sWellPlan.

The experimental investigations shows that the water fraction in oil based drilling fluids has a significant impact when the drilling fluid was formulated with 7 kg/m3 organophilic clay. The fluid sample with the lowest water fraction showed a behavior of a viscoelastic liquid, while the remaining samples showed viscoelastic gel characteristics. These characteristics are shown to be time and temperature dependent. The characterizations also included a yield stress evaluation based upon five different measurements techniques, which reveal that the apparent yield stress is much lower than estimated by theHerschel-Bulkley model and Bingham Plastic model. Hydraulic simulation showed that a higher water fraction impose an increase in ECD and pump pressure, while the cuttings transport simulation showed a beneficial effect from the increased water fraction. This is primarily caused by the higher viscosity.