Effect of Drilling Muds Wall Slip on Reliable Frictional Pressure Loss Estimations in Offshore Drilling

Abstract

The large variations in temperature and pressure conditions that prevail in deepwater operations have drastic effects on the rheology of drilling fluids used. Under such conditions, rheological properties of oil-based drilling fluids could be detrimentally affected. The main reasons of the aforementioned include; the high sensitivity of its organic phase thermal expansion (compression) coefficient and its large viscosity buildup observed with decreasing temperatures. The consequences on drilling hydraulics may include high surge/swab pressures, poor hole cleaning, and the potential of substantial errors in estimated static and equivalent circulation densities; thereby compromising well integrity. Rheological models used to fit flow curve data of drilling fluids from commonly used field instruments such as the Fann viscometer, do not capture wall slip phenomenon prevalent in highly concentrated suspensions and emulsions as a result of the no-slip boundary condition in their constitutive equations. This could mean significant errors in the derived drilling hydraulic parameters for fluids displaying wall slip. In this study, water-based (WBM) and oil-based mud (OBM) samples were prepared using different surfactants and their rheology was characterized using a Fann 35 viscometer and MCR 52 rheometer at temperatures ranging from 10 – 75 °C. This was done to highlight differences in frictional pressure loss estimations between a commonly used field instrument and a higher precision rheometer for muds displaying wall slip behavior. The shearing gaps used in the investigation were 1.17 mm for the Fann 35 viscometer and 0.5 – 2.0 mm for the MCR 52. Wall slip behavior of the mud samples, as well as discontinuous shear thickening due to jamming of dispersed particles, was found to affect agreement in flow curve measurements between both instruments. For set well depth and environmental conditions, lower frictional pressure losses and cuttings transport efficiency were determined for samples showing high levels of wall slip, particularly at high temperatures. The importance of the outcome is underscored by its importance to the successful and timely completion of the wells, as well as total equipment cost.