Experimental Study on the Phenomenon of Barite Sag
Authors: Parvizinia Ali , Ahmed Ramadan Mohammed , Osisanya Samuel Olusola 
Published: March 2015

Increasing global energy demand and diminishing petroleum reserves have continually elevated the significance of extended reach deepwater drilling. The exorbitant cost of such specialized technologies necessitates efficient operations and the shortest time to target possible. Settling of barite particles (barite sag) while drilling often results in undesirable equivalent circulating density (ECD) and pressure fluctuations in directional wells. The sedimented particles of barite form a bed on the low side of a wellbore, causing density differences in the cross section, which generate pressure imbalance and downhole ECD fluctuations. Barite sag phenomenon can lead to a variety of drilling problems such as lost circulation, stuck pipe, poor cement job and wellbore instability. The phenomenon is likely to occur under dynamic conditions, elevated temperatures and inadequate annular flow velocities. Barite sag is also exacerbated in the absence of drillstring rotation and low annular diameter ratios.

This article presents an experimental study conducted on barite sag behaviors of oil-based fluids. A cylindrical sagtesting cell has been developed to measure the level of barite sag at different shear rates (from 0 to 0.82 1/s) and temperatures (80°F and 120°F). The cell has a rotating dick at the top to create the shear field in the cylinder, which is filled with test fluid.

Pressure sensors were mounted on the wall of the cylinder to measure sag tendencies of the sample as a function of pressure gradient. Several tests were conducted on OBM fluid at different temperatures and disk rotation speeds. Density and rheological properties of the samples were measured before and after the test.

The experimental results indicate significant barite sag, especially in fluids subjected to high shear rates and elevated temperature. For the fluids tested, the change in temperature had the greater influence over sag behavior in comparison to shear rate. Results suggest that the viscosity of the oil-phase, which is very sensitive to temperature, has more pronounced effect on sag than the rheology of the mud system. The outcomes of this investigation are very useful for wellbore pressure management and drilling optimization.

1. Introduction

During drilling of oil and gas wells, mud is required to perform many tasks including hole cleaning, lubrication and cooling of the drill bit, stabilization of the wellbore and bottom hole pressure control. Stability of the mud is critical for successful completion of a drilling operation. Sedimentation of barite particles causes density variations in the cross section of a wellbore. This generates pressure imbalance that induces secondary flow (i.e. downward sliding of heavy barite bed and upward flow of lighter fluid layer). The secondary flow results in a convection current that in turn accelerates the settling process. Barite sag can lead to a variety of drilling problems such as lost circulation, stuck pipe, poor cement job, excessive fluid losses and wellbore instability (McLean & Addis, 1996). Although a number of studies have been conducted to understand the mechanisms responsible for initiation and exacerbation of barite sag, this phenomenon is still not fully understood.

The principal objective of this study is to examine the effects of fluid rheology, shear rate and temperature on static and dynamic barite sag. Sedimentation theory suggests that the fluid temperature enhances the rate of barite sag by reducing the viscosity of the fluid. The shear rate is also expected to increase the rate of barite sedimentation due to drilling fluid's shear thinning behavior and subsequent viscosity reduction. The shearing of the fluid could also encourage the separation of heavy solids particles from the mud structure, which may in turn increase the sagging rate. Moreover, heavy components of drilling mud have the tendency to aggregate and settle out of the fluid.