(383c) Settling Characteristics of Weighting Material in the OIL-Based Drilling Fluids

Maintaining a uniform drilling fluid density along the downhole and controlling the downhole pressure are important tasks for successful and safe operation of the drilling process. Settling of the barite particles, which act as the weighting material, can lead to density variations in the downhole as high as 0.5 kg/L, and severe operational problems such as loss of control, lost circulation and stuck pipe, etc. Further, the local density of the drilling fluid or mud plays an important role in preventing gas entrainment and fluid formation in the downhole. Despite continuous efforts by the drilling industry, mitigation of the sagging of weighting materials in drilling fluids continues to be a challenge faced by the oil industry. Therefore, there is a need to understand the settling characteristics of barite particles in oil-based drilling muds. Though there are several publications reported on this topic, no publication has reported on the relation between barite sag and rheological properties of the drilling fluid. Hence in the present work, an attempt has been made to relate the static and dynamic sag potentials with the rheological properties of the oil-based drilling fluid.

Rheological properties of the drilling fluid have been measured using the ARES G2 tensiometer (TA instruments) with a cup-and-bob geometry. A barite sag phenomenon in oil-based drilling fluids was experimentally investigated under static and dynamic conditions. The settling characteristics of the barite particles have been quantified using pressure drop method and gamma densitometry, for the first time. Experiments were performed in a static cell (with no shear) and in a Taylor Couette cell (with low shear stress, to simulate the real operating conditions in the drilling process). Preliminary results indicated the shear thinning behavior of the drilling fluids with the existence of yield stresses at all temperatures (10-70^oC), as evidenced by the decreasing slope of the stress profile with an increase in the shear rate (0.01-1000 S^-1). The temperature dependence was also clearly visible as the viscosity decreased exponentially with increasing temperature. Gamma densitometry experiments using a static cell revealed that the fluid density decreased at an axial location of 6 cm (near the top of the test section) with time. The decrease in the fluid density near the top of the test section occurred due to the gravitational settling of high-density barite particles. On the other hand, the fluid density at lower axial locations (1, 3 and 5 cm from the bottom) increased with time. Interestingly, the increment in the fluid density was approximately the same at all three axial locations, and could be attributed to the gel forming nature of the drilling fluid. The rate of change in the density could be correlated to the sedimentation rate in both static and Taylor Couette cells. Additionally, an attempt has also been made to relate the rate of barite particle settling to the rheological properties of the drilling fluid.