Experimental and modeling studies on dynamic barite sag and effects of particle size, shear rate, and fluid rheology

Abstract

Barite is often utilized to control the weight of drilling muds. However, in inclined wellbores, barite particles settle out of the mud in a lateral direction and quickly reach the wellbore wall. Over time, deposited particles form unstable barite beds that slide intermittently downward, causing downhole pressure fluctuations. The observation is often referred to as barite sag, which occurs prominently when the system is in a dynamic state. Barite sag leads to density variation in the mud system, resulting in drilling issues such as stuck pipe, fluid loss and lost circulation, loss of well control, and wellbore instability. Various experimental studies have been conducted on barite sag, but limited mathematical modeling has been undertaken.This article presents the results of experimental and mathematical modeling studies on barite sag. First, particle size distribution tests were performed with API barite. Then, different oil-based muds (OBMs) were prepared using barite samples with various particle size distributions. Next, the apparent viscosities of the muds were measured with a rheometer. Then, the Viscometer Sag Shoe Test (VSST) method was used to study barite sag at 49 °C varying rotational speed and barite particle size distribution. Furthermore, the settling behavior of barite particles under dynamic conditions was simulated using a sedimentation-consolidation model. The model is solved numerically by applying a fully implicit finite difference method. The accuracy of the numerical solution is validated using simulation results presented in a previous study. The model has been improved to handle non-Newtonian fluids and simulate dynamic conditions.According to the apparent viscosity measurements, barite particle size has a minor effect on OBM rheology. The VSST results also show that barite sag increases with particle size and rotational speed. The improved model predicts VSST results fairly well. Additionally, a model-based parametric study indicates that barite sag decreases with increasing OBM rheological parameters (consistency index, yield stress, and fluid behavior index).