The occurrence of barite sag in drilling fluids has relatively often been the cause for gas kicks in oilwell drilling. The subsequent absorption of gas into drilling fluid could lower the density and reduce the viscosity of the drilling fluid, thereby aggravating both pressure control and hole cleaning. In this paper, we present experimental measurements of rheological properties and barite sag in a typical North Sea oil-based drilling fluid at downhole pressure and temperature conditions. A new experimental apparatus was setup for barite sag measurements at static condition with operational temperature and pressure capabilities up to 200 °C (392°F) and 1000 bar (14,503.8 psi), respectively. Rheometry measurements were conducted on fluid samples with and without barite particles at operating conditions up to 90 °C and 100 bar. We observed that at a typical shear rate of 250 s⁻¹, which is experienced in 8.5″ hole annulus, the viscosity of fluid sample with barite increased nearly three times as that of the fluid sample without barite as the temperature and pressure increased. However, temperature effect on viscosity dominates at high shear rates compared to pressure effect. Furthermore, the fluid samples showed more shear-thinning effect with increasing yield stress as the temperature increased. On the other hand, barite sag measurements revealed that whereas fluid samples under high pressure are less prone to sag, high temperature fluid samples, however, promote sag significantly. The data from this study are useful to validate extrapolations used in computational models and to improve understanding and operational safety of sag phenomena at downhole conditions. We also discuss the importance of this study in optimizing drilling operations.

钻井液中重晶石流挂的出现相对经常是油井钻井中气涌的原因。随后气体被吸收到钻井液中会降低钻井液的密度和粘度，从而加重压力控制和井眼清洁。在本文中，我们介绍了在井下压力和温度条件下典型北海油基钻井液的流变特性和重晶石垂度的实验测量结果。设置了一种新的实验装置，用于在静态条件下测量重晶石垂度，工作温度和压力能力分别高达 200 °C (392°F) 和 1000 bar (14,503.8 psi)。在高达 90 °C 和 100 bar 的操作条件下，对含有和不含重晶石颗粒的流体样品进行了流变测量。⁻¹, 8.5″井眼环空经历, 随着温度和压力的升高, 含重晶石流体样品的粘度比不含重晶石流体样品的粘度增加了近三倍。然而，与压力效应相比，温度对粘度的影响在高剪切速率下占主导地位。此外，随着温度的升高，流体样品随着屈服应力的增加而表现出更大的剪切稀化效应。另一方面，重晶石下垂测量显示，虽然高压下的流体样品不易下垂，但高温流体样品会显着促进下垂。这项研究的数据有助于验证计算模型中使用的外推，并提高对井下条件下垂现象的理解和操作安全性。