Barite Sag is an oilfield term used to describe significant density variations while circulating bottoms up after a trip, logging run or other operations that require the mud to remain static for an extended period of time. Sag is caused by static and/or dynamic settling followed by slumping of the weighted material. Static sag, as the name suggests is caused when circulating is stopped for an extended period of time, and the weighting agents begin to settle under the influence of gravity. Due to slumping, dynamic sag occurs frequently in inclined holes. However, the problem has been exacerbated by the increased frequently of high-angle wells with the associated increase in particle settling rate which occurs in an inclined fluid column. The settling of solids is enhanced by convective currents created by density differences in the fluid across the annulus cross section. This effect is frequently referred to as Boycott effect.
This paper presents the most effective ways to avoid the occurrence of barite sag in high-pressure high temperature (HPHT) drilling operations, and in high-angle wells. The ways such as replacing barite by other weighted materials such as Ilmenite or Manganese tetraoxide. Furthermore, it is suggested to use a cesium formate based drilling fluid or polymer-coated ultra-fine barite for drilling HPHT wells. Moreover, different technologies to predict dynamic sag such as, flow loop tests and modified rotational viscometer tests, are introduced.