Abstract To be able to keep a well under control during drilling it is important to have the weight material suspended during the whole operation. Especially in oil based, or synthetic based drilling fluids, controlling sag is difficult.A large number of wells have experienced lost circulation or well control problems with the associated high costs relating to sag. There are limited field proven methods available to predict the potential for sag for a particular drilling fluid. Therefore, barite sag continues to be one of the most difficult tasks to handle for a drilling fluid engineer. It is well known that standard viscosity readings cannot be used to predict the sag potential of a drilling fluid.
The current paper discusses in detail the mechanisms of sag in oil based drilling fluid and how to treat sag. It is shown that at ultra-low shear rates the oil based drilling fluid is shear thickening as opposed to the shear thinning effect observed at higher shear rates. It is described how this shear thickening behaviour leads to sag if the drilling fluid composition is incomplete. It is described how organophilic clay should be used to minimize sag and why different types of clay have different effect onto hindering sag although none of these differences have to be visible on standard viscometer readings. Furthermore, it is suggested that "modifiers" generally have little effect on preventing sag. Different methods to monitor or predict the sag potential of drilling fluids are also outlined. Dilution of the drilling fluid normally results in increased sag. Good practice is that the dilution with new drilling fluid should be minimized to control the sag properties of the oil based drilling fluid.
Introduction Settling of weighting materials in drilling fluids may cause several problems during drilling and completing of a well. While drilling, these problems range from having insufficient drilling fluid density for well control to fracturing the formation when re-suspending a weight material bed. In addition the settling of weighting agents may hinder the running of casing as well as causing insufficient displacement efficiency during cementing operations.Settled weight material may also cause problems during completion operations.
Within the petroleum industry the serious study of the weight material sag phenomenon started in the very late 1980's, including the three important early contributions from Jamison and Clements, Hanson et al. and Jefferson.Jamison and Clements used an inclined tube to study barite sag in static drilling fluids. Analysis of their data shows a tendency of increased sag potential with reduced viscosity or gel strength. There was, however, a significant scatter in their data.Based on this scatter they concluded that it was not possible to relate the sag potential of a static drilling fluid to YP, PV, 10 second or 10 minute gel strength as measured by the standard API methods.
The study by Hanson et al. and Jefferson focused on practical guidelines to prevent sag. They also emphasized the importance of dynamic sag; the sag occurring while circulating drilling fluid. They recognised that preventing dynamic sag is more difficult than preventing static sag.
Boycott settling, the effect of enhanced gravity driven settling in deviated pipes, was found by all the early investigators to give a major contribution to barite sag in drilling fluids. When particles settle downwards in a deviated pipe or annulus, a layer with low-density fluid will be created at the upper part of the flow area. A higher density fluid will be developed at the lower part of the flow area. In this way, a high-density portion of the fluid higher up in a deviated well will overlay a lesser dense fluid layer further down in the well. The existence of dense fluid over less dense is unstable, and a mixing motion is created; accelerating the separation process.