Investigation of Barite "Sag" in Weighted Drilling Fluids in Highly Deviated Wells

Abstract

SPE Members Abstract Significant mud weight variations greater than 4 Ibm/gal have been measured in flowline returns when circulating bottoms up after trips. This phenomenon, called sage, has preceded pack off, lost circulation, failure to get logging preceded pack off, lost circulation, failure to get logging tools to bottom, stuck pipe, and other drilling problems on some directional wells. Two key findings have emerged from an extensive project to investigate and minimize sag in highly deviated wells. First, the density variations are caused by the slumping of beds which form when weight material settles to the low side of the hole. Second, most of the bed formation occurs while the mud is circulating. Previous efforts to investigate sag have focused on static settling. Case histories, results from laboratory flow-loop studies, and operational guidelines to minimize sag and its consequences are presented in this paper. Introduction The extraordinary demands placed on drilling fluids in highly deviated wells are well known. However, it is not well known that lost circulation, stuck pipe and logging tools, and other drilling and completion problems sometimes encountered can be the consequence of barite "sag". Although an informal poll of major operators confirmed sag is a widespread occurrence, lack of documentation in the literature indicates sag has been underestimated as a contributor to these problems. Sag is an oilfield term used to describe significant density variations while circulating bottoms up after a trip, logging run, or other operation requiring the mud to remain static for an extended period. Fig. 1 is a plot of flowline mud weight versus circulation time after a trip on a Green Canyon directional well. Samples from the shaker underflow were weighed on both rig and pressurized mud balances. The variance between the two measurements caused by gas entrainment illustrates one of the difficulties in identifying sag in the field. Historically thought to be more severe in oil muds, sag has been observed in all types of muds from 12–20 Ibm/gal. Differences between the maximum and minimum mud weights have been measured greater than 4 Ibm/gal in the Gulf of Mexico and reported greater than 7 Ibm/gal in oil-base muds in the North Sea. It is highly probable that some degree of sag has occurred in most directional wells. Unfortunately, records are incomplete since density measurements after trips usually are limited to bottoms-up samples. Heavy mud off bottom frequently has been attributed to slugs, dehydration, and/or dispersion. In many cases where sag has been identified, it simply has been tolerated, presumably because the incidence of sag-related problems was thought to be low. P. 223