Abstract

Settling of weighed drilling mud has always been a problem, but in directional drilling the problem is magnified through rapid settling of barite and cuttings on the lower side of the borehole, which after some accumulation start sliding downwards. Sagging of barite may disrupt safety and lead to operational problems during drilling and completion.

To study the effect of rheology on settling, a laboratory tool. the sag tester, was designed, consisting of a small pipe, 45 degree inclined, and a vertical collector pipe attached to the low side of the inclined tube. The pressure was measured at the bottom of each pipe. With a stable mud both pressures remained constant. For an unstable mud, the settling of barite particles caused a reduction of the fluid density and a change in pressure. After some accumulation, the barite bed in the upper part of the inclined pipe slid and fell into the vertical collector pipe increasing the pressure here, but not in the inclined tube. The difference between the two pressures can be applied as a measure of mud instability.

The behavior of several mud's were studied at simulated dynamic (tripping, pressure surges and circulation) and static conditions The results show that sagging is most severe during laminar flow. Stable and unstable mud's can clearly be differentiated through their pressure behavior in the Sag Tester.

Introduction

In weighted drilling mud barite tends to segregate slowly. In directional drilling operations the settling process is accelerated. Barite settles in the lower side of the borehole and starts sliding (or sagging) when the borehole has an inclination > 30 °. Sagging can lead to drilling and completion problems; a density variation or unlinear hydrostatic pressures gradients which can lead to pressure control problems, while thick and tight barite beds can lead to high torque and drag, stuck pipes and plugged boreholes and even lost circulation.

The sag problem is related to the so called Boycott Effect1, first described in 1920. Hanson et. al.2 have investigated the phenomenon and found that most of the sagging occurs while the mud is circulating. The same conclusion was reached by Bern et. al.3, the sagging tendency is highest at low annular velocities. Zamora and Jefferson4 presented a method for tracking drilling fluid density variations, which helps to detect, but not to predict drilling mud instability. Jamison and Clements5 developed a test method to characterize settling/sag tendencies in static drilling fluids, however their equipment was not able to distinguish between settling and sliding. They also found that their data, based upon standard API rheological parameters like PV and YP, were unsuitable for prediction of sagging behavior. It is apparent from previous works that the most difficult part of the problem is the prediction of drilling mud instabilities for both static and flowing drilling fluids. To date there are no API test procedures for sag testing.

A new simple lab tool was therefore designed for the purpose of study and develop a method to predict sagging under ambient conditions

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