Barite Segregation In Inclined Boreholes

Abstract

Settling and sagging of barite in inclined boreholes may lead to safety and operational problems. To study the effect of rheology on settling, a laboratory tool was designed, consisting of two connected tubes, one inclined and one vertical. The hydrostatic pressure was measured at the bottom of each pipe. Stable and unstable muds can clearly be differentiated through their pressure behaviour. Several muds were studied at simulated static and dynamic conditions. The results show that sagging is most severe during laminar flow and also indicate that the rheological parameters may be used for predicting stability problems. 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 when the borehole has an inclination above 30 °. This phenomenon is known as barite sagging. Sagging can lead to drilling and completion problems; a density variation or non-linear hydrostatic pressure 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 effect(1), 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 Jefferson(4) presented a method for tracking drilling fluid density variations, which helps to detect, but not to predict drilling mud instability. Jamison and Clements(5) developed a test method to characterise settling and 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 behaviour. 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 laboratory tool was therefore designed for the purpose to study and develop a method to predict sagging. Drilling Mud Separation Settling of Particles In vertical wells the settling of weighting material is generally not a problem due to the long settling distance. In horizontal wells the distance to the lower side of the wall is only about 0.2 m, which leads to rapid generation of solids beds. The settling velocity of a single spherical particle, vs,l, in a fluid is expressed by Stoke's law(6): Equation 1 (available in full paper) A barite particle (ρp = 4,200 kg/m3) with a diameter (dp) of 20 µm in a fluid of density (ρfluid) 1,500 kg/m3 and a viscosity (µ??of 40 cP will settle at a rate of 53 mm/h.

Enhancement of Static and Dynamic Sag Performance of Water-Based Mud Using a Synthetic Clay

“…At 0.5 lb/bbl of laponite, the vertical and inclined sag factor decreased from 0.569 and 0.58 to 0.532 and 0.542, respectively, and these values are still above the acceptable value, while adding 0.75 lb/bbl brought the sag factor to the safe range with a sag factor of 0.502 and 0.51; therefore, less sag tendency is anticipated at that conditions. It was observed that the drilling fluid always performed better in vertical conditions than in inclined conditions because the inclination angle contributes significantly to the sagging phenomenon, and the critical range of inclination is between 30 and 60° as reported in previous studies. , Similarly, the base fluid, without laponite, showed unfavorable sag tendency at dynamic conditions with a sag factor (VSST) of 2.3 (Figure . Adding 0.5 lb/bbl of laponite significantly improved the mud homogeneity and reduced the VSST to 1.54, which is still above the acceptable value (below 1). , Increasing the concentration of laponite to 0.75 lb/bbl substantially reduced the VSST to 0.17.…”

Prevention of Barite Sag in Oil-Based Drilling Fluids Using a Mixture of Barite and Ilmenite as Weighting Material

“…Higher concentration (>40 wt.%) is not required, and it will increase the cost of the drilling fluid. The static sag factor at an inclination of 45 • was slightly higher than at vertical conditions because solids settlement is accelerated at inclination between 30 • and 60 • [20,42]. The optimum ilmenite concentration, however, may vary with the used drilling fluid formulations and well conditions which require optimizing the ratio of ilmenite to barite for each formulation before field applications.…”

A Combined Barite–Ilmenite Weighting Material to Prevent Barite Sag in Water-Based Drilling Fluid

“…A static sag test was performed to study the effect of ilmenite on sag tendency of barite-weighted drilling fluid in static conditions. The test was conducted in vertical and inclined conditions (45°) because the solids sag issue becomes more serious at an inclination angle above 30° [33]. Figure 3 shows the experimental set-up used to perform the static sag test.…”