Measuring and Predicting Dynamic Sag

Abstract

Summary Weighting-material sag is a reoccurring problem with many oil-based drilling fluids. Attempts to correlate sag tendencies to various rheological properties commonly used to benchmark drilling fluids have had limited success in prevention and anticipation of sag problems in the field. This paper presents a new testing apparatus for dynamic and static settling-rate (sag) measurements, which has proved to provide a better understanding of the sag phenomena and a better means to characterize fluid performance. This apparatus greatly expands the precision of sag measurements over previous techniques and allows testing conditions similar to those experienced downhole. Good correlation has been found between settling-rate measurements and performance of drilling fluids in the field. Introduction Sag is a variation in density of a drilling fluid caused by settling of suspended particles or weighting material in a wellbore. Laboratory and field experience suggests that sag is often worse in dynamic situations caused by pumping, pipe rotation, and tripping. However, sag can occur in either static or dynamic conditions. In the presented apparatus, measurements are performed at prescribed shear rates, elevated temperatures to 177°C (350°F), and pressures to 690 bar (10,000 psi). Additionally, the apparatus requires only a 50-cm3 sample for complete analysis. The settling-rate measurements obtained are useful in planning and as a diagnostic tool for sag performance in active drilling-fluid systems. Preliminary Laboratory Studies A typical way to control the shear of a non-Newtonian drilling fluid is to use a concentric-cylinder configuration with the sample fluid occupying the annulus. If either the outer or inner cylinder is rotated relative to the other, the annular fluid is subjected to an approximately uniform shear field that can be modeled easily. The configuration is comparable to the common oilfield viscometer and is commonly referred to as "Searle geometry" if the inner cylinder rotates relative to a stationary outer cylinder or as "Couette geometry" if the outer cylinder rotates relative to a stationary inner cylinder. Cylinder rotation combined with axial flow of the annular fluid would more closely resemble the borehole configuration, but would greatly complicate the computational modeling and control. Flow loops usually expose the sample to a range of shear rates in contrast to the constant shear rates possible in the simpler system. A flow loop also would require a high-pressure pumping system, as well as added unnecessary bulk, sample volume, and system complexity. A preliminary study apparatus was assembled (Fig. 1), which consisted of a clear-plastic outer cylinder approximately 2 m (6 ft) long and 7.62 cm (3 in.) internal diameter (ID), with sealing caps closing the ends. Bushings in the caps supported a rotatable concentric inner stainless-steel tube of 3.81-cm (1.5-in.) outside diameter. This gave a diameter ratio of 0.50. In later studies, another clear tube was centered in the original outer tube with an internal diameter of 5.08 cm (2 in.), giving a diameter ratio of 0.75. The narrower annular gap more closely approximates ideal Searle flow. The entire apparatus was pivoted on a bench-mounted knife edge, near the center, and tilted at 45° from vertical. A pivoted strut from the top end of the tube rested on a electronic laboratory digital scale, setting the angle of tilt and allowing the measurement of the imbalance force. A gear motor mounted on the upper end of the outer tube was arranged to belt drive the inner cylinder. The motor speed was adjustable by an electronic drive. A temperature-controlled bath was connected to the inner rotating tube in a way that allowed the tube to rotate while fluid from the temperature controlled bath circulated through it. When the annulus of the tubes was filled with a sample of drilling fluid, changes in the center of gravity could be tracked by monitoring the scale readings. Sample taps at intervals along the bottom side of the sloped outer tube allowed measurements of the density of the fluid at that those points.

Barite–Micromax mixture, an enhanced weighting agent for the elimination of barite sag in invert emulsion drilling fluids

“…Because of the solids settlement, the fluid density will vary, causing a severe well control issue (Saasen et al 2002;Zamora and Bell 2004), and interference with the drilling operations (Mohamed et al 2017). This phenomenon occurs in both directional and vertical wells, whether under dynamic or static conditions, most commonly with dynamic conditions at low shear rates (Murphy et al 2008).…”

“…Several experimental studies were performed to simulate and measure the barite sag tendency in the laboratory under static and dynamic conditions. One of the common methods to evaluate the sag tendency of the drilling fluid is measuring the weight difference between the top and bottom of the fluid column using flow loops, sag cell setup, and viscometer sag show test (Hanson et al 1990;Aldea et al 2001;Saasen et al 2002;Temple et al 2004;Murphy et al 2008;Alabdullatif et al 2015;Davis et al 2017;Basfar et al 2018;Elkatatny 2018Elkatatny , 2019Boyou et al 2019;Mohamed et al 2020). Another method of barite sag measurement is using the rheological and viscoelastic properties obtained by different rheometers and correlates the data with sag tendency (Chilingarian et al 1983;Saasen et al 1995;Power and Zamora 2003;Maxey 2007;Bern et al 2010).…”

New Lab Tests and Models Improve Planning for Complex Cementing Jobs Offshore New Zealand

“…An apparatus that is able to measure sag in drilling fluids is the DHAST (Dynamic High Angle Sag Test) as shown in Figure 3 below. Murphy et al reported on this new lab device for testing drilling fluids in SPE-103088-PA-P (Murphy et al 2008). To our knowledge, testing of other fluids such as cement spacers (Figure 4) and slurries has not been discussed in the literature prior to the application described below in this paper.…”

“…The DHAST apparatus used for the phase two investigation was the second version described by Murphy et al (Murphy et al 2008) which can apply a wide range of shear rates to dynamic fluids in the test cell's annulus at high angles, temperatures up to 350ºF (177ºC), and pressures up to 10,000 psi (690 bar). As further discussed below, testing showed a "soft" settling issue in both the spacer and the mud, determining that neither fluid was optimum for highly deviated wells, but could be used without serious effect on the ability to place all of the cement slurry in the annulus by pumping all of the displacement mud.…”

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

“…Moreover, the accumulated solid particles and drilled cuttings increase the possibility of pipe sticking and decrease the rate of penetration [20]. Barite sag phenomenon may occur in both vertical and directional wells at static and dynamic conditions, and it is more serious under dynamic conditions with a low shear rate [21]. However, maintaining and monitoring drilling fluid rheology and training the drilling rig personnel would mitigate the consequences of solids sag [22].…”