Barite Sag: Measurement, Modeling, and Management
- P.A. Bern (BP-Amoco) | Eric van Oort (Shell) | Beatrice Neustadt (Shell) | Hege Ebeltoft (Statoil) | Christian Zurdo (Elf) | Mario Zamora (M-I Drilling Fluids) | K.S. Slater (M-I Drilling Fluids)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2000
- Document Type
- Journal Paper
- 25 - 30
- 2000. Society of Petroleum Engineers
- 1.7.5 Well Control, 1.6.10 Running and Setting Casing, 1.6 Drilling Operations, 1.6.1 Drilling Operation Management, 1.14.1 Casing Design, 1.11.4 Solids Control, 1.12.6 Drilling Data Management and Standards, 2.7.1 Completion Fluids, 1.11 Drilling Fluids and Materials, 1.1 Well Planning, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.10 Drilling Equipment, 2 Well Completion
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A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behavior. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field.
Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60 to 75°. Drillpipe rotation was particularly beneficial in minimizing barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole.
The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in high-pressure/high-temperature and extended-reach drilling operations.
Barite sag is the undesirable fluctuations in mud weight that occur due to downhole settling of the weighting agent. The problem has been exacerbated by the increased frequency of high-angle wells with the associated increase in particle settling rate which occurs in inclined fluid columns (Boycott effect1). Barite settlement, referred to as sag, occurs both statically and dynamically.
Operational consequences of barite sag can be severe. Potential problems include mud-weight variations (in and out), well-control difficulties, downhole mud losses, induced wellbore instability, and stuck pipe. The situation is particularly acute in high-angle wells where the allowable mud weight window can be restricted by mechanical wellbore stability considerations.2
A special flow loop was used to quantify barite sag under simulated field conditions of annular flow and drillpipe rotation. Tests were conducted on 20 different fluids provided by project participants. Data were used to develop a simple empirical relationship to predict sag behavior with time. Also, an existing sag model was found that is helpful for correlating laboratory and field results.
Most importantly, results from the study were combined with field observations to develop guidelines on minimizing barite sag. Case histories are given where the operational guidelines have been successfully employed to mitigate barite sag problems.
Laboratory Measurement of Sag Potential
Sag potential was measured in over 130 tests using the purpose-built flow loop shown schematically in Fig. 1. It was shown previously that sag potential is directly related to the weight loss of the circulating fluid in this flow loop.3 Mud-weight measurements were adjusted for temperature effects to accurately determine sag-induced density changes. Additional investigations included the effects of chemical- and clay-based rheology modifiers, fluid density, weighting agent density, particle size, and low-shear rheology.
Test parameters included angle (0 to 90°), annular velocity (25 to 200 ft/min), pipe rotation (0 to 250 rpm), eccentricity (0 to 0.8), and time. The standard test protocol shown graphically in Fig. 2 was established to evaluate the combined effects of these key factors under a common set of conditions. Designed to observe multiple interactions among key parameters, the protocol was based on seven time steps between which flow rate and pipe rotation were changed systematically. Annular velocities were recorded as nominal values. Other test protocols were used on a few fluids to examine certain characteristics in greater detail.
Flow rate and rotation were maximized during the first and last time segments to stabilize the test fluid, clean up the flow loop, and qualify each test. Segments 2 to 3 lowered flow rate in two steps without rotation in order to induce bed formation. Segments 4 to 6 measured the potential for removing sag beds, first by increasing pipe rotation, and then flow rate. All standard tests were run at 0.8 eccentricity since preliminary tests showed less response when the pipe was centralized.
The 20 test fluids represented a variety of mud types, formulations, weights, suppliers, and geographical sources. One of the project goals was to select muds from active directional wells (>30°) using weighted muds (>12 lbm/gal) in order to provide immediate feedback to operations. No other stipulations were made on field muds. Some laboratory-modified and laboratory-prepared fluids were also tested. Physical properties of all test fluids are listed in Tables 1 and 2.
Fig. 3 shows results for Mud 6 using the standard protocol at four inclinations (45, 60, 75, and 90°). Circulating fluid density change (corrected to 120°F) is plotted vs. time. All fluids tested responded similarly although the magnitudes varied.
Most, if not all, of the sag-bed formation occurred at low flow rates and no rotation. Initial pipe rotation to 75 rpm consistently had the greatest effect on removing the beds. Doubling the rotary speed and then the annular velocity helped as expected, but to a much lesser extent.
Time-segment 3 in Fig. 3, during which the sag was greatest, most clearly demonstrates the effects of angle. For Mud 6, the order of decreasing sag severity was 60, 45, 75, and 90°. While each individual mud tested exhibited slightly different behavior, the angle at which the maximum sag occurred was consistently in the range 60 to 75°. This is consistent with previously reported data.3
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