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Barite sag
Barite or weight material sag is a problem of drilling mud and it occurs when weighting material (barite, calcium carbonate, etc) separate from liquid phase and settle down. The method is based in part on continuously measuring fluid density during the first circulation after the fluid has been static after some time. However it can occur in a dynamic condition with low annular velocity. The barite sag can result in big variations in mud density in well bore. The light density is on top and heavy density at the bottom.[1]
In vertical wells, barite sag happens when circulation is stopped. Whereas, in deviated wells, the complex setting mechanism called “Boycott settling” can cause quick settling. Heavier particles settle at the low side of the wellbore and lighter fluids are at the high side.[2]
Causes
Barite sag can occur in high-angle wells (possibly at 35°, but increasingly likely at ≥ 50°, then diminishing as the interval approaches 75 to 90°). The most severe sag incidents typically occur in the 45 to 65° range. Sag causes a decrease in drilling-fluid density for fluids closer to the surface and an increase in density in fluids closer to bottom, as compared to the previous circulating mud weights. Barite sag is particularly noticeable when circulating bottoms up after a long noncirculating period, where lighter mud weights are observed followed by heavier mud weights close to bottoms-up. The barite particles settle to the low side of the wellbore and then this heavier slurry slides towards the bottom of the well, creating an accumulation of heavier weighted fluid around the lower part of the annulus. Barite sag occurs most often in low viscosity/low gel strength fluids, in low shear rate conditions, in freshly prepared fluids with minimal drill solids, and in high temperature wells (where the dowhole viscosity is reduced due to increased temperature).
Measuring barite sag
There have been numerous attempts to measure dynamic barite sag, both in the lab and the field, under high temperature high pressure (HTHP) conditions.
- Dynamic High Angle Sag Tester (DHAST)
- Measure density differences in the fluid as the angle of drilling change.
- Used under dynamic conditions, in which the fluid can be subjected to shear or observed statically.
- Has temperature and pressure specifications of 350°F and 10,000 psia.
- High-angle sag test (HAST)
- Measure density differences in the fluid as the angle of drilling change.
- Used with fluids under static conditions.
Rheological measurements
A rheometer is used to perform non-Newtonian rheological measurements on drilling fluids. Rigorous rheological measurements and sag testing performed on the drilling fluid samples in tandem could help understand the fluid’s propensity towards sag. [3]
Flow curve analysis
From viewing the flow curves as viscosity versus shear stress some significant differentiation between the fluids can be made including the slope of the viscosity. Viscosity increases as the shear stress approaches a yield point is likely a good indicator of the probability of sag events being observed with a fluid.[3]
Gel strength test analysis
Another set of tests that can be performed are gel strength tests and structural breakdown analysis. Analysis of that decay can be useful in determining the behavior of the mud system.[3]
Oscillatory amplitude sweep test analysis
Strain amplitude sweep, the complex modulus, G* (the geometric average of the storage modulus, G’, and loss modulus, G’’) is examined as a function of stain amplitude. This has been previously identified as a possible correlation to probability for barite sag to occur, and indeed a general trend similar to that observed in the viscosity/stress flow curves is observed, with a higher G* in the linear viscoelastic region (LVE, where G* is not a function of strain amplitude) correlating with better static sag behavior in sag testing while G* in the nonlinear region correlating with dynamic sag.[3]
Oscillatory frequency sweep test analysis
Measure commonly referred to when relating rheological properties to dynamic sag behavior is the value of the damping function, tan(δ), which is the ratio of G’’ to G’ and indicative of the degree of elastic or viscous behavior in a material. When tan(δ) is less than unity, the material behavior is dominated by elastic contributions, while a value greater than unity indicates viscous dominance in the system. For a drilling fluid to provide support of solids in suspension (both drilled cuttings and barite) under near stagnant conditions (when hydraulic transport is negligible) then the fluid should be elastically dominant. Comparisons of this are presented for oscillatory frequency sweeps and oscillatory time sweeps.[3]
Oscillatory time sweep test analysis
An additional test can determine the rapidity of structural growth and long term stability of the fluid. Structural growth is observed through the initial exponential decrease in tan(δ), coincident with an exponential rise in η*, where the formation of microstructural bonds results in increasing elastic behavior and increased complex viscosity.[3]
Ways to prevent barite sag
Barite sag can lead to well-control issues and stuck pipe; and can aggravate hole cleaning problems. A well-designed sweep program can help prevent or minimize the occurrence of sag. Recent field results indicate significant success in preventing sag using properly formulated weighted sweeps. When using an emulsion-based synthetic fluid that contains no commercial clays, operators have experienced little or no detectable barite sag, based on data retrieved from downhole pressure-sampling tools and drilling-fluid-density measurements recorded while circulating bottoms up.[4] [5]
Sag index
A well site method (sag index) for determining sag was developed to help quantify or pilot test the impact of potential fluid adjustments in the field. A Fann viscometer is used to shear the mud for dynamic test conditions.
The ability of a mud to suspend barite under dynamic conditions certainly is important; however, other factors may be equally significant. The sag index is the product of the mud weight difference measured in the viscometer and four constants based on key operational factors affecting sag: well bore inclination, flow regime, hole diameter, and length of the inclined well bore.
The sag index is useful during well planning because it can estimate sag potential qualitatively. Studies are currently under way to correlate the sag register and the sag index.[6]
References
- ↑ Drilling-mud. 2011. Barite sag – Drilling mud problem. http://www.drilling-mud.org/barite-sag-drilling-mud-problem/.
- ↑ Barite Sag – Drilling Mud Problem. 2011. Drilling Mud Problems. Drilling-Mud.org. http://www.drilling-mud.org/barite-sag-drilling-mud-problem/
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Savar, Sharath, Kullami, Sandeep, Maxey, Jason, and Teke, Kushabhau. 2013. A Comprehensive Approach to Barite Sag Analysis on Field Muds. American Association of Drilling Engineers. AADE-13-FTCE-30.https://www.aade.org/application/files/3415/7261/2277/AADE-13-FTCE-30.pdf Cite error: Invalid
<ref>tag; name "r3" defined multiple times with different content Cite error: Invalid<ref>tag; name "r3" defined multiple times with different content Cite error: Invalid<ref>tag; name "r3" defined multiple times with different content Cite error: Invalid<ref>tag; name "r3" defined multiple times with different content Cite error: Invalid<ref>tag; name "r3" defined multiple times with different content - ↑ Burrows, K., Carbajal, D., Kirsner, J. et al. 2004. Benchmark Performance: Zero Barite Sag and Significantly Reduced Downhole Losses with the Industry's First Clay-Free Synthetic-Based Fluid. Presented at the IADC/SPE Drilling Conference, Dallas, Texas, 2-4 March. SPE-87138-MS. http://dx.doi.org/10.2118/87138-MS.
- ↑ Hsia, R. and Patrickis, A. 2004. Case History: Zero Whole Mud Losses Achieved During Casing and Cementing Operations on Challenging Deepwater Well Drilled with Clay-Free Synthetic-Based Fluid. Presented at the AADE Drilling Fluids Conference, Houston, 6–7 April. AADE-04-DF-HO-36. http://www.aade.org/app/download/7238832156/AADE-04-DF-HO-36.pdf (PDF)
- ↑ Controlling barite sag can reduce drilling problems. 1994. Oil and Gas Journal. Vol 92:7 http://www.ogj.com/articles/print/volume-92/issue-7/in-this-issue/general-interest/controlling-barite-sag-can-reduce-drilling-problems.html
Noteworthy papers in OnePetro
Dye, W. M., Mullen, G. A., & Gusler, W. J. 2006. Field-Proven Technology To Manage Dynamic Barite Sag. Society of Petroleum Engineers. http://dx.doi.org/10.2118/98167-MS
Scott, P. D., Zamora, M., & Aldea, C. 2004. Barite-Sag Management: Challenges, Strategies, Opportunities. Society of Petroleum Engineers. http://dx.doi.org/10.2118/87136-MS
External links
Monitoring of barite sag important in deviated drilling
The Halliburton Baroid Ecosystem - Barite sag (Video)