Performance Report: Success of North Sea Well Attributed to Reduced Equivalent Circulating Density with no Barite Sag

Challenge: An operator in the central North Sea wanted to abandon an existing platform well and drill a sidetrack from the 9⅝‐in. casing. In the 8½‐in. section of the hole, it was important to control wellbore instability in reactive Eocene shales by managing the mud weight and equivalent circulating density (ECD). The 6‐in. section of the hole was to be drilled parallel to the existing wellbore with a very tight drilling window. ECD management was critical to success. It was also essential to accurately place the well and to ensure a quality cement job on the 7‐in. liner.

Solution: M‐I SWACO suggested the use of the VERSACLEAN low toxicity oil mud formulated with M‐I BAR ULTRA FINE product. This combination allowed a low‐viscosity fluid to be used with reduced ECD without the risk of barite sag.

Result: Both sections of the hole were successfully drilled and completed without incident. In the 8½‐in. hole, the Fann viscometer 6 rpm dial readings were maintained at 9‐10 (measured at 120°F [49°C]) with a mud weight of 12.0 to 12.6 lb/gal (1438 to 1510 kg/m3). Even though the viscosity was slightly higher than planned, the hydraulics were improved when compared to those achieved in offset wells. Pump rates were higher, and there was an estimated reduction in pump pressure of 200 to 500 psi (13.8 – 34.5 bar). There was no evidence of barite sag while the 7‐in. liner was being run and cemented. In the 6‐in. hole, a 6‐rpm dial reading was maintained at 8 with a mud weight of 9.5 to 10.2 lb/gal (1138 to 1222 kg/m3). There was an estimated 200‐ to 600‐psi (13.8 – 41.4 bar) reduction in pump pressure in comparison to that achieved in the offset wells. Higher rates of penetration–up to 70 ft/hr (21 m/hr), a record for this platform in a 6‐in. hole–were obtained as a result of the lower viscosity and improved hydraulics. No barite sag was noticed.

Well Information
Location UK Central North Sea
Interval Drilled 8½‐in. hole for a total of 5500 ft (1676 m), 8,100‐13,600 ft (2469‐4145 m)
Interval Drilled 6‐in. hole for a total of 4290 ft (1308 m), 13,600‐17,890 ft (4145‐5453 m)

Because the operator had previous experience with sag in extended-reach wells, the normal specification for oil mud viscosity was set at a 6-rpm reading of 12 or higher at 180°F (82°C). Following these specifications led to unnecessarily high plastic viscosity. M-I SWACO conducted a lab study comparing standard mud with a lower viscosity fluid weighted with M-I BAR ULTRA FINE product. A comparison of the two fluids, mixed at 11.4 lb/gal (1366 kg/m3) with an oil-to-water ratio of 75/25, is shown in the table below along with the pressure drop and ECD for the 6-in. hole section as predicted using M-I SWACO VIRTUAL HYDRAULICS software.

The use of M-I BAR ULTRA FINE product allows finer shaker screens to be used without the risk of removing the weight material. On this well, a combination of 270- and 300-mesh TBC screens were run in the 8.-in. hole. There was improved centrifuge performance (better removal of low-gravity solids compared to barite) in the 6-in. section.

Fann‐35 Data‐°F (°C) 120 (49) 180 (82) 120 (49) 180 (82)
PV 32 23 24 13
YP 37 26 10 13
100‐rpm 40 32 19 16
6‐rpm 17 12 8 7
Predicted hydraulics data for 6‐in. hole (225 gal/min [0.85 m3])
Total P drop 4553 psi (314 bar) 3917 psi (270 bar)
ECD at total depth‐lb/gal (kg/m3) 13.82 (1656) 12.58 (1507)

Measured properties from the field application are shown in the following table.

Density lb/gal 9.55 – 11.4 9.55 10.2 9.75ESD
Plastic Viscosity (120°F [49°C]) cP ALAP 12 26 18
Yield Point (120°F [49°C]) lb/100 sq ft 15 ‐ 25 10 21 14
6 RPM (180°F [82°C]) lb/100 sq ft > 8 4 10 8
Gels (10s/10m @ 180°F [82°C]) lb/100 sq ft ≥ 9 / 13 5 / 6 10 / 19 9 / 15
HTHP (300°F [149°C]) mL < 4.0 2.6 4.4 3.6
Oil:Water Ratio % 75:25 77:23 72:28 76 / 24

Download: Success of North Sea Well Attributed to Reduced Equivalent Circulating Density with no Brine Sag (0.48 MB PDF)

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“We saw no evidence of barite sag in either of the two sections drilled. Use of the M‐I BAR ULTRA FINE product allowed us to control wellbore instability in reactive shales.”
Mike Hodder
UK Technical Services Manager
M‐I SWACO Colombia