Performance Report: Weighting Material Meets Low-Rheology Requirement in North Sea Reservoir

Challenge: An operator working in the North Sea wanted to sidetrack a well from inside the 7‐in. casing at 15,257 ft (4650 m) and drill a 6‐in. hole to a total depth (TD) of 21,290 ft (6489 m). Drilling the sidetrack required a long horizontal well path through Upper Triassic sands. Because of the narrow annulus and the depleted nature of the reservoir, the sidetrack required a very low rheology profile, which greatly increased the possibility of experiencing barite sag during the expected long trip times.

Solution: The M‐I SWACO solution was to use the VERSACLEAN low‐toxicity oil mud weighted to 10.5 lb/gal (1258 kg/m3) with M-I BAR ULTRA FINE weighting material. This approach allowed a lower viscosity fluid to be used with reduced equivalent circulating density (ECD) with less risk of barite sag. This approach required an 80:20 oil‐to‐water ratio.

Result: Although the drilling team encountered minor barite sag following the displacement, the first sidetrack was drilled to 16,469 ft (5020 m), with less time spent conditioning mud than was necessary for the offset well. At this point, the well had to be abandoned because of downhole losses. The second sidetrack was drilled successfully with improvements to ECD and no significant occurrence of barite sag.

Well Information
Location UK Northern North Sea
Interval Drilled 6‐in. hole for 1230 ft (375 m), 15,239‐16,469 ft (4645‐5020 m)
Interval Drilled 6‐in. hole for 5195 ft (1583 m), 15,239‐20,434 ft (4645‐6228 m)

Some sag occurred (maximum delta mud weight 1.2 lb/gal [144 kg/m3]) during the initial circulation after setting the whip stock. The sag was due in part to the use of new, partly-sheared mud and seawater contamination picked up during the displacement. The drilling team added base oil and barite to obtain a consistent mud weight. Additions of TRUVIS product were made to obtain more progressive gels (8/12 lb/100 sq ft). The milling assembly was pulled to pick up a rotary steerable system to drill ahead.

Initial returns after the trip showed minor sag, with return weights starting at 10.1 lb/gal (1210 kg/m3) increasing to 10.8 lb/gal (1294 kg/m3) at bottoms up. No further problems with barite sag resulted from further treatments. Even as the hole angle increased toward horizontal, there was very little weight loss upon breaking circulation post trip. This fluid performance was markedly improved over previous wells in the field, requiring much less circulating and conditioning to achieve a consistent fluid weight. The high-temperature/high-pressure (HTHP) fluid loss was maintained at approximately 1.2 mL using ECOTROL additive.

The well was drilled to 16,469 ft (5020 m), at which point losses occurred. Four fracture closure stress pills were pumped in an attempt to strengthen the formation. When the pills proved unsuccessful, two cement plugs were set and the hole abandoned.

A second sidetrack was drilled from the original depth of 15,239 ft (4645 m) to TD at 20,434 ft(6228 m) in 240 rotating hr. The 6 rpm Fann dial reading was maintained in the 4-6 range, and the gel strengths varied from 5/6 to 6/8 (lb/100 sq ft). The average flow rate was 253 gal/min (958 L/min). Surface losses for the original sidetrack were 0.224 bbl/ft. Losses for the second sidetrack were 0.056 bbl/ft.

The reduction in PV/YP obtained by using M-I BAR ULTRA FINE weight material resulted in significant improvements in ECD.

Comparison of PV/YP Data with Offset well using API barite
Well PV YP
Min Max Ave Min Max Ave
Original 10 14 11.5 7 14 11
Sidetrack 7 18 13 6 14 10
Offset 17 23 20 12 18 15

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“The possibility of experiencing barite sag on these sidetracks was a concern. We were able to successfully overcome any occurrence of barite sag and maintain a very low rheology profile.”
Mike Hodder
UK Technical Services Manager
M‐I SWACO Colombia