CASE STUDY
Success of North Sea Well Attributed to
Reduced Equivalent Circulating Density with
no Barite Sag
“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
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)
The Situation
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.
The 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.
The Results
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.
The Details
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.
API Barite
M-I Bar Ultra Fine
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
17
12
8
7
6-rpm
Predicted hydraulics data for 6-in. hole (225 gal/min [0.85 m3/min])
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.
Property
Unit
Program
Minimum
Maximum
Typical
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
Solids-Control Equipment Performance
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 8½-in. hole. There was improved centrifuge performance (better removal of
low-gravity solids compared to barite) in the 6-in. section.
Questions? We’ll be glad to answer them.
If you’d like to know more about the M-I Bar Ultra Fine product and how it’s performing for our other customers, please call the M-I SWACO
office nearest you.
This information is supplied solely for informational purposes and M-I SWACO makes no guarantees or
warranties, either expressed or implied, with respect to the accuracy and use of this data. All product
warranties and guarantees shall be governed by the Standard Terms of Sale. Nothing in this document
is legal advice or is a substitute for competent legal advice.
©2007 M-I L.L.C. All rights reserved. *Mark of M-I L.L.C.
FPR.1301a.0710.R1 (E) P.O. Box 42842
Houston, Texas 77242-2842
www.miswaco.slb.com
Email: questions@miswaco.slb.com