MANAGED PRESSURE DRILLING
New well design and MPD
deployed to manage pressure
uncertainties in Gulf of Thailand
3-casing design coupled with MPD not only
provided flexibility in pressure management
but also lowered well construction costs
BY STEPHEN WHITFIELD, ASSOCIATE EDITOR
While well designs with four casing
strings are typical in the Gulf of Thailand,
some wells with pressure uncertain-
ties call for a different approach. In 2019,
PTTEP called on Weatherford to help
devise and execute a new three-string
design in two wells that would be drilled
using managed pressure drilling (MPD).

Not only did MPD provide the pressure
management flexibility that the opera-
tor needed in these wells, but they also
reduced the costs associated with having
to use a fourth string of casing.

This marked the first time that
Weatherford had used such a design with
an operator in the region, and Harpreet
Kaur Dalgit Singh, MPD Project Engineer
with Weatherford, discussed the plan-
ning process, equipment modifications
and drilling results at the most recent
SPE/IADC Managed Pressure Drilling and
Underbalanced Operations Conference,
held in Kuala Lumpur in September.

“Given the uncertainties of these two
wells, we wanted to do things a little
bit differently in terms of drilling opti-
mization and the cost optimization,” Ms
Kaur said. “Instead of forcing a four-string
design, let’s incorporate these uncertain-
ties into our planning and execution.

Let’s think of a different way to drill this
hole section. That’s where we thought of
the using the three-string with the MPD
because we know the benefits of MPD in
managing bottomhole pressure.”
Weatherford’s work scope involved the
drilling of 6 1/8-in. sections in two wells
– Well C and Well H – with the lowest
and safest mud weights to allow a reduc-
38 tion in equivalent circulating density
(ECD) and added surface backpressure
(SBP) when drilling and making connec-
tions. This would help to minimize the
potential for the pore pressure to balloon
beyond the range of 1.46 and 1.50, which
had been indicated in initial formation
integrity testing, and safely stay below
the maximum 1.60-SG rating of the cas-
ing strings.

To accommodate the small platform
of the tender assist rig used in this drill-
ing campaign, Weatherford made sev-
eral alterations to its MPD system before
deployment. This included adding a split
skid choke manifold, comprising an MPD
choke skid and detection skid. The split
was needed as the rig-up crane was
unable to lift the MPD choke manifold
to the BOP deck, where it is normally
located. The split also allowed the MPD skids
to be lifted from the supply vessel onto
the edge of the BOP deck using the tender
crane, then moved to the deck center
with the help of hydraulic jacks and roller
skates. Ms Kaur noted that this approach
could be a good option for other MPD proj-
ects on rigs with small platform decks.

Weatherford also considered alternative
rig-up solutions for the rotating control
device (RCD) due to the limited space out
between the RCD and the rig floor on this
rig. A modified 21 ¼-in. bell nipple inner
barrel was installed on top of the RCS in
lieu of a flowline, and then an RCD rub-
ber leak containment line was added that
diverted RCD leak flow directly to the mud
trough. Planning, drilling
and execution
Ms Kaur noted that MPD planning typi-
cally is guided by the drilling window,
with the pore pressure as the lower limits
and the fracture pressure as the upper
limit. Planning also considers drilling
mud weights with wellbore pressures that
“walk the line” within a given drilling
window. Prior to drilling the two Gulf of Thailand
wells, Weatherford ran two simulations
on each well. The worst-case scenario
planned for each simulation was a bal-
looning gradient, which would lead to
higher-than-expected pore pressures and
a narrow drilling window (approximately
0.1 SG for both wells) to drill the 6 1/8-in.

hole section. The best-case scenario was a
gradient that allowed the pore pressure to
fall within the parameters of the formation
integrity test.

The simulation also took into account
PTTEP’s plan to drill the section with two
BHAs – a motorized BHA and an adjust-
able gauge stabilizer (AGS) BHA. For Well
C, the operator planned to use the motor-
ized BHA to drill out the 7-in. casing shoe
to a 3,318.5-m measured depth, then drill
the AGS BHA from 3,318.5-m to the sec-
tion total depth, 4,830 m. For Well H, the
motorized BHA would drill out to 3,306.26
m, then the AGS BHA would drill from that
point to 4,625.69 m, the section total depth.

The simulations helped Weatherford to
determine that drilling with an underbal-
anced 1.4-1.45 SG mud weight would be
sufficient for both wells while keeping the
ECD within the drilling window. At that
mud weight, the expected pore pressure
peak of 1.5 SG for Well C would happen at
3,930-m measured depth. For well H, the
expected pore pressure peak of 1.54 SG
would be seen at 3,605-m measured depth.

For Well C, the static pore pressure test
performed at the end of the section total
depth recorded 1.58 SG, higher than the
maximum expected pore pressure of 1.5
SG. Thanks to the MPD system, the well
was successfully drilled to total depth –
Ms Kaur noted that the system allowed
for higher flow rates to drill the section at
points where it encountered unexpectedly
high pressure.

She also said the pressure measured in
Well C confirmed the limitations of the
JAN UARY/FEB RUARY 2023 • D R I LLI N G CO N T R ACTO R