I N NOVATI N G WH I LE DR I LLI N G
Preventing lost circulation,
instability offshore Malaysia
BY STEPHEN WHITFIELD, ASSOCIATE EDITOR
From an operator’s perspective, minimiz-
ing downhole losses while maintaining
wellbore stability is one of the biggest
challenges to successfully drilling an
extended-reach well. Michael Yao, Senior
Rock Mechanics Advisor at Hess, noted
that extended laterals tend to have more
breakouts than vertical or less deviated
wells. This means a higher mud weight is
often required to keep the wellbore stable.
Equivalent circulating density (ECD) is
also typically higher. Moreover, there is a
greater uncertainty in how the cuttings
transport will behave, which can make
hole cleaning more difficult.
If ECD, mud weight and hole cleaning
are not managed properly, the operator
runs the greater risk of wellbore instabil-
ity, borehole collapse and lost circulation,
all of which can increase the potential for
the well to miss its target depth and to add
significant costs to the drilling program.
“If you’re drilling a less deviated well,
the drilling margin is typically wide. We
have much more freedom to navigate mud
weight and ECD between the pore pres-
sure and the fracture gradient. It’s like
driving in light traffic,” Dr Yao said, noting
that drilling an extended-reach well is
more akin to driving in a traffic jam. “Hole
cleaning becomes more challenging, and
the difference between the ECD and mud
weight is much larger due to the hydrau-
lics of these wells. There are much more
challenging conditions to maneuvering
the mud weight and ECD.”
Further, addressing the dual challenges
of lost circulation and wellbore instability
in extended-reach wells requires a holistic
approach to strengthening the wellbore
if the formation is depleted, he said. This
has to be a collaborative effort among the
operator’s drilling engineers, subject mat-
ter experts and rig site teams, as well as
the service companies.
Dr Yao highlighted one such effort on
a shallow gas field offshore Malaysia at
an IADC Drilling Engineers Committee
Technology Forum in November. In its
most recent extended-reach well in the
formation, the company had experienced
lost circulation in the reservoir section,
which showed unexpected depletion from
production, and well startup was delayed.
Hess drilled the 8 1/2-in. section of an extended-reach well offshore Malaysia with
an ECD ranging from 12.0-12.6 lb/gal and a 10.8-gal mud weight. By maintaining
these parameters, the operator was able to drill the section without any losses.
18 With this well, which had a measured
depth (MD) of 12,642 ft and true vertical
depth (TVD) of 3,925 ft, Hess aimed to pre-
vent losses and maintain wellbore stabil-
ity in the 8 ½-in. lateral section. Modeling
showed the level of breakout at different
mud weights ranging from 10.6 lb/gal to
11.2 lb/gal. Understanding limited level of
breakouts need to be tolerated, Hess chose
a 10.8-lb/gal mud weight for the section as
a reconciliation between wellbore stability
and lost circulation concerns.
Hydraulics modeling showed that ECD
for the section would range between 12.3-
12.9 lb/gal at the 10.8-lb/gal mud weight.
However, even at that mud weight and
ECD, the section could still see losses if the
pore pressure fracture gradient was on the
low side due to depletion. Hess formulated
a “stress cage” to prevent losses in case
of low pore pressure fracture gradient. A
stress cage is a wellbore strengthening
model of the mechanism that boosts up
the fracture gradient to be above expected
ECD through the addition of mud additives.
The 8 ½-in. section was drilled at an
average ROP of 50 ft/hr, with a pump rate
of 450 gal/min and a rotation of 140-150
rev/min. The ECD stayed primarily within
the range indicated by hydraulics mod-
eling (12.0-12.6 lb/gal). Stress cage was
implemented prior to drilling into depleted
reservoir as precaution.
Reservoir pressure was then measured
by the Formation Pressure While Drilling
(FPWD) tool once the sensor was in con-
tact with permeable formation. Based on
that, the fracture gradient was expected
to be higher than the ECD. The stress cage
materials were deemed unnecessary and
then screened out. The well was subse-
quently drilled to total depth (TD).
Dr Yao noted the importance of collabo-
ration between different stakeholders in
drilling the well without any losses. “We
put tremendous scrutiny into our plans for
the wellbore strengthening material ahead
of time, and during the operations we had
a lot of eyes looking at screens while the
drilling was happening to see how every-
thing was performing. Communication
and collaboration were key during the
whole process.”
The well also provides a valuable blue-
print for handling similar wells in the
future, he added. DC
M A R C H/A P R I L 2023 • D R I L L I N G C O N T R AC T O R