Summary
The field-development plan for a Sarawak Shell Bhd.-operated gas field,
located in the South China Sea offshore Sarawak, Malaysia, specified drilling
of horizontal wells into the Tertiary-Miocene carbonate reservoir. The wells
were planned as high-capacity producers with a big-bore, long-casing flow
design.
The traditional well design dictated that before entering the reservoir a
casing had to be installed to stabilize the hole in soft shale. The uncertainty
of detecting the formation top resulted in a premature casing commitment of at
least 30 ft true vertical depth (TVD) above the top of the reservoir and the
need to use an expandable liner to cover 300 ft of exposed shale above the
reservoir.
To obviate this problem, the capability of one of the components in the
logging-while-drilling (LWD) tool array, specifically the
electromagnetic-wave-resistivity (EWR) forward-modeling technique, was used to
detect the top of the carbonate formation (i.e., the top of reservoir),
immediately before drilling into it. A standard LWD tool is configured to
prioritize EWR forward-model response as the carbonate-formation top is
approached. This configuration, together with an appropriately designed
bottomhole assembly (BHA) and well trajectory, enabled the successful
implementation of the plan to stop drilling approximately 1 ft TVD above the
carbonate top.
At this point, a conventional 9⅝-in. casing string was set at an optimum
depth. This eliminated potential well-control problems, costly remedial
actions associated with lost circulation, and inferior cementation of the
9⅝-in. casing string.
Thereafter, the wells were drilled horizontally in a conventional manner
into the carbonate-gas reservoir.
This paper compares predrilling EWR forward modeling of the proposed well
trajectory with the actual well data while drilling. The predrilling- and
post-drilling-modeled data are presented.
The cost savings from employing this technique are variable, ranging from
substantial—in the event of a well-control situation and attendant high
losses—to moderate if the need to set an expandable liner is eliminated. A
minimum of U.S. $1 million per well was saved with this technique.
Introduction
Sarawak Shell Bhd. operates numerous gas fields in the central Luconia area
located in the South China Sea, offshore Sarawak, Malaysia. The
field-development plan for the M4 field specified drilling two horizontal wells
into the Tertiary-Miocene carbonate reservoir. The wells were planned as
high-capacity producers with a big-bore, long-casing flow design.
In the central Luconia area, the drilling of development wells is
challenging because frequent mud losses or total loss of circulation are
encountered as a result of the karst phenomenon, which is the secondary erosion
of limestone formation producing subterranean fissures, conduits, and caverns
as well as fractures in the carbonate reservoir.
The carbonate reservoir is overlaid with a thick layer of soft shale, which
needs to be drilled with high mud-weight drilling fluids: 13.27 ppg. A 9⅝-in.
casing string has to be set before drilling into the carbonate reservoir to
avoid borehole collapse when drilling into the reservoir with lower mud weight
to prevent severe mud losses. Because the shale section above the
carbonate reservoir lacks any geological character or marker beds to help
delineate the top carbonate, it was common practice to set the casing point at
least 30 ft TVD above the top carbonate. Subsequent drilling left
approximately 300 ft of exposed soft shale along the borehole before entering
the carbonate reservoir, which necessitated the installation of an expandable
liner to isolate the soft shale.
Therefore, a new methodology was sought to minimize the length of the
openhole shale section.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
27 May 2004
- Revised manuscript received:
15 September 2005
- Manuscript approved:
15 September 2005
- Version of record:
15 December 2005
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