Summary
A post-mortem analysis of the Gnu-1 well was conducted to help us to
understand drilling experiences in the context of the pore-pressure and stress
profiles. The post-mortem involved a review of the drilling experiences and an
analysis of CAST image data, wireline-log data, and the logging-while-drilling
(LWD) logs. This information was used to refine and verify a geomechanical
model (in-situ stress, pore pressure, and rock-mechanical properties) in the
vicinity of the Gnu-1 well. Of prime concern was the verification of the
predrill pore-pressure prediction previously undertaken using
3D-seismic-velocity data and offset-well data. Wellbore-failure and
natural-fracture analyses were integral parts of the post-mortem. Wellbore
breakouts seen in the image data allowed the pore pressure in the 8.5-in. hole
section of Well Gnu-1 to be constrained. Modeling using image data collected in
the Athol formation indicates that the pore pressure does not increase as
rapidly as was estimated in the predrill study. Pore pressures in the North
Rankin formation and below were consistent with the predrill study. The
geomechanical model was able to explain the losses seen in the Athol formation
in Well Gnu-1 when using the mud weights experienced by the open hole at the
time of drilling.
Introduction
The Gnu prospect is situated in the northern portion of Block WA-209-P in
the Dampier subbasin, Australian northwest shelf (Fig. 1). The prospect is
located within the Reindeer gas field. A number of offset wells exist in the
region, the closest wells being Well Reindeer-1 (approximately 1.5 km to the
northeast) and Well Caribou-1 (2 km to the southeast).
Well Gnu-1 was designed as an exploration well. The anticipated overburden
stratigraphy at the location of Well Gnu-1 consists of Tertiary and Upper
Cretaceous carbonates, marls and siltstones that overlie Cretaceous claystones,
siltstones and minor sandstones, and greensands. The primary aim was to drill
vertically to intersect the Muderongia australis glauconitic sandstone
and then to build angle and continue drilling a deviated hole through the main
Reindeer field gas appraisal within the Legendre formation and into the North
Rankin, Brigadier, and Mungaroo formations.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
18 September 2007
- Meeting paper published:
30 October 2007
- Revised manuscript received:
8 October 2008
- Manuscript approved:
14 November 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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