Summary
Remote real-time pore pressure monitoring using a combination of
Logging-While-Drilling (LWD) services coupled with a predrill pore-pressure
model reduces risk and cost by providing significant insight into wellbore
stability and allowing for casing seat optimization. This paper presents the
results of a shelf job in the Gulf of Mexico (GoM) that allowed an operator to
drill successfully in a very tight hydraulic envelope and even eliminate a
string of casing.
The uncertainty in the pore-pressure prediction ahead of the bit can be
significantly reduced by model updating. The LWD measurements allow the
predrill velocity-to-pore-pressure transforms to be updated while drilling
using the velocities from the sonic tool and pressures from the LWD formation
pressure tool. This calibrated transform is then applied to revise the predrill
pore-pressure model while drilling, and thus provide an estimate ahead of the
bit. In this case study, the predrill model used interval velocities extracted
from a 3D mechanical earth model of the northern GoM based on velocities
derived from checkshots and sonic logs. These velocity data were kriged to give
a 3D velocity model over the entire northern GoM with uncertainty
estimates.
Using state of the art LWD technologies, a new methodology was used to
optimize drilling performance on a well in Vermillion Block 338. Continuously
updated LWD annular pressure measurements effectively gauge wellbore pressures
and help the driller rapidly intervene in pressure and/or geomechanical
wellbore stability issues. A complete understanding of the hydraulic forces
acting on a borehole can increase the rate of penetration, provide greater
safety, minimize casing strings, reduce or eliminate kicks and formation
fracturing, and allow faster and less expensive completions.
The techniques described in this paper using real-time measurements allowed
the operator to extend both the 9 5/8-in. intermediate casing and 7 in. liner
to TD. As a result, a critical casing string was pushed 1,287 ft deeper than
planned and a pre-planned 5 in. liner eliminated. This saved casing expense, as
well as slim-hole drilling and completion costs.
Introduction
Introducing real-time LWD formation pressure measurements and pore-pressure
prediction to the drilling workflow can reduce wellbore stability uncertainty
during the well construction process. Failure to maintain annular pressures
within the hydraulic operating range of the pore-pressure and the fracture
gradient can compromise the ability to push casing seat points as deep as
possible. If this limitation is not properly mitigated, it can and may
significantly increase project cost.
The drilling performance on a well in Vermillion Block #338 was optimized by
applying a method that sends real-time streaming data received from the
downhole tools through mud-pulse telemetry via satellite to an Operation
Support Center (OSC) for real-time drilling operations. OSC personnel monitored
and updated the wellbore hydrodynamics and mechanical earth model,
incorporating observations made during drilling. The results along with mud
weight recommendation were then sent to the rig, and the appropriate action was
taken to ensure that the surface mud weight, the equivalent circulating density
(ECD), and the equivalent static density (ESD) were kept within the limits of
the pore pressure and fracture gradient.
In general, most operators use wireline formation pressures and interval
velocity data to update the compaction-driven pore pressure model at the end of
the drilled section. This technique is often used in areas such as the GoM
where compaction disequilibrium is the most important cause of overpressure
(Bourgoyne et al. 1986). This methodology works well in young, fine-grained
sediments, particularly where the lithology remains similar (in composition and
grain size) throughout the section drilled.
The limitations of using this technique on wireline are:
1. The formation pressure data are not available in real-time while
drilling.
2. An operator may not account for variations and/or divergence in the
actual pore pressure from pre-drill pressure prediction models. When this
happens, operations are often constrained by an approval for expenditure (AFE)
that does not include an extra logging run to validate real-time pore
pressures. Drilling without this information often results in well stability
problems, kicks, increased stuck-pipe potential, and often, a substantial
increase in well costs.
3. The computed pore pressure can be either under- or over-estimated by a
significant amount, if the velocity-to-pore-pressure transform is not correctly
established. The former can lead to avoidable well control incidents, while the
latter can impede optimal casing point selection and reduce the chances of
completing the well in the target reservoir.
The critical success factor for the Vermillion well was the combination of
real-time LWD technology and continuous communication between the rig and the
OSC. Rapid analysis of the LWD sonic and formation pressure data allowed for
re-calibrating the velocity to pore-pressure transform, thus reducing the
uncertainty of the look ahead geopressure model.
© 2008. Society of Petroleum Engineers
View full textPDF
(
605 KB
)
History
- Original manuscript received:
8 June 2006
- Meeting paper published:
31 August 2006
- Revised manuscript received:
8 February 2008
- Manuscript approved:
20 March 2008
- Version of record:
15 September 2008
View Cart
