Summary
A common major challenge in deepwater drilling is the narrow margin between
pore pressure and fracture gradient. In many basins, deepwater wells are
becoming even more challenging, with subsalt and deeper horizons introducing
problems that can lead to well abandonment before the target depth (TD) is
reached. The key to drilling these wells safely and cost- effectively is to use
technologies, methods, and procedures that allow the mud-weight window to be
widened.
This paper describes two technologies that, when combined, can widen the
effective mud-weight window significantly. One is the Micro-Flux Control (MFC)
method. This is a closed-loop drilling system that allows the safe use of a mud
weight as close as possible to the pore pressure. The other technology is an
ultralow invasion-drilling fluid (ULIF) that increases the leakoff pressure of
the openhole section, thereby opening the mud-weight window. Several field
cases involving the drilling fluid are described, with increases of more than
1.5 lbm/gal in the leakoff pressure observed in wells in the Gulf of Mexico
(GOM).
Results obtained with the MFC method at a test facility in the USA are
described. These show the ability of the method to deal very effectively with
any influx or loss.
Significant operational savings have already been obtained with the drilling
fluid. However, the combination of the two technologies will produce a step
change in safety, technical capability, and economics in many deepwater
operations.
Introduction
Wells with a narrow margin between pore pressure and fracture gradient are
becoming more common. Not only is the geological environment where new reserves
are often located becoming more complex, but the depletion of current fields is
also creating increasing problems in development and infill wells. The
situation becomes more complex in deep water, especially when operators are
trying to locate reserves below salt layers, and in
high-pressure/high-temperature (HP/HT) environments.
In many deepwater prospects, several casing strings (including contingency
strings) have to be considered. However, many of these wells are still
abandoned before reaching TD. These wells are extremely expensive,
sometimes costing close to USD 100 million each. One operator in the GOM
abandoned a well after spending more than USD 80 million. Another spent USD 120
million before abandoning the well. It is clear that the technical limit of
conventional drilling is often encountered before achieving the ultimate goal
of reaching TD to access hydrocarbon reserves. New technologies and methods
must therefore be introduced to allow these new frontiers to be added to
producible reserves in the near future.
This paper focuses on wells with narrow margins between the pore pressure
and fracture gradient, highlighting two technologies that can reduce
significantly the problems faced when drilling those wells. Even though the
technologies can be used separately, the combination of the two in the same
well can provide the optimum drilling solution.
The paper first addresses the MFC method and describes some of its unique
capabilities for detecting influxes or losses at a very early stage. This
allows the operator to trigger actions to detect and control a problem as it
begins, and to prevent it from escalating to the point where the solution is
costly and time consuming (sometimes forcing early abandonment). With the MFC
method, the mud weight can be reduced as close as possible to the pore pressure
in a very safe way. Thus, MFC deals with the left side of the mud-weight
window—the pore-pressure curve.
This paper will then describe how to increase the leakoff pressure of the
well, effectively widening the mud-weight window. The method of doing this by
using a ULIF has been applied in many wells with extremely good results, while
also giving big savings to operators. Field cases where the ULIF approach has
been used are described.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
2 May 2006
- Revised manuscript received:
16 January 2007
- Manuscript approved:
5 February 2007
- Version of record:
20 September 2007
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