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SPE 143226
Riser Less Well Intervention for Subsea Work OverTerje Skeie,
yvind Hjorteland & Michael Macdonald Arnskov, Welltec
Copyright 2011, Society of Petroleum Engineers
This paper was prepared for presentation at the Brasil Offshore
Conference and Exhibition held in Maca, Brazil, 1417 June 2011.
This paper was selected for presentation by an SPE program
committee following review of information contained in an abstract
submitted by the author(s). Contents of the paper have not
beenreviewed by the Society of Petroleum Engineers and are subject
to correction by the author(s). The material does not necessarily
reflect any position of the Society of Petroleum Engineers,
itsofficers, or members. Electronic reproduction, distribution, or
storage of any part of this paper without the written consent of
the Society of Petroleum Engineers is prohibited. Permission
toreproduce in print is restricted to an abstract of not more than
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Abstract
The number of subsea wells has increased steadily over the years
and is estimated to have exceeded 5500 by the end of 2010.
Subsea wells do in general have substantially lower recovery
rates than what is normally achieved from comparable non-
subsea wells. This is due to the high intervention costs which
are directly related to the rates of the rigs required to carry
outsuch operations if the traditional and conventional approach is
adopted. Hence the incentive to increase recovery rates has
been
limited as the balance between cost and revenue has been
unfavorable, even with increasing oil and gas prices. However,
thishas also stimulated the development of alternative methods
which can enhance recovery rates and not least address
thechallenges created by more fields passing maturity and
exploration moving to more demanding areas.
Lightweight and riserless intervention well intervention has in
this context become a proven approach with considerablepotential
for driving the future development. The ability to substitute large
rigs with smaller and dynamically positioned light
well intervention vessels has a major impact on cost and time.
Riserless intervention as such has been done regularly for a
number of years and has set excellent HSE standards. However, in
recent years the use of riserless well intervention hasbecome
increasingly instrumental in the effort to enhance recovery rates
and general performance.
The paper gives an account of a scale milling operation carried
out at a Norwegian field in the North Sea where two wells
bothsuffered from a significant drop in production.
Introduction
The average recovery rate for a subsea well is considerably
lower than that of a comparable surface well due to the
relativelymore complicated well intervention and maintenance
issues. Using heavy and traditional rigs for subsea intervention is
costly
and time consuming due to the high day-rates and mobilization
aspects. The base costs are therefore considerable higher
ascompared to surface well intervention where tools can be deployed
directly through the risers from the production unit.
As a consequence subsea wells are less often subjected to well
intervention measures which in the long term results in
lowerrecovery rates than what is achievable from non-subsea wells,
where 50% or even more is achievable depending on the
field,technology and reservoir management exercised.
The number of fields approaching the end of their lifespan is
rapidly increasing. It is estimated that approximately 30.000
wells will have to undergo plug and abandonment within the next
fifteen years. Approximately 30% of the wells are subseawells. In
the North Sea alone it is estimated that about 6.000 wells will be
subjected to plug and abandonment including 1400subsea wells
depending on aging profiles. In Asia the average age of subsea
wells are more than 5 years and in Malaysia 70-80% of the subsea
wells are mature and on average 20-25 years old. Similar in the
number of wells passing maturity is rapidlygrowing in the Gulf of
Mexico. Hence, with more and more subsea fields reaching or passing
maturity and explorationbecoming even more challenging the
incentive to improve recovery rates increase, not least as
technology today provides
alternative but also proven - solutions which eliminate the need
for heavy rigs.
