Summary
A method to accurately measure seafloor subsidence away from platform
locations is presented. The method is based on seafloor water pressure, which
is measured on top of predeployed benchmarks visited one after another using a
remotely operated vehicle (ROV) and is at the same time measured continuously
throughout the survey at one or more reference locations. Because no
significant subsidence is expected during a few days of data acquisition,
high-precision relative depths representative for the average time of the
survey can be obtained. Accurate subsidence estimates between seafloor surveys
are found assuming negligible subsidence at benchmarks located outside the
field.
Results from six seafloor surveys performed at two gas fields in the North
Sea are presented. For an area of 1 km2 at 80 m water depth,
single-measurement relative depth precision (standard deviation) of 0.4 cm was
obtained. Correspondingly, for an area of 700 km2 at 295 to 345 m
water depth, 0.6 cm was obtained. Single-station subsidence accuracy down to 1
cm is achieved from the two most recent pressure surveys at the large field. A
subsidence signal is seen for this difference, and it is compared with modeled
subsidence. Error budgets for depth precision and subsidence, incorporating
instrumental and environmental errors, are discussed.
Introduction
Reservoir compaction caused by the extraction of hydrocarbons usually
results in surface subsidence. The most vulnerable fields are those with thick
and soft reservoir formations being exposed to a large drop in pore pressure.
For such fields, reservoir compaction can be a major energy drive for
production that enhances the recovery of the field (Merle et al. 1976).
Reservoir compaction can also lead to a reduction in permeability. When
significant subsidence occurs, it can cause well failures and costly repairs of
surface structures such as platforms and pipelines (Bruno 1992).
Subsidence monitoring can improve the safety of field installations, and it
may also be used to estimate reservoir behavior and formation properties.
Marchina (1996) reported examples of high-accuracy leveling data from the
Groningen gas field used to estimate pressure depletion in the aquifer by
solving a linear inverse problem, and Nagel (1998) used bathymetry data from
the Ekofisk field to achieve estimates of overburden properties.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
25 May 2005
- Revised manuscript received:
6 February 2006
- Manuscript approved:
16 February 2006
- Version of record:
20 September 2006
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