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.
The Large-Field Bright-Star High-Precision CCD Photometer of BAO