To monitor water influx, seafloor gravity and subsidence measurements have been acquired at 21 seafloor stations above the Mikkel gas field in the years 2006 and 2011. Time-lapse repeatability is on average 2.2 uGal in gravity and 3.1 mm in depth (standard deviation). This paper compares the gravity data with forward gravity estimates from the Mikkel pre-survey simulation model. It shows how an updated model containing significantly less water gives an excellent gravity match. Knowledge of water influx is important for long-term planning and can affect the ultimate recovery of the field. This updated simulation model also shows a significantly better subsidence match than the older model. Using the subsidence data, the pore compressibility has been estimated. The results show that gravity monitoring is feasible not only on huge fields, but on a modest size gas field at medium depth.


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