A new reservoir management application uses precise time‐lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3–5‐year period. We have constructed and deployed a new instrument—the ROVDOG (Remotely Operated Vehicle‐deployed Deep‐Ocean Gravimeter) system—for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea‐floor benchmarks. We have built an instrument based upon the Scintrex CG‐3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties. The instrument prototype was deployed in the North Sea during June 1998; 75 observations were made at 32 stations. The standard deviation of repeated gravity measurements was 0.026 mGal; the standard deviation of pressure‐derived heights, for repeated measurements, was 1.4 cm. A refined instrument was deployed in August 2000 in a three‐sensor configuration. Multiple sensors improved the precision by averaging more samples without incurring additional survey time. A total of 159 measurements were made at 68 stations. The standard deviation of repeated measurements was 0.019 mGal; the standard deviation of pressure‐derived heights was 0.78 cm. A ROVDOG pressure case rated to 4500 m depth has also been constructed. This system was deployed with the Alvin manned submersible in November 2000 to a depth of 2700 m.
Thirty seafloor gravity stations have been placed above the carbon dioxide [Formula: see text] injection site and producing gas reservoir at the Sleipner Øst Ty field. Gravity and depth measurements from 2002 and 2005 reveal vertical changes of the permanently deployed benchmarks, probably caused by seafloor erosion and biologic activity (fish). The original gravity data have been reprocessed, resulting in slightly different gravity-change values compared with earlier published results. Observed gravity changes are caused by height variances, gas production and water influx in the Ty Formation, and [Formula: see text] injection in the Utsira Formation. Simultaneous matches to models for these effects have been made. The latest simulation model of the Ty Formation was fitted by permitting a scale factor, and the gravity contribution from the [Formula: see text] plume was determined by using the plume geometry as observed in 4D seismic data and varying the average density. The best-fit vertical gravity gradient is [Formula: see text], and the response from the Ty Formation suggests more water influx than expected in the presurvey simulation model. The best-fit average density of [Formula: see text] is [Formula: see text]. Estimates of the reservoir temperature combined with the equation of state for [Formula: see text] indicate an upper bound on [Formula: see text] density of [Formula: see text]. The gravity data suggest a lower bound of [Formula: see text] at 95% confidence.
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