Since 1998 we have been making repeated relative gravity measurements onseafloor benchmarks to monitor temporal and spatial density change in producingnatural gas reservoirs and in a CO2 sequestration reservoir. We simultaneouslycollect ambient ocean pressure data to infer benchmark height changes. We haveobtained repeatability in gravity and height of 2 to 3 ?Gal and 2 to 5 mmrespectively. To date we have completed 16 surveys of six reservoirs; each hasbeen repeated at least once. The data provide information useful for reservoirmanagement and tracking injected CO2.
Maximizing the recovery of hydrocarbons in producing reservoirs is an importantactivity currently being addressed by several geophysical techniques. One ofthese — time lapse gravity — offers a unique vision into a reservoir wheredensity is being changed because of production. Our collaboration has beenusing changes in gravity, observed on the seafloor above offshore reservoirs. Because of the low density of gas compared to water or oil, gravity has beenmost effective in monitoring gas reservoirs.
In the simplest model, water flows into a gas reservoir during production toreplace the gas that has been removed. The resulting net increase in densityproduces a positive change in the value of gravity observed over the reservoir. Depending on the geometry and characteristics of the gas reservoir, gravitymeasurements can be made precise enough to detect a rise in reservoir waterlevel of less than 1 m. Compared to the 4D seismic approach, this can be moresensitive to small movements of large fronts.
Seafloor surveys require the use of a Remotely Operated Vehicle (ROV) and asurvey vessel equipped with the infrastructure to support it. We have found itmost efficient to use three seafloor gravity meters simultaneously. The metersare Scintrex model CG5s which we have adapted for seafloor use. Each meterconsists of a CG5 mounted in a motor-controlled gimbal frame housed in a deepocean pressure cylinder. A microprocessor in the package controls gimballeveling motors to orient the sensor with the local vertical. Each pressurecase also contains a quartz pressure gauge (Paroscientific DigiQuartz model31k) to record ambient seawater pressure. Gravity, pressure, tilt angles, compass heading, sensor temperature, ambient temperature and other housekeepingparameters are acquired and formatted by the microprocessor and telemetered tooperators on the ship in real time via the ROV's umbilical. The operatorscontrol each sensor through a LabView interface program operating on a PC inthe ROV control room on the vessel. This enables smooth communication betweenthe gravity meter operators and the ROV pilots.
The three gravity meter systems are mounted into a single deployment framewhich isolates them from shocks generated by the ROV launch and recoveryactivities. The frame is lifted by a hydraulic arm on the ROV, speciallyconstructed for these surveys.