4D Gravity & Seafloor Subsidence Surveys for Cost-Effective Monitoring of Offshore Gas Reservoirs Martha Lien, Hugo Ruiz, Martin Vatshelle, OCTIO AS
Introduction Optimizing hydrocarbon recovery in offshore hydrocarbon fields involves decisions involving costly investments, like drilling infill wells or installing compression facilities. A good understanding of the dynamical behaviour of the reservoir over the lifetime of the field is of key importance to support and reduce the risk related to such investments. Time-lapse seismic is generally accepted as a suitable monitoring technology. However, it involves considerable operational costs and it remains difficult to process 4D seismic information in a timely manner for reservoir management decisions. Time-lapse gravity and subsidence surveys can provide complimentary information to 4D seismic in a timely manner at a cost of typically 10% of that of seismic surveys. This approach has been used on the NCS for two decades, and field cases demonstrate that 4D gravity and subsidence monitoring can sometimes provide information beyond the reach of seismic, and timely enough to guide important reservoir management decisions. Time-lapse gravity changes at the seafloor are sensitive to fluid redistribution in the reservoir. As an example, vertical movements of water-gas contacts smaller than a meter can be detected under some circumstances (Ruiz et al., 2015). That is possible because of the high accuracy of the time-lapse gravity measurements, which is at the level of a few μGal (Agersborg et al., 2017).
Seafloor subsidence monitoring uses precise water pressure measurements at the seafloor as a starting point. After processing which involves for example correcting for tides and oceanographic effects, the method reaches accuracies as low as 2 mm. Subsidence is not only a required correction for the interpretation of gravity results, but also a valuable monitoring tool by itself, as it is sensitive to important reservoir and overburden properties. The lateral development of a subsidence bowl is related to pressure depletion and lateral compartmentalization, and in some cases, it is a key factor for the safety of the installations.
The marine 4d gravimetry and subsidence method Gravity and seafloor subsidence data are acquired simultaneously in combined surveys. A sensor frame containing three relative gravimeters and three pressure sensors is used for the measurements. Gravity and water pressure are measured with the sensor frame at a number of lateral positions across the field. The sensor frame is placed on a semi-permanent concrete platform at the seafloor. The number of stations ranges from 20 to 120, depending on the size of the field, with station spacing being typically similar to the reservoir depth below the seabed. The top surface of the platforms is circular with a diameter of approximately 1 m, and their role is to guarantee time-lapse repeatability in the measurement location. The concrete platforms are left on the seafloor during the field lifetime and can be retrieved at the end of production. During a survey, a vessel is positioned sequentially above the concrete platforms, and
a remotely operated vehicle (ROV) deploys the sensor frame to perform the 20-minute measurements on top of each of them. The duration of a survey ranges from one to five weeks. Surveys are organized in loops starting and ending at one or two centrally-placed platforms, called base stations. Repeat measurements at the base stations are used as a reference to model instrumental drifts, by constraining the measurements at the different visits to have the same value after the correction. Stations are located both above and surrounding the hydrocarbon field. Zero-level stations are placed in peripheral locations were production is expected to induce neither a change in gravity nor vertical seafloor deformation. That allows using those stations as a calibration reference in time-lapse computations. The survey aperture, including the offset of zero-level stations from the field outline is a function of reservoir depth. In a subset of stations, tide gauges are deployed during the whole survey, as a means for correcting raw pressure measurements for tides and other oceanographic effects. By applying these corrections, pressure measurements obtained with the frame at the concrete platforms are converted in measurements of platform depths. Depth differences between surveys provide a measurement of subsidence with a precision of a few millimeters. Gravity changes are computed after correcting measurements for instrumental drift, tides, water density and the effect of seafloor subsidence. Gravity data is easy to integrate in history 67
