Information obtained from 4D gravity and subsidence monitoring provides improved decision-making in the exploitation of offshore reservoirs. Field cases demonstrate the impact of this technology in the estimation of hydrocarbon volumes, the evaluation of risk of water breakthrough, understanding of drive mechanisms and identification of undrained compartments.
The additional information provides increased hydrocarbon recovery in later phases of the projects, through the identification of infill-well targets or by the optimization of compression facilities. The cost of this technology, which is typically 10﹪ of that of 4D seismic, makes it feasible in a large range of offshore fields. The technology has been used in eight fields on the Norwegian continental shelf (NCS).
4D gravity is sensitive to changes in the mass distribution in the reservoir. Gas depletion and water influx from surrounding aquifers produce an observable time-lapse gravity signal. The observed signals are independent of seismic velocities, which makes this technology complementary to seismic monitoring. In addition, gravity provides a more precise quantification of mass changes than 4D seismic, and it can provide a better sensitivity to the movement of the gas-water contact.
Seafloor subsidence is also sensitive to important reservoir and overburden properties. It is directly related to compartmentalization, and it can be a key factor for the safety of installations. It is an observable effect of geomechanical changes. Seafloor subsidence has been used to identify non-depleted compartments; determine the drilling-window for in-fill wells; understand aquifer properties; and improve the geomechanical model hence the interpretation of seismic time-shifts.
In this abstract, we review the principles of the 4D gravity and subsidence monitoring technology. We then discuss some of the case studies from the NCS that illustrate the value these data provide for reservoir management. Finally, we discuss the main cost drivers of the technology, and which steps are taken by the industry to reduce cost and hence extend its feasibility to a wider range of fields.
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.