Gravity changes measured at the seafloor are sensitive to fluid redistribution in hydrocarbon reservoirs. As an example, vertical movements of water-gas contacts smaller than a meter can be detected under some circumstances. Seafloor subsidence monitoring uses water pressure measurements at the seafloor as a starting point. Once the required tide corrections are applied, the method reaches accuracies as low as 2 mm, depending on the field conditions. Subsidence is a required correction for the interpretation of gravity results, and it is by itself a valuable monitoring tool, sensitive to important reservoir and overburden properties. It is directly related to pressure depletion and lateral compartmentalization, and in some cases, it is a key factor for the safety of the installations. In this abstract, we review the principles of the 4D gravity and subsidence monitoring technology. We then discuss some field cases from the Norwegian Continental Shelf that illustrate the value that this type of data provides for reservoir management. Finally, we discuss what are the drivers of the cost of the technology and what steps are taken by the industry to reduce cost, and hence to make it feasible in more fields.


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