Accurate Measurements of Seabed Subsidence above North Sea fields

Summary High-precision field-wide seafloor subsidence measurements have been applied to nine hydrocarbon fields in Norway. These measurements provide lateral information on the product of pore compaction and pressure depletion in the reservoir. The method uses water pressure measurements at the seafloor and reaches survey precision of 2 – 5 mm. Cases demonstrate that the lateral distribution of subsidence can be used to identify reservoir compaction, undrained compartments, and to calibrate the geomechanical model. In this abstract we discuss precision limitations and particularly temperature effects. Temperature stabilization of the sensors has recently been claimed to eliminate temperature-induced effects and provide superior accuracy at the Ormen Lange field. We argue that this is not consistent with the data provided. Temperature stabilization does not improve the depth repeatability when using 20 minutes recording time. The subsidence results indicate significant unaccounted errors or effects, and it is premature to conclude on the impact of the temperature stabilization on the subsidence errors after one such survey only. For surveys on the shelf, the measurement time can probably be shortened for pressure-only surveys compared to the common practice of 20 minutes for gravity surveys, irrespective of any system for stabilizing sensor temperatures.