A METHOD FOR MONITORING SEAFLOOR SUBSIDENCE AND FOR GRAVITY MONITORING AN UNDERGROUND HYDROCARBON RESERVOIR

摘要

The invention is a method for monitoring subsidence of the sea-bed (14) of asurvey area (8) caused by compaction ofan underground hydrocarbon reservoir (1), and comprises the following steps:Conducting at least two series (S1, ..,Si,..,Sm) of time-indexed depth measurements (13a,.., 13n), with separation in time .DELTA.between the measurement series characteristic of a significantlydetectable long-term change of seafloor elevation due to compaction to takeplace in the reservoir. Measurements are time-indexedand corrected for tidal depth variations. Depth measurements (13) areconducted on survey stations (2) arranged on benchmarks (6)which have settled in the locally consolidated seabed (14). To handle short-term depth variations several stationary time-indexedshort-time local reference depth measurement series (19r) are conducted atshort-term local reference stations (18r) at benchmarks(6) during each separate measurement series (Si). The reference depthmeasurement series (19r) are continuous for correcting eachdepth measurement (13) for short-time tidal depth variations occuring duringeach separate measurement series (Si). To monitorcompaction or seafloor subsidence during the series of measurements, the depthmeasurements (13, 19r) are conducted relative to adepth measurement (13r) during each series (S) at reference station (9)arranged on the seabed (14r) at a distance from the reservoir(8) being sufficiently far to be unaffected by long-term effects taking placedue to compaction in reservoir (1).