초록 ▼

The invention is a method for monitoring subsidence of the sea-bed ( 14 ) of a survey area ( 8 ) caused by compaction of an underground hydrocarbon reservoir ( 1 ), and comprises the following steps: Conducting at least two series (S 1 , . . . ,S i , . . . ,S m ) of time-indexed depth measurement

The invention is a method for monitoring subsidence of the sea-bed ( 14 ) of a survey area ( 8 ) caused by compaction of an underground hydrocarbon reservoir ( 1 ), and comprises the following steps: Conducting at least two series (S 1 , . . . ,S i , . . . ,S m ) of time-indexed depth measurements ( 13 a , . . . ,13 n ), with separation in time Δ between the measurement series characteristic of a significantly detectable long-term change of seafloor elevation due to compaction to take place in the reservoir. Measurements are time-indexed and corrected for tidal depth variations. Depth measurements ( 13 ) are conducted 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-indexed short-time local reference depth measurement series ( 19 r ) are conducted at short-term local reference stations ( 18 r ) at benchmarks ( 6 ) during each separate measurement series (S i ). The reference depth measurement series ( 19 r ) are continuous for correcting each depth measurement ( 13 ) for short-time tidal depth variations occuring during each separate measurement series (S i ). To monitor compaction or seafloor subsidence during the series of measurements, the depth measurements ( 13, 19 r ) are conducted relative to a depth measurement ( 13 r ) during each series (S) at reference station ( 9 ) arranged on the seabed ( 14 r ) at a distance from the reservoir ( 8 ) being sufficiently far to be unaffected by long-term effects taking place due to compaction in reservoir ( 1 ).

대표청구항 ▼

1. A method for monitoring possible subsidence of a seabed ( 14 ) of a survey area ( 8 ) caused by compaction of an underground hydrocarbon reservoir ( 1 ), comprising:conducting at least two measurement series (S 1 , . . . ,S i , . . . ,S m ) each comprising at least one time-indexed depth measu

1. A method for monitoring possible subsidence of a seabed ( 14 ) of a survey area ( 8 ) caused by compaction of an underground hydrocarbon reservoir ( 1 ), comprising:conducting at least two measurement series (S 1 , . . . ,S i , . . . ,S m ) each comprising at least one time-indexed depth measurement ( 13 a , 13 b , . . . , 13 n ), with a separation in time Δt between the at least two measurement series on the order of months or years;conducting each depth measurement ( 13 a , 13 b , . . . , 13 n ) on a survey station ( 2 a , 2 b , . . . , 2 n ) arranged on a benchmark ( 6 a , 6 b , . . . , 6 n ) having fixed vertical and horizontal position relative to the local sea-bed ( 14 a , 14 b , . . . , 14 n );within each measurement series (S i ), conducting at least one stationary time-indexed short-time local reference depth measurement series ( 19 r ) on at least one short-term local reference station ( 18 r ) on at least one benchmark ( 6 ), for correcting each depth measurement ( 13 a , 13 b , . . . , 13 n ) for short-time depth variations;conducting the depth measurements ( 13 , 19 r ) relative to at least one depth measurement ( 13 r ) at a reference station ( 9 ) arranged on the seabed ( 14 r ) outside the survey area ( 8 ) at least once during each measurement series (S i ), the reference station ( 9 ) essentially being unaffected by long-term effects taking place due to compaction in the reservoir ( 1 ) during the series of measurements (S 1 , . . . ,S i , . . . , S m ). 2. Method according to claim 1, comprising interpreting a difference of relative depth values (Δd) or ( 15 2 , . . . , 15 m ) having occured during the time (Δt) in terms of compaction in the reservoir ( 1 ). 3. Method according to claim 1, the time between at least the first and the latest measurement series (S 1 , . . . ,S m ) being characteristic of a significantly detectable change of seafloor elevation due to compaction to take place in the reservoir ( 1 ). 4. Method according to claim 1, the depth measurements ( 13 a , 13 b , . . . , 13 n ) being deducted from pressure measurements ( 23 a , 23 b , . . . , 23 n ). 5. Method according to claim 1, the stationary short-time local reference depth measurement series ( 19 r ) being continuous. 6. Method according to claim 1, the reference station ( 9 ) essentially being close to the survey area ( 8 ) and situated at a depth comparable to the depth of the survey area. 7. Method according to claim 1, comprising the stationary short-time local reference depth measurement ( 19 r ) series being conducted by at least one separate depth meter ( 17 r ). 8. Method according to claim 1, comprising at least one stationary short-time local reference depth measurement ( 19 r ) series being conducted at a short-term local reference station ( 18 r ) being identical to or co-located with the reference station ( 9 ) arranged on the seabed ( 14 r ) outside the survey area ( 8 ). 9. Method according to claim 1, comprising arranging three or more separate depth meters ( 17 r 1 , 17 r 2 , 17 r 3 ) at separate short-term reference stations ( 18 r 1 , 18 r 2 , 18 r 3 ) distributed geographically over the survey area ( 8 ) to monitor the tidally varying series of depth measurements ( 19 r ), using the separate continuous and time indexed depth measurement series ( 19 r 1 , 19 r 2 , 19 r 3 ) with a tidal model for interpolating the local tidal depth for correcting each time-indexed depth measurement ( 13 ) at each station ( 2 ). 10. A method according to claim 1, further comprising:including, in at least two of the measurements series (S 1 , . . . ,S i , . . . S m ), relative gravimetric ( 11 a , 11 b , . . . , 11 n ) measurements simultaneously with the depth measurements ( 13 a , 13 b , . . . , 13 n );conducting each gravity measurement ( 11 a , 11 b , . . . , 11 n ) on the survey stations ( 2 a , 2 b , . . . , 2 n ) arranged on the benchmarks ( 6 a , 6 b , . . . , 6 n );conducting the gravity measurement ( 11 ) relative to at least one reference gravity measurement ( 11 r ) at the reference station ( 9 );correcting the relative gravimetric measurements ( 11 ) for the corresponding long-term and short-term relative depth measurements ( 13 , 19 r ) producing depth corrected relative gravity values ( 21 a , 21 b , . . . , 21 n );correcting the relative gravity values ( 21 ) for the effect of seabed subsidence ( 15 2 , . . . , 15 m ) as calculated on the basis of the relative depth measurements ( 13 a , 13 b , . . . , 13 n ) during the long-term time Δt; andinterpreting a difference of depth corrected relative gravity values (Δg) or (Δ 21 2 , Δ 21 3 , . . . ,Δ 2 l m ) having occurred during the long-term time (Δt) in terms of parameters representing a mass density change and/or a mass displacement in the reservoir ( 1 ). 11. A method according to claim 5, in which during each measurement series (S 1 , S 2 , . . . , S m ) the gravity sensor ( 10 ) and the depth sensor ( 12 ) are arranged with a fixed relative elevation in their operational position on the station ( 2 , 9 , 18 ). 12. A method according to claim 5, in which the gravity sensor ( 10 ) and the depth sensor ( 12 ) are carried by means of an ROV ( 5 ) from one station ( 2 , 9 ) to another station ( 2 , 9 ) between one pair of a relative gravimetric and depth measurements ( 11 , 13 ) and the next pair of a relative gravimetric and depth measurement ( 11 , 13 ) in one measurement series (S). 13. A method according to claim 12, in which the ROV ( 5 ) is separate from the gravity sensor ( 10 ) during each relative gravimetric measurement ( 11 ) in order not to affect the gravimetric measurment ( 11 ). 14. A method according to claim 12, in which the relative gravimetric measurement ( 11 ) from the gravity sensor ( 10 ) and the depth measurement ( 13 ) from the depth sensor ( 12 ) are transferred to the ROV ( 5 ). 15. A method according to claim 14, in which the relative gravimetric ( 11 ) and depth measurements ( 13 ) are transferred via an ROV umbilical cable ( 53 ) to a surface vessel ( 7 ). 16. A method according to claim 10, in which the gravimetric measurements ( 11 ) are time-indexed and corrected for the gravity sensor's ( 10 ) drift with respect to time by other time-indexed gravimetric measurements ( 11 , 11 r ) taken before and/or later with the same gravity sensor ( 10 ) at a survey station ( 2 ) or reference station ( 9 , 18 ) during the same actual period of time (t i ), giving drift-corrected gravimetric measurements ( 11 t ) for further processing to produce corrected gravity values ( 21 ). 17. A method according to claim 4, wherein the depth measurements ( 13 ) arise from pressure measurements ( 23 ) converted according to the actual water density and optionally to the measured water density distribution depth profile. 18. Device for monitoring possible subsidence of a seabed ( 14 ) of a survey area ( 8 ), and gravity changes in an underlying petroleum reservoir, comprising:a depth sensor ( 12 ), adapted for being carried by an ROV ( 5 ), to be placed on survey stations ( 2 ) on benchmarks ( 6 ) at the seabed ( 14 ) for conducting depth measurements ( 13 );a water-tight pressure housing ( 34 ) with a gravity sensor ( 10 ) for making relative gravity measurements ( 11 );in which the gravity sensor ( 10 ) and the depth sensor ( 12 ) are arranged in a mutually fixed elevation in their operational position, and adapted for being carried by means of an ROV ( 5 ) from one seabed station ( 2 , 9 ) to another seabed station ( 2 , 9 ) between one pair of a relative gravimetric and depth measurements ( 11 , 13 ) and the next pair of a relative gravimetric and depth measurements ( 11 , 13 ). 19. Device according to claim 18, the depth sensor ( 12 ) compris ing quartz pressure gauges ( 22 ). 20. Device according to claim 18, the number of pressure gauges ( 22 ) being three. 21. Device according to claim 18, comprising a clock ( 16 ) for time-indexing the depth measurements ( 13 ). 22. Device according to claim 21, comprising double gimbal frames ( 36 x , 36 y ) orthogonally arranged for adjusting the verticality of the gravity sensor ( 10 ) around two axes ( 37 x , 37 y ). 23. Device according to claim 22, comprising actuators ( 38 x , 38 y ) for turning the gravity sensor ( 10 ) in the gimbal frames ( 36 x , 36 y ). 24. Device according to claim 21, comprising coarse and fine tilt sensors ( 64 , 68 ) for giving a coarse and fine tilt signal ( 65 , 69 ) for the deviation from verticality for the gravity sensor ( 10 ). 25. Device according to claim 21, comprising a signal cable ( 53 ) between the depth sensor ( 12 ) and the gravity sensor ( 10 ), and the ROV ( 5 ), arranged for transmitting the depth measurement ( 13 ) and the relative gravity measurement ( 11 ) from the sensors ( 10 , 12 ) to the ROV. 26. Device according to claim 25, the signal cable ( 53 ) also conducting the tilt signals ( 65 , 69 ) from the tilt sensors ( 64 , 68 ) of the gravity sensors to the ROV ( 5 ). 27. Device according to claim 18, at least the gravity sensor ( 10 ) adapted for being released from the ROV ( 5 ) during each relative gravimetric measurement ( 11 ) in order not to be disturbed by the ROV ( 5 ). 28. A device according to claim 18, in which the relative gravimetric measurement ( 11 ) from the gravity sensor ( 10 ) and the depth measurement ( 13 ) from the depth sensor ( 12 ) are transferred to the ROV ( 5 ) by an umbilical cable ( 53 ). 29. A device according to claim 14, in which the relative gravimetric ( 11 ) and depth measurements ( 13 ) are transferred via an ROV umbilical cable ( 51 ) to a surface vessel ( 7 ).