Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-10-15
2004-11-02
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C073S38200R
Reexamination Certificate
active
06813564
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention concerns a method and an apparatus for monitoring fluid movements and seafloor subsidence/reservoir compaction in hydrocarbon-producing fields by repeated relative seafloor gravity and depth measurements. Monitoring of hydrocarbon reservoir changes (as saturation, pressure, and compaction) during production is traditionally done by well measurements integrated through dynamic reservoir modelling. Geophysical techniques for measuring changes between the wells has emerged as useful technology in recent years, particularly repeated seismic measurements. The benefits of such observations and improved understanding of the reservoir behaviour during production are many: among others to optimize production/reservoir management, optimizing drilling of infill wells and improving estimates of remaining reserves.
TECHNICAL BACKGROUND, STATE OF THE ART
A new system comprising an instrument for use in seafloor gravity observations has been designed and deployed. The system is called “Remotely Operated Vehicle Deep Ocean Gravimeter” (ROVDOG). The aim of the project was to perform repeated measurements of gravity and pressure in an oilfield to monitor the development of the reservoir. The actual field in question is the Troll field in the North Sea. Because of the requirement for accurate location of the measurement points (each to within one cm of the previous observation) a gravimeter was required which could be handled by the arm of an ROV and placed atop sea floor benchmarks. Such an instrument has been designed around a Scintrex CG-3M land gravimeter. Motorized gimbals within a watertight pressure case are used to level the sensor. An assembly of 3 precise quartz pressure gauges
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provides pressure measurements which can be transformed to depth information. The instrument may be operator-controlled via a serial datalink to the ROV. A view of the data stream for recording can be monitored. In one embodiment of the invention, the serial datalink is according to the RS-232 standard. In a test run of the system, the instrument was first deployed in the Troll field during June 1998. A total of 75 observations were made at 32 seafloor locations over a period of 120 hours. The repeatability figure of merit is 0.027 milliGal for the gravity measurement and 2 cm for pressure-derived heights.
Scope of Work
The Troll field licence partners decided in 1996 to try to perform measurements on the Troll field in order to montitor changes caused by the gas production and the influx of water from the aquifier in particular. Among different solutions comprising well monitoring and repeat seismic monitoring to perform such measurements, repeated gravity measurements on the sea floor above the field was proposed by the inventors. In an internal study in 1997 the changes of the gravity field were identified as comprising the following factors:
I) water influx in the gas reservoir;
II) seafloor subsidence; and
III) gas density reduction.
I) The expected increase in gravity caused by water influx.
II) The expected seafloor subsidence due to reservoir compaction. This seafloor subsidence will cause a change in the gravity field, as measured at the seafloor, being proportional to the subsidence, due to the vertical gradient of the Earth's gravity field. Thus it is necessary to monitor the gravity changes or “the gravity signal” to within a resolution of the gravity corresponding to an elevation difference of a few centimeters.
III) Gas density reduction will give a reduction of the mass density of the reservoir, causing a reduction of the gravity field, i.e. of opposite sign with respect to gas/water rise.
Fujimoto et al. in “
Development of instruments for seafloor geodesy
”, Earth Planets Space, vol. 50, pp. 905-911, 1998, describes instruments for monitoring differential displacements across a fault zone in the seabed, and examines their resolutions through seafloor experiments at relatively short baselines. The horisontal differential displacement is measured by an acoustic ranging system using a linear pulse compression technique being able to measure distances on the order of 1 km between markers with an accuracy of 1 cm. The leveling or vertical displacement monitoring of the seabed is planned to use an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. Fujimoto et al describes how ocean bottom pressure measurements can be used in two ways to detect vertical movements of the seafloor. An ocean bottom pressure array acts as a monitoring system of relative vertical movements. Variations of atmospheric pressure are mostly compensated at the sea surface. By simulating pressure and gravity one can discriminate between a pressure change due to vertical seafloor displacements and a pressure change due to vertical sea surface displacements:
Consider the seafloor rising by 1 cm. The pressure value will decrease by 1 cm of water column. Gravity will decrease by 2.2 microGal (−3.068 microGal due to height change and +0.864 microGal due to reduced gravitational attraction of the global sea water).
Next, Consider the sea surface lowering 1 cm. The pressure in this case will also decrease by 1 cm of water column. The gravity in this case will increase by 0.432 microGal due to the reduced gravitational attraction of the local seawater.
In both of the above mentioned cases, pressure monitored at the seabed decreases, but the gravity changes differently. If measurements are performed with high accuracy, simultaneous measurements of pressure and gravity can discriminate between the two effects: sea surface level change and seabed level change.
Fujimoto et al. do not propose any method for monitoring changing parameters representing density and/or mass distribution of an underground sub-sea reservoir by means of gravimetric measurements with a gravity sensor on the sea-bed. Fujimoto proposes conducting series of relative gravimetric measurements with a gravity sensor and relative depth measurements with a depth sensor on survey stations arranged on a benchmark having fixed vertical position relative to the local sea-bed in a survey area over a suspected fault zone, the gravimetric measurements being relative to gravimetric measurements and depth measurements taken on a reference station on land. Fujimoto proposes correcting the relative gravimetric measurements for the corresponding relative depth measurements, producing corrected relative gravity values. The corrected gravimetric values are then used for interpreting seabed vertical motions, and not used for comparison between series of observed corrected gravimetric values with later series of observed corrected gravity values and interpretion of a difference of corrected gravimetric values in terms of a change of parameters representing density and/or mass displacement in the underground sub-sea reservoir. Although seabed subsidence monitoring over the reservoir zone is one major issue of the present invention, the gravity change represented by seabed subsidence is noise with respect to detecting gravity changes due to mass movements and density change in the reservoir. Thus, in the present invention, the gravity measurement due to seabed subsidence (or rise) must, in addition to tidal and drift corrections, be corrected for by corresponding water column pressure changes at the seabed.
Presentation of Relevant Known Art
Gravity monitoring has previously been applied in exploiting hydrothermal energy (Allis, R. G, and Hunt, T. M., Geophysics 51, pp 1647-1660, 1986
: Analysis of exploitation-induced gravity changes at Wairakei geothermal field
.; San Andres, R. B, and Pedersen, J. R., Geothermics 22, pp 395-422, 1993
, Monitoring the Bulalo geothermal reservoir, Philippines, using precision gravity data
.), and in volcanology (Rymer, H. And Brown, G. C., J. Volcanol. Geotherm. Res. 27, pp. 229-254, 1986
, Gravity fields and the interpretation of volcanic str
Eiken Ola
Hildebrand John
Zumberge Mark
Barlow John
Den Norske Stats Oljeselskap A.S.
Taylor Victor J.
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