Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-02-09
2001-01-16
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06175809
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to a device for carrying out operations of seismic monitoring of an underwater zone by means of elastic wave sources permanently installed against the bottom of the water mass.
2. Description of the Prior Art
During production of a reservoir, it is well-known to carry out cyclic seismic exploration operations with transmission in the formation of elastic waves, recording of the waves reflected by the subsoil discontinuities and processing of the acquired seismic data in order for example to follow the evolution of the reservoir with time.
A conventional method referred to as VSP consists in coupling the wall of a well to a string of seismic receivers placed at intervals along a conventional logging cable in order to pick up the waves reflected by the discontinuities of the surrounding formation in response to the waves emitted by a source outside the well.
In order to simplify and to accelerate these periodic seismic well exploration or monitoring sessions which require drilling operation interruptions for logging, it is well-known to install permanently one or preferably several sources with different offsets and/or azimuths in relation to the well, as described for example in patent French Patent-2,728,973.
Onshore, the source can be readily placed in contact with the ground or at the bottom of a cased hole deep enough for the source to be in contact with the formation below the weathered layer (WZ). Several independent sources coupled with the ground with different offsets in relation to the well can be successively activated for each position of the VSP logging tool.
Offshore, in order to carry out similar multi-offset, multi-azimuth VSP type exploration sessions, it is well-known to tow an immersed impulsive source (air, water or explosive gun, marine vibrator, etc.), to displace it in a zone around the well to a succession of “shooting” positions and to carry out, in each one of them, a succession of emission-reception cycles. It is clear that the duration of these interposed acquisition sessions is therefore considerably longer. It is faster and obviously much more expensive to use several workboats each towing a marine source and to trigger them in sequence for each position of the logging tool. Prospecting methods using towed immersed sources suffer from a common drawback. In fact, it is not possible to repeat exactly the same geometry of the emission-reception system from one exploration session to the next notably because of the ocean currents which contribute to changing the effective positions occupied by the source during the shots. The same uncertainty also exists for boats that are not equipped with dynamic positioning means allowing to stabilizing the position of the source.
SUMMARY OF THE INVENTION
A device designed for seismic exploration of an underwater formation according to the invention allows operations of monitoring of an underwater zone to be readily and rapidly carried out.
The method and the device of the invention has applications in many fields, notably in the field of seismic exploration or monitoring of an underwater formation such as a reservoir for example.
The device comprises seismic wave reception devices designed to be coupled with the formation, at least one self-contained emission unit immersible at the bottom of a water body combining at least one source of seismic waves suited to be directly pressed against the surface of the bottom, a local energy storage which supplies the at least one source it, a surface central station laterally distant from a point of immersion of each self-contained emission unit, a multifunction connection connecting each self-contained emission unit to a surface relay unit provided with bidirectional communication with the central station, suited for transmission of control signals triggering each source and of synchronization signals emitted from each emission unit.
The device preferably comprises a device associated with each self-contained emission unit allowing location of the position thereof at the bottom of the water body.
According to an embodiment of the invention, the device comprises at least one self-contained emission unit including several elastic wave sources supplied with fluid under pressure, the local energy storage comprises tanks for the fluid under pressure, and the multifunction connection, providing connection with the surface relay unit, comprising a hydraulic line connected to the tanks and control signal transmission and feeder lines.
The communication connection comprises for example a transmission set including for example a first module on the central control installation and a second module on each relay unit. each being associated with a radio emission-reception antenna.
According to an embodiment, the elastic wave reception means comprise at least one seismic receiver lowered in a well that can be associated with a well drill string, notably in the neighborhood of the drill bit or of stabilizers, so that it can be acoustically coupled to a formation.
According to another embodiment of the invention, the elastic wave reception device comprises at least one seismic receiver in contact with the bottom of the water body.
The method designed for seismic exploration of an underwater formation according to the invention comprises positioning, at the bottom of a water body, at least one self-contained emission unit comprising at least one seismic wave source suited to directly contact-and against the surface of the bottom, local energy storage for supplying the wave source, a multifunction connection connecting each self-contained emission unit to a surface relay unit provided with a communication device communicating with a central station, installation of seismic wave receivers in at least one well drilled in the formation from a production platform, determination of the position of each emission unit in relation to the production platform and carrying-out of cycles of seismic monitoring of the underground formation with periodic triggering of each source from the central station, and acquisition of the signals picked up by the receivers in response to the waves received, reflected by the discontinuities of the underground formation.
The method of the invention comprises positioning, at the bottom of the water body, several self-contained emission units in different azimuth and offset positions in relation to the central production platform, and carrying out cycles of acquisition of the signals picked up by the receivers in response to the triggering of the various emission units successively.
The method of the invention further comprises associating at least one receiver with a well drill string in the neighborhood of a drill bit and/or of a stabilizer of the string, and carrying out seismic prospecting operations with triggering of at least one seismic source and reception of the waves reflected by the formation, during instants when the drill string is motionless and in close contact with the formations surrounding the well.
Such a coupling is for example obtained by applying a moderate weight to the drill string that pushes the bit against the hole bottom and presses the stabilizers positioned at intervals along the string against the walls.
The method of the invention further comprises using receivers coupled with the bottom of the water mass and carrying out seismic prospecting operations with triggering of at least one seismic source and reception of the waves reflected by the formation.
The method of the invention further comprises displacing a single workboat to each of the surface relay units for control and energy supply of each emission unit, then removal and recovery thereof after use.
Such a permanent source layout has many advantages. A single workboat is sufficient to immerse the emission units successively in well-located spots corresponding to offsets and azimuths selected in relation to the well, and to provide periodic energy supply and maintenance. Each emission u
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
McElheny Jr. Donald E.
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