Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Patent
1999-03-30
2000-08-08
Oda, Christine K.
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
367 72, G01V 130
Patent
active
06101446&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method for mapping drillable areas in an on-shore or off-shore oil field, without the risk of encountering anomalous zones, such as surface gases.
We know that surface gases are located between the surface of the ground and a depth at which the well bore has not yet been equipped with devices that make it possible to stop a potential gas inlet. These gases are a potential and very serious danger when drilling for oil and their location and significance must be evaluated prior to installing a well bore.
Until now, the detection of surface gases was carried out by using the exploration seismic reflection method called two-dimensional (2D). This method consists of gathering seismic traces resulting from the reflection of acoustic waves by the subsoil using an acquisition apparatus that consists of at least one acoustic wave emitting device and one line of acoustic receivers, and that is moved along the surface of the ground (land shooting) or the surface of the sea (marine shooting).
In particular, in 2D marine seismic exploration, the data acquisition equipment consists of a boat equipped with an acoustic impulse emitting source that tows a line or streamer on which are assembled a plurality of equally spaced sensors designed to receive the acoustic waves reflected by the various layers of the subsoil and where each one delivers a signal that represents the amplitude of the waves received in relation to time, this signal is recorded in order to provide a record called seismic trace. The length of the streamer may vary from approximately 500 m to approximately 6000 meters and the sensors fitted on the streamer are spaced from a few meters to several tens of meters apart, for example from around 12.5 meters to 50 meters. The marine area to be studied in the 2D seismic exploration method can range from several hundreds of square kilometers and can, for example, have a surface of 200 km.times.200 km. The boat covers this area following straight parallel paths, several kilometers apart, for example 5 to 6 km apart. During its movement, the seismic source emits impulses at regular time intervals, for example every 5 seconds. After treating the seismic recordings, for each straight path of the boat we obtain a 2D seismic section consisting of a plurality of vertical traces. This section represents the vertical cut of the subsoil in a system of coordinates X (direction of the boat's movement) and T (depth expressed in time).
In the case of shallow sands saturated with gas, the seismic energy that is reflected can take on significant values that are translated on the recorded seismic section by peaks of high amplitude (bright spots).
In order to reconstruct the image of the subsoil of the area being studied, we must put the various recorded seismic sections side by side and imagine interpolations between the bright spots of such sections. However, these interpolations are lengthy operations that are somewhat subjective. Indeed, it is not unusual for two interpreters to carry out the same interpolations in different ways and come up with maps with different risks.
We also know the three dimensional (3D) seismic acquisition method. This method uses an acquisition device that consists of at least one source that emits acoustic waves and a plurality of lines of acoustic sensors that is moved along the surface of the ground (3D land seismic acquisition) or the surface of the sea (3D marine seismic acquisition). In using this method in marine seismic acquisition, a boat equipped with at least one source of emitting acoustic waves tows several lines or streamers arranged parallel to each other, whose number can reach 8, where each line has a plurality of acoustic wave sensors. The lines are shorter than in the previous 2D seismic acquisition, are separated from each other for example by approximately 50 m and carry sensors that are equally spaced, for example every 25 m. The width of the area covered by the boat increases with the number of lines of sensors and is, fo
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des Vallieres Thierry
Kuhn Helmut
Lafet Yves
Parron Didier
Elf Aquitance Production
Oda Christine K.
Taylor Victor J.
Total
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