Acoustics – Geophysical or subsurface exploration – Well logging
Reexamination Certificate
1994-12-29
2001-12-25
Lobo, Ian J. (Department: 3662)
Acoustics
Geophysical or subsurface exploration
Well logging
C367S025000, C367S057000
Reexamination Certificate
active
06332507
ABSTRACT:
FIELD OF THE INVENTION
1. Background of the Invention
The present invention relates to a method and to a device for performing prospecting operations in a well, allowing filtering of certain undesirable waves likely to be propagated in a subsurface formation from a source of elastic waves, whether, induced or not, to pickups coupled with the formations crossed by these wells.
2. Description of the Prior Art
The prospecting method according to the invention is for example applicable to the VSP type (vertical seismic prospecting) or of the RVSP type (reversed vertical seismic prospecting). In the first case, it may be implemented by means of an emission-reception assembly including seismic pickups (geophones or hydrophones) placed in a well and a source of elastic waves arranged at the surface. In the second case, the assembly includes a source placed in a well and seismic pickups arranged either at the surface or in another well. The method according to the invention may also be implemented in a well by means of sonic sondes.
The method and the device according to the invention is, for example, applicable to seismic operations where the noises emitted by a drill bit at work at the bottom of a well are recorded (drilling noise monitoring).
It is well-known that wave conversions occur in the upper part of a well crossing the zone called the weathered layer. The ground vibrations generated by a source at the surface are converted into waves referred to as tube waves, which are guided along the well to the pickups arranged at a given depth when a VSP type method is implemented.
It is the same for the reversed configuration (RVSP), where the waves generated by a source arranged in a well are guided along the well towards the weathered layer and converted into surface waves which disturb the signals received by the pickups arranged at the surface.
SUMMARY OF THE INVENTION
The method according to the invention allows significant reduction of the waves referred to as tube waves propagating along a well containing a liquid which are responsible for parasitic signals picked up by seismic receivers coupled with a subsurface formation crossed by the well, in response to waves applied to the ground.
The invention performing prospecting operations while a gas substantially insoluble in said liquid and compressible enough to absorb at least part of the energy conveyed by the tube waves towards the seismic receivers has been dispersed in the liquid in the upper part of the well, for example by setting off a chemical reaction, or an explosion.
The dispersion in the well of a gas likely to absorb at least part of the energy conveyed by the tube waves may also be achieved, for example, while the well is drilled by a drill bit associated with a drill string, by injecting the gas in a drilling fluid circulation feeding the bit. The elastic waves may be emitted by the drill bit working at the well bottom or by a source of waves distinct from the bit, which is actuated for example after the bit has been removed. The injected gas flowing up through the annular space between the well and the string absorbs the tube waves in this case.
Seismic operations may be performed with emission and reception devices arranged in at least one well sonde.
The device according to the invention includes for example a source producing elastic waves, receivers coupled with the formations and a source such as an explosive charge, to create in situ, at a determined depth of the well, a chemical reaction generating the absorbent gas. This charge may constitute the source of elastic waves.
According to another embodiment, the device includes a device for injecting gas under pressure in a drilling fluid circulation established during the drilling of a well by a drill bit, and a source which may be the drill bit itself or which may be distinct therefrom, and actuated for example after the drill bit has been removed to produce elastic waves.
The method according to the invention has been proven to be highly effective. It has been established that the attenuation factor on seismic records of disturbances caused by the conversion of the guided waves obtained by this gas dispersion ranges for example between 8 and 20.
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patent: 4849945 (1989-07-01), Widrow
patent: 4858718 (1989-08-01), Chelminski
patent: 4873675 (1989-10-01), Barr, Jr. et al.
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patent: 5171943 (1992-12-01), Balogh et al.
patent: 5185503 (1993-02-01), Fortin
Laurent Jean
Naville Charles
Ollivier Yves
Pignard Guy
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Lobo Ian J.
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