Communications – electrical: acoustic wave systems and devices – Wellbore telemetering – Through drill string or casing
Reexamination Certificate
1996-11-15
2001-07-17
Moskowitz, Nelson (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Wellbore telemetering
Through drill string or casing
C367S081000, C367S083000, C181S102000, C181S106000, C340S853300, C340S854400, C175S040000, C702S006000
Reexamination Certificate
active
06262941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for seismic prospecting using a source of seismic vibrations at the bottom of a well.
2. Description of the Prior Art
According to one technique in common use, a well is drilled by a powered drive table at the surface driving a drill string composed of a set of interconnected pipes will a drilling tool attached at its lower end.
Drilling tools of this type produce powerful seismic waves, and the idea of using these waves to measure the rock traversed by the tool during drilling, particularly in seismic prospecting (these operations are known as MWD), with a view to decreasing oil prospecting costs has already been conceived, since the wells are not out of action while measuring is being conducted.
Various embodiments in this field are described in French Patent 1,584,915 and U.S. Pat. Nos. 4,965,774, 4,964,087, 4,926,391, 4,718,048, 4,862,423, 4,675,852, etc.
However, this type of operation is difficult to implement for numerous reasons. One of them is that the characteristic of the signals emitted and the emission spectrum of the source are generally poorly known because of the great distance between the emission location and the surface station. Another is that the vibrations are usually picked up by accelerometers placed in contact with the tubular drill string at the head thereof, after being propagated mechanically over its entire length. Under these conditions, the transmitted signals are strongly altered by complex resonance and absorption phenomena along the string and depend on the structure of the surface installation. Since the transfer function of the entire assembly is poorly known, there is no strict reference for processing the seismic signals.
A signal truly representative of the vibrations emitted by the tool can be obtained if the accelerometer is attached to the bottom of the drill string provided it is possible to transmit the measurements to the surface at a sufficient rate. It is possible to do this using modulated waves such that the sound waves propagate in the fluid contained in the well provided the data transmission rate remains relatively slow.
For applications where the data transmission rate is high, as is the case in prospecting operations, very large down-hole data storage devices can be used, but they must be brought up to the surface regularly by withdrawing the drill string or pulling up the cable in order to dump their contents.
Another technique available for high-speed transmission of signals picked up by one or more down-hole accelerometers is described for example in the Assignee's French 2,688,026 and 2,700,018. It consists essentially of using a drill string of which at least a part is equipped with a line for transmitting reference signals from the down-hole sensors to the surface station.
Difficulties of interpretation also occur due to the fact that the vibrations are not emitted solely at the drilling tool. It has been found that the drill string itself acts as a source of secondary vibration. This is also the case in a surface drilling platform that transmits mechanical energy to the rock through its feet, the transmitted energy being propagated along the drill string and acting as another source of secondary vibration.
It is known and easily verified that the guided waves propagated by a drill string are less quickly attenuated, as a function of the distance from the source, than the volume waves propagated in the formation. Since the guided waves radiate in the formation and create parasitic secondary vibrations, it is justifiable to install appropriate drilling assembly elements to attenuate these effects.
The above-mentioned French Patent 2,700,018 shows that absorbing elements, and possibly resonating elements or acoustic discontinuity elements favoring resonance in the seismic frequently range, can be interposed in the drill string to damp the vibrations propagating directing to the surface along the drill string.
The use of down-hole sensors, possibly associated with in situ data storage devices, is not always possible or advisable for the following reasons:
the down-hole measuring equipment is not designed to operate at the high temperatures prevailing at the bottom of the borehole in geothermal applications or at great depths, and
the depth at which the down-hole equipment must operate renders inoperable the wave transmission devices designed to transmit the measurement signals to the surface recording and control station.
Other than the above-noted drawbacks, use of a sensor at the top of the drill string often proves impossible in directional wells because of the multiple reflections that occur along the part of the drill string below the bend, which often distort still further the reference signals used to correlate the recordings.
SUMMARY OF THE INVENTION
The method for seismic prospecting of a geological formation traversed by a well according to the invention involves application of vibrations to the formation by means of a seismic vibration source comprising, in operation, a down-hole assembly comprising a drilling tool advancing at the bottom of a well and connected to the surface drilling equipment by a drill string comprising a drill collar assembly and a set of pipes, the vibration source including a decoupling device disposed in the vicinity of the drilling tool that allow the latter to remain substantially in contact with the well bottom, the method comprising detection of reference signals indicating vibrations of the drill string, reception of waves coming from the formation in response to the vibrations communicated to the rock by a set of seismic receivers coupled with the formation traversed by the well, recording of the waves received at the seismic receivers by a system for acquisition and collection of the seismic signals detected by the seismic receivers, and correlation of the seismic signals thus recorded with the reference signals.
The method of the invention is particularly useful in the case where the vibration source at the bottom of the well is a drilling tool associated with a drilling assembly progressing to the bottom of a well, at the end of a drill string.
The method involves the correlation of reference signals filtered by the decoupling device, furnished by the sensors disposed in contact with the set of pipes beneath the surface of the geological formation and above the decoupling device.
When the well has a bend, reference signals coming from sensors disposed between the bend and the damping device are preferably used for correlation.
In the case where the drilling power comprises a down-hole motor with built-in damping device for driving the drilling tool, the correlation uses reference signals coming from the sensors disposed in contact with the string, receiving the vibrations filtered by these damping device.
According to one embodiment, an additional decoupling device interposed into the string are used to reduce parasitic emissions in the formation, and a filter is used that includes at least one portion of the string whose diameter is intermediate between the diameter of the drill collars and that of the set of pipes, the length of this string portion being chosen as a function of the wavelengths in a seismic frequency range, to attenuate in this range the multiple reflections due to diameter discontinuities intermediate between the drill collars and the pipes in the set of pipes, the sensors being disposed beneath the additional decoupling device.
The device for seismic prospecting of a geological formation according to the invention includes a seismic vibration source comprising a drilling tool advancing at the bottom of a well and connected to surface drilling equipment by a drill string comprising a drill collar assembly and a set of pipes, a drive device for driving the drilling tool, a set of seismic receivers coupled with the formation traversed by the well to receive waves coming from the formation, and a system for acquisition and collection of the seismic
Antonelli Terry Stout & Kraus LLP
Institute Francais du Petrole
Moskowitz Nelson
LandOfFree
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