GB2235046A

GB2235046A - A downhole acoustic source for tomography acquisitions

Info

Publication number: GB2235046A
Application number: GB8917321A
Authority: GB
Inventors: Alain Ducatel
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1989-07-28
Filing date: 1989-07-28
Publication date: 1991-02-20
Also published as: GB8917321D0

Abstract

The acoustic source comprises an anvil in the form of a membrane (20) the top surface of which is struck by a hammer in a prechosen time sequence and the bottom surface of which faces a diffusion chamber (3) having apertures (32) from which acoustical signals penetrate into the formation layers surrounding the borehole. Acoustic reflector (41) serves to direct the signals perpendicularly to the borehole surface. <IMAGE>

Description

A DOWNHOLE ACOUSTIC SOURCE FOR TOPOGRAPHY ACQUISITIONS The present invention relates to a downhole acoustic source for tomography acquisitions.

In particular the present invention relates to a downhole acoustic source for tomography acquisitions which produces acoustical signals for surveying formation layers surrounding a borehole and which comprises substantially cross-axially to the borehole an anvil being struck by a hammer.

From US patent specification No. 2 933 144 it is known to use waves originating from the drilling zone which reveal valuable information about the character of the formation around the borehole and below the drill head. During a drilling process an electrical actuator applies vertical impacts to the drill head at a constant frequency to effect percussion drilling. A number of pick-up devices are located at the earth surface and distributed about the top of the well. On an oscilloscope screen changes in wave patterns received can be monitored, e.g. a transition from a soft to a hard formation layer. However, the signals received are derived from the drilling process as such and thus they are strongly dependent on the in situ drilling conditions.

Recently a seismic generator has been introduced which comprises a hammer-anvil construction and which is coupled to the surrounding formation layers by means of a pressurizing device in order to create specifically shear (or SH-, or transversal) waves (as such known to those skilled in the art and not explained in detail). Said pressurizing device consists of a fluidum filled three cell unit on which the seismic generator is arranged.

However, said composed generator system results in a very bulky and cumbersome source arrangement.

Thus, it is an object of the invention to give an acoustic source construction by means of which predefined signals will be generated.

It is a further object of the invention to build an acoustic source having a simple construction.

It is another object of the invention to build an acoustic source which can easily be coupled to a drill string thereby substituting the drill bit.

The invention therefore provides a downhole acoustic source having an anvil as a membrane, the top surface of which being struck by a hammer in a prechosen time sequence and the bottom surface of which facing a diffusion chamber from which acoustical signals penetrate into surrounding formation layers.

The invention further provides a diffusion chamber which comprises sidewalls facing the borehole wall and provided with apertures and a bottom provided with a raised central bottom portion in order to reflect the acoustical signals originating from the anvil throughout said apertures onto the borehole wall.

In a further embodiment the anvil has a corrugated cross-section form provided with at least one raised central anvil portion. Furthermore the anvil can comprise concentric corrugations.

In an advantageous embodiment the diffusion chamber has the form of a cylinder, and said apertures are arranged at equal heights from the bottom of said diffusion chamber and are equidistant.

In another embodiment the raised central bottom portion has the form of a curved solid of revolution, e.g. a cone, a truncated cone or a multilateral pyramid. Moreover the cone has a top angle of 90".

Advantageously the diffusion chamber is detachable from the anvil, for example said chamber is screwably connectable on the anvil.

The invention will now be described by way of example in more detail with reference to the accompanying drawings, in which: fig. 1 shows a cross-sectional view of the acoustical source in accordance with the invention taken along its central axis, and fig. 2 shows an elevational view of the cross-section of the source as shown in fig. 1 and taken along the line I-I of fig. 1.

In the figs. 1 and 2 similar reference numerals are used for corresponding elements.

In fig. 1 a cross-sectional view of a downhole acoustical source taken along its central axis of said source, to be used for tomography acquisitions, is shown. A drill string head member 1 comprises an anvil 2 on which a hammer (not shown for reasons of clarity) will strike. Said head member 1 comprises upstanding wall elements 11 for mounting hammer driving means. Generally the hammer is a plunger being an end part of a piston. The piston can be driven either electromagnetically in a solenoid arrangement or pneumatically. Furthermore, a drill bit unit can be connected to the lower part of the head member 1.

The anvil 2, as used for our purpose of generating acoustical signals for tomography acquisitions, comprises an anvil body 20 suitable for oscillating axially.

First, the anvil body 20, functioning as a membrane, has at least a raised central portion being struck by the above mentioned hammer. Furthermore, in order to obtain the desired oscillating conditions, the anvil body 20 advantageously has a corrugated form as shown in fig. 1. The corrugated form results from grooves 23 in top and bottom surface of the anvil body 20, respectively 21 and 22, arranged in such a way that the grooves have staggered positions to each other. The resulting corrugations can extend both parallel to the raised central portion in the middle, in case of a cylindrical head member 1 parallel to the diameter, and concentrically around a point-like raised central portion. It will be clear that other forms for the anvil body are possible provided that the oscillating features of the anvil body are guaranteed.

Joining to the above to obtain a good transfer of the acoustical energy to the surrounding formation layers the downhole acoustic source comprises a diffusion chamber 3 from which acoustical signals penetrate into said layer.

Referring to both fig. 1 and fig. 2 the diffusion chamber 3 comprises sidewalls 31 facing the borehole wall. Said sidewalls 31 are provided with apertures 32. The diffusion chamber 3 is further provided with a bottom 4 having a raised central bottom portion 41 as a reflector means, reflecting the acoustical signals originating from the anvil 2 throughout said apertures 32 onto the surrounding borehole wall.

Advantageously the diffusion chamber 3 has the form of a cylinder as shown in fig. 1 and fig. 2. The apertures 32 can be arranged in different ways and they can have many possible forms.

As shown in fig. 1 and fig. 2 the apertures 32 have the form of axially extending slots which are extending over equal heights and which are arranged at equal distances to each other. In particular for both figures twelve slots 32 have been shown.

As sketched above for grooves 23 extending parallel to a diameter two opposite apertures 32 could be sufficient.

Closely related to the above the bottom 4 of the diffusion chamber 3 has to be formed in such a way to give an optimal reflection characteristic. Generally the central raised bottom portion 41 has the form of a curved solid of revolution, e.g. a cone or a truncated cone. From experiments it has appeared that the anvil 2 provided with concentric grooves 23 as shown in fig. 1, produces acoustical signals which will be reflected to the borehole wall in a sufficient way when a cone having a top angle of 90" is used.

Dependent on other arrangements of anvil and/or sidewall slots the central raised bottom portion 41 may have the form of a wedge or a multilateral pyramid.

As described above the diffusion chamber 3 is connectable to the head member 1 by means of a connection portion 33 at the top end of the diffusion chamber. Advantageously the connection portion 33 has the form of a rim provided with a thread 34 which is screwed on a thread arranged on the head member 1 as shown in fig. 1.

Those skilled in the art will appreciate that dependent on the downhole conditions the source in accordance with the invention has to be arranged in a suitable way.

Preferably said source is free standing in the borehole normally filled up with borehole fluid. In that case, since there is no direct source-to-borehole wall coupling, generally so-called pressure (or P, or longitudinal) waves (as such known to those skilled in the art and not explained in detail) will be generated.

However, if necessary, for example in dipping boreholes, in order to avoid direct support of the downhole source upon a part of the borehole wall, a clamping device will be employed.

The source in accordance with the invention has been tested extensively. The source as used had a diameter of 92 mm. Hammer striking frequencies up to 30 Hz were used whereas P-wave frequencies in a frequency range from 1.4 to 1.8 kHz were applied.

It will be clear to those skilled in the art that modified P-wave frequencies may be chosen, for example frequencies up to 10 kHz as a consequence of correspondingly modified source dimensions.

Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims

1. A downhole acoustic source for tomography acquisitions which produces acoustical signals for surveying formation layers surrounding a borehole and which comprises substantially cross-axially to the borehole an anvil as a membrane, the top surface of which being struck by a hammer in a prechosen time sequence and the bottom surface of which facing a diffusion chamber from which said acoustical signals penetrate into said formation layers.

2. The apparatus as claimed in claim 1 wherein the anvil has a corrugated cross-section form provided with at least a raised central anvil portion.

3. The apparatus as claimed in claim 2 wherein the anvil comprises concentric corrugations.

4. The apparatus as claimed in claim 1 wherein the diffusion chamber comprises side walls facing the borehole wall and provided with apertures, and a bottom provided with a raised central bottom portion in order to reflect the acoustical signals originating from the anvil throughout said apertures onto the borehole wall.

5. The apparatus as claimed in claim 4 wherein the diffusion chamber has the form of a cylinder.

6. The apparatus as claimed in claim 5 wherein the apertures are arranged at equal heights from the bottom of said diffusion chamber and are equidistant.

7. The apparatus as claimed in claim 6 wherein the diffusion chamber comprises at least two apertures.

8. The apparatus as claimed in any one of the claims 4 to 7 wherein the apertures have the form of slots.

9. The apparatus as claimed in claim 8 wherein the apertures have the form of axially extending slots.

10. The apparatus as claimed in claims 2 and 4 wherein the central raised bottom portion has the form of a wedge.

11. The apparatus as claimed in claims 3 and 4 wherein the central raised bottom portion has the form of a curved solid of revolution.

12. The apparatus as claimed in claim 11 wherein the central raised bottom portion has the form of a cone.

13. The apparatus as claimed in claim 12 wherein the cone has a top angle of 90 .

14. The apparatus as claimed in claim 11 wherein the central raised bottom portion has the form of a truncated cone.

15. The apparatus as claimed in claims 3 and 4 wherein the raised central bottom portion has the form of a multilateral pyramid.

16. The apparatus as claimed in any one of the preceding claims wherein the diffusion chamber is detachable from the anvil.

17. The apparatus as claimed in claim 16 wherein the diffusion chamber is screwably connectable on the anvil.

18. Downhole acoustic source for tomography acquisitions substantially as described in the description with reference to the appended drawings.