Dipole logging tool

Details Dipole logging tool Dipole logging tool

2000-03-29

2002-11-05

Nappi, Robert E. (Department: 2837)

Acoustics

Geophysical or subsurface exploration

Well logging

C181S104000, C181S111000, C181S121000, C367S025000, C367S117000, C367S153000, C367S176000, C367S189000

Reexamination Certificate

active

06474439

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to acoustic borehole logging tools having dipole transmitters. In particular, the invention relates to the use of a novel dipole transmitter for use in a borehole logging tool which addresses certain shortcomings of prior art designs.
BACKGROUND OF THE INVENTION
The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. In some cases, but not all, techniques and technologies are transferable between these domains. A summary of the general techniques involved in borehole acoustic logging can be found in GEOHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopaedia of Electrical and Electronic Engineering 1999, pp 340-365.
One example of a sonic logging tool is Schlumberger's Dipole Sonic Imaging tool (DSI) which is shown in schematic form in FIG.
1
. The DSI tool comprises a transmitter section
10
having a pair of (upper and lower) dipole sources
12
arranged orthogonally in the radial plane and a monopole source
14
. A sonic isolation joint
16
connects the transmitter section
10
to a receiver section
18
which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge
20
is connected at the top of the receiver section
18
and allows communication between the tool and a control unit
22
located at the surface via an electric cable
24
. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire (digitised) waveforms. The modes are: upper and lower dipole modes (UDP, LDP)—waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole mode—waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&S)—monopole waveforms from high frequency firing of the monpole transmitter; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source.
Various types of dipole signal source have been proposed and used in the past. These include:
i) Electro-magnetic transducer devices such as is used in Schlumberger's DSI tool. (see for example Hoyle et al; U.S. Pat. No. 4,862,991 and Kitsunezaki in U.S. Pat. No. 4,207,961 or Ogura in U.S. Pat. No. 4,383,591).
ii) Linked mass vibrators driven by magnetostricitve actuators. (see for example Cohick and Butler, “Rare-earth Iron Square Ring Dipole Transducer”, J. Acoustical Society of America, 72(2), August, 1982)
iii) Piezo-electric bender devices such as are used in the XMAC tool of Baker Atlas. (see for example Angona et al, U.S. Pat. No. 4,649,525)
iv) Magnetic repulsion transducers driving a plate in contact with a fluid in an acoustic wave guide system such as are used in the MPI XACT tool. (see for example Gill et al, U.S. Pat. No. 5,852,262)
v) Eccentric orbital masses as proposed by Cole in U.S. Pat. No. 4,709,362, Meynier in U.S. Pat. No. 5,135,072 and others, mainly for seismic uses.
Dipole sonic sources of types i) to iv) described above typically comprise a heavy, stiff tool body having the actuator (piston or plate) mounted therein via a transducer or drive mechanism, the actuator contacting the borehole fluid through ports in the tool body. In use, the tool body acts a reaction mass against which the transducer acts to oscillate the actuator. However, the effect of this is to excite tool body recoil vibrations, which interfere with the dipole flexural signal in the borehole. In these sources the tool body is used as the reaction mass. Its large vibrating surface is a very efficient radiator of noise into the borehole flexural mode. This means also that increasing the excitation force increases recoil vibrations in the same proportion as the signal. Another problem is that the dipole signals couple into the tool structure and travel along it directly to the receivers where they interfere with the detection of the signals of interest from the formation. Various measures have been used or proposed to deal with this problem, for example: locating the source and receivers in separate sondes connected by a flexible cable; the use of isolation joints which include structures for attenuating or delaying signals travelling along the tool; or adopting a structure which does not include any continuous mechanical structure along the length of the tool so as not to provide a signal path; or the use of housings around the receivers which delay the arrival of tool signals. Transmitters of these types are imperfect dipoles, because of the limited azimuthal extent of the active radiating surface (i.e. the ports in the tool body). Strong hexapole aliasing can be produced by such sources, and possibly strong monopole contamination when the source is eccentered in the borehole. Furthermore, bender (piezoceramic bimorph) sources (type iii) are inherently band limited in frequency, and radiate over a small azimuthal extent.
Orbital vibrators and counter-rotating eccentric mass devices (type v) are typically appropriate only for lower frequencies, typically below 500 Hz., and are often found in seismic applications. Such sources are not normally considered as suitable for broad band or higher frequency use such as is encountered in sonic logging.
If the full flexural dispersion curve (phase slowness vs. frequency) is to be used to make measurements at different depths of investigation for a wide variety of petrophysical, geophysical, and geomechanical applications, wideband dipole sources of high purity will be required. Moreover, in dipole tools, where the flexural impedance of even the most “rigid” tool body is low, recoil vibrations of the tool body, wave propagation along the tool body, and reflections at tool joints and within acoustic isolation sections, have all been found to radiate noise into the borehole and formation, contaminating the flexural signal. Consequently, the transmitter and tool body should be designed as a system to minimize these noise sources.
SUMMARY OF THE INVENTION
The present invention provides a logging tool comprising a tool body, which can be positioned in a fluid-filled borehole, having a receiver section and a dipole transmitter; wherein the dipole transmitter includes a transducer comprising a shell having a reaction mass and a motor located therein, the motor operatively connecting the shell and the reaction mass such that only an outer surface of the shell is in contact with the fluid in the borehole.
The present invention concerns a new type of dipole source for well logging. Specifically, the new source involves the idea of shaking all or part (axially) of a dipole tool body to produce a pure, broadband acoustic dipole signal while at the same time coupling as little energy as possible into the tool body. Important variations on this idea include a linear phased array of shaker sources, and active cancellation of tool borne noise.
The dipole transducer is preferably mounted in the tool body by means of spring mountings, the resonant frequency of which is such that coupling of flexural vibrations from the receiver to the tool body in a predetermined frequency range is inhibited. Typically, the resonant frequency of the spring mountings is less than the lower limit of the predetermined frequency range. It is also preferred that the spring mountings are relatively flexible in the direction of oscillation of the dipole transmitter and relatively stiff in a direction orthogonal thereto.
The resonant frequency of the shell also preferably falls outside a predetermined frequency range, and the weight of the shell is less than the weig

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