EP1247060A2 - Procede et dispositif de focalisation d'image pour imagerie de resistivite de trou de sonde - Google Patents

Procede et dispositif de focalisation d'image pour imagerie de resistivite de trou de sonde

Info

Publication number
EP1247060A2
EP1247060A2 EP01901934A EP01901934A EP1247060A2 EP 1247060 A2 EP1247060 A2 EP 1247060A2 EP 01901934 A EP01901934 A EP 01901934A EP 01901934 A EP01901934 A EP 01901934A EP 1247060 A2 EP1247060 A2 EP 1247060A2
Authority
EP
European Patent Office
Prior art keywords
measure
electrodes
formation
current
distance
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01901934A
Other languages
German (de)
English (en)
Inventor
Martin T. Evans
Richard A. Burt
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Baker Hughes Holdings LLC
Original Assignee
Baker Hughes Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Baker Hughes Inc filed Critical Baker Hughes Inc
Priority claimed from PCT/US2001/000742 external-priority patent/WO2001051880A2/fr
Publication of EP1247060A2 publication Critical patent/EP1247060A2/fr
Withdrawn legal-status Critical Current

Links

Definitions

  • the Gianzero patent discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole.
  • the measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole.
  • the measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations.
  • a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode.
  • a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole.
  • they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies
  • the Dory patent discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole.
  • the present invention is a method and apparatus for obtaining resistivity images of a borehole.
  • the apparatus includes an array of measure electrodes separated from a pad or the body of the instrument by guard electrodes.
  • the guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto.
  • the current from the measure electrode initially diverges as it enters the formation, then converges (focuses) and then finally diverges again to define a depth of investigation.
  • This arrangement makes it relatively insensitive to borehole rugosity.
  • circumferential and vertical overlap may be obtained either by the arrangement of the measure electrodes to provide the overlap, or by relying on the broadening of the measure beam in a region of investigation within the formation.
  • Fig. 1 is a perspective and block diagram view of a borehole investigating tool with pads
  • Fig. 3 illustrates the actual flow of current using the tool of Fig. 1;
  • Fig. 4 is a schematic cross section illustrating the arrangement of electrodes and the flow of currents in an embodiment of the present invention;
  • Figs 5A - 5D are illustrations of arrangements of electrodes in various embodiments of the present invention.
  • Fig. 1 shows a pad 30 and array 70 often circular measure electrodes 36 as closely spaced as possible in a multiple number of rows 74, 76.
  • the electrodes 36 are surrounded by insulator rings 44.
  • Measure electrodes 36 are mounted flush on pad 30 whose surface 42 is conductive. Insulators 44 electrically isolate measure electrodes 36 from the conductive pad surface 42 while in the operation of a pad 30 the electrical potential of the several measure electrodes 36 and conductive surface 42 is effectively the same.
  • the currents flowing from each of the electrodes 36 is indicative of the resistivity of the formation in contact with the electrode.
  • the pad 30 or the body of the instrument acts as an effective electrode.
  • Fig. 2 shows a schematic cross section of a resistivity array device including a measure electrode. Shown is a device 100 in contact with a borehole wall 106 with a formation 102. The device includes a pad of which portions 101a, 101b are shown, along with a measure electrode 103 and gaps 107 electrically separating the measure electrode 103 from the pad. For simplifying the illustration, the connections between a source of electrical current and the measure electrode 103 and the pad 101a, 101b are not shown.
  • the current flow is directly into the formation and the flow lines are parallel for a distance indicated by 105 after which the current paths start diverging.
  • the current will ultimately return back to the source through a return electrode (not shown).
  • the distance from the borehole wall 106 to 105 is the depth of investigation of the tool.
  • the magnitude of the current flowing from the measure electrode 103 is indicative of the electrical conductivity of the formation adjacent to the measure electrode 103 up to the depth of investigation.
  • the currents from the individual measure electrodes are measured to give an indication of the conductivity of the formation adjacent to the electrodes.
  • Fig. 3 the real situation is indicated. Due to the geometry of the pads, the conductive portion of the tool body in the vicinity of the pads and the geometry of the electrically isolating section of the tool between the pad section and the aforementioned return electrode, the electrical current from pad and the measure electrodes diverges quite rapidly. The is indicated by the paths 109a', 109b' and 109c' of current from the measure electrode 103 and the rapid divergence of the currents Ilia' . . . 11 If from the pad.
  • the current flowing from the electrode 103 depends upon the conductivity of the formation in a first region in contact with the measure electrode as well as portions of the formations surrounding this first region. This reduces the resolution of the instrument.
  • the current paths will be effected by irregularities in the borehole wall as indicated by 106'. Both of these effects (the divergence and the effects of rugosity) can give erroneous indications of formation conductivity.
  • This technique causes the measure beam (current from the measure electrode) to be defocused as it leaves the instrument and then refocus again approximately 1/4 to V ⁇ inch (.625 to 1.25 cm.) away from the pad carrying the measure electrode. This desensitizes the image to the effects of borehole wall rugosity. This is described below.
  • the arrangement of the electrodes is unchanged but the current flows into the measure electrodes.
  • the magnitudes of the voltages are the same as discussed above. This configuration is not discussed further but would be a straightforward variation of the discussion here on current flowing out of the measure electrodes.
  • the lower potential of the primary guard 203a, 203b causes the current from the measure electrode 215 to diverge as it leaves the measure electrode. The more divergent the beam, the less the sensitivity to that element of the length of the measure beam. As the measure beam progresses further into the formation, the higher potential of the pad 201a, 201b causes the beam from the measure electrode 215 and the guard 203a, 203b to be pinched in, thereby increasing the sensitivity from that portion of the measure beam.
  • the beam Since the greatest divergence of the measure beam is closest to the borehole wall, the beam is quite insensitive to the effects of borehole rugosity. At large distances from the borehole wall such as 205b, the measure beam again diverges.
  • the region between 205a and 205b includes the region of greatest sensitivity of the tool.
  • the region of greatest sensitivity is roughly defined by the portion between 205c and 205b where the diameter of the measure beam is small, with the smallest value being attained at a distance between 205c and 205b.
  • By suitable adjustment of the electrical potentials it is possible to obtain a measure beam having a diameter within the sensitive region that is larger than or smaller than the diameter of the measure electrodes.
  • the electrodes and the pad are connected to sources of electrical current to maintain the desired voltages and, as in prior art devices, the current from the measure electrode is indicative of formation conductivity.
  • One advantage of the present invention over prior art devices is the ability to make accurate measurements in irregular boreholes. As may be seen in Fig. 4, there is no requirement of physical contact between the entire measure electrode and the formation: all that is necessary is good electrical contact, even through any intervening borehole fluid. Due to the defocusing of the beam near the measure electrode, the present device is relatively insensitive to the presence of borehole fluid between portions of the electrode and the formation. Similarly, the invention also functions properly when there is incomplete physical contact between the guard electrode and the formation.
  • the word "pad” is used here only for convenience and the invention would work equally well if the body of the instrument is used instead of the pad for the purpose of carrying measure electrodes and/or injecting currents into the formation as described below.
  • the pad in the present invention does not actually have to contact the borehole wall.
  • the primary function of the pad is to focus the measure beam after the defocusing caused by the guard electrode. This can be accomplished by a tool body that is not in actual contact with the borehole wall almost as well as by a pad in contact with the borehole wall. Accordingly, use of the word "pad” hereinafter and particularly in the claims is intended to include the body of the instrument as well.
  • Fig. 5a illustrates an array of measure electrodes 315a, 315b, 315c. . . set within a substantially rectangular guard electrode 303 with gaps 307a (that contain insulating material therein).
  • the guard electrode 303 is separated from the pad or body 301 by a substantially rectangular insulating gap 307b.
  • the spacing between the measure electrodes is selected as in the Gianzero patent to provide overlap in azimuth and depth, i.e., the diameter D of the measure electrode is greater than the horizontal spacing di of the electrodes 315b, 315c in adjacent rows and the vertical spacing d 2 between the rows of electrodes.
  • the electrodes do not have this azimuthal an and vertical overlap , but due to the broadening of the measure beam discussed above in reference to Fig. 4, overlap in azimuth and borehole depth of the region of investigation is obtained.
  • Fig. 5b depicts another arrangement of electrodes in the present invention.
  • the measure electrodes 415 are concentric with the guard electrodes 403. Insulating gaps 407 and 419 are also indicated.
  • the measure electrodes may or may not have overlap in azimuth and depth. When the measure electrodes themselves do not overlap, the broadening of the measure beam provides overlap of measurements at the depth of investigation.
  • Fig. 5c illustrates an arrangement of measure electrodes 515 separated from a guard electrode 519 by insulating gap 503.
  • the guard electrode 503 is, in turn, separated form the pad by insulating gap 519.
  • Fig. 5d illustrates a configuration of the measure electrodes measure electrodes 615 are arranged in two groups with insulating gaps 607. As in Fig. 5c, the guard electrode 603 is separated from the pad by another gap 619.
  • a secondary guard electrode located between the pad and the guard electrode is used to provide the focusing of the beam in the region of investigation.

Landscapes

  • Geophysics And Detection Of Objects (AREA)
  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

Abstract

L'invention concerne un dispositif permettant d'obtenir des images de résistivité d'un trou de sonde, qui comprend un réseau d'électrodes de mesure séparées d'une plaque ou du corps de l'instrument par une électrode de garde. L'électrode de garde est maintenue à un potentiel légèrement inférieur à celui de la plaque, et l'électrode de mesure présente un potentiel intermédiaire. Ce dispositif permet au courant provenant de l'électrode de garde de défocaliser le courant de mesure provenant de l'électrode de mesure lorsqu'il pénètre dans la formation, et, à de plus grandes distances, le courant provenant de l'électrode de mesure est refocalisé grâce au courant provenant de la plaque. Ces focalisation et refocalisation définissent une région d'investigation éloignée de la paroi de trou de sonde, qui est relativement insensible à la rugosité. Un chevauchement circonférentiel et vertical peut être obtenu par un aménagement des électrodes de mesure, ou par un élargissement du faisceau de mesure lorsqu'il pénètre dans la formation.
EP01901934A 2000-01-11 2001-01-10 Procede et dispositif de focalisation d'image pour imagerie de resistivite de trou de sonde Withdrawn EP1247060A2 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US17558500P 2000-01-11 2000-01-11
US175585P 2000-01-11
PCT/US2001/000742 WO2001051880A2 (fr) 2000-01-11 2001-01-10 Procede et dispositif de focalisation d'image pour imagerie de resistivite de trou de sonde

Publications (1)

Publication Number Publication Date
EP1247060A2 true EP1247060A2 (fr) 2002-10-09

Family

ID=22640812

Family Applications (1)

Application Number Title Priority Date Filing Date
EP01901934A Withdrawn EP1247060A2 (fr) 2000-01-11 2001-01-10 Procede et dispositif de focalisation d'image pour imagerie de resistivite de trou de sonde

Country Status (2)

Country Link
EP (1) EP1247060A2 (fr)
AU (1) AU2001227785A1 (fr)

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO0151880A3 *

Also Published As

Publication number Publication date
AU2001227785A1 (en) 2001-07-24

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