Schlumberger Technology Corporation

Dylan H Davies

Jean-Claude Trouiller

Jan W Smits

Dominique Benimeli

Pierre-Marie Petit

Jacque R Tabanou

A logging method and apparatus for investigating the geometrical characteristics of a borehole comprises an array of circumferentially spaced apart azimuthal current electrodes (Aaz.sub.i) disposed on a sonde (21), and an annular current electrode (A) longitudinally spaced apart from the array of azimuthal electrodes. In a method of investigating the geometrical characteristics of the borehole, currents (Ic.sub.i) are emitted between the azimuthal electrodes and the annular electrode (A). Monitor electrodes (M, Maz.sub.i) are respectively associated with the current electrodes (A, Aaz.sub.i). The potential differences (Vc.sub.i) between annular monitor electrodes (MAz.sub.i) are detected. In response to the detected potential differences, output signals (Rc.sub.i) are generated indicative of the distances between the sonde and the wall of the borehole in a plurality of directions around the sonde. The technique can be applied to azimuthal resistivity measurements of earth formations for correcting said measurements for sonde eccentring and/or the irregular shape of the section of the borehole.

Logging method and apparatus for investigating geometrical

The invention relates to a logging method and apparatus using passive azimuthal electrodes. The apparatus includes an array of azimuthal electrodes (M'az.sub.i) that are circumferentially spaced apart from one another on a body together with two annular guard electrodes (A2) disposed on either side of the array of azimuthal electrodes (M'az.sub.i). The guard electrodes (A2) emit currents. Two annular monitor electrodes (M3, M4) are associated with the annular guard electrodes (A2). The potential difference (.DELTA.Vaz.sub.i) between the interconnected monitor electrodes (M3, M4) and each of the azimuthal electrodes (M'az.sub.i) are detected. In response to the detected potential differences, output signals (R'az.sub.i) representative of the resistivity of the formations in a plurality of directions around the axis of the borehole are generated. The apparatus of the invention also relates to apparatus having two alternative modes of operation, one corresponding to a passive azimuthal electrode technique and the other corresponding to a technique in which currents (Iaz.sub.i) are emitted by the azimuthal electrodes.

Well logging method and apparatus using passive azimuthal electrodes

A logging method for measuring the resistivity of formations having a borehole passing therethrough, in which a logging sonde is suspended inside the borehole by means of a cable, the sonde comprising electrodes for injecting alternating currents into the formations, these electrodes comprising a central electrode A.sub.0 and focusing electrodes, together with a return electrode B and a potential reference electrode N, one of which is placed above the sonde and the other of which is placed at the surface, and producing a resistivity log from the current I.sub.0 emitted by the central electrode and the potential difference between the sonde and the reference electrode. The method further comprises the step of producing a second log of the resistivity with the return and potential reference electrodes being placed as for the first log and under conditions such that the investigation depth is different from that applicable to the first log. The invention is applicable to correcting the Groningen effect.

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Viroflay, France

Dylan H Davies