GB701498A - Improvements in or relating to apparatus for electrical logging of wells - Google Patents

Abstract

701,498. Geographical exploration. SCHLUMBERGER WELL SURVEYING CORPORATION. Sept. 12, 1952 [Nov. 20, 1951], No. 22965/52. Class 37 Apparatus for investigating the earth formations traversed by a bore hole containing a column of conductive liquid comprises means for locating a body containing a plurality of symmetrically assembled electrodes in the bore hole, and means for measuring the electrical resistivity in zones at different lateral depths from the wall. One embodiment for measuring the resistivity in zones relatively close to the wall of a bore hole 10, Fig. 1, consists of an electrode array 13 comprising a series of point electrodes of which A<SP>0</SP>, B<SP>1</SP>, B<SP>2</SP> are current emitting electrodes, B<SP>1</SP>, B<SP>2</SP> being interconnected, fed from a constant current source 14, and M<SP>1</SP>, M<SP>1</SP>1, M<SP>2</SP>, M<SP>1</SP>2 are symmetrically'located potential electrodes, with M<SP>1</SP>, M<SP>2</SP> connected together, and M<SP>1</SP>1, M'2 connected together. The P.D. between M<SP>1</SP>/M<SP>2</SP> and M<SP>1</SP>1/M<SP>1</SP>2 due to current flow in the bore hole fluid 12 is fed to an amplifier 25 whose output is taken to electrodes B<SP>1</SP>/B<SP>2</SP> and to a further pair of electrodes A<SP>1</SP>, A<SP>2</SP> disposed symmetically about the other electrodes and also connected together, the output current from amplifier 25 being such as to maintain the P.D. between M<SP>1</SP>/M<SP>2</SP> and M<SP>1</SP>1/M<SP>1</SP>2 at zero, whereby the current from A<SP>0</SP> is prevented from travelling vertically and is substantially restricted to the horizontal direction. The depth of current penetration into the wall is determined by the relative positions of A<SP>0</SP> and B<SP>1</SP>/B<SP>2</SP> and is fixed to allow only small depths of penetration. The P.D. between electrodes M<SP>1</SP>/M<SP>2</SP> and an earth point 28 is measured by a voltmeter 27 and indicates the resistivity of this part of the wall. The dimensions of one embodiment are given for a penetration of five times the diameter of the bore hole. To reduce the effect of the bore hole fluid 12 on the indications, the electrodes A‹, M<SP>1</SP>, M<SP>2</SP>, M<SP>1</SP>1, M<SP>1</SP>2 are made circular and embedded in a rubber cushion 111 (Fig. 2) and maintained in close proximity with the wall by being carried on springs 115 attached to a central support 110 carrying the remaining electrodes A<SP>1</SP>, A<SP>2</SP>, Bi, B<SP>2</SP>. Various other forms of electrode arrays are shown in which the electrodes are concentric circles and may be reduced in number by combining A‹ with M<SP>1</SP>, M<SP>2</SP> and M<SP>1</SP>1/M<SP>1</SP>2 with A<SP>1</SP>/A<SP>2</SP> on the face of a rubber pad, or the electrodes may constitute a plurality of cylinders vertically disposed along the axis of the bore hole. The output of the amplifier 25 may alternately be fed to electrodes A<SP>1</SP>/A<SP>2</SP> and earth, when the voltmeter reads the resistivity of zones at a larger depth of penetration. The two readings may be obtained concurrently by switching the amplifier output and the power input intermittently from electrodes B<SP>1</SP>/B<SP>2</SP> to an earth point, or two amplifiers may be used and two recording voltmeters. Alternatively, two A.C. sources of different frequencies may be used together with suitable filters, when switching is unnecessary.