Spontaneous potential measurement apparatus connected between metallic

Abstract

A spontaneous potential (SP) sub is interconnected between two metallic housings of a wellbore apparatus and is lowered into a wellbore. A spontaneous potential (SP) of an earth formation trasversed by the wellbore is measured by the SP sub, and a log is generated from the sub which is representative of the spontaneous potential of the formation, the log being substantially free of a special type of distortion which typified the prior art SP measurements taken by prior art SP measurement apparatus. The SP sub includes a measurement electrode, monitoring electrodes placed on both sides of the measurement electrode, a bucking electrode placed on both sides of the monitoring electrodes, current driving circuitry connected to the bucking electrodes, and receiving circuitry connected to the monitoring electrodes. The current driving circuitry delivers a current of sufficient magnitude to each bucking electrode such that the voltage potential between the monitoring electrodes is approximately equal to zero. The current from each bucking electrode is grounded via its own respective metallic housing and not via a surface oriented electrode. The receiving circuitry monitors the voltage between each of the monitoring electrodes and looks for a zero potential difference between the monitoring electrodes. When the receiving circuitry locates the zero potential difference at a moment in time between its monitoring electrodes, the measurement of the spontaneous potential taken by the measurement electrode at that same precise moment in time is received and recorded. A plurality of such measurements made by the measurement electrode represents a log of spontaneous potential of the earth formation traversed by the wellbore which is substantially free of the distortion which typified the prior art SP log measurements.