Spontaneous potential measurement apparatus connected between metallic housings of a wellbore apparatus for measuring the spontaneous potential of an earth formation

A method of measuring a spontaneous potential of a formation traversed by a wellbore, comprising the steps of:

• (a) interconnecting a housing of a spontaneous potential measurement apparatus between a first metallic housing and a second metallic housing;
• (b) lowering said first metallic housing, said spontaneous potential measurement apparatus, and said second metallic housing into said wellbore, said spontaneous potential measurement apparatus including a measurement electrode, a bucking electrode disposed on both sides of said measurement electrode, and at least one monitoring electrode disposed between each said bucking electrode and said measurement electrode;
• (c) driving each of the bucking electrodes with a first current, each of the bucking electrodes generating a second current, the second current generated by each of the bucking electrodes being returned to a ground potential via either said first metallic housing or said second metallic housing;
• and
• (d) when a potential difference between each of the monitoring electrodes and the measurement electrode is approximately zero, reading said spontaneous potential relative to a reference potential from said measurement electrode,
• said spontaneous potential read from said measurement electrode during the reading step (d) representing said spontaneous potential of said formation traversed by said wellbore.

An apparatus adapted to be disposed in a wellbore for measuring a spontaneous potential of a formation traversed by the wellbore, a plurality of naturally occuring currents flowing in said formation, comprising:

• a first housing;
• a second housing;
• and
• a measurement apparatus including a housing, the housing of said measurement apparatus being interconnected between said naturally occuring currents flowing in said formation adjacent the measurement electrode, the first and second metallic housings and the bucking electrodes being substantially cancelled when the first and second potential differences are approximately equal to zero,
• said measurement electrode measuring said spontaneous potential of said formation adjacent said measurement electrode when said naturally occurring current flowing in said formation adjacent said measurement electrode is substantially cancelled.

An apparatus adapted to be disposed in a wellbore for measuring a spontaneous potential of a formation traversed by the wellbore, a plurality of naturally occuring currents flowing in said formation, comprising:

• a first metallic housing;
• a second metallic housing;
• and
• a spontaneous potential measurement apparatus including a housing, the housing of said spontaneous potential measurement apparatus being interconnected between the first metallic housing and the second metallic housing, said spontaneous potential measurement apparatus including,
• a measurement electrode adapted for measuring said spontaneous potential of said formation adjacent said measurement electrode, said naturally occuring currents flowing in said formation adjacent said measurement electrode,
• a bucking electrode disposed on both sides of said measurement electrode, said naturally occuring currents flowing in said formation adjacent the metallic housings and the bucking electrodes,
• a pair of monitoring electrodes disposed between each of the bucking electrodes and the measurement electrode, and
• current source means connected to the bucking electrodes for providing a first current to each of said bucking electrodes, said bucking electrodes generating a second current in response thereto, said second current from said bucking electrodes flowing in said formation and returning to a ground potential via either said first metallic housing or said second metallic housing,
• a first potential difference existing between each of the pair of monitoring electrodes, said first potential difference being approximately equal to zero in response to a predetermined magnitude of said second current from said bucking electrodes,
• a second potential difference existing between each of the bucking electrodes, said second potential difference being approximately equal to zero in response to said predetermined magnitude of said second current from said bucking electrodes,
• whereby substantially no current flows in the formation adjacent said measurement electrode when said potential difference is approximately equal to zero.

An apparatus adapted to be disposed in a wellbore for measuring a spontaneous potential of an earth formation penetrated by said wellbore, comprising:

• a first metallic housing;
• a second metallic housing;
• and
• a spontaneous potential sub including a sub housing, said sub housing being interconnected between said first metallic housing and said second metallic housing, said spontaneous potential sub including,
• a measurement electrode adapted for measuring the spontaneous potential of said formation, a naturally occurring current flowing in the formation adjacent said measurement electrode,
• a first electrode adapted for generating a counteracting current and flowing said counteracting current in said formation for counteracting and cancelling said naturally occurring current in said formation, said counteracting current in said formation returning to a ground potential via either said first metallic housing or said second metallic housing, and
• a monitoring electrode disposed between said measurement electrode and said first electrode, a potential difference existing between said monitoring electrode and said measurement electrode,
• said potential difference being approximately zero when said counteracting current from said first electrode counteracts and cancels said naturally occurring current,
• said measurement electrode measuring said spontaneous potential of said formation when said potential difference is approximately zero.

A method of measuring a spontaneous potential of an earth formation penetrated by a wellbore, comprising the steps of:

• interconnecting a housing of a spontaneous potential sub between a housing of a first tool string and a housing of a second tool string thereby producing a system;
• lowering said system into said wellbore, said spontaneous potential sub including a measurement electrode adapted for measuring said spontaneous potential of said formation penetrated by said wellbore, a current emitting electrode disposed on one side of said measurement electrode adapted for emitting a counteracting current into said formation, and a monitoring electrode disposed between said current emitting electrode and said measurement electrode, a potential difference existing between said monitoring electrode and said measurement electrode, a naturally occurring current flowing in said formation in the vicinity of said measurement electrode;
• emitting said counteracting current from said current emitting electrode, said counteracting current returning to a ground potential via the housing of said first tool string, said counteracting current counteracting and cancelling at least a portion of said naturally occurring current flowing in said formation in the vicinity of said measurement electrode, said potential difference being approximately equal to zero when said counteracting current cancels said at least a portion of said naturally occurring current in the vicinity of said measurement electrode;
• and
• when said potential difference is approximately equal to zero, reading said spontaneous potential of said formation from said measurement electrode.