Spontaneous Potential Well Log

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Exploration Technique: Spontaneous Potential Well Log

Exploration Technique Information
Exploration Group: Downhole Techniques
Exploration Sub Group: Well Log Techniques
Parent Exploration Technique: Well Log Techniques
Information Provided by Technique
Lithology: SP technique originally applied to locating sulfide ore-bodies.
Stratigraphic/Structural: -Formation bed thickness and boundaries

-Detection and tracing of faults

-Permeability and porosity

Hydrological: Determination of fluid flow patterns: electrochemical coupling processes due to variations in ionic concentrations, and electrokinetic coupling processes due to fluid flow in the subsurface.
Thermal: Location of thermal anomalies: thermoelectric coupling processes due to variations in temperature in the subsurface.
Cost Information
Low-End Estimate (USD): 0.4040 centUSD
4.0e-4 kUSD
4.0e-7 MUSD
4.0e-10 TUSD
/ foot
Median Estimate (USD): 0.4747 centUSD
4.7e-4 kUSD
4.7e-7 MUSD
4.7e-10 TUSD
/ foot
High-End Estimate (USD): 1.00100 centUSD
1.0e-3 kUSD
1.0e-6 MUSD
1.0e-9 TUSD
/ foot
Spontaneous Potential Well Log:
A passive electrical resistivity method in which an electrode is lowered down a borehole and natural electrochemical reactions in the subsurface cause voltage fluctuations which can be detected by the electrode.
Other definitions:Wikipedia Reegle

Spontaneous Potential (SP) also called Self Potential is a passive electrical resistivity technique. For SP logging an electrode is lowered down the borehole and another electrode is grounded on the surface. The voltage difference between the two electrodes is measured.[1] The voltages are created by naturally occurring electrical currents in the subsurface. See Self Potential.

SP logging was accidentally discovered from DC potential that was affecting electric logs. It was one of the first logging measurements and has been in use for over 50 years. SP logging requires very simple instruments and the SP electrode is normally combined with other well logging tools.

Use in Geothermal Exploration
SP logging is standard practice when logging a well and normally done in conjunction with other well logging techniques. SP logs can help with characterization of faults and porous mediums that provide flow for geothermal fluids. Steep SP anomaly gradients are sometimes indicators of Electrokinetic and thermoelectric coupling from fluid flow in porous rock and/or temperature gradients across rock bodies.[2]

Field Procedures
The SP logging tool is a very simple tool and it is normally standard to integrate it along with a gamma ray log or sonic log.

An SP Log requires one electrode to be lowered into the borehole and one ground electrode on the surface. The voltage difference between the two electrodes is measured.[1]

Physical Properties
Data Access and Acquisition
Best Practices
Conductive fluids are needed in the borehole for SP electrodes to receive electrical measurements, so a downhole SP survey cannot be done in non-conductive drilling muds or a borehole filled with air.[1]
Potential Pitfalls
SP measurements are susceptible to noise interference from power lines, buried pipelines, and other metallic objects, so the downhole electrode must be below the well casing and the surface electrode must be away from any object which would cause interference.

  1. 1.0 1.1 1.2 N/A. N/A. Spontaneous Potential (book section). N/A: N/A. N/Ap.
  2. R. F. Corwin,D. B. Hoover. 2005. The Self-Potential Method in Geothermal Exploration. Geophysics. 44(2):226-245.

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