WELLOG SPONTANEOUS POTENTIAL (SP)
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WHAT IS SPONTANEOUS POTENTIAL?
There are at least three sources of spontaneous potential.
In boreholes, at shale/sand interfaces, a potential is produced because of membrane potential.
Electrochemical activity involving metallic minerals in and around massive sulfide ore bodies containing conductive ions creates a measurable potential.
In areas where significant mineralization occurs, potentials of hundreds of millivolts can be measured.
It is believed that a potential is produced by the movement of water in the subsurface. Subsurface water flow around water containment areas and dams can be evaluated by measuring spontaneous potential.
HOW IS SURFACE SPONTANEOUS POTENTIAL MEASURED?
Spontaneous potential is a DC voltage. Equipment designed to measure small DC voltages and having the ability to filter low frequency noise are used to measure SP. Specialized probes are used to connect the instrumentation to the surface of the earth. Probes must be non-metallic because they may produce a “contact” voltage thru electrochemical action of the probe itself. Normally, non-metallic probes using a “porous pot” and containing a metal salt (copper sulfate) solution are used for this purpose.
What is SSP?
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WHERE IS SPONTANEOUS POTENTIAL USED?
Water and oil well exploration:
Water and oil exploration require definition of sandstone/shale sequences in the earth. The thickness of porous oil producing zones in an oil well can be determined using spontaneous potential. In sand/shale zones, membrane action causes segregation of ionic charges.
Minerals contained in the earth react electrochemically with moisture within the earth. In areas containing concentrations of sulfide ore, oxidized zones produce a voltage as a result of the redox of minerals. The electrochemical action produces ions. The ions are electrically charged by definition and are attracted or repelled by other ions resulting in ion flow thru the earth. SP may be used as a surface geophysical technique or subsurface (borehole) technique. In boreholes, SP is measured and can provide important information about the vertical extent of massive sulfide deposits in and around the borehole. Core drilling may miss a nearby sulfide. An SP log of the borehole gives information that can be combined with other SP profiles to vertically map a sulfide structure. Deeper sulfide deposits can be indicated on a spontaneous potential log that would not be shown thru core drilling.
Combination of surface and subsurface SP profiles provide a 2D/3D image of conductive zones.
Dam site studies can be conducted using SP. Underground water flow can be measured and evaluation of water flow around containment areas can be made.
WHAT CAN SPONTANEOUS POTENTIAL REVEAL?
SP will reveal the presence of conductive ore bodies that may or may not be visible at the surface of the earth. Because the currents produced radiate outward around conductive ore bodies, the voltage may be measured at a great distance from it. Surface and borehole profiles may be combined to reveal the true extent of a buried conductor.
Spontaneous Potential can be measured in exploratory or other open-hole boreholes. The method requires a water or mud filled borehole in order to make electrical contact with surrounding formations.
The method provides vertical SP definition of sulfide targets/mineralized contacts and non-mineral rock interfaces. It also provides definition of sandstone/shale sequences.
WELLOG performs both surface and borehole SP surveys.
Surface SP surveys are conducted using digital measurements that are logged onto a PC with GPS positional information. Measurements are made over a selected area and data points representing a grid are logged. The data points may be presented in linear form or as a contour map.
Borehole SP is performed using a 1 ¼ inch caliper mounted SP probe or with 2 inch E-log mandrel. The caliper mounted SP makes direct contact with the borehole wall and does not require a fluid filled hole. The 2 inch E-log mandrel requires a water filled borehole for proper operation.
Presentation of borehole SP is done using a log format with SP voltage on the horizontal axis and depth on the vertical axis. A data file containing the logged data with depth information is created while logging. The data is in the standard log ASCII (LAS) format.
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