Carlos Torres-Verdín, Ph.D. Assistant Professor PGE368 Fall 2003 Semester September 23 and 25, 2003
• Tounderstand the physical principles governing borehole spontaneous potential (SP) phenomena, • To learn how to interpret SP logs in terms of lithology and petrophysical properties, and • To understand whatcorrections are normally applied to SP logs before using them for petrophysical interpretation.
Course Plan, I
Borehole Environment Mud-Filtrate Invasion
The Anatomy of a Log LogFormats and Plots
Caliper and Tension Logging Temperature Logging Spontaneous Gamma Ray Logging Spontaneous Potential (SP) Logging
Complementary Reading Assignments:
1.Bassiouni, Z., 1994, Theory, Measurement, and Interpretation of Well Logs, Chapter 6 “The Spontaneous Potential Log.” 2. Introduction to Wireline Log Analysis, Western Atlas Logging Services, Pages 75-76,and 147-149. 3. Schlumberger’s Computer Animated Presentation on SP available from our course web site.
Open Hole SP Measurement Principle
The Historical Origin of the SP Log
SP Log,Gradient SP Log, and Interpretation
• The SP is measured in millivolts, mV. • The scale on the log shows a number of mV per division for example 20mV/division. This gives a total for the trackof 200mV. • The scale across the track is variable and depends on the conditions in the well. • The scale is set during logging to have the SP curve in the track over the zone of interest and as muchof the rest of the log as possible.
Spontaneous Potential (mV) ->10mV< + Tracks 2/3
PRACTICAL USES OF SP LOGS
• Differentiate potentially porous and permeable reservoir rocks from impermeableshales. • Define bed boundaries. • Give an indication of shaliness (maximum deflection is clean sand; minimum is shale). • Determine Rw in both salt and fresh muds.
SP LOG EXAMPLE
Rmf > Rw...