Non-NaCl Static Self Potential from Ion Activities Formula
Non-NaCl Static Self Potential from Ion Activities calculates static self potential for electrical properties workflows in petrophysics.
How engineers use this formula
Use this formula when the listed inputs (K, a_Na, a_Ca, a_Mg, a_Nam, a_Cam, a_Mgm) are known and the assumptions behind the cited electrical properties relationship match the engineering case being checked.
Assumptions
- Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
- The declared units match the field-unit constants used in the formula.
- The cited formula applies to the selected petroleum engineering workflow.
Limitations
- The calculation does not replace a full engineering model or operating procedure.
- Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.
Common mistakes
- Mixing unit systems without converting the inputs.
- Using default example values as field recommendations.
- Applying the formula outside the source assumptions.
Default example
Using the default inputs, E_ssp equals -0.762434 mV.
70
0.08
0.01
0.005
0.12
0.004
0.002
Inputs
K
mVSP Equilibrium Constant
a_Na
dimensionlessActivity of Sodium Ions in Formation Water
a_Ca
dimensionlessActivity of Calcium Ions in Formation Water
a_Mg
dimensionlessActivity of Magnesium Ions in Formation Water
a_Nam
dimensionlessActivity of Sodium Ions in Mud Filtrate
a_Cam
dimensionlessActivity of Calcium Ions in Mud Filtrate
a_Mgm
dimensionlessActivity of Magnesium Ions in Mud Filtrate
Outputs
E_ssp
Static Self Potential
K
SP Equilibrium Constant
Source and review
reviewedTheory, Measurement, and Interpretation of Well Logs, Bassiouni, Z. (1994)
Bassiouni, Z. 1994. Theory, Measurement, and Interpretation of Well Logs. SPE Textbook Series Vol. 4, Chapter 6, Page 135.
Source