Rw from SP Calculator

Calculate formation water resistivity (Rw) from spontaneous potential (SP) log deflection. Essential for determining water salinity and calculating water saturation in clean formations.

SP Log Water Resistivity Petrophysics Salinity

Input Parameters

SP Log Reading

mV
Negative value indicates clean sand (typical: -60 to -120 mV)

Formation Temperature

°C
Formation temperature at measured depth
°F
Automatically converted from °C

Mud Filtrate Properties

ohm-m
At formation temperature (typically 0.1 - 2.0 ohm-m)

Quick Presets

Results

Theory & References

SP Equation

SP = -K × log₁₀(Rmf / Rw)

K = 61 + 0.133 × T (°F)

Solving for Rw:

Rw = Rmf × 10^(-SP/K)

Where:

  • SP: Spontaneous potential deflection (mV)
  • K: Temperature-dependent coefficient (mV/decade)
  • Rmf: Mud filtrate resistivity (ohm-m)
  • Rw: Formation water resistivity (ohm-m)
  • T: Formation temperature (°F)

Temperature Correction

Arps Formula:

Rw@75°F = Rw@T × (T + 6.77) / (75 + 6.77)

Temperature correction is essential because resistivity is strongly temperature-dependent. The standard reference temperature is 75°F (23.9°C).

Key Points:

  • Higher temperature → Lower resistivity
  • Temperature correction factor typically ranges from 0.5 to 2.0
  • Always report Rw at both formation and standard temperatures

Assumptions & Applications

Required Conditions:

  • Clean formations: No conductive shale or clay
  • Permeable zones: Must allow mud filtrate invasion
  • Water-based mud: SP develops from ionic activity
  • Salinity contrast: Rmf ≠ Rw for SP deflection

Best Applications:

  • Determining formation water salinity
  • Input for Archie water saturation calculation
  • Identifying permeable zones and fluid contacts
  • Correlation with core analysis

Limitations:

  • Not valid in shaly formations
  • Requires accurate Rmf measurement
  • Oil-based mud produces no SP deflection
  • Assumes complete flushing in invaded zone