TDS Estimator from Geophysical Logs

From log readings to estimated TDS

Enter one depth per row, with formation resistivity and porosity. Temperature is estimated from mean surface temperature, bottom-hole temperature, and total depth. Tortuosity (a) and cementation exponent (m) remain editable assumptions.

1. Compute formation factor: F = a / φᵐ, using porosity as a fraction.
2. For a water-saturated interval, estimate water resistivity: Rw = Rt / F.
3. Estimate formation temperature: Tf = AMST + (BHT − AMST) × depth / total depth.
4. Correct water resistivity to 77°F: Rw77 = Rw × (Tf + 6.77) / (77 + 6.77).
5. Estimate specific conductance: SC = 10,000 / Rw77, in µS/cm.
6. Apply the project relationship: TDS = 0.6874 × SC − 76.901, in mg/L.

The calculator accepts pasted tab-delimited rows and displays TDS against depth. Porosity is supplied in percent or calculated from sonic measurements using the displayed relation and its parameters.

GGS3114 worked example

The supplied calculation workbook uses a = 1.0, m = 1.8, mean surface temperature = 75°F, bottom-hole temperature = 148°F, and total depth = 6,668 ft. Its sonic relation is φ = (5/8) × (DT − 49) / DT, with DT in µs/ft. These are example-specific settings.

At 814.96 ft, Rt = 3.9637 ohm-m and DT = 98.9099 µs/ft give estimated TDS of approximately 12,709 mg/L: moderately saline water in the salinity transition zone. All 26 calculation rows have been independently reproduced.

This calculation is intended for suitable water-saturated formations. Shale, hydrocarbons, invasion, thin beds, and mismatched tool responses can invalidate the assumptions. The interpretation must distinguish reliable intervals from apparent salinity spikes. The empirical SC–TDS relationship requires project-specific justification.