Archies Equation for Water Saturation

Reference

Archies equation converts electrical resistivity into water saturation, the calculation every hydrocarbon volume ultimately rests on.

The equation

Sw = ((a Rw) / (φ^m Rt))^(1/n)
  • Sw is water saturation of the pore space
  • Rw is formation water resistivity, Rt the measured deep resistivity
  • a, m, n are the tortuosity, cementation, and saturation exponents, commonly 1, 2, 2

Worked example

With Rw of 0.05 ohm meters, porosity 0.20, and Rt of 10 ohm meters, Sw = √(25 × 0.05 / 10) = 0.35. The remaining 65 percent of the pore space holds hydrocarbon.

When to use it

Built for clean rocks. Clay conducts, so shaly sands read too much water unless you switch to a shaly sand model. The exponents deserve local calibration: m alone can move reserves by tens of percent.

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