Question

The resistivity reading of a flushed zone across a permeable formation (drilled with water-based mud) is 20 $\Omega\!\cdot$m. Laboratory analysis shows that the resistivity of the core plug (100% saturated with a NaCl brine) from the same formation is 6 $\Omega\!\cdot$m. The resistivity of the NaCl brine is 0.6 $\Omega\!\cdot$m. If the resistivity of the mud filtrate is 0.9 $\Omega\!\cdot$m and Archie's saturation exponent is 2, then the estimated residual hydrocarbon saturation (in percentage) in the flushed zone is ............. (rounded to two decimal places).

Hint:

In flushed zones, use $S_{xow}=\left(\dfrac{F\,R_{mf}}{R_{xo}}\right)^{1/n}$; the residual hydrocarbon saturation is simply $1-S_{xow}$.

Answer 1

- From the 100% brine-saturated core: $F=\dfrac{R_{o}}{R_{w}}=\dfrac{6}{0.6}=10$.
- In the flushed zone (mud filtrate replaces most pore water): using Archie, \[ S_{xow}=\left(\frac{F\,R_{mf}}{R_{xo}}\right)^{1/n} =\left(\frac{10\times0.9}{20}\right)^{1/2} =\sqrt{0.45}=0.67082. \] - Residual hydrocarbon saturation in the flushed zone: \[ S_{xo\,hc}=1-S_{xow}=1-0.67082=0.32918 \Rightarrow 32.92%. \]