Formation Water and Mud Filtrate
Learning objectives
- Explain why formation-water resistivity Rw controls the saturation calculation
- Relate Rw to salinity and temperature
- Apply the Arps temperature correction
- Distinguish mud-filtrate resistivity Rmf from Rw and explain why their contrast matters
The Conductive Brine
Saturation is read from resistivity, but the rock matrix and the hydrocarbons barely conduct at all; the only thing carrying current through the pore space is the salty formation water. So before resistivity can mean anything, we need the resistivity of that water, Rw. It is the single most important input to every saturation equation, and the one most often gotten wrong.
Salinity and Temperature
Two things set Rw. Salinity: more dissolved salt means more ions to carry current, so Rw falls as the water gets saltier. Temperature: ions move more freely when hot, so Rw also falls as the rock gets deeper and warmer. The temperature effect is large and must always be corrected to the depth of interest, using the Arps relation
for temperatures in degrees Fahrenheit. A Rw quoted at surface temperature can be several times too high for a deep, hot reservoir.
Mud Filtrate and the Contrast
The mud has its own water, and its filtrate has a resistivity Rmf measured at the surface and corrected the same way. Water-based muds are usually made up fresher than the formation brine, so Rmf is higher than Rw. That contrast is the whole reason invasion is detectable: where mud filtrate has flushed the near-wellbore rock, that zone reads at Rmf-controlled resistivity while the deep, virgin rock still reads at Rw. The next sections turn that contrast into a picture of the invaded zone.
References
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
- Bateman, R. (2012). Openhole Log Analysis and Formation Evaluation, 2nd ed. SPE.