Basic Petrophysics · Module B5
Saturation Essentials
Archie's equation from the formation factor up, the effect of each parameter on water saturation, and the checks that keep a saturation honest: the Pickett plot, irreducible water saturation, and where Archie stops working as shale volume rises.
- Level
- Beginner
- Position
- Module 5 of 7
- Video
- About 20 minutes, narrated
- Format
- Narrated video lesson with chapters; with the certified option, a graded module quiz.
What the module covers
- Formation factor and Archie's equation
- How Rw, m and n move the answer
- Pickett plots for Rw and the cementation exponent
- Pore-volume-weighted averaging
- Irreducible water saturation as a sanity check
- When shale makes Archie unreliable
Learning outcomes
After the module you should be able to:
- Compute water saturation with Archie's equation and explain each parameter
- Use a Pickett plot to check Rw and the cementation exponent
- Recognise when a shaly-sand model is needed
A short preview
Archie's equation, Sw = (a·Rw / (φᵐ·Rt))^(1/n), has one property worth remembering: with n = 2, water saturation goes as the square root of Rw. Double Rw from 0.04 to 0.08 ohm·m and Sw rises by a factor of √2, about 41 percent, with nothing else changed. A water resistivity taken from the wrong sample or at the wrong temperature is therefore one of the largest single errors in a saturation, and the module spends time on how to estimate it and how to check it.
Who it is for
Geoscientists, reservoir and production engineers, and graduates who need to read a standard log suite and build a first-pass interpretation they can defend.
Prerequisites: None beyond basic geology and algebra.
Take the module in the Learning Hub
Basic Petrophysics: Seven foundational modules with narrated lessons and chapter-based navigation. The lesson opens in the Learning Hub once the track is purchased; graded quizzes and the certificate come with the certified option.