Permeability on the Ogbon-1 Well
Learning objectives
- Build a Timur permeability log from porosity and Sw
- Compare the log estimate with the core truth down the well
- Recognize where the irreducible-water assumption breaks
- Read the net-reservoir flag from a permeability cutoff
A Permeability Log
The chapter ends on the well. The permeability track carries two curves: the core truth, the measured permeability, and a Timur log estimate built foot by foot from the porosity and the computed water saturation, , the quickest transform in the toolbox. Laid side by side, they show exactly how far a log permeability can be trusted.
Where the Estimate Breaks
In the clean oil pay the two curves lie almost on top of each other, because there the computed really is the irreducible water Timur assumes. But follow them down toward the oil-water contact and the Timur curve collapses: as the water saturation climbs above irreducible, the term drives the estimate to a tiny, wrong value for rock that is in fact perfectly permeable, just water-wet. That is the caveat of every irreducible-water estimator, made visible on a real log: it is valid only where the rock is at irreducible water.
Net Reservoir, and Onward
A cutoff line marks the net-reservoir permeability floor, and the flag strip lights up the feet that beat it, the rock that can actually flow. With porosity, saturation, and now permeability all in hand and all honestly bounded, the evaluation is nearly complete. What remains is to connect saturation to height through capillary pressure, and then to gather porosity, saturation, and net pay into a hydrocarbon volume, the work of the chapters ahead.
References
- Timur, A. (1968). An investigation of permeability, porosity, and residual water saturation relationships. SPWLA 9th Annual Logging Symposium.
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.