Absolute Permeability by the Timur Correlation
Estimates the absolute permeability of a sandstone from porosity and irreducible water saturation, when no core sample is available for laboratory testing. The Timur correlation (1968), fitted on 155 Alaskan sandstone samples, is k = 0.136 × porosity^4.4 ÷ irreducible saturation², with permeability in millidarcy. The result tells how well the reservoir can flow: below 1 mD the formation is considered tight, between 10 and 100 mD it is moderate and above 500 mD it is excellent. The convention that causes most errors here is the unit: the constants 0.136 and 4.4 were fitted with porosity and saturation in percent, not as fractions — entering 0.22 instead of 22 drops the result by several orders of magnitude, which is why both fields ask for percent. Enter the effective porosity and the irreducible water saturation.
Result
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Timur Permeability: Estimating k Without Core Data
Core is expensive and it arrives late. Between the logging run and the first formation test there is a window where the wellsite geologist and the reservoir engineer must decide where to perforate, which interval deserves a test and what thickness enters the model — and none of that follows from porosity alone. What is missing is permeability, the property telling you whether the rock will actually flow. The Timur correlation fills that gap with what the log suite already provides: effective porosity and irreducible water saturation.
The expression is k = 0.136 × φ^4.4 ÷ Swi², with k in millidarcy. Here φ means effective porosity and Swi the irreducible water saturation, the water capillarity holds back and production never recovers. Timur fitted the constants in 1968 on 155 Alaskan sandstone samples with φ and Swi expressed in percent — the convention this screen keeps, which is why the fields ask for 22 and 25 rather than 0.22 and 0.25. At that point the answer reads 175.52 mD. Below 1 mD a formation counts as tight, 10 to 100 mD as moderate, and anything above 500 mD flows very well.
The 4.4 exponent punishes porosity error hard: one extra percentage point, from 22 to 23, lifts k by 21.6%. On top of that, the correlation grew out of sandstone with intergranular porosity and assumes the zone really sits at irreducible saturation. Inside a transition zone, or in limestone, dolomite and fractured rock where flow travels through vugs and fractures, the estimate loses any grip on reality. Treat the figure as an order of magnitude for ranking intervals within one well, never as an absolute number for sizing production.
Frequently asked questions
Do I enter porosity as a percentage or a fraction?
Is 175.52 mD a good permeability?
Does it work for limestone or fractured rock?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.