New NMR tool defines near-wellbore fluid density differences ///
In a rhetorical thesis by a long-departed pioneer in the art of well log interpretation, the author asked, “What’s a milli-mho worth?” He was trying to make the point that even a small measurement, if done inaccurately, can make a significant difference when it is magnified to reservoir scale. Today, we ask, “What’s a p.u. worth?”
In this case, every acre-foot of a typical reservoir of 30% porosity and 20% water saturation contains about 1,862 bbl of oil, or 62 bbl per porosity unit (p.u.). At today’s prices, that’s a little more than $3,100 worth. Insignificant? Not when you figure-in the number of acre-feet in a typical commercial reservoir.
Looking at it another way, typically we compute porosity from the density log by dividing the difference between the density of 100% matrix rock and the log reading by the difference between the density of 100% matrix rock and the density of water using the familiar porosity equation. The density of the matrix rock can be obtained from cores or spectroscopy logging.
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