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tapered cutoff

1. n. [Formation Evaluation]
In a nuclear magnetic resonance measurement, the use of a gradual rather than a sharp cutoff to distinguish between bound water and free water. A sharp cutoff at, for example T2 = 33 ms in sandstones, is normally used to distinguish free water (all T2s above 33 ms) from bound water (all T2s below 33ms). In a water-filled rock, in the fast diffusion limit, T2 is directly related to pore size. The distinction between bound and free water is based on the assumption that all free water resides in large pores, and all bound water in small pores. However, in rocks with large pores, a significant volume of bound water exists on the surface of the grains around a large pore. Being part of a large pore, it gives a long T2 and will be incorrectly counted as free water. One solution is the tapered cutoff, in which the bound water is the sum of all the T2 below a minimum, for example 5 ms, and is then a progressively smaller fraction of the volume at T2s up to a maximum, for example 500 ms. All signal above 500 ms represents free water. The form of the taper is usually empirical, but is based on some model of pore shape, such as a bundle of tubes. See Kleinberg RL and Boyd A: 'Tapered Cutoffs for Magnetic Resonance Bound Water Volume' paper SPE 38737, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5-8, 1997.