Determination of NMR T2 cut off for ductile, low permeability shaly sandstone

Abstract

For the past decades nuclear magnetic resonance (NMR) technology has gained acceptance as petrophysical tool for evaluating reservoir quality. Comprehensive formation evaluation requires determination of irreducible fluids, movable fluids, and permeability. The presence of clay, their occurrences and distributions, however, in some reservoir rocks tends to introduce complexity in any formation evaluation activities. This can also cause problem for NMR log interpretation. In the presence of clays the most commonly used T2 cut off values, a constant value throughout a formation, seem to eventually yield inaccurate permeability estimates. Therefore, NMR measurements should be integrated with other measurements from conventional cores for a comprehensive formation evaluation, in which T2 cut off may vary for reservoir with different reservoir qualities. This paper presents results of a study that focuses on NMR measurements on Tirrawarra shaly sands taken from 3 wells situated in Cooper Basin, South Australia. The study suggests that the T2 cut off values for the samples vary significantly in order for NMR-derived irreducible water to match core-derived irreducible water. This is also true for NMR-derived permeability estimates when compared to measured permeability values. Comparisons between estimates produced using the normally used `constant T2 cut off' and the suggested `varied T2 cut off', as well as their effect on formation evaluation, are also discussed. In general, the results highlight the need to study T2 cut off values more directly for specific reservoir rocks before their practical uses in the field.