Exceptions to Default Assumptions

Interpretation of NMR well logs is usually made with the assumption that the formation is water-wet such that water occupies the smaller pores and oil relaxes as the bulk fluid. Examination of crude oil, brine, rock systems show that a mixed-wet condition is more common than a water-wet condition, but the NMR interpretation may not be adversely affected [47]. Surfactants used in oil-based drilling fluids have a significant effect on wettability and the NMR response can be correlated with the Amott-Harvey wettability index [46]. These surfactants can have an effect on the estimation of the irreducible water saturation unless compensated by adjusting the T2 cut-off [48]. 

Estimation of pore size distribution is based on the assumption that there is no diffusional coupling between pores of different sizes. Microporous carbonate grain-stones have been identified as an example where this assumption is not valid [31]. 

NMR has proven to be a valuable tool for formation evaluation by well logging, downhole fluid analysis and laboratory rock characterization. It gives a direct measure of porosity as the response is only from the fluids in the pore space of the rock. The relaxation time distribution correlates with the pore size distribution. This correlation makes it possible to estimate permeability and irreducible water saturation. When more than one fluid is present in the rock, the fluids can be identified based on the difference in the fluid diffusivity in addition to relaxation times. Interpretation of NMR responses has been greatly advanced with the ability to display two distributions simultaneously. 

The author acknowledges the contributions of many graduate students and colleagues and also the financial sponsorship of the industrial consortium on Process in Porous Media, NSF CTS-9321887, DOE DE-AC26-99BC15201 and DOE DE- 

Coates, L. Xiao, M. G. Prammer 1999, NMR Logging Principles ul Applications, Halliburton Energy Services, Houston. 

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