C. Straley, C. F. Morriss, W. E. Kenyon
Schlumberger
J. J. Howard
Yale University, Dept. of Geology
Abstract
Laboratory NMR measurements of longitudinal relaxation were carried out on low-permeability clay-rich sandstone samples from formations in Western Canada (five wells) and from the Wilcox formation of the US Gulf Coast. NMR measurements were made with the samples fully water-saturated, and at residual water saturation obtained by centrifuging. Measurements at various air/water and kerosene/water saturations were made on additional samples.
The lab NMR measurements were converted to T1 distribution curves. In this form, the NMR response at partial saturation is readily seen to be closely related to the physics of capillary drainage. As a result, the amount of water removed by centrifuging can be estimated in a simple way from the water saturated T1 distribution curve.
The Free fluid Index (FFI) obtained from borehole Nuclear Magnetism Logs (NML) is then discussed. First, FFI values from 5 wells are compared directly with lab-measured values of centrifugeable water on the corresponding core samples, and found to agree well. Second, the FFI values can be readily understood by comparison with laboratory-measured T1 distribution curves and the underlying physical principles.
The relationship between the principles of FF1 interpretation and the laboratory measurements is shown. Five log examples briefly illustrate the application of some of these interpretation principles.