A new method is presented for analysis of the nuclear magnetic (NML)resistivity log combination. It is shown that a crossplot on logarithmic coordinates of effective porosity (φe), as determined from neutron and density logs. minus free fluid porosity ((φ[fluid) as determined from the NML log vs. true formation resistivity should result in a straight line with a negative slope equal to the water saturation exponent, n, for intervals which are at irreducible water saturation.
Extrapolation of the straight line toφe - φr = 1.0 yields the product a in the true resistivity scale. With these data, values of water saturation are readily calculated.
Intervals with movable water fall distinctly below the straight line. Gas-bearing zones plot above the straight line.
The method is illustrated with the use of a high-porosity sand-shale sequence. Estimates of formation permeability and water cut are included in the analysis. A word of caution is raised with respect to the concept that the product φeSw should remain constant for intervals of the same lithology at irreducible water saturation.
A major finding is that water resistivity can be calculated from the NML Resistivity combination in reservoirs which (1) produce clean oil with no water. (2) have a poor SP development, (3) do not have a well established aquifer, and (4) do not have enough data so as to allow generation of: water resistivity catalogues.
The nuclear magnetic log (NML) has an approximate radius of investigation of 1 inch. It measures the earth's field proton free induction decay of formation fluids. As such the tool responds only to hydrogen protons associated with free or movable fluids in the formation1.
This characteristic allows the NML, when combined with other porosity logs, to calculate irreducible water saturation. Sw1 in turn can be used for estimating permeability based on empirical correlations.
A comparison of water saturation as determined from Archie's equation2 and irreducible water saturation as determined from the NML permits a forecast of fractional water cut for an untested interval.
In carbonates, the NML tool reads close to total porosity due to the carbonates small surface activity3. The tool is not affected by hydrogen protons in the matrix. This makes it valuable for determination of porosity in unusual lithologies containing extensive water of hydration (for example gypsom) where nuclear tools tend to give very large values of porosity1. In formations with light oil the NML tool gives porosity that includes this free oil. In formations with very heavy oil it reads the free water filled porosity. Since the tool radius of investigation is only about 1 inch the NML reading might allow determination of hydrocarbon saturation in the flushed zone.
Neumann4 has reported on the use of the NML for estimation of residual oil saturation. In this approach the oil zone is invaded by mud filtrate containing paramagnetic ions. The idea is to suppress the water in the formation from the NML signal, in such a way that the residual oil is the only one that contribute to the fluid porosity, φ[fluid.
