A New Approach For Analysis Of The Nuclear Magnetic Log - Resistivity Log Combination

Abstract

Abstract A new method is presented for analysis of the nuclear magnetic (NML)-resistivity log combination (NML is a Schlumberger Trade Mark). It is shown that a crossplot on logarithmic coordinates of effective porosity (Φe), as determined from neutron and density logs, minus free fluid porosity (Φft) 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 to Φe- Φft = 1.0 yields the product aRw in the true resistivity scale. With these data, values of water saturation are readily calculated. Intervals with moveable 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 sandshale 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 ΦeSwi 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. Introduction The nuclear magnetic log (NML) has an approximate radius of investigation of one 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 moveable fluids in the formation(l). This characteristic allows the NML, when combined with other porosity logs, to calculate irreducible water saturation, Swi in turn can be used for estimating permeability based on empirical correlations. A comparison of water saturation as determined from Archie's equation(2) 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 activity(3). 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 gypsum) where nuclear tools tend to give very large values of porosity(l). In formations with light oil, the NML tool gives porosities that include this free oil. In formations with very heavy oil, the NML reads the free water filled porosity. Because the tool radius of investigation is only about 1 in, the NML reading might allow determination of hydrocarbon saturation in the flushed zone. Neumann(4) 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.