NML-The Hope for Production Prediction
Authors: Hilchie Douglas W. 
Published: December 2007

The nuclear magnetism log (NML) ha recently been upgraded to a workable and reasonably trouble-free service. Although the first NML was run commercially in 1960 by Perforating Guns Atlas Corp., the service did not become routinely adequate until 1987. The NML has been one of the great hopes for significant improvement in log interpretation and should now provide the next significant improvement in log interpretation.

The NML instrument measures the nuclear magnetism of protons. The proton is a particle identical to the nucleus of the hydrogen atom. The -proton is Mm a magnet; when placed in a magnetic field, it tends to slip with the magnetic field like a compass needle. The NML instrument in its simplest form has a large coil that produces a magnetic field when electrical current is passed through this coil. The p in the formations around the wellbore are aligned by this magnetic field. The borehole (mud) is "doped" with something like magnetite to eliminate the borehole influence. When the magnetic field is released by stopping current flow m the coil, the protons tend to reslip with the earth's magnetic field. During the process of realigning, the protons give back the energy that it took to slip them. This energy induces a signal in the same coil used to create the magnetic field. The process is very inefficient.

Free-Fluid Index

The most important parameter measured by the NML is the free-fluid index (FFI). The initial magnetization of the formation is a function of the number of aligned protons. This is the FFI, which is the number of protons that are free to be oriented, scaled to porosity units. Protons adsorbed on, or in close proximity to, a solid surface like sand and shale are not free to be oriented.

FFI is obviously related to the porosity and surface area of the solid rock material. Intuitively, FFI must then be related to porosity and the amount of water that is coating the surfaces of the reservoir rock-the irreducible water saturation. In rocks, FFI may be thought of as a true effective porosity because it is a measure of the pore space through which fluids will flow.

Although irreducible water saturation is influenced by hydrocarbon density (particularly gas), it is a useful parameter. In physical terms, irreducible water on is the water saturation you would obtain in the laboratory if you passed oil or gas through an initially water-saturated permeable rock for weeks or months. The water remaining in the rock is held by surface and capillary forces. Irreducible water saturation is controlled by the pore sizes and the surface area of the rock. Artus proposed that the irreducible water saturation (Siw), porosity (0), and FFI (IFF) were related as

..........................................(1)

Siw is usually related to permeability and porosity by one of several empirical correlations. Unfortunately, these correlations require an increase in porosity with increased permeability, which does not always occur.

Siw is the parameter that indicates the maximum water saturation a reservoir rock can contain and produce only oil or gas. A high Siw implies that the well will produce few, if any, fluids. A low Siw implies that the reservoir rock can produce hydrocarbons if hydrocarbons are present. Siw is related to effective permeability, which is the permeability to a particular fluid with more than one fluid permit in the formation. This is the condition we always have in the reservoir and is thus the important key to hydrocarbon productivity.

A knowledge of Siw makes a good estimate of the fluids to be produced from a reservoir possible. For example when a calculated water saturation, Sw, from a conventional set of well logs is 40% and the Siw is 40%, the interval will produce only hydrocarbons. If, for the same formation, an Siw, of 50% is calculated, you will probably produce hydrocarbons and water. If, for the same formation, you calculate an Sw of 70% or more, you will probably produce water. SW, is an indication of the magnitude of Sw, needed to produce water-free production. It is also indirectly an indication of the median grain size of a granular-type rock (see Hilchie).

An independent source of Sw could help define the parameters needed to calculate Sw from conventional logging suites in reservoir where no water zone is present. For example, a conventionally calculated Sw, that is less than Siw in an interval that produced only hydrocarbons indicates a need for better log-interpretation parameters.

Fig 1 is a good example. This published log, unfortunately, is not complete (no spontaneous potential), and is apparently a Cretaceous section (probably from Alberta). Each horizontal lineon this metric depth scale represents 5 m (16.4 ft). The sandstone Intervals A and B are the main concern. The formation water resisitivity, Rw, is quoted as 0.2 to 0.5 . m . which is not of much value. The water saturations for Intervals A and B, using the resistivity values from the deep induction and the porosity values from the density and neutron log, calculate about the same, as the resistivities and porosities are about the same.

P. 273^