NMR logging of natural gas reservoirs

The phenomenon causing reduced NMR porosities in gas reservoirs, the so called "gas effect", has lately become a subject of great interest in the petrophysical community. Contrary to the industry wide belief, NMR logging tools can detect gas provided that the pulse sequences are chosen properly and that the logging tool has adequate depth-of-investigation. Furthermore, gradient-based logging tools such as the MRIL-C can be used to unambiguously identify the gas phase in the reservoir. Failure to recognize gas may result in gas being misinterpreted as bound fluid, which in turn may result in excessively high irreducible water saturations and incorrect permeability estimates. The NMR properties of gas are quite different from those of water and oil under typical reservoir conditions and this can be used to quantify the gas phase in a reservoir. A new NMR-only interpretation approach based on this principle, called the Differential Spectrum Method (DSM), has been developed and successfully tested in the Gulf of Mexico. This method utilizes properly selected NMR pulse sequences and does not require resistivity or other porosity logs. The DSM can be used in reservoirs containing gas and/or oil. Another technique exploiting the diffusion properties of gas, called the Shifted Spectrum Method (SSM), is also introduced. Hydrocarbon saturations computed using the Differential and Shifted Spectrum Methods show very good agreement with those obtained conventionally. The methods are mineralogy independent and insensitive to clay bound water, and ideal for shaly sand applications.