With conventional NMR T2 analysis, it is often difficult to make a positive distinction between gas and water in low permeability or unconventional reservoirs. This makes NMR fluid typing a very difficult issue for the end user in these types of reservoirs. The recent innovation of T1 logging has introduced a robust method to better identify fluid types.
Natural gas overlaps water in the T2 spectrum due to its high diffusivity. In order to separate gas from water in the spectrum, traditional analysis methods require the acquisition of multiple T2 measurements, each taken at different levels of polarization. The differences between these measurements are then interpreted to determine the fluid type. In tight zones, the NMR signals are small; and the resulting differences do not provide robust interpretation. This new, T1 logging method, however, takes advantage of the fact that T1 is insensitive to diffusion and the fact that the gas signal is clearly separate from water in the T1 spectrum in these types of reservoirs. In addition to the opportunity to more confidently carry out fluid typing, there is still another benefit to the new method, namely significant improvement in the logging speed for T1 vs. T2 measurements.
Examples comparing T1 and T2 measurements from the Rocky Mountains and the Mid-Continent are discussed to support this novel application of NMR logging in low permeability gas sands and unconventional reservoirs.