The NMR signature of methane saturated Barnett shale core plugs measured in a non-gradient magnetic fieldis dramatically different from what would be seen if the methane relaxed as a bulk fluid.At laboratory temperatures, the methane T2 relaxation spectra have modes on the order of 10 ms. This is consistent with a significantpercentage of the methane being located in nanometer scale hydrophobic organic pores, and relaxing by a dipole-dipole interaction with the hydrogen molecules in the organic material. For pores of this size any gradient fielddiffusion induced relaxation would be strongly suppressed, consequently the effects of internal gradients should be of second order. Methane in these pores would not be displaced by invasion of water-based mud, so an NMR log should register the total gas stored in these pores within its signal.
The NMR water signals in Barnett core plugs have T2 relaxation spectra with a mode on the order of 1 ms. This observation is consistent with either the water being clay bound, or at the least the water is located in very small pores that are at least partly water wet.
This study shows that NMR logging measurements should be able to provide quantitative estimates of the free gas storage and information on the percentage of free gas stored in organic pores. However, this will require development of new interpretation methodologies, and application of traditional methodologies, based on the gas signature in conventional reservoirs, will produce erroneous results.
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