Unconventional reservoirs require a rigorous set of decision principles to achieve cost-effective and efficient completions. The oilfield definition of "unconventional reservoirs" includes any that exhibit permeability in the 0.01 md and tighter range. These permeability definitions become very tight as the reservoir includes a greater component of shale. Ultimately, shale resource reservoirs have permeability in the nanodarcy range, several orders of magnitude less than other unconventional resources.
Nuclear magnetic resonance (NMR) relaxation, or T2 measurement, has been used to establish permeability by direct application of the Bray-Smith permeability. There have been numerous comparisons of these permeability calculations to laboratory permeability and production rates. These comparisons have included wells in the unconventional range. The results have been quite favorable.
This paper explores the possibility that this calculation of permeability can be extended from the millidarcy to the nanodarcy range, proving useful in shale reservoirs worldwide. The basis of work was accomplished by comparing NMR results to laboratory core results. Additional refinement to the changes of calculation was performed using comparison to production rates from wells in the study.
The results illustrate the changes required to derive a useful solution of permeability directly from the NMR data in these shale reservoirs. A comparison of core results to this calculated permeability is also included.