Determination of NMR T2 cut-off for clay bound water in shales: A case study of Carynginia Formation, Perth Basin, Western Australia

Abstract

Low-field Nuclear Magnetic Resonance (NMR) has proved to be a valuable tool for the petrophysical characterization of conventional reservoirs, but its effective application to unconventional reservoirs is still under research. Pore structure characterization of shales is particularly challenging due to the complexity of the pore network and the small size of pores.Using low-field NMR, we performed transverse relaxation (T2) experiments on samples from the Perth Basin, Western Australia. The samples were initially saturated with KCl brine to obtain the total NMR porosity and T2 distribution, then centrifuged and finally oven-dried at increasing temperatures. T2 spectra were also acquired after centrifuging and heating the samples. Our results indicate that most of the transverse relaxation occurs below 3ms in saturated samples and that a conventional centrifuge cannot remove water from the smaller pores, making the commonly accepted clay bound water cut-off unsuitable for shales. Furthermore, the results from NMR experiments performed on the oven-dried shale samples suggest that the water content remains relatively constant after heating them above 65°C. The calculated T2 cut-off for clay bound water is between 0.22 and 0.26ms for the samples studied.The methodology presented in this paper can be replicated in other formations to find a suitable T2 value for clay bound water, which can be a good indication of potentially producible porosity and can also be used for permeability estimation.