Characterization of shales using T-1-T-2 NMR maps

Nuclear Magnetic Resonance (NMR) relaxation methods are key techniques for evaluating shales, from both cores and logging data. First, very small pore sizes, down to nano-meter length scales, can be detected and quantified if the NMR tool has the ability to measure relaxation times in the range [0.1-1 ms], and second the different proton populations (water, oil, gas, and kerogen) can be distinguished using 2D T-1-T-2 maps. We use a NMR instrument working at a higher frequency (23 MHz) that yields a much higher sensitivity than standard plug-size benchtop apparatus. In this work, we show the position of each proton population in this map: hydroxyls from the clay (T-2 < 0.1 ms, 10< T-1/T-2 < 100), water (Ti/T2 similar to 2), and in certain situations kerogen (10 < T-1/T-2< 100). Methane can be clearly distinguished with T-1/T-2 similar to 10. One can use the T-1/T-2 contrast to separate the different contributions. As an additional characterization, deuterium tracer techniques are used to quantify the connectivity of the pore network system, providing a measure of tortuosity. Due to high diffusivity, the T-2 distribution is affected by diffusive pore coupling and therefore, pores smaller than about 800 nm are theoretically all gathered at a single relaxation time. (C) 2015 Elsevier BY. All rights reserved.