Characterization of Shales with Low Field Nmr

Nuclear Magnetic Resonance (NMR) relaxation methods are key techniques for evaluating shales, both from 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, organic matter) can be distinguished using 2D T1-T2 maps. In this work, we show two techniques to characterize shales: 2D relaxation techniques to identify fluids, and deuterium tracer techniques to identify connectivity of the pore network system, providing also a measure of tortuosity. We used a NMR instrument working at a higher frequency (20 MHz) that yields a much higher sensitivity than standard plug-size benchtop apparatus. T1-T2 maps were acquired on different samples at different saturation states (dry, 100% water and 100% gas saturated). In such maps, the fluid typing is based on the T1/T2 ratio and T2 values. One can detect different populations of protons: hydroxyls from the clay (T2<0.1 ms, 10<T1/T2<100), water (T1/T22), and in certain situations organic matter (10< T1/T2<100). Methane can be clearly distinguished with T1/T210. INTRODUCTION The characterization of gas shales remains a challenge and cannot be performed using conventional petrophysical techniques. Even for basic properties such as porosity and permeability, appropriate protocols and instrumentation are required. For the visualisation of the pore space, we rely on recent microscopy techniques (Scanning Electronic Microscopy coupled with Focused Ion Beam) that are appropriate at nanometre scale. NMR relaxation techniques are also very appropriate in this context because it can detect small quantity of water or gas, in very small pore sizes. For example in the interlayer space of smectites, it is possible with an appropriate instrument to detect and quantify the water content even when the pore width is only one or two water layers thick [1]. However, for complex porous media such as shales, standard T2 relaxation distributions provide limited information. There are many recent efforts to use advanced and/or multidimensional NMR techniques for separating the different proton contributions [2–