NMR Relaxometry in Shale and Implications for Logging

The quantity of light hydrocarbon and natural gas in tight-oil and gas shales respectively is one of the primary indicators of reservoir quality (RQ). The measurement of RQ therefore depends on the ability to distinguish the quantity of the light oil or gas from other fractions of the total organic carbon, namely the immobile hydrocarbons such as kerogen, bitumen, heavy oil and formation water. Additionally, the separation of the oil into fractions hosted in organic versus inorganic porosity is important for determining the potentially producible fraction. Although multidimensional diffusion-relaxation correlation experiments can distinguish hydrocarbons from other uids in conventional reservoirs, their use is restricted owing to the presence of short relaxation times in most tight-oil and gas-shale plays. We demonstrate the feasibility of nuclear magnetic resonance (NMR) relaxometry for determining the different constituents in shale based on the frequency dependence of their relaxation times, using 2D NMR T-1-T-2 experiments. We determine the 2D T-1-T-2 relaxation distributions of the different fluid fractions in unconventional shale plays, and comprehend the T-1/T-2 ratios based on the intrinsic relaxation mechanisms of each component. We also discuss the practical implications of these relaxation mechanisms by determining which fractions are identiable in low-eld NMR logs, thereby setting limits to fluid typing and saturation determinations, downhole. The NMR experiments discussed here together with associated log analysis could enable in-situ estimates of the potentially producible fluid volumes, thereby aiding well placement, completion methodologies and production predictions.