Nuclear magnetic resonance measurement of methane diffusion in organic-rich shales

Organic-rich shales have pore size on nanometer scale and permeability on the order of nano-Darcy. These properties give rise to significant challenges in understanding and modeling flow processes during the hydrocarbon production from shale reservoirs. During production, light hydrocarbons move from nanopores in the rock matrix to fractures generated by hydraulic fracturing to the well. Diffusion becomes a key mechanism in controlling hydrocarbon transport within matrix and from matrix to fractures. However, reliable experimental data for hydrocarbon diffusion coefficients in organic-rich shales are still lacking in the literature. This study presents experimental measurements of in situ methane diffusion coefficients in shales using a pulse-field-gradient nuclear magnetic resonance method. This method measures the diffusion coefficient at equilibrium state and thus eliminates the possibility of fluid flow effects in laboratory measurement generated from pressure or concentration gradient. The measured methane diffusion coefficients in shales are two orders of magnitude smaller than those in the bulk state in the pressure range from 75 to 6000 psi. The measured data can be used to more accurately evaluate and model the production from shale reservoirs.