CO2 Enhanced Hydrocarbon Recovery in Organic-Rich Shales: Insights from 500 MHz 1H NMR

CO2 huff-and-puff has emerged as a promising approach for enhanced oil recovery in unconventional source rock shales, yet the underlying recovery mechanisms remain difficult to isolate and quantify. Here, high-field 1H nuclear magnetic resonance (NMR) spectroscopy was used to investigate in situ CO2–shale interactions in a preserved, intact slim source rock plug. CO2 was stepwise loaded from 0 to 6000 psi and unloaded back to 0 psi at 40 °C. At each pressure step, 500 MHz 1H spectra were acquired and deconvoluted into two Lorentzian peaks representing pore fluid and one Gaussian peak representing kerogen. Pressure-dependent changes in the integrals and line widths of these peaks were used to investigate hydrocarbon depletion from the rock sample and CO2-induced changes in molecular mobility of kerogen and pore fluids. In control experiments, helium injections up to 2500 psi produced no measurable changes in the NMR spectra. In contrast, CO2 injections caused pronounced, pressure-dependent changes in intensities and line widths of 1H NMR signals, revealing mixing with pore fluids and penetration into the kerogen matrix. Both pore fluid and kerogen spectra changed markedly as CO2 entered the supercritical regime above 1000 psi at 40 °C, indicating a distinct transition in molecular mobility. Above 4000 psi, substantial light hydrocarbon evaporation from the plug left heavier, less-mobile components in the pores, resulting in broader NMR peaks. Analysis of the complete loading–unloading cycle indicated that approximately 24.5% of the initial pore fluid was produced, dominated by late-stage high-pressure CO2 loading. These results demonstrate that high-field 1H NMR can quantify hydrocarbon recovery while resolving molecular-scale CO2 interactions with different components in source rock shales that are not easily accessible by conventional mass-balance methods.