Quantification of CO2 Flow in Organic-Shale Formations Using NMR Saturation Profile Measurements

Abstract Reliable evaluation of CO2 transport in porous rocks is essential for several subsurface applications such as oil recovery and gas storage. Visualizing CO2 front advancement is crucial for oil shales. The goal of this work is to evaluate the front movements during gas Huff-and-Puff (HnP) in shales. A Mancos shale core sample of 3.51% porosity and 112 nD permeability is used. The HnP cycles were performed in a Hassler core holder, with HnP pressure applied only at the sample inlet. Nuclear Magnetic Resonance (NMR) T2 and high-resolution 1D saturation profile measurements were acquired after each cycle to monitor the spatial distribution of kerosene and track the advancement of the CO2 front along the core length. The core is fully saturated with kerosene prior to conducting three CO2 HnP cycles, each performed at an injection pressure of 3,000 psi and varying soaking times of 1 hr, 4.5 hrs, and 12 hrs. The results demonstrated a clear dependence of CO2 front advancement (xf) on soaking time. Specifically, the CO2 front advances by 2.67 mm following the 1-hr soaking period, 8.03 mm after 4.5 hrs, and reaches 28.05 mm after 12 hrs of soaking. These front advancements corresponded to oil recovery factors of 21%, 63%, and 94%, respectively, indicating substantial mobilization of kerosene with increasing exposure time to CO2. Additionally, diffusion coefficients were calculated from the experimental CO2 front locations using a 1D diffusion model, revealing an increase from 3.66 × 10⁻¹¹ m2/s at shorter soaking times to 7.47 × 10−10 m2/s at the longest soaking duration. This trend suggests that effective CO2 diffusivity within the tight shale matrix is not constant, but dynamically increases with extended soaking, likely due to changes in oil composition during the HnP process.