NMR relaxometry insights into pore-level sweep efficiency for methane infusion IOR in unconventional reservoirs

Abstract

Oil recovery factors (RF) remain under 10% in the major tight oil basins of the United States. This study examined the Charlotte 1–22H well in McKenzie County, North Dakota. Our study utilized nuclear magnetic resonance (NMR) logging to assess the methane infusion (MI) process as an improved oil recovery (IOR) method to mobilize oil at near-miscibility pressure. We propose a new nanoscale-centric method to characterize pore size distribution (PSD) based on our examination of T₂ relaxation time distribution. The authors used this methodology and its correlation with Field Emission Gun – Scanning Electron Microscope (FEG-SEM) imagery analysis to create the proposed PSD method, providing compatibility with nanoscale petrophysical analysis. In core samples, the T_{2_cutoff} pores sizes measured in UB-23 at 6.807 nm, MB-27 at 11.909 nm, and LB-44 at 3.253 nm. NMR relaxometry was used to study the pore-level sweep efficiency of methane infused into shale and tight formations. Variable infusion times allowed for a comparison of near-miscibility conditions per cycle with RFs of UB-23 at 60.78%, MB-27 at 42.97% and LB-44 at 45.18%. The RF of the infused hydrocarbon solution increased when interfacial tension (IFT) and viscosity were reduced. Additionally, the effects of rock-surface-to-volume ratio, soaking period, and their variable interactions with gas compositions were compared across the three core samples. The efficacy of MI at near-miscibility was substantial in all three core samples. The authors believe this research can serve as a foundation for a broader and more comprehensive study of MI as an effective IOR process.