Abstract

The objective of this study was to design an optimal Huff-n-Puff enhanced oil recovery (EOR) scheme for a field test in unconventional reservoirs. Using an integrated workflow for process assessment, our study indicates that carbon dioxide (CO2) Huff-and-Puff may be a technically feasible EOR method for unconventional reservoirs. The main recovery mechanisms are:

  1. vaporization of lighter oil components, and

  2. interfacial tension (IFT) reduction at pressures above the minimum miscibility pressure (MMP).

Moreover, the presence of hydraulic and natural fractures may provide a large contact area for injected gas to penetrate into the ultra-low permeability matrix. The effect of molecular diffusion was also investigated, but this did not appear to have a major impact on recovery for the modeled conditions at the field scale. The cycling scheme evaluation indicates that oil recovery is proportional to the mass of CO2 injected, and longer soak times do not greatly affect the amount of oil recovered from a Huff-and-Puff cycle. We found that the time to switch a well back to injection was when oil production returns to the pre-treatment base decline rate. The modeling also indicates that optimizing the cycles can result in a reasonable increase in oil production at acceptable utilization ratios. These results suggest that miscible Huff-and-Puff could be a technically feasible EOR method for unconventional reservoirs.

Introduction

Miscible Enhanced Oil Recovery (EOR) mechanisms in unconventional, tight reservoirs may be substantially different than those associated with miscible gas flooding in conventional reservoirs [1]. Due to potentially low cross-well throughput in ultra-low permeability rock, the single-well Huff-n-Puff process is being considered within the industry for application in unconventional reservoirs [1–7]. Below the minimum miscibility pressure (MMP), CO2 improves oil recovery through the mechanisms of oil phase swelling, viscosity reduction, and gas-oil displacement. At pressures above the MMP, miscibility results in high displacement efficiency due to ultra-low capillary forces [8–9].

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