Volume 57

Fundamental Experimental Study on Hot CO2 Fluid Injection in Ultra-Deep Heavy Oil Reservoirs Jiajing Chang, Zhaojie Song, Yibin Qi, Yongqiang Tang, Suobing Zhang, Zengmin Lun, Bingyu Ji

Abstract

The development of ultra-deep heavy oil reservoirs currently faces challenges including steam injection difficulties, poor fluid mobility, and significant thermal losses in oil layers. The target reservoir had undergone various enhanced oil recovery (EOR) methods such as steam thermal recovery and viscosity reducer huff-and-puff stimulation. However, none of these approaches had achieved breakthrough in productivity. This highlights the critical importance of exploring hot CO2 fluid injection technology specifically tailored for ultra-deep heavy oil reservoir development. This study systematically conducted the following experimental investigations: (1) Three-stage (10 %, 30 %, 60 %) CO2 gas injection expansion experiments with heavy oil, coupled with rheological characterization under varied temperature-pressure conditions. (2) Precise measurements of CO2 diffusion coefficients and solubility parameters in heavy oil systems, including thermal conductivity determination across temperature/pressure gradients. (3) Experimental analysis of pressure-time dependent CO2 extraction effects on heavy oil component fractionation. (4) Microscopic visualization experiments elucidating the CO2 displacement mechanisms in ultra-deep heavy oil reservoirs at pore-scale resolution. Experimental observations revealed that increasing the CO2 injection ratio leads to a systematic decrease in crude oil density (from 1.0646 to 0.9863 g/cm³ at 100 °C) accompanied by a 4.65-12.47 % expansion in volume coefficient. The native heavy oil exhibited shear-thinning behavior consistent with the Herschel-Bulkley model, whereas CO2-saturated samples transitioned to Newtonian flow characteristics with viscosity reductions exceeding 75 % at 10 s⁻¹ shear rate. Notably, diffusion coefficients in ultra-deep heavy oil reservoirs (depth > 2000 m, viscosity > 1×10⁴ mPa·s) were measured to be approximately one order of magnitude lower than those in conventional crude oil. Thermal conductivity analysis of the CO2-heavy oil mixture showed progressive reductions with elevated temperature and pressure. Compositional monitoring identified selective extraction of C₅ – C₇ hydrocarbons during initial CO2 interaction, while higher carbon-number components (C₁₂ – C₂₀) required increased pressure for effective mobilization. The pressure-driven enhancement of CO2 extraction efficiency in ultra-deep heavy oil necessitated extended contact durations (> 24 h) to reach equilibrium. In the process of heavy oil displacement, CO2 was prone to gas channeling, while CO2 synergist had the effect of emulsifying and dissolving viscosity reduction and forming foam oil to expand sweep efficiency in the process of displacement.

Keywords Ultra-deep, heavy oil, supercritical CO2, thermal-CO2‚ synergistic, mechanism of action