Low-Temperature Air/Solvent Injection for Heavy-Oil Recovery in Naturally Fractured Reservoirs

Summary

Limited studies on oil recovery from naturally fractured reservoirs using low-temperature air injection show that the process is strongly dependent on oxygen (O2)-diffusion coefficient and matrix permeability, both of which are typically low. A new approach (i.e., the addition of hydrocarbon solvent gases into air) is expected to improve the diffusivity of the gas mixture and to accelerate the oxidation. To study this new idea, called low temperature air/solvent injection, laboratory tests were performed by soaking heavy-oil-saturated cores in air/solvent filled reactors to determine the critical parameters on recovery. Laboratory tests were complemented by conducting experiments using air at different O2 concentrations: zero (i.e., nitrogen), 21.0 mol% (air), and 37.3 mol% (O2-enriched air). For safety reasons, it is imperative that enough time be given for air diffusion before the injected air breaks through a highly permeable fracture network. This implies that the huff ‘n’ puff type of injection is a plausible option as opposed to the continuous injection of air. A high recovery factor was obtained by soaking a single matrix in an air/solvent chamber at static conditions rather than with air only. The period of pressure stabilization was faster for the air/solvent mixture than in 100% solvent. The asphaltene content was lower in the air/solvent case than in the 100%-air injection case. Instead of pure-hydrocarbon solvent, injection of an air/solvent mixture yields a better recovery with less asphaltene. This is expected to reduce the cost of the process compared with pure-solvent injection. At low temperatures (75C), O2 consumption in the matrix oil was low, while at high temperatures, the O2 was partially (150C) or totally (200C in the presence of propane) diffused and consumed in the matrix.