Estimation of Recovery Factor in Light-Oil Air-Injection Projects

Abstract

Summary In deep, high-pressure light-oil reservoirs, air injection provides several advantages compared with other improved recovery injection processes. The combination of rapid pressurization, spontaneous ignition, complete oxygen utilization, stripping of the light hydrocarbons [natural gas liquids (NGL's)], and near miscibility of the in-situ-generated combustion gases with the reservoir oil results in improved recovery. The availability of the injectant should allow wide application of air injection. Estimation of the recovery factor for this process is subject to uncertainties and requires history matching. To date, computer models have shown limited predictive capability owing to the complexity of the process. Also, an estimate of the fuel laydown at high pressure is not available. In this paper, a method is proposed for estimating the recovery factor on the basis of the producing gas/oil ratio (GOR) of the field. The field results of two ongoing air-injection projects, Medicine Pole Hills Unit (MPHU) in North Dakota and Buffalo Red River Unit (BRRU) in South Dakota, are used to illustrate this technique. MPHU and BRRU are unique applications because they use high-pressure air injection into deep, high-temperature, low-permeability carbonate reservoirs producing light oils. The produced-gas analysis from MPHU is used to arrive at the fuel laydown and the H/C ratio of the fuel. This is the first time the fuel laydown has been estimated for a high-pressure air-injection project in a light-oil reservoir. Finally, the results of the produced-gas analysis are used to arrive at the NGL content of the effluent stream. The combined oil recovery and NGL production provide the basis for estimating the total recovery factor for this field. Such an estimate can be used by other operators to assess the economic viability of this process in their fields.