Performance of Air Injection vs. CO2/Water Injection in a Tight, Light-Oil Reservoir: A Laboratory Study

Abstract

Summary This paper outlines the results of an early-stage comparative study of air-injection-based and immiscible-carbon-dioxide (CO2)/water-injection-based enhanced-oil-recovery (EOR) processes for a 30+ °API tight, light-oil reservoir. This was accomplished by embedding multiple low-permeability core plugs in crushed reservoir-core material to create a composite core that was contained in a 1.84-m-long core holder. The objectives of this unscaled experimental work were to understand the suitability of each EOR process for a low-permeability reservoir; to define process parameters before a potential field pilot; and to understand the relative merits of each EOR process to mobilize light oil from a tight matrix to a fracture network. A detailed experimental investigation was conducted at realistic reservoir conditions to evaluate the feasibility of an air-injection-based EOR process. The air-injection results were compared with those from an immiscible CO2/water-injection EOR experiment using the same experimental setup and reservoir conditions. Both the air- and CO2/water-coreflood tests were performed at 10 340 kPag (1,500 psig) and 99°C in a 100-mm diameter, 1.84-m-long composite-core holder using 38-mm-diameter reservoir core plugs (that represented the matrix) mounted within the crushed reservoir-core material (that represented the fracture); inert helium gas was used to pressure up the core holder to reservoir pressure. Permeability of the core plugs was from 0.3 to 3 md, while the permeability of the crushed core material was 1 to 3 darcies. Air injection was performed as a standard, one-dimensional combustion-tube test with injection of 2.3 pore volumes (PVs) of air to burn 71% of the packed core length (including helium, a total of 4.3 PV of gas injected). The CO2/water coreflood was performed with the injection of 2.86 PV of CO2 followed by an extended soak period, then a second injection of an additional 2.86 PV of CO2, followed by the injection of 2.6 PV of water. The pretest and post-test core-plug measurements of oil saturation show that the air-injection process removed significantly larger quantities of hydrocarbons than the immiscible-CO2/water-injection process. Relative to the initial conditions of the core plugs for the air-injection experiment, more than 95% of the hydrocarbons were removed (note that some fraction of original oil was consumed as fuel). In the post-test CO2/water-injection core plugs, oil recovery was in the range of 30 to 55% of initial oil in place (IOIP). These findings suggest that, under an appropriate field design, both processes have the potential to recover incremental oil from tight reservoirs. However, the air-injection process might be better suited to mobilize oil, because of thermal expansion, than the CO2/waterflood process.