Recently, High Pressure Air Injection (HPAI) is considered to be an effective EOR method to light oil reservoirs. HPAI has some remarkable merits as follows. (1) Injection gas source is air, which can be supplied anywhere. (2) Initial and operation costs are inexpensive because the main facility required is only air compressor and air is free. (3) Air can be applied even in low permeable reservoirs where water cannot be injected. However, it is pointed out that the evaluation method for HPAI is difficult, because oxidation and combustion reactions are complicated. A Japanese domestic oil field has been investigated to apply HPAI since 1999.
A series of experiments is essential in HPAI study. Accelerating Rate Calorimeter (ARC) test and Combustion Tube (CT) test are generally conducted to evaluate a potential of air injection in fields. ARC test is used to determine the extent and continuity of reactions in the low and high temperature range. The purposes of the CT test are (1) to assess the overall burning characteristics of oil, (2) to measure incremental oil production, air and fuel requirements and other parameters and (3) to measure produced gas compositions and oil and water production to provide benchmarks for monitoring future field operations.
The results of ARC tests suggest that the ignition temperature of the oil may be higher than the reservoir temperature. It means an artificial ignition method should be conducted for HPAI in the field. Additional ARC tests were carried out to assess the influence of oxygen slug, NO2 addition and linseed oil on ignition temperature as the ignition method. The experimental results suggest that the linseed oil method is the most promising, because the ignition temperature with high concentration of linseed oil is below the reservoir temperature. Two CT tests were also conducted, in low water saturation case and high water saturation case after waterflooding, to estimate applicability of HPAI including on tertiary mode. Both tests show stable combustion performance in terms of temperature, pressure response, produced gas compositions and incremental oil production.
On the other hand, numerical simulation is important to predict field production performance. The appropriate reaction kinetics on the combustion has to be used in the numerical simulation. History matching with CT tests was conducted to obtain the kinetics. Two kinds of reactions, “oxidation reaction” and “combustion reaction” were used in the study. In the former reaction, oxygen is consumed by the oil to form oxidation compounds. In the latter one, oxygen reacts with the compounds to form carbon dioxide and water. Reaction parameters were used as matching parameters. Good results of history matching with both CT tests using same reaction kinetics were obtained.
