Abstract Since 1950s In-Situ Combustion (ISC) has been applied to mainly heavy-oil reservoirs. In recent years, High Pressure Air Injection (HPAI) which is a displacement process categorized as ISC, is applied to light-oil reservoirs. And it has proven to be a valuable Enhanced Oil Recovery (EOR).
Reduction of oil viscosity is very important for ISC process. In contrast, it is not so essential for HPAI because original viscosity of light-oil is not as high as that of heavy oil. HPAI is considered as flue gas injection, since a flue gas sweep is one of the most important recovery factors of HPAI. However a flue gas sweep recovery factor is not effective for highly water saturated light-oil reservoirs, while thermal effects become an important recovery mechanism.
This paper discribed the feasibility study of HPAI for watered out light-oil reservoirs, oil recovery mechanism and several simulation studies to establish maximum oil recovery factor.
For the feasibility study, Combustion Tube tests (CT tests) and simulation studies were conducted. The oil recovery was observed in the CT test with crushed core which was flooded out by water. This result suggests that HPAI can be applied to highly water saturated light-oil reservoir. The results of simulation studies also indicate its feasibility. They also made clear that distillation process that was one of thermal effects of HPAI was a main recovery factor for a HPAI in this case.
In order to maximize the oil recovery, controlling a channeling of injected air is important because early breakthrough of the air reduces oil production period significantly. The results of our studies show that not only a design of well completion but also an adjustment of air injection rate enable to increase vertical sweep efficiency and that line drive injection is effective to increase areal sweep efficiency.
Introduction High Pressure Air Injection for light-oil reservoirs. HPAI is one of EOR techniques applied for light-oil reservoirs which is a displacement process with air injection into a reservoir, such as ISC. In the reservoir, oxygen contained in the air reacts with hydrocarbon to create a high temperature combustion front that is propagated through the reservoir. In recent years, HPAI has been drawn attention as an effective EOR technique in virtue of the many successful projects located on Williston Basin,[1–4] and some HPAI projects are planned for waterflooded reservoirs in North Sea[5] and Argentina.[6]
In comparison with ISC, reduction of oil viscosity which is a significant recovery factor of ISC is less important for HPAI, because the original viscosity of light-oil is not as high as heavy oil. In addition, there is a difference in the combustion behaviors between light-oils and heavy-oils, those of light-oils are preferable for successful air injection process.
According to Moore et al.,[7] oxidation processes of both light-oils and heavy-oils have two temperature regions and two reaction pathways. Figure 1 shows the temperature regions of both oils, these two regions are Low Temperature Oxidation (LTO) and High Temperature Oxidation (HTO). In the case of light-oil, oxygen uptake rate becomes high in LTO, while it occurs in HTO for heavy-oil case. Two reaction pathways of oxidation process mean oxygen addition process and combustion process. The former is that the oxygen atoms chemically bound into the molecular structure to produce various oxygenated compounds which further react and polymerize with each other. It results in not only increasing oil viscosity, density and boiling range but also shrinking vapor phase significantly. The latter is that the oxygen breaks up the hydrocarbon molecules to principally produce carbon dioxide and water, about 15% of carbon dioxide is generally observed in production gas while the process successfully occurs in reservoirs. Therefore, oxygen addition process is harmful to air injection process, and combustion process is a critical to successful air injection EOR.
In the case of light-oils, the oxygen addition process occurs at the temperature below 150°C, the combustion process principally occurs in the temperature range of 150–300°C. Both processes occur in LTO where oxygen uptake rate of light-oil is high, the oxidation process favorably changes to and reacts in combustion process. In the case of heavy oils, combustion process will not be dominant at the temperature below 450°C where HTO occurs. Once LTO occurs in heavy-oil reservoirs, oxygen addition process become dominant, and it is difficult to change to combustion process occurring in HTO.
Indeed, the history of ISC without much success arises from operation with oxygen addition process in LTO. In the case of HPAI, combustion process is a favored reaction pathway, and successful displacement process is highly expected.