Historically, the air injection literature has stated that the fuel for the in-situ combustion (ISC) process is the carbon-rich, solid-like residue resulting from distillation, oxidation, and thermal cracking of the residual oil near the combustion front, commonly referred to as “coke.” At first glance, that assumption appears sound, because many combustion tube (CT) tests reveal a “coke bank” at the point of termination of the combustion front. However, when one examines both the laboratory results from tests conducted on various oils at reservoir conditions and historical field data from different sources, the conclusion may be different from what has been assumed. For instance, CT tests performed on light oils, at elevated pressures, rarely display any significant sign of coke deposition, which would make them poor candidates for air injection; nevertheless, they have been some of the most successful ISC projects.
An analysis of different ramped temperature cracking (RTC) and ramped temperature oxidation (RTO) experiments performed on oil samples ranging from 6.5 to 38.8 °API, at reservoir pressures, indicates that thermal cracking and oxidation do not tend to generate enough coke to sustain the ISC process in light oil reservoirs. Similarly, peak temperatures observed during RTO and CT tests performed on lighter oils, at high pressures, were below the levels required to be associated with the combustion of coke. Instead, RTC and RTO experiments indicate that flammable vapors are also generated and can be consumed as fuel, in a behavior that is consistent across the entire oil density spectrum. Such vapor fuel appears to be the result of oxidative and thermal cracking of original and oxidized oil fractions. Therefore, while coke may form as a result of low-temperature oxidation of heavy oil fractions, and while thermal cracking of those fractions on the pathway to coke may produce vapor components that may themselves burn, the coke itself is not necessarily the main fuel for the process, particularly for lighter oils.
This paper presents new insights regarding the nature of the fuel utilized by the ISC process and the role played by the different hydrocarbon phases present. It discusses the fundamental aspects associated with the proposed theory, and it summarizes the laboratory evidence and the field evidence which support the concept. This new theory does still share much common ground with the current understanding of the ISC process, but with a twist. The new insights result from the analysis of laboratory tests performed on lighter oils at reservoir pressures; data that were not available at the time the original ISC concepts were developed.
This material suggests a change to one of the most important paradigms related to the ISC process, which is the nature of the fuel. This affects the way we understand the process but provides a unified theory, which is important for the modeling efforts and overall development of the technology.