In-situ combustion is the oldest thermal recovery technique. It has been used for more than nine decades with many economically successful projects. In-situ combustion is regarded as a high-risk process by many, primarily because of the many failures of early field tests. Most of those failures came from the application of a good process to the wrong reservoirs or the poorest prospects. The objective of this page is to describe the potential of in-situ combustion as an economically viable oil recovery technique for a variety of reservoirs. For a more complete review, the work of Sarathi, Prats, and Burger et al.
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Should be consulted. Contents. Process description In-situ combustion is basically injection of an oxidizing gas (air or oxygen-enriched air) to generate heat by burning a portion of resident oil. Most of the oil is driven toward the producers by a combination of the following:. Gasdrive (from the combustion gases). This process is also called fire flooding to describe the movement of a burning front inside the reservoir.
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Based on the respective directions of front propagation and air flow, the process can be either of the following:. Forward (when the combustion front advances in the same direction as the air flow). Reverse (when the front moves against the air flow) Reverse combustion This process has been studied extensively in and tried in the field. The idea is that it could be a useful way to produce very heavy oils with high viscosity. In brief, it has not been successful economically for two major reasons. Combustion started at the producer results in hot produced fluids that often contain unreacted oxygen. These conditions require special, high-cost tubulars to protect against high temperatures and corrosion.
More oxygen is required to propagate the front compared to forward combustion, thus increasing the major cost of operating an in-situ combustion project. Unreacted, coke-like heavy ends will remain in the burned portion of the reservoir. At some time in the process, the coke will start to burn, and the process will revert to forward combustion with considerable heat generation but little oil production. This has occurred even in carefully controlled laboratory experiments. In summary, reverse combustion has been found difficult to apply and economically unattractive. Forward combustion Because only forward combustion is practiced in the field, we will only consider this case. Forward combustion can be further characterized as either of the following:.
“dry,” when only air or enriched air is injected. “wet,” when air and water are coinjected Dry combustion The first step in dry forward in-situ combustion is to ignite the oil. In some cases, autoignition occurs when air injection begins if the reservoir temperature is fairly high and the oil is reasonably reactive. This often occurs in California reservoirs. Ignition has been induced with:. Downhole gas burners. Injection of pyrophoric agents or steam injection After ignition, the combustion front is propagated by a continuous flow of air.
Rather than an underground fire, the front is propagated as a glow similar to the hot zone of a burning cigarette or hot coals in a barbecue. As the front progresses into the reservoir, several zones exist between injector and producer as a result of:. Heat. Mass transport. Chemical reactions Fig. 1 is an idealized representation of the various zones and the resulting temperature and fluid-saturation distributions. In the field, there are transitions between zones; however, the concepts illustrated provide insight on the combustion process.