In-situ combustion processes are largely a function of oil composition and rock mineralogy. Laboratory studies, using crude and matrix from a prospective in-situ combustion project, should be performed before designing any field operation. A more recent and more accurate kinetics model has been developed. Only two reactions are used, but in addition, the geometry of the reacting residual fuel in the pore spaces is taken into account, as indicated in Figure 1. LTO can be described as oxygen addition to the crude oil.
Heavy oil is defined as liquid petroleum of less than 20 API gravity or more than 200 cp viscosity at reservoir conditions. No explicit differentiation is made between heavy oil and oil sands (tar sands), although the criteria of less than 12 API gravity and greater than 10,000 cp are sometimes used to define oil sands. The oil in oil sands is an immobile fluid under existing reservoir conditions, and heavy oils are somewhat mobile fluids under naturally existing pressure gradients. Unconsolidated sandstones (UCSS) are sandstones (or sands) that possess no true tensile strength arising from grain-to-grain mineral cementation. Many heavy oil reservoirs are located in unconsolidated sandstones.
Predicting the production response to in-situ combustion (ISC) has been the topic of various studies. Complete numerical simulation of in-situ combustion is difficult because of the complex reactions and the thin burning front that requires small gridblocks for representation. The easiest method is essentially a tank balance, adapted by Prats. The oil and water produced are given by If the volumes are in acre-ft and the production terms are in bbl, a multiplication factor of 7,758 must be used.
Many useful and reasonably accurate calculations can be made on in-situ combustion to predict the behavior of a proposed project. This page discusses the calculation process involved with behavior prediction. In-situ combustion prediction calculations will be explained in the following diagrams and example calculations. They start with a very simple heat balance and are then extended to more closely represent what happens in the reservoir. Start by assuming that no combustion data are available to get an initial idea of the feasibility of a project.
In-situ combustion requires standard field equipment for oil production, but with particular attention to air compression, ignition, well design, completion, and production practices. Air-compression systems are critical to the success of any in-situ combustion field project. Past failures often can be traced to poor compressor design, faulty maintenance, or operating mistakes. See Compressors for a detailed discussion of compressors and sizing considerations. Other discussions are available in Sarathi.