Air injection is developing as a broad based IOR technology, applicable over a wide range of reservoir conditions, including heavy, medium and light oils. In remote and offshore locations, air represents a cheap source of injection gas for implementing a number of different recovery processes. In light oil reservoirs, there are three possibilities: if the oil-rock matrix reactivity is low, then the oxidation reactions will be limited to low temperature oxidation (LTO) and either the net thermal effect in the reservoir will be close to zero, i.e. isothermal at a low gas rate, or a thermal displacement will develop at significantly higher temperatures (still LTO), but only at a much higher gas injection rate. If, on the other hand, the oil-rock reactivity is high, high temperature oxidation (HTO) reactions will take place which, with a sufficiently high air injection rate, an in-situ combustion front can be propagated through the reservoir. A necessary requirement for all air injection processes is that the oxygen be completely removed via oxidation reactions, or at least to a sufficiently low level.

One problem observed in light oil reservoirs is the tendency to de-saturate the oil layer downstream of the moving front, particularly with thermal IOR processes, because of the additional vaporisation which occurs and high oil mobility. Low pressure ISC tests were carried out in a 3-D cell equipped with an extensive thermocouple array. In order to restrict the de-saturation of the oil layer, a modified horizontal producer well, incorporating a ‘sleeve-back’ principal was used. The idea was to mimic the ‘Toe-to-Heel’ displacement process occurring during heavy oil recovery, wherein the downstream oil is essentially immobile due to its high viscosity. The ‘sleeve-back’ of the well was achieved using a co-aligned, two well assembly, so that the upstream section of the horizontal producer well is active, and continually adjusted during propagation of the combustion front.

Significant improvement in overall stability of the process was observed using the ‘sleeve-back’ modification, as attested by sustained propagation of the combustion front, high levels of produced CO2 and high oil recovery.

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