Abstract
The recovery mechanisms for steam injection in naturally fractured reservoirs and the characteristic recovery times for these mechanisms are reviewed. Laboratory studies suggest that steam injection can recover as much as 50% more oil than waterflooding in fractured reservoirs. This recovery is essentially independent of oil gravity, making steam injection equally attractive in light or heavy oil fractured reservoirs.
One of the most important incremental recovery mechanisms is the differential thermal expansion between the oil and the matrix pore volume. At high temperatures, chemical reactions within the matrix block can generate gases, which will displace oil. In matrix blocks containing organic carbon, additional oil can be generated. Oil can also be recovered during cyclic processes by dropping the pressure and flashing the water in the matrix blocks to steam. The effects of steam injection on capillary imbibition of water are less clear, although the reduced viscosity at the higher temperatures will speed the imbibition process. Other mechanisms like gravity drainage, solution gas drive, and distillation are thought to play a less important role in oil recovery.
The characteristic expulsion times for each of these recovery mechanisms are identified. Most of the oil in matrix blocks as large as 10 ft wide can be recovered within a few months.
