Primary and secondary production from reservoirs containing heavy, low-gravity crude oils is usually a very small fraction of the initial oil in place. This is due to the fact that these types of oils are very thick and viscous and, as a result, do not migrate readily to producing wells. Figure 11.10 shows a typical relationship between the viscosity of several heavy, viscous crude oils and temperature.
Figure 11.10 Typical viscosity-temperature relationships for several crude oils.
As can be seen, viscosities decrease by orders of magnitude for certain crude oils, with an increase in temperature of 100°F to 200°F. This suggests that, if the temperature of a crude oil in the reservoir can be raised by 100°F to 200°F over the normal reservoir temperature, the oil viscosity will be reduced significantly and will flow much more easily to a producing well. The temperature of a reservoir can be raised by injecting a hot fluid or by generating thermal energy in situ by combusting the oil. Hot water or steam can be injected as the hot fluid. Three types of processes will be discussed in this section: steam cycling, steam drive, and in situ combustion. In addition to the lowering of the crude oil viscosity, there are other mechanisms by which these three processes recover oil. These mechanisms will also be discussed.
Most of the oil that has been produced by tertiary methods to date has been a result of thermal processes. There is a practical reason for this, as well as several technical reasons. In order to produce more than 1% to 2% of the initial oil in place from a heavy-oil reservoir, thermal methods have to be employed. As a result, thermal methods were investigated much earlier than either miscible or chemical methods, and the resulting technology was developed much more rapidly.26,27
Leave a Reply