The steam-drive process is much like a conventional waterflood. Once a pattern arrangement is established, steam is injected into several injection wells while the oil is produced from other wells. This is different from the steam stimulation process, whereby the oil is produced from the same well into which the steam is injected. As the steam is injected into the formation, the thermal energy is used to heat the reservoir oil. Unfortunately, some of the energy also heats up the entire environment, such as formation rock and water, and is lost. Some energy is also lost to the underburden and overburden. Once the oil viscosity is reduced by the increased temperature, the oil can flow more readily to the producing wells. The steam moves through the reservoir and comes in contact with cold oil, rock, and water. As the steam contacts the cold environment, it condenses, and a hot water bank is formed. This hot water bank acts as a waterflood and pushes additional oil to the producing wells.
Several mechanisms have been identified that are responsible for the production of oil from a steam drive. These include thermal expansion of the crude oil, viscosity reduction of the crude oil, changes in surface forces as the reservoir temperature increases, and steam distillation of the lighter portions of the crude oil.
Most steam applications have been limited to shallow reservoirs because, as the steam is injected, it loses heat energy in the wellbore. If the well is very deep, all the steam will be converted to liquid water.
Steam drives have been applied in many pilot and field-scale projects with very good success. Oil recoveries have ranged from 0.3 bbl to 0.6 bbl of oil per barrel of steam injected. A process that was developed in the 1970s and has become increasingly popular is the steam assisted gravity drainage (SAGD) process. This process involves the drilling of two horizontal wells (see Fig. 11.11) a few meters apart. Steam is injected in the top well and heavy oil, as it heats up from the steam, drains into the bottom well. The process works best in reservoirs with high vertical permeability and has received much attention by companies with heavy-oil resources. There have been many applications of this process in Canada and Venezuela.19,20,24
Figure 11.11 Schematic of the steam assisted gravity drainage process (courtesy Canadian Centre for Energy Information).
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