The recovery of heavy crude oils is impeded by a viscous resistance to flow at reservoir temperatures. The heating of heavy crudes markedly improves their mobility and promotes their recovery. Heat may be introduced into the reservoir by injecting a hot fluid, such as steam or hot water, or by burning some of the in the reservoir (a process referred to as in situ combustion or fire flooding).
A common method involving the use of steam to recoveris known as steam soak, or steam cycling. It is essentially a well-bore stimulation technique in which steam generated in a boiler at the surface is injected into a production well for a number of weeks, after which the well is closed down for several days before being put back into production. In many cases there is a significant increase in output. It is sometimes economic to steam-soak the same well several times, even though recovery usually declines with each succeeding treatment. Steam soaks are economically effective only in thick permeable reservoirs in which vertical (gravity) drainage can occur.
Continuous steam injection heats a larger portion of the reservoir and achieves the most efficient heavy oil recoveries. Known as steam flooding, this technique is a displacement process similar to waterflooding. Steam is pumped into injection wells, which in some cases are artificially fractured to increase reservoir permeability, and the oil is displaced to production wells. Because of the relatively high cost of steam, water is sometimes injected at an optimum time to push the steam toward the production wells. Because the steam serves two functions, the heating and the transporting of the oil, some steam must always be circulated through the rock formation without condensing. Even in some of the most favourable reservoirs, it is necessary to consume an amount of energy equivalent to burning roughly 25 to 35 percent of the heavy oil produced in order to generate the required amount of steam.
In situ combustion
The mechanics of heavy oil displacement in an in situ combustion operation is similar to that in the steam-flooding process except for one difference. Unlike in the latter, steam is produced by vaporizing water already in the rock formation or water that has been injected therein with heat from the in situ combustion of some of the oil in the reservoir. After the in-place heavy oil has been ignited, the burning front is moved along by continuous air injection. In one variation of the in situ combustion process known as forward combustion, air is injected into a well so as to advance the burning front and heat and displace both the oil and formation water to surrounding production wells. A modified form of forward combustion incorporates the injection of cold water along with air to recover some of the heat that remains behind the combustion front. The air-water combination minimizes the amount of air injected and the amount of in-place oil burned (to between 5 and 10 percent). In another variation of in situ combustion called reverse combustion, a short-term forward burn is initiated by air injection into a well that will eventually produce oil, after which the air injection is switched to adjacent wells. This process is used for recovering extremely viscous oil that will not move through a cold zone ahead of a forward-combustion front.
The costs associated with the generation of heat within a heavy oil reservoir and the success of the recovery process are influenced by the depth of the reservoir. In general, shallower reservoirs are candidates for steam soaks and steam floods, deeper reservoirs for in situ combustion.
Solvent extractions also have been used to recover heavy oils. In this process a solvent or emulsifying solution is injected into a heavy oil reservoir. The fluid dissolves or emulsifies the oil as it advances through the permeable reservoir. The oil and fluid are then pumped to the surface through production wells. At the surface the oil is separated from the fluid, and the fluid is recycled.
The bitumen in tar sands can be recovered by surface mining. Open-pit mining methods can be employed where thick deposits occur near the surface. Earth-moving equipment is used to strip and stockpile the topsoil, remove and dispose of the overburden, and excavate the tar sand. The recovery efficiency of surface mining tar sands is estimated at roughly 90 percent. A mill is required to separate the bitumen from the sand in order to upgrade it to commercial quality. This process includes crushing the tar sand and separating the bitumen by mixing the crushed ore with steam and hot water. The bitumen is concentrated by floating and is then treated with a solvent for final separation from the sand and water. The cleaned crude bitumen is upgraded in a delayed coking unit, which produces a blend of lighter hydrocarbon fractions that yield synthetic crude oil, naphtha, kerosene, and gas oil. While there are a large number of heavy oil fields in production throughout the world, few commercial tar sand surface-mining and synthetic-oil processing operations exist.
Economic and technical constraints
Unfortunately, there are problems associated with the exploitation of heavy hydrocarbons. Costs for tar sand mining and upgrading are considerably higher than for producing conventional oil, even in most frontier areas. The tar sand that is mined and milled along with the bitumen is very abrasive and causes rapid wear of equipment. Also, mills and upgrading (coking) facilities are very expensive.
Likewise, the heavy oils are a less desirable energy resource than the lighter crudes, for they are much more costly to extract and to process. An average of about one barrel of oil is combusted (or its energy equivalent expended) to produce the heat necessary to net two barrels of recovered heavy oil. This reduces the recoverable oil in a heavy oil reservoir by one-third.
If the heavy oil is transported by pipeline, direct heating is often required before it will flow at an acceptable rate, necessitating the combustion of additional fuel. The refining of heavy oil results in low yields of distillate products (e.g., naphtha, kerosene, jet fuel, gasoline, oil, and diesel) and correspondingly high yields of sulfur and high-viscosity residues (e.g., asphalt and coke) with metals concentrated in them.
Even under thermal stimulation, heavy oil production ranges only from about 5 to 100 barrels per day per well. This can be compared with recoveries in giant conventional oil fields on the order of 10,000 barrels per day per well. Consequently, even though heavy oil fields are exploited at much slower rates than conventional fields, many more wells are still required. This considerably increases development costs.