Primary Recovery — When the natural pressure of the oil (or gas) is enough to get it out of the ground. The recovery factor is usually low (10-20%). The vast majority of oil production happens this way.
Secondary Recovery — In order to keep the pressure elevated or to force the oil out, water (sometimes gas) is pumped into the reservoir. Recovery of the reservoir can reach as high as 45%. More and more oil fields are being exploited this way because of years of using just the reservoirs natural pressure which has since dropped so low that primary recovery rates became un-economical.
Tertiary Recovery — Also known as Enhanced Oil Recovery (EOR), includes methods which inject fluids to make the oil easier to flow. Solvents could be injected to dissolve into the oil, hot gases or fluids could be injected to make it 'lighter' or chemicals may be injected to reduce oil tension with the rock and water. Depending on the EOR method, another 5 - 20% could be added to the recovery. A small percentage of oil production, though growing, happens using EOR.
CHOOSING THE EXTRACTION METHOD:
However as oil prices rise and more unconventional sources of oil become profitable, oil companies find it in their interest to consider different extraction methods as early as the initial production phase.
A useful illustration of when certain methods are used can be found below:
The x-axis measures the heterogeneity of the reservoir. That is to say, the 'uniformity' of the rock, oil and water properties and compositions. If some parts of the reservoir allow for flow to pass easier than other parts, then the heterogeneity of the reservoir is low.
The y-axis measures the 'quality' of the oil. If the oil is light and flows easily, the quality is high and production is easier. If the oil is heavy and viscous (doesn't flow as readily) then production is more complex.
Cold Production
When the solid and fluid composition of the reservoir is fairly uniform and the oil quality is high, primary production is employed. For reservoirs with heavier oil, cold heavy oil production with sand (CHOPS) is used. This type of production is mostly prevalent in the heavy oil fields of Venezuela. There is also the option of drilling a very high concentration of multilateral wells in a specific area so as to make sure even the least readily flowing oil is produced.
Less prevalent of a method, but one which produces 20% of Texan oil, is CO2 injection. The CO2 dissolves into the oil and changes it's chemical composition making it easier to flow. Other fluids, such as light hydrocarbon gases, could be used as solvents. However CO2 is more readily available. This type of extraction is called Miscible EOR because the CO2 and Oil are miscible with one another; that is to say they can 'mix'. Immiscible EOR involves injecting fluids which do not combine with the oil and simply 'push' it along to the collector well.
All these processes have in common the fact that they can operate at reservoir temperature.
Hot Production
When the oil quality is low thermal methods must be used. When heavier oils are heated the lighter oil fractions within it evaporate. Heavy oils are made up of long hydrocarbon chains, so the remaining heavy residue will be chemically cracked into light components. Thus, thermal EOR has the advantage that it can 'upgrade' the oil quality within the reservoir itself.
Most heavy oil deposits are relatively shallow, which usually means reservoir heterogeneity is high. This means that if a hot gas (be it steam or air) is injected to upgrade the oil, it might not reach the majority of the oil because it is channeled to regions of the rock which are more permitting to fluid flow. To cut a long story short, THAI (toe to heel air injection) and SAGD (steam assisted gravity drainage) work around the problem of heterogeneity by injecting hot air and steam horizontally. This way gravity does most of the work of in terms of distributing the fluid and sweeps the reservoir uniformly. SAGD and THAI are new methods but they are very promising and are being pioneered in Canada.
Finally CSS (cyclic steam stimulation) is a popular technique in heavy oil fields which works at a short range and thus is able to overcome heterogeneity problems.
Final thoughts
If you realize that I have oversimplified a lot of aspects of this post, please don't be too critical. It's not meant to be addressed to those who are familiar with these methods, but rather to those who really have no idea how oil is extracted.
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