The beginning of the 20th century in the United States was marked by what historians call the Texas oil boom. Its other name was the Gusher Age. The gushers were oil wells where the oil and gas were under so much pressure; they sprang out when the well got drilled. Today, the gusher age is a thing of the past. We know how to handle gas and liquids under massive pressure. Also, oil producers have developed ways to extract as much oil and gas from a reservoir as possible even after the initial pressure subsides.
The three stages of oil recovery
The hydrocarbon resources associated with gushers of the Texas oil boom would now be called primary oil recovery. The gushing itself would now be called a blowout and drillers nowadays have blowout preventers to stop the oil from springing up in a fountain. The initial production of oil from a well that relies on the internal pressure of the reservoir to push the hydrocarbons to wellhead is called primary recovery.
This primary recovery can yield about a quarter to 35% of a reservoir’s reserves on average. After that, internal pressure declines and the well needs additional stimulation so the oil can rise to the surface. This extra stimulation comes in the form of water or gas. Waterflooding, as the name suggests, involves sending substantial amounts of water down the well to push the lighter oil to the surface. Gas injection does the same: the gas expands in the well and displaces the oil, which gets forced to flow up.
These are both production approaches that work well at conventional wells in formations with excellent permeability characteristics, meaning it is easy to extract the oil from the rock. They also work well with lighter (less viscous) crude oil grades. But not all oil-bearing geology is the same and not all oil is the same. For heavier oil and oil trapped in less permeable rock, drillers have developed other, more complex, methods of extraction.
All the methods used to boost oil production after the primary phase are collectively known as enhanced oil recovery (EOR).
What is enhanced oil recovery or EOR?
Enhanced oil recovery involves injecting gas or chemicals into a well to change the makeup and location of the oil in it to improve recovery rates. EOR methods tend to be costly, but they can increase the amount of oil produced from a reservoir to as much as 75%. For some oil reservoirs, EOR is the only way to extract the oil because it is too heavy to flow.
The Canadian oil sands are a case in point. These deposits hold bitumen, which is oil in a solid form that cannot flow up a well. Some of the bitumen deposits are on the surface, so it can get mined, but some are located deeper underground, and mining does not make economic sense. Producers inject steam into the reservoir, to melt the bitumen into a liquid and send it up the well to the surface.
There are three established enhanced oil recovery methods, and a fourth one was recently added. One thing all four shares is the cost. EOR is an expensive way to boost the output from an oil well, which is why it is not employed for every well. Before a producer decides to use an EOR method, a detailed evaluation of the economics of such a move is made and only if the technique does prove economical, the producer goes ahead with EOR.
Water injection, also called waterflooding, is perhaps the simplest enhanced oil recovery method. A well is drilled near the production well, and water gets pumped down to sweep the oil from the reservoir and send it up the production well. Often, depleted production wells are used as injection wells.
Besides the simplest, water injection is also probably the cheapest way to enhance oil production. At offshore fields, water is abundantly available and at onshore fields drillers often use the water that comes out of the well along with the oil instead of disposing of it.
While cheap and straightforward, waterflooding is not the most universally efficient enhanced recovery method. It cannot be used in natural gas reservoirs, for example. It also cannot move bitumen to the production well. That makes its application limited to conventional reservoirs of crude oil. It is also slow: the effects of waterflooding could take up to two years before it becomes evident, which is another drawback of the method.
Gas injection is the practice of sending gas into an oil and gas deposit to push the hydrocarbons to the surface when the internal pressure of the deposit wanes. Gas injection is commonly used during the stage of secondary recovery.
There are two types of gases used in this method of recovery when used for crude oil. Miscible gases can mix with the oil and since they are lighter than the oil and expand under pressure, they make the flow of oil to the production well easier. Methane and ethane are commonly used as miscible gases.
Immiscible gases, those that cannot mix with oil in a homogenous substance, are used to increase the pressure in the reservoir and push the oil out. Nitrogen and dry gas—a gas that is found without condensates or other liquid hydrocarbons—are commonly used as immiscible gases in gas injection recovery.
The most popular gas for gas injection recovery is carbon dioxide. It is a miscible gas that, when it mixes with crude oil, reduces its viscosity and makes it flow into the production well. The use of carbon dioxide has been hailed as a productive way to utilise greenhouse gas instead of letting it into the atmosphere. However, there is a cost issue to consider, and it is a significant issue, including capture and transportation costs.
These costs can be limited if the source of the carbon dioxide is close to the reservoir where it will get injected and some sources are indeed close to the fields, mostly gas processing plants that release CO2 as part of their operation. If the CO2 has to be brought to the reservoir from hundreds of miles, however, the cost may become prohibitive.
Gas injection, however, is a cost-effective solution to the gas flaring problem many oil drillers have on their hands. Gas flaring is the practice of burning associated gas that comes out of the reservoir together with the oil and has nowhere else to get sent or stored. The Permian shale play in the United States, for example, became a record-breaker in gas flaring last year due to a shortage of gas pipelines and storage facilities since the region focuses on oil rather than gas production.
Yet this excess gas can be injected into the reservoir and enhance the recovery of oil from it. It is much cheaper than having to bring in carbon dioxide from somewhere else, and it is also more environmentally conscious than flaring.
First used in the 1960s in Venezuela, whose oil reserves are overwhelmingly comprised of superheavy—highly viscous—oil, today thermal recovery is one of the most popular EOR techniques. Put simply; it involves heating the oil to reduce its viscosity and force it up to the surface.
The heating process often involves steam produced by either burning natural gas or, more recently, solar power. The solar-powered steam gets produced by concentrating mirrors that reflect sunlight onto a solar collector that then turns the light into heat. The technique has a smaller carbon footprint than gas-produced steam. Steam flooding could be pretty efficient for tertiary oil recovery at a conventional field, but it is not as suitable for bitumen deposits.
Steam-assisted gravity drainage
This technique is an improvement on the steam flooding and uses two wells. The steam gets injected into one of these, and once the bitumen melts, it gets pushed up the second one. The two horizontal wells are drilled parallel to each other close to the bottom of the reservoir. Steam is sent continuously down the top well, and it heats the bitumen around. Thanks to gravity, this heated bitumen seeps into the lower well. From there, it gets pumped up to the surface.
Some companies have started adding solvents to the steam they inject into the bitumen formation to improve recovery rates. The solvents include natural gases such as propane and butane, as well as mixtures the companies develop themselves and that reduce the surface tension between the liquids and the solids in bitumen. Some producers are now doing tests to see if they can replace steam with solvent mixtures altogether, which would reduce the cost of EOR.
Microwave heating is not a particularly popular technique for heating viscous oil mostly because of its high cost and the disproportionately low rate of success historically. Yet the method has been revived in recent years.
Microwave heating that might replace steam injection involves sending radio waves into a bitumen deposit where the waves heat the water molecules present in the oil. This water turns into steam and melts the oil. The oil flows into a production well and is ‘sucked up’ by pipes like a milkshake through a straw. It is an emerging technology that has yet to prove that it is cost-effective.
This method of oil recovery, which is even older than the steam injection, combines production and refining in one. Combustion occurs at temperatures high enough to crack the crude oil into fractions—the same process that takes place in a refinery—so in situ combustion does not just bring the oil up to the surface. Still, it also brings it up in a form that would then require much less refining.
In situ combustion can be a much more economical method of extracting oil than alternatives because it involves primary refining. It can also get deployed across a wide variety of formations. However, since the process is a lot more complicated than the steam injection, safety requirements are more stringent, which increases costs.
Chemical injection into an oil reservoir works by either enhancing the effect of waterflooding with the addition of chemicals or by manipulating the surface tension between the oil and the formation to make the oil flow more easily to the surface.
The addition of water-soluble polymers to the water used to flood the reservoir is one way to improve oil recovery by making the water more viscous. The more viscous it is, the more effective its sweeping action in the reservoir that pushes the oil up to the surface. What’s more, the addition of polymers that boost the water’s viscosity makes it applicable in reservoirs with heavier oil.
Other chemicals, called surfactants and alkaline substances, when added to the water, act on the surface tension between the oil and the formation. They reduce it, the way soap reduces the surface tension of the objects it is applied to, and this helps the oil flow more easily out of the reservoir.
Chemical injection has proven itself a promising EOR method because of relatively low costs and high efficiencies. It does have drawbacks, and these have mostly to do with the loss of their effectiveness under certain circumstances. For example, polymer-enriched water tends to suffer loss of viscosity in reservoirs that feature brines besides the water and hydrocarbons. Water with surfactants and alkali, for its part, has strong adsorption properties, meaning it sticks to the rock pores.
A subtype of chemical injection involves bacteria. In microbial EOR the bacteria are flushed into the reservoir where they consume some of the hydrocarbons, producing solvents. These solvents mix with the oil to reduce its viscosity, which makes the method particularly suitable for heavy oil reservoirs.
The bacteria can also produce natural surfactants that have the same effect on oil as the artificial ones introduced through chemical injection. A third way microbial EOR works is with the bacteria producing gases as part of their metabolism and boosting the internal pressure of the reservoir.
The costs of using microbial EOR, however, are prohibitively high and there are also environmental concerns related to the use of this method, which has made it unpopular for the time being.
The plasma pulse technology, developed in Russia, uses a tool with a 3,500 W generator to create plasma arcs in the well. These plasma arcs emit enormous amounts of heat and pressure but only for a fraction of a second.
The heat and the pressure, in turn, create acoustic waves that resonate across the reservoir and this resonance eventually breaks up the large hydrocarbon molecules into smaller ones, which makes them more mobile and hence easier to extract.
According to proponents of the technology, it is much cheaper than alternatives, much easier to use, and more environmentally friendly. However, it has yet to garner popularity to become a mainstream enhanced oil recovery method.
Enhanced oil recovery is not standard practice for every oil field. Sometimes producers decide it would be cheaper to simply plug the well rather than try to extract any more oil and gas from it. Yet with the amount of undiscovered oil and gas in the world steadily dwindling because they are both finite resources, and with EOR techniques continuously improving, at some point some of them may become standard practice if the cost falls sufficiently without affecting results.