Enhanced Oil Recovery
Oil and Gas refers to the naturally occurring liquid and natural gas specifically made up of long chain hydrocarbons and various organic compounds found beneath the surface of the earth in entrapments called reservoirs; the presence of oil and gas in these reservoirs is the reason humans survive everyday and carry out their daily activities effectively. Different activities are usually carried out to ensure that the oil and gas present in the reservoirs continue to support humans through their day-to-day activities; such activities include exploration, development, production and finally, abandonment and reclamation.
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This process is what is referred to as “the oil and gas process”. On completion of this process, numerous efforts are made to increase the quantity of oil that can be extracted again from the oil well and we refer to these efforts as “recovery”. There are three main types of recovery, primary recovery (solution gas, gas cap and natural water drive) secondary recovery (gas injection and water flooding) and tertiary recovery (enhanced oil recovery EOR, polymer flooding and steam flooding).
In this article, analysis on the “enhanced oil recovery” technology and trends will be emphasized upon. Enhanced oil recovery becomes very vital when oil production has to be increased to obtain a recovery percentage of at least 75% (Rigzone, 2009). This can only achieved by using any of the four basic methods of EOR; these methods include chemical flooding, miscible gas displacement, thermal recovery and microbial EOR. Among these four mentioned, “Thermal methods are the oldest EOR methods, they have been developed over the last thirty years” (Elsevier, 1981).
Miscible gas displacement also called “gas injection” by some engineers refers to the process of injecting CO2, natural gas and Nitrogen into a reservoir; “in miscible gas displacement, the gas is injected at or above the minimum miscibility pressure (MMP) which causes the gas to be miscible in oil” (Bandar, 2007). Chemical flooding also called “chemical injection” involves injecting chemicals such as a polymer directly into the reservoir to enhance the oil recovery. Thermal recovery introduces heat to the reservoir to reduce the viscosity of the oil.
Many times, steam is applied to the reservoir, thinning the oil and enhancing its ability to flow (Rigzone, 2009). Over 50% of the tertiary recovery method employed by the United States of America is the thermal recovery method. The most common gas employed when “gas injection” is being used is the CO2 (carbon dioxide) gas. The two major reasons why carbon dioxide is used are because it is miscible with crude oil and also it is the cheapest of all the other miscible fluids present.
Carbon dioxide injection into the reservoir based on previous experiences has shown that a recovery of up to 15% of the oil that was originally in the reservoir is achievable. With the injection of the carbon dioxide gas, changes occur in the reservoir such and temperature and pressure changes. Through the changes in temperature and pressure, carbon dioxide can form a solid, liquid, gas and supercritical fluid (Teledyne, 2007).
Furthermore, when carbon dioxide has been injected into the reservoir, it begins to form a homogeneous mixture with the crude oil thus, the light hydrocarbons, which are present in the crude, mix with the carbon dioxide gas and this dissolves the oil. Upon miscibility of the carbon dioxide gas and the crude, the physical forces separating the liquid phase and the gaseous phase gives way and this helps the carbon dioxide gas move the oil from the rocks towards the wells for production. According to the USDOE, a very good example of an applied carbon dioxide gas injection technique is the Wasson.
Field’s Denver Unit CO2 EOR project which has resulted in more than 120 million incremental barrels of oil through 2008 (2010). Moreover, carbon dioxide is not the only gas employed when the gas injection technique is employed. Nitrogen is another gas commonly used for gas injection basically to maintain the reservoir pressure; it can also be employed when gas cycling and gas lifting are to be employed. The minimum miscibility pressure of nitrogen depends largely on the pressure, temperate and the composition of the reservoir fluid.
According to Schlumberger, “above the MMP, nitrogen injection is a miscible vaporizing drive. Miscibility of nitrogen can be achieved only with light oils that are at high pressures; therefore, the miscible method is suitable only in deep reservoirs” (2011). Both methods of miscible injection from past experiences have proved very effective. When oil has been effectively recovered from these reservoirs using the miscible gas, the gas that has mixed with the recovered oil is separated from the oil and re-injected into the reservoir to enhance the oil recovery once again.
This process can be repeated until the reservoir is completely depleted and cannot produce any more oil. Note that nitrogen gas injection is used when dealing with reservoir containing light oils and also very economical allowing a recovery of up to 40%. A practical example of nitrogen gas injection is in the Cantarell oil field in Mexico. Upon application of this enhanced oil recovery methods, there was a noticeable increase in the oil production from 1. 9 million bpd to 2. 1 million bpd. Fig 1: Typical illustration of the miscible gas process using CO2 and water (University of Kansas, 1999)
Chemical flooding can also be referred to as chemical injection and is the process whereby chemicals and polymers are injected into a reservoir to increase its “recovery factor”. EOR methods involving the use of polymers usually yield a recovery range of ~6% to ~30%. When these chemicals are injected into the reservoir, the oil that is trapped there is released thus it can flow freely into the production wells. Furthermore, “polymers help increase the effectiveness of water-floods and boost the efficiency of surfactants” (Duncan, Ferguson, Sudip and Neptune, 2011).
The use of polymers in chemical injection further helps to enhance the competence of the surfactants and also the efficiency of the water flooding; to be precise, water soluble polymers are best used for this process. The polymer that is regularly utilised during the process is the partially hydrolyzed polyacrylamide and with development in technology, the quality has improved over time. In order to achieve the best results from the chemical injection process, there are certain characteristics that need to be in place.
These characteristics include “high remaining oil saturation, low water-flood residual saturation, high permeability and porosity, high polymer concentration and slug size, fresh water and soft water and reservoir temperatures of less than 220F” (Gary, 2007). Favourable characteristics are not limited to these that have just been mentioned but if these characteristics are present, then the expected recovery from the reservoir in question would be satisfactory as up to ~30% recovery is achievable.
Moreover, economics have shown that chemical flooding is not profitable when the price of the surfactants is compared with that of the price of oil; it usually cost a lot to obtain the surfactants needed to recover oil from reservoirs and in some cases, the chemicals which are injected into the reservoir are more expensive than the oil which they extract from that reservoir. Chemical flooding can prove to be very pricy and at the same time provide the engineers with the needed results.
Thermal recovery is usually applied to reservoirs with depths lesser than 3000ft. These reservoirs usually contain very viscous oil. There are four main types of thermal flooding that can be applied to any reservoir; they include: Fig 3: The steam injection process (The Piping Guide, 2009) 1. Cyclic stem injection: Popularly known as the “huff and puff” technique, it differs from all other types of thermal flooding in that it uses a central well to carry out every function from the injection of steam to the extraction of oil.
The technique is quite straightforward, steam is pumped into the well and the well is covered up for some time so that the steam can get to the reservoir rocks and upon arrival, the steam enhances the oil flow. To improve the oil flow further, this process can be repeated over again which leads to a new process called “steam drive”. 2. Steam drive: In this process, steam is re-injected into the reservoir constantly. Moreover, this process cannot be as effective as required if the reservoir rocks have fractures.
If there were fractures, “the steam would simply head straight through those into the producing wells instead of working its way through the reservoir rock” (Anon. , 2008). After the steam is injected, underlying heat is formed and this eventually aids the oil to flow with ease. 3. Thermally assisted gas-oil gravity drainage: this process is best suited for highly fractured reservoirs. In this process, the steam is injected directly into these fractures to reduce the viscosity of the oil and allow it flow easily.
According to the petroleum development Oman, the steam “just needs to get the oil flowing more easily. This then allows gravity to take effect, causing the oil to drain down into the fractures and then into horizontal producing wells that are situated towards the bottom of the reservoir” (2008). 4. In-situ combustion: In this process, a reasonable quantity of the oil in the reservoir is actually set on fire to act as a steam and gas generator. According to the petroleum development Oman “it’s the trickiest of the four main thermal EOR methods to get right.
It is generally only used in a reservoir that has high permeability (i. e. fluids can flow easily through the reservoir rock) and even then only as a last resort” (2008). Some of the oil might be lost during burning but more can be recovered as a result of the steam that has been generated that drives the oil towards the producing well. MICROBIAL ENHANCED OIL RECOVERY (MEOR) Microbial enhanced oil recovery also known as microbial flooding refers to the process whereby microorganisms are injected into the reservoir to increase oil production from a well.
The process of MEOR is a biological based technology in which these microorganisms react with the reservoir fluids in order to mobilize the oil and move it towards the production wells. Usually, MEOR is used in the third stage of EOR after 12% to 20% of the oil in the well has already been recovered. The characteristics of the reservoir in which the MEOR process is being applied will greatly have an effect on its success. There are two types of MEOR, “those in which the microorganisms react with reservoir fluids to generate surfactants and those that react with microorganisms to generate polymers” (Ronald, 2001).
MEOR is an economical way of increasing oil production and as genetic technology keeps improving, it would become the most attractive and cheapest method of enhanced oil recovery as the improvement lowers the cost of oil production. Conclusively, certain strains of bacteria that exist and are used for the MEOR method sometimes do not survive due to the harsh conditions they encounter in the reservoirs but as mentioned earlier, microbiologists are working on a different strain of bacteria that can survive in the reservoirs irrespective of these harsh conditions.
Enhanced oil recovery has an outstanding economic potential to increase oil recovery thereby maximizing profit. EOR also has it disadvantages that have negative effects on the ecosystem. Steam injection causes serious air pollution and also, polymer flooding which falls under chemical injection also leads to contamination of underground water which eventually makes it unfit for consumption. Furthermore, during the EOR process, a substance called “brine” is produced at the surface of the well; this substance is highly toxic and also radioactive. On exposure to humans, it can be very dangerous and highly detrimental.
In my opinion, more research on how the chemical injection process can be carried out without affecting the underground water supply and also how the thermal process of steam injection can be done safely without a threat to the atmosphere. Regulations put in place to control the environmental impacts of EOR should be as strict as possible so that eventually, the eco-system is preserved and oil and gas companies can still maximize profit.