EOR final

ENHANCED OIL RECOVERY 1

INTRODUCTION

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. 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

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.

MISCIBLE GAS DISPLACEMENT

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 supercritical1 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

1 “A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist” (Wikipedia).


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

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 factor2”. 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 surfactants3” (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. A very

good example of a successful application of chemical flooding was in Daqing oil

2 Recovery factor is the percentage of the total barrels of oil that can be recovered from a reservoir3 A surfactant can be defined as a substance that tends to lower the surface tension; in this case, the surface tension of the reservoir to which it is applied.


field in China, they were able to produce 8.16 million tons of crude oil as a result

of polymer flooding (Anon., 2010)

Fig 2: Typical Illustration of the chemical “micellar-polymer” flooding process (The Energy Lab)


THERMAL FLOODING

Thermal flooding is a process whereby heat is introduced into a reservoir

in order to reduce the viscosity of that reservoir so that in turn, it increases the

recovery factor. 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

combustion4: 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

4 Fig 4. The diagram above shows the “in-situ combustion” process using an air and a water injection system. Air is pumped into the reservoir and in the presence of oxygen, burning occurs. Water is used to stop the burning. (The Piping Guide, 2009)


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)5

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.

5 Fig 5 above is a microscopic image of typical oil bacteria. “Hydrocarbon-degrading bacteria growing at a crude oil-water interface” (Gunhild, 2011)


PROPOSITION

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.

ENHANCED OIL RECOVERY10

REFERENCES

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