Carbon dioxide (CO
2) flooding is a process whereby
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
is injected into
oil reservoirs in order to increase output when extracting
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
, especially in reservoirs where production rates have declined over time. The process was first attempted in 1977 in
Scurrey County,
Texas
Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
. Since then, the process has become extensively used in the
Permian basin region of the US and is now more recently begun to be pursued in many different states, but still remains fairly uncommon outside of
the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
.
Overview
When a
reservoir's pressure is depleted through primary and secondary production, carbon dioxide flooding can be an ideal
tertiary recovery method. It is most effective and most commonly used in
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
and
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
(such as
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
or
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
) reservoirs and reservoirs that produce medium to light oil. By injecting CO
2 into the reservoir, the
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
of any
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
will be reduced, and hence will be easier to sweep into the production well. Sandstone and carbonate reservoirs are preferred for this method over reservoirs with ultra-low permeability such as
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
due to the risk of CO
2 gas breaking through hydraulic or natural fractures in the rock.
CO
2 flooding is still sometimes used in these instances, but usually using the "huff and puff" CO
2 injection method, which allows the CO
2 to soak in a reservoir after being pumped in through the injection well for a period of time before the production well is opened and put back into functionality. This method reduces the chances of unwanted gas breaks, and increased amount of oil recovered as opposed to the more common CO
2 injection water alternating gas process (WAG).
As an
oil field
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.
Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence ...
matures and production rates decline, there is a growing incentive to intervene and attempt to increase oil output utilizing tertiary recovery techniques (also termed improved or
enhanced oil recovery
Enhanced oil recovery (abbreviated EOR), also called tertiary recovery, is the extraction of crude oil from an oil field that cannot be extracted otherwise. EOR can extract 30% to 60% or more of a reservoir's oil, compared to 20% to 40% using ...
). The decline of efficacy of primary or secondary methods could be due to several factors, such as poor management, reduction of internal pressure due decreasing amounts of oil, or because of heterogeneity of the reservoir. Reservoir heterogeneity could be caused by fractures or faults in the rock, or barriers produced by highly cemented areas or shale.
Most of the time, around 60 to 70% of oil cannot be procured conventionally, and secondary or tertiary recovery techniques must be used.
It is also generally true that the transition from secondary recovery techniques to tertiary recovery techniques happens well before the reservoir becomes completely unproductive.
Petroleum engineers assess available options for increasing well productivity, options that include chemical injection, thermal/steam injection, and CO
2 injection. Based on data-gathering and computer simulations, the best enhanced oil-recovery technique to maximize well-productivity is determined. However, in all cases of reservoirs that have declining productivity, to increase the rate of oil production, the pressure within the reservoir must be increased. The method employed, however varies on a case-by-case basis.
Method
In CO
2 flooding, the first step is an injection of water into the reservoir, which will allow the pressure in the reservoir to return to productive levels. Once the reservoir has sufficient pressure, the next step is to pump the CO
2 down through the same
injection well
An injection well is a device that places fluid deep underground into porous rock formations, such as sandstone or limestone, or into or below the shallow soil layer. The fluid may be water, wastewater, brine (salt water), or water mixed with indus ...
. The CO
2 gas is forced into the reservoir to come into contact with the oil. This creates a
miscible
Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...
front of CO
2 and low-boiling hydrocarbon derivatives.
An oil bank forms in front of the miscible zone, and is more easily moved out of the reservoir via the production well. Additionally, as the gas dissolves into the oil, the
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
of the oil with CO
2 dissolved in it decreases.
Normally the CO
2 injection is alternated with water injection and the water acts to sweep the oil towards the production zone, this is called water alternating gas process (WAG).
Pros and Cons
CO
2 flooding is a method preferred for medium to light oils due to the "mobility ratio" between the gas and the oil. The mobility ratio refers to the ratio of the mobility of the gas or fluid injected into a reservoir for secondary or tertiary production versus the mobility of the oil.
For medium or light oils with a high
API gravity
The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks ...
, fluids or gases that are less viscous themselves can be used. However, if an injection fluid or gas that had lower viscosity was used on a heavy crude oil or
bitumen
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
, the injection fluid or gas would bypass the oil and result in a poorly swept reservoir.
While CO
2 and water injection are more useful for lighter oils, co-solvents are still often added to make them more viscous so that they will have a more productive sweep of the reservoir.
In cases where the reservoir is filled with extremely heavy oil or bitumen,
steam injection, or other methods that employ heat, are much more commonly favored so that the mobility or viscosity of the oil can be lowered and the extraction will become easier.
Generally, reservoirs with lighter oils will have higher recovery percentages with primary and secondary recovery methods, but reservoirs with heavier oils or bitumen will have much lower recovery with primary and secondary recovery methods and the transition from secondary to tertiary methods will have to occur much earlier in the reservoirs lifespan.
CO
2 flooding is the second most common tertiary recovery technique and is used in facilities around the world. In connection with
greenhouse gas emissions
Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and lar ...
and
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, CO
2 flooding is advertised to
sequester
Sequestration or sequester may refer to:
Law and government
* Sequestration (law), the seizure of property for creditors or the state. See also Committee for Compounding with Delinquents
* Jury sequestration, the isolation of a jury
* Bankruptcy, ...
CO
2 underground and therefore offset CO
2 emissions elsewhere.
This and the fact that it can usually be applied in a wide range or geologic conditions makes it a very popular method to recover residual oil, however, there are several drawbacks to the method. For one, because the gas alone has very low viscosity, there is very little control over it and this causes the possibility of it breaking through the producer well or fractures in the rock, and this is why the WAG technique is used. Another drawback is that this method has high gas requirements, and while most reservoirs attempt to use natural, locally sourced CO
2, sometimes this is not feasible and the CO
2 must either be transported long distances using a pipeline, or made from artificial industrial sources such as
natural gas processing.
Of course, all methods of sourcing CO
2 for CO
2 flooding have the possibility for accidental release of CO
2, resulting in increased greenhouse gas emissions.
See also
*
Petroleum Industry
The petroleum industry, also known as the oil industry or the oil patch, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The larges ...
References
External links
Mississippi Oil Journal Map Oil Well Map of EOR field in Brookhaven Mississippi
Enhanced Oil Recovery InstituteEnhanced Oil Recovery Institute
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Wikipedia Student Program
Oil wells
Greenhouse gases