Carbon Dioxide Flooding
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Carbon dioxide (CO2) flooding is a process in which
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
is injected into an
oil reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in Porosity, porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by t ...
to increase the 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) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...
. This is most often used in reservoirs where production rates have declined due to depletion.


Overview

When the amount of recoverable oil in an
oil reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in Porosity, porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by t ...
is depleted through primary and secondary production, around 60 to 70% of oil that was originally in the reservoir may still remain. In some cases, carbon dioxide (CO2) flooding may be an ideal tertiary recovery method to recover more of the recoverable oil than could be produced using secondary oil recovery methods. Because of its special properties, CO2 improves oil recovery by lowering
interfacial tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to ...
, swelling the oil, reducing
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
of the oil, and by mobilizing the lighter components of the oil. When the injected CO2 and residual oil are
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). Such substances are said to be miscible (etymologically ...
, the physical forces holding the two fluids apart effectively disappears. This results in a viscosity reduction of the
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 Hydrophobe, hydrophobic; their odor is usually fain ...
and makes it easier to displace the crude oil from the rock pores and sweep it to the production well. In other cases where the CO2 and residual oil are immiscible, the injected CO2 may still be used to drive the crude oil through the formation to be produced. One reason this occurs is because the injected CO2 can flow into the minute pores that are unavailable to oil and water.


Process

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 prese ...
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 after primary and secondary recovery methods have been completely exhausted. Whereas primary and se ...
).
Petroleum engineers Petroleum engineering is a field of engineering concerned with the activities related to the production of hydrocarbons, which can be either crude oil or natural gas or both. Exploration and production are deemed to fall within the ''upstrea ...
assess available options for increasing reservoir productivity. The options include chemical flooding, thermal/steam injection, and CO2 injection. One of the criteria for determining if CO2 flooding is a candidate for the recovery of oil from the formation is the pressure of the formation. The miscibility of the CO2 and the crude oil is dependent upon the pressure and the temperature. However, since it is difficult to change the temperature of the reservoir, the pressure of the reservoir may be adjusted, to an extent, to bring the reservoir to a pressure that keeps the CO2 in a supercritical state. If a miscible flood is found to be feasible, the pressure is kept above the minimum miscibility pressure (MMP). The pressure may be below the MMP if an immiscible flood is desired. A petroleum engineer will then determine a method of using CO2 flooding to recover petroleum from the reservoir. This may be a continuous injection method, a water alternating gas (WAG) method, or some combination. The amount, or amounts of CO2 will be determined by the amount of the pore volume of the formation that is filled with oil. This is known as the hydrocarbon pore volume (HCPV). The petroleum engineer will also decide if the flood will be a pattern flood or a line drive flood. In a pattern flood, CO2 is usually injected into a number of
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 ind ...
s surrounding a producing well. Alternatively, CO2 may be injected into injector wells surrounded by producing wells. This is called an inverted pattern. In a line drive, the injection wells are located in a straight line parallel to the production wells. Optimally, a slug of CO2 will mobilize a flood front where the mixture of oil and CO2 will mobilize more oil. This flood front will radiate from each injection well towards the surrounding producing wells where the oil will be produced. The formation of a front is dependent upon the rate that the CO2 is injected, how fast it mobilizes the oil, and the porosity of the formation. Injecting the CO2 too fast will allow the CO2 to channel from the injector directly to a producing well without mobilizing any oil. Injecting CO2 too quickly may fracture the formation, which may again allow channeling from the injector to any or all of the producing wells. Also, injecting CO2 may migrate fines, which are small particles of clay and minerals, may plug the pores and prevent the mobilization of oil through the formation. In a continuous flood, a slug of CO2 will be continuously injected and not followed by any other fluid. The amount of CO2 is usually calculated to be around 100% of the HCPV of the field or pattern. In a water alternating gas (WAG) process, slugs of CO2 are followed by slugs of water. The overall amount of CO2 may be between 40% and 50% of the HCPV. The WAG process is known to reduce channeling of the CO2.


Formations and Oil

Sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
and
carbonate A carbonate is a salt of carbonic acid, (), 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 group ...
reservoirs (such as
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
or dolomite) 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 mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
due to the risk of CO2 channeling through hydraulic or natural fractures in the rock. CO2 flooding is still sometimes used in these instances, but usually using the "huff and puff" CO2 injection method, which allows the CO2 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 channeling, and increases the amounts of oil that may be recovered as opposed to the more common CO2 injection water alternating gas process (WAG) or by following a soak of CO2 with steam. Miscible CO2 flooding is a method preferred for medium to light oils due to the mobility ratio between the CO2 and the oil. The mobility ratio refers to the ratio of the mobility of the CO2 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 Bitumen ( , ) is an immensely viscosity, viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American Engl ...
, the injection fluid or gas would bypass the oil and result in a poorly swept reservoir. For reservoirs filled with extremely heavy oil or bitumen, steam injection, or other methods that employ heat, are much more commonly favored to reduce the oil's mobility or viscosity, thus easing extraction. Generally, reservoirs with lighter oils will have higher recovery percentages with primary and secondary recovery methods, but reservoirs with heavier oils or
bitumen Bitumen ( , ) is an immensely viscosity, viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American Engl ...
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 reservoir's lifespan.


History

Using CO2 for enhanced oil recovery was first investigated and patented in 1952. In 1964, a field test was conducted at the Mead Strawn Field, which involved the injection of a large slug of CO2 (25% of the hydrocarbon pore volume or HCPV) followed by
carbonated water Carbonated water is water containing dissolved carbon dioxide gas, either artificially injected under pressure, or occurring due to natural geological processes. Carbonation causes small bubbles to form, giving the water an effervescent quali ...
at reservoir conditions. Results indicated that 53 to 82 percent more oil was produced by the CO2 flood than was produced by water in the best areas of the waterflood. The process was first commercially attempted in 1977 in
Scurry County Scurry County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 16,932. Its county seat is Snyder, which is the home for Western Texas College. Scurry County is named for Confederate General William S ...
,
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
. Since then, the process has become extensively used in the Permian basin region of the US and is now more recently is being pursued in many different states. It is now being more actively pursued in China and throughout the rest of the world.


Sequestration of Carbon Dioxide

In connection with
greenhouse gas emissions Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (), from burning fossil fuels such as coal, petroleum, oil, and natural gas, is the main cause of climate chan ...
and
global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
, CO2 flooding may be used to sequester CO2 underground and therefore offset CO2 emissions elsewhere.


See also

*
Petroleum Industry The petroleum industry, also known as the oil industry, includes the global processes of hydrocarbon exploration, exploration, extraction of petroleum, extraction, oil refinery, refining, Petroleum transport, transportation (often by oil tankers ...
*
Supercritical carbon dioxide Supercritical carbon dioxide (s) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or a ...


References


External links



PetroWiki - Miscible flooding

PetroWiki - Enhanced Oil Recovery (EOR) {{Authority control Petroleum production