Underground coal gasification

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Underground_coal_gasification 

 

Underground coal gasification

Process type	chemical
Industrial sector(s)	oil and gas industry coal industry
Feedstock	coal
Product(s)	coal gas
Leading companies	Africary Linc Energy Carbon Energy
Main facilities	Angren Power Station (Uzbekistan) Majuba Power Station (South Africa) Chinchilla Demonstration Facility (Australia)
Inventor	Carl Wilhelm Siemens
Year of invention	1868
Developer(s)	African Carbon Energy Ergo Exergy Technologies Skochinsky Institute of Mining

Underground coal gasification (UCG) is an industrial process which converts coal into product gas. UCG is an in-situ gasification process, carried out in non-mined coal seams using injection of oxidants and steam. The product gas is brought to the surface through production wells drilled from the surface.

The predominant product gases are methane, hydrogen, carbon monoxide and carbon dioxide. Ratios vary depending upon formation pressure, depth of coal and oxidant balance. Gas output may be combusted for electricity production. Alternatively, the gas output can be used to produce synthetic natural gas, or hydrogen and carbon monoxide can be used as a chemical feedstock for the production of fuels (e.g. diesel), fertilizer, explosives and other products.

The technique can be applied to coal resources that are otherwise unprofitable or technically complicated to extract by traditional mining methods. UCG offers an alternative to conventional coal mining methods for some resources. It has been linked to a number of concerns from environmental campaigners.

History

The earliest recorded mention of the idea of underground coal gasification was in 1868, when Sir William Siemens in his address to the Chemical Society of London suggested the underground gasification of waste and slack coal in the mine. Russian chemist Dmitri Mendeleyev further developed Siemens' idea over the next couple of decades.

In 1909–1910, American, Canadian, and British patents were granted to American engineer Anson G. Betts for "a method of using unmined coal". The first experimental work on UCG was planned to start in 1912 in Durham, the United Kingdom, under the leadership of Nobel Prize winner Sir William Ramsay. However, Ramsay was unable to commence the UCG field work before the beginning of the World War I, and the project was abandoned.

Initial tests

In 1913, Ramsay's work was noticed by Russian exile Vladimir Lenin who wrote in the newspaper Pravda an article "Great Victory of Technology" promising to liberate workers from hazardous work in coal mines by underground coal gasification.

Between 1928 and 1939, underground tests were conducted in the Soviet Union by the state-owned organization Podzemgaz. The first test using the chamber method started on 3 March 1933 in the Moscow coal basin at Krutova mine. This test and several following tests failed. The first successful test was conducted on 24 April 1934 in Lysychansk, Donetsk Basin, by the Donetsk Institute of Coal Chemistry.

The first pilot-scale process started 8 February 1935 in Horlivka, Donetsk Basin. Production gradually increased, and, in 1937–1938, the local chemical plant began using the produced gas. In 1940, experimental plants were built in Lysychansk and Tula. After World War II, the Soviet activities culminated in the operation of five industrial-scale UCG plants in the early 1960s. However, Soviet activities subsequently declined due to the discovery of extensive natural gas resources. In 1964, the Soviet program was downgraded. As of 2004 only Angren site in Uzbekistan and Yuzhno-Abinsk site in Russia continued operations.

Post-war experiments

After World War II, the shortage in energy and the diffusion of the Soviets' results provoked new interest in Western Europe and the United States. In the United States, tests were conducted in 1947–1958 in Gorgas, Alabama. The experiments were carried out in a partnership between Alabama Power and the US Bureau of Mines. The experiments at Gorgas continued for seven years until 1953, at which point the US Bureau of Mines withdrew its support for them after the US Congress withdrew funding. In total 6,000 tons of coal were combusted by 1953 in these experiments. The experiments succeeded in producing combustible synthetic gas. The experiments were reactivated after 1954, this time with hydrofracturing using a mixture of oil and sand, but finally discontinued in 1958 as uneconomical. From 1973–1989, extensive testing was carried out. The United States Department of Energy and several large oil and gas companies conducted several tests. Lawrence Livermore National Laboratory conducted three tests in 1976–1979 at the Hoe Creek test site in Campbell County, Wyoming.

In cooperation with Sandia National Laboratories and Radian Corporation, Livermore conducted experiments in 1981–1982 at the WIDCO Mine near Centralia, Washington. In 1979–1981, an underground gasification of steeply dipping seams was demonstrated near Rawlins, Wyoming. The program culminated in the Rocky Mountain trial in 1986–1988 near Hanna, Wyoming.

In Europe, the stream method was tested at Bois-la-Dame, Belgium, in 1948 and in Jerada, Morocco, in 1949. The borehole method was tested at Newman Spinney and Bayton, United Kingdom, in 1949–1950. A few years later, a first attempt was made to develop a commercial pilot plan, the P5 Trial, at Newman Spinney Derbyshire in 1958–1959. The Newman Spinney project was authorised in 1957 and comprised a steam boiler and a 3.75 MW turbo-alternator to generate electricity. The National Coal Board abandoned the gasification scheme in summer 1959. During the 1960s, European work stopped, due to an abundance of energy and low oil prices, but recommenced in the 1980s. Field tests were conducted in 1981 at Bruay-en-Artois, in 1983–1984 at La Haute Deule, France, in 1982–1985 at Thulin, Belgium and in 1992–1999 at the El Tremedal site, Province of Teruel, Spain. In 1988, the Commission of the European Communities and six European countries formed a European Working Group.

In New Zealand, a small scale trial was operated in 1994 in the Huntly Coal Basin. In Australia, tests were conducted starting in 1999. China has operated the largest program since the late 1980s, including 16 trials.

Process

The underground coal gasification process.

Underground coal gasification converts coal to gas while still in the coal seam (in-situ). Gas is produced and extracted through wells drilled into the unmined coal seam. Injection wells are used to supply the oxidants (air, oxygen) and steam to ignite and fuel the underground combustion process. Separate production wells are used to bring the product gas to the surface. The high pressure combustion is conducted at temperature of 700–900 °C (1,290–1,650 °F), but it may reach up to 1,500 °C (2,730 °F).

The process decomposes coal and generates carbon dioxide (CO
₂), hydrogen (H
₂), carbon monoxide (CO) and methane (CH
₄). In addition, small quantities of various contaminants including sulfur oxides (SO
_x), mono-nitrogen oxides (NO
_x), and hydrogen sulfide (H
₂S) are produced. As the coal face burns and the immediate area is depleted, the volumes of oxidants injected are controlled by the operator.

There are a variety of designs for underground coal gasification, all of which provide a means of injecting oxidant and possibly steam into the reaction zone, and also provide a path for production gases to flow in a controlled manner to the surface. As coal varies considerably in its resistance to flow, depending on its age, composition and geological history, the natural permeability of the coal to transport the gas is generally not adequate. For high pressure break-up of the coal, hydro-fracturing, electric-linkage, and reverse combustion may be used in varying degrees.

The simplest design uses two vertical wells: one injection and one production. Sometimes it is necessary to establish communication between the two wells, and a common method is to use reverse combustion to open internal pathways in the coal. Another alternative is to drill a lateral well connecting the two vertical wells. UCG with simple vertical wells, inclined wells, and long deflected wells was used in the Soviet Union. The Soviet UCG technology was further developed by Ergo Exergy and tested at Linc's Chinchilla site in 1999–2003, in Majuba UCG plant (2007) and in Cougar Energy's failed UCG pilot in Australia (2010).

In the 1980s and 1990s, a method known as CRIP (controlled retraction and injection point) was developed (but not patented) by the Lawrence Livermore National Laboratory and demonstrated in the United States and Spain. This method uses a vertical production well and an extended lateral well drilled directionally in the coal. The lateral well is used for injection of oxidants and steam, and the injection point can be changed by retracting the injector.

Carbon Energy was the first to adopt a system which uses a pair of lateral wells in parallel. This system allows a consistent separation distance between the injection and production wells, while progressively mining the coal between the two wells. This approach is intended to provide access to the greatest quantity of coal per well set and also allows greater consistency in production gas quality.

A new technology has been announced in May 2012 by developer Portman Energy wherein a method called SWIFT (Single Well Integrated Flow Tubing) uses a single vertical well for both oxidant delivery and syngas recovery. The design has a single casing of tubing strings enclosed and filled with an inert gas to allow for leak monitoring, corrosion prevention and heat transfer. A series of horizontally drilled lateral oxidant delivery lines into the coal and a single or multiple syngas recovery pipeline(s) allow for a larger area of coal to be combusted at one time. The developers claim this method will increase syngas production by up to ten (10) times above earlier design approaches. The single well design means development costs are significantly lower and the facilities and wellheads are concentrated at a single point reducing surface access roads, pipelines and facilities footprint.[9] The UK patent office have advised that the full patent application GB2501074 by Portman Energy be published 16 October 2013.

A wide variety of coals are amenable to the UCG process and coal grades from lignite through to bituminous may be successfully gasified. A great many factors are taken into account in selecting appropriate locations for UCG, including surface conditions, hydrogeology, lithology, coal quantity, and quality. According to Andrew Beath of CSIRO Exploration & Mining other important criteria include:

• Depth of 100–600 metres (330–1,970 ft)
• Thickness more than 5 metres (16 ft)
• Ash content less than 60%
• Minimal discontinuities
• Isolation from valued aquifers.

According to Peter Sallans of Liberty Resources Limited, the key criteria are:

• Depth of 100–1,400 metres (330–4,590 ft)
• Thickness more than 3 metres (9.8 ft)
• Ash content less than 60%
• Minimal discontinuities
• Isolation from valued aquifers.

Economics

Underground coal gasification allows access to coal resources that are not economically recoverable by other technologies, e.g., seams that are too deep, low grade, or that have a thin stratum profile. By some estimates, UCG will increase economically recoverable reserves by 600 billion tonnes. Lawrence Livermore National Laboratory estimates that UCG could increase recoverable coal reserves in the US by 300%. Livermore and Linc Energy claim that UCG capital and operating costs are lower than those for traditional mining.

UCG product gas is used to fire combined cycle gas turbine (CCGT) power plants, with some studies suggesting power island efficiencies of up to 55%, with a combined UCG/CCGT process efficiency of up to 43%. CCGT power plants using UCG product gas instead of natural gas can achieve higher outputs than pulverized-coal-fired power stations (and associated upstream processes), resulting in a large decrease in greenhouse gas (GHG) emissions.

UCG product gas can also be used for:

• Synthesis of liquid fuels;
• Manufacture of chemicals, such as ammonia and fertilizers;
• Production of synthetic natural gas;
• Production of hydrogen.

In addition, carbon dioxide produced as a by-product of underground coal gasification may be re-directed and used for enhanced oil recovery.

Underground product gas is an alternative to natural gas and potentially offers cost savings by eliminating mining, transport, and solid waste. The expected cost savings could increase given higher coal prices driven by emissions trading, taxes, and other emissions reduction policies, e.g. the Australian Government's proposed Carbon Pollution Reduction Scheme.

Projects

Cougar Energy and Linc Energy conducted pilot projects in Queensland, Australia based on UCG technology provided by Ergo Exergy until their activities were banned in 2016. Yerostigaz, a subsidiary of Linc Energy, produces about 1 million cubic metres (35 million cubic feet) of syngas per day in Angren, Uzbekistan. The produced syngas is used as fuel in the Angren Power Station.

In South Africa, Eskom (with Ergo Exergy as technology provider) is operating a demonstration plant in preparation for supplying commercial quantities of syngas for commercial production of electricity. African Carbon Energy has received environmental approval for a 50 MW power station near Theunissen in the Free State province and is bid-ready to participate in the DOE's Independent Power Producer (IPP) gas program where UCG has been earmarked as a domestic gas supply option.

ENN has operated a successful pilot project in China.

In addition, there are companies developing projects in Australia, UK, Hungary, Pakistan, Poland, Bulgaria, Canada, US, Chile, China, Indonesia, India, South Africa, Botswana, and other countries. According to the Zeus Development Corporation, more than 60 projects are in development around the world.

Environmental and social impacts

Eliminating mining eliminates mine safety issues. Compared to traditional coal mining and processing, the underground coal gasification eliminates surface damage and solid waste discharge, and reduces sulfur dioxide (SO
₂) and nitrogen oxide (NO
_x) emissions. For comparison, the ash content of UCG syngas is estimated to be approximately 10 mg/m³ compared to smoke from traditional coal burning where ash content may be up to 70 mg/m³. However, UCG operations cannot be controlled as precisely as surface gasifiers. Variables include the rate of water influx, the distribution of reactants in the gasification zone, and the growth rate of the cavity. These can only be estimated from temperature measurements, and analyzing product gas quality and quantity.

Subsidence is a common issue with all forms of extractive industry. While UCG leaves the ash behind in the cavity, the depth of the void left after UCG is typically greater than that with other methods of coal extraction.

Underground combustion produces NO
_x and SO
₂ and lowers emissions, including acid rain.

Regarding emissions of atmospheric CO
₂, proponents of UCG have argued that the process has advantages for geologic carbon storage. Combining UCG with CCS (Carbon capture and storage) technology allows re-injecting some of the CO
₂ on-site into the highly permeable rock created during the burning process, i.e. the cavity where the coal used to be. Contaminants, such as ammonia and hydrogen sulfide, can be removed from product gas at a relatively low cost.

However, as of late 2013, CCS had never been successfully implemented on a commercial scale as it was not within the scope of UCG projects and some had also resulted in environmental concerns. In Australia in 2014 the Government filed charges over alleged serious environmental harm stemming from Linc Energy's pilot Underground Coal Gasification plant near Chinchilla in the Queensland's foodbowl of the Darling Downs. When UCG was banned in April, 2016 the Queensland Mines Minister Dr Anthony Lynham stated "The potential risks to Queensland's environment and our valuable agricultural industries far outweigh any potential economic benefits. UCG activity simply doesn't stack up for further use in Queensland."

Meanwhile, as an article in the Bulletin of Atomic Sciences pointed out in March 2010 that UCG could result in massive carbon emissions. "If an additional 4 trillion tonnes [of coal] were extracted without the use of carbon capture or other mitigation technologies atmospheric carbon-dioxide levels could quadruple", the article stated, "resulting in a global mean temperature increase of between 5 and 10 degrees Celsius".

Aquifer contamination is a potential environmental concern. Organic and often toxic materials (such as phenol) could remain in the underground chamber after gasification if the chamber is not decommissioned. Site decommissioning and rehabilitation are standard requirements in resources development approvals whether that be UCG, oil and gas, or mining, and decommissioning of UCG chambers is relatively straightforward. Phenol leachate is the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. The US Dept of Energy's Lawrence Livermore Institute conducted an early UCG experiment at very shallow depth and without hydrostatic pressure at Hoe Creek, Wyoming. They did not decommission that site and testing showed contaminants (including the carcinogen benzene) in the chamber. The chamber was later flushed and the site successfully rehabilitated. Some research has shown that the persistence of minor quantities of these contaminants in groundwater is short-lived and that ground water recovers within two years. Even so, proper practice, supported by regulatory requirements, should be to flush and decommission each chamber and to rehabilitate UCG sites.

Newer UCG technologies and practices claim to address environmental concerns, such as issues related to groundwater contamination, by implementing the "Clean Cavern" concept. This is the process whereby the gasifier is self-cleaned via the steam produced during operation and also after decommissioning. Another important practice is maintaining the pressure of the underground gasifier below that of the surrounding groundwater. The pressure difference forces groundwater to flow continuously into the gasifier and no chemical from the gasifier can escape into the surrounding strata. The pressure is controlled by the operator using pressure valves at the surface.

Coalbed methane

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Coalbed_methane 

 

A German methane detector used for coal mining in the 1960s.

Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Australia, and other countries.

The term refers to methane absorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lack of hydrogen sulfide. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal (called the matrix). The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water.

Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural-gas condensate. It often contains up to a few percent carbon dioxide. Coalbed methane is generally formed due to thermal maturation of kerogen and organic matter. However, coal seams with regular groundwater recharge see methane generated by microbial communities living in situ.

History

Coalbed methane grew out of venting methane from coal seams. Some coal beds have long been known to be "gassy," and as a safety measure, boreholes were drilled into the seams from the surface, and the methane allowed to vent before mining.

Coalbed methane as a natural-gas resource received a major push from the US federal government in the late 1970s. Federal price controls were discouraging natural gas drilling by keeping natural gas prices below market levels; at the same time, the government wanted to encourage more gas production. The US Department of Energy funded research into a number of unconventional gas sources, including coalbed methane. Coalbed methane was exempted from federal price controls, and was also given a federal tax credit.

In Australia, commercial extraction of coal seam gas began in 1996 in the Bowen Basin of Queensland.

Reservoir properties

Gas contained in coal bed methane is mainly methane and trace quantities of ethane, nitrogen, carbon dioxide and few other gases. Intrinsic properties of coal as found in nature determine the amount of gas that can be recovered.

Porosity

Coalbed methane reservoirs are considered as a dual-porosity reservoirs. Dual porosity reservoirs are reservoirs in which porosity related to cleats (natural fractures) are responsible for flow behavior and reservoir porosity of the matrix is responsible for the storage of gas. The porosity of a coalbed methane reservoir can vary from 10%-20%; However, the cleat porosity of the reservoir is estimated to be in the range of 0.1%-1%

Adsorption capacity

Adsorption capacity of coal is defined as the volume of gas adsorbed per unit mass of coal usually expressed in SCF (standard cubic feet, the volume at standard pressure and temperature conditions) gas/ton of coal. The capacity to adsorb depends on the rank and quality of coal. The range is usually between 100 and 800 SCF/ton for most coal seams found in the US. Most of the gas in coal beds is in the adsorbed form. When the reservoir is put into production, water in the fracture spaces is pumped off first. This leads to a reduction of pressure enhancing desorption of gas from the matrix.

Fracture permeability

Fracture permeability acts as the major channel for the gas to flow. The higher the permeability, the higher the gas production. For most coal seams found in the US, the permeability lies in the range of 0.1–50 milliDarcys. The permeability of fractured reservoirs changes with the stress applied to them. Coal displays a stress-sensitive permeability and this process plays an important role during stimulation and production operations. Fracture permeability in Coalbed methane reservoir tends to increase with depletion of gas; in contrast to conventional reservoirs. This unique behavior is because of shrinking of coal, when methane is released from its matrix, which results in opening up of fractures and increased permeability. It is also believed that due to shrinkage of coal matrix at lower reservoir pressures, there is a loss of horizontal stress in the reservoir which induces in-situ failure of coal. Such a failure has been attributed to sudden decrease in the fracture permeability of the reservoir

Thickness of formation and initial reservoir pressure

The thickness of the formation may not be directly proportional to the volume of gas produced in some areas.

For example, it has been observed in the Cherokee Basin in Southeast Kansas that a well with a single zone of 1 to 2 feet (0.3 to 0.6 m) of pay can produce excellent gas rates, whereas an alternative formation with twice the thickness can produce next to nothing. Some coal (and shale) formations may have high gas concentrations regardless of the formation's thickness, probably due to other factors of the area's geology.

The pressure difference between the well block and the sand face should be as high as possible as is the case with any producing reservoir in general.

Other properties

Other affecting parameters include coal density, initial gas-phase concentration, critical gas saturation, irreducible water saturation, relative permeability to water and gas at conditions of Sw = 1.0 and Sg = 1-Sw irreducible respectively.

Extraction

Diagram of a coalbed methane well (US DOE)

 

Typical production profile of a coalbed methane well (USGS)

 

Main article: Coal bed methane extraction

To extract the gas, a steel-encased hole is drilled into the coal seam 100 to 1,500 metres (330 to 4,920 ft) below ground. As the pressure within the coal seam declines due to natural production or the pumping of water from the coalbed, both gas and produced water come to the surface through tubing. Then the gas is sent to a compressor station and into natural gas pipelines. The produced water is either reinjected into isolated formations, released into streams, used for irrigation, or sent to evaporation ponds. The water typically contains dissolved solids such as sodium bicarbonate and chloride but varies depending on the formation geology.

Coalbed methane wells often produce at lower gas rates than conventional reservoirs, typically peaking at near 300,000 cubic feet (8,500 m³) per day (about 0.100 m³/s), and can have large initial costs. The production profiles of CBM wells are typically characterized by a "negative decline" in which the gas production rate initially increases as the water is pumped off and gas begins to desorb and flow. A dry CBM well is similar to a standard gas well.

The methane desorption process follows a curve (of gas content vs. reservoir pressure) called a Langmuir isotherm. The isotherm can be analytically described by a maximum gas content (at infinite pressure), and the pressure at which half that gas exists within the coal. These parameters (called the Langmuir volume and Langmuir pressure, respectively) are properties of the coal, and vary widely. A coal in Alabama and a coal in Colorado may have radically different Langmuir parameters, despite otherwise similar coal properties.

As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and restricting gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.

The potential of a particular coalbed as a CBM source depends on the following criteria. Cleat density/intensity: cleats are joints confined within coal sheets. They impart permeability to the coal seam. A high cleat density is required for profitable exploitation of CBM. Also important is the maceral composition: maceral is a microscopic, homogeneous, petrographic entity of a corresponding sedimentary rock. A high vitrinite composition is ideal for CBM extraction, while inertinite hampers the same.

The rank of coal has also been linked to CBM content: a vitrinite reflectance of 0.8–1.5% has been found to imply higher productivity of the coalbed.

The gas composition must be considered, because natural gas appliances are designed for gas with a heating value of about 1,000 BTU (British thermal units) per cubic foot, or nearly pure methane. If the gas contains more than a few percent non-flammable gases such as nitrogen or carbon dioxide, either these will have to be removed or it will have to be blended with higher-BTU gas to achieve pipeline quality. If the methane composition of the coalbed gas is less than 92%, it may not be commercially marketable.

Environmental impacts

Methane

As with all carbon-based fossil fuels, burning coalbed methane releases carbon dioxide (CO₂) into the atmosphere. Its effect as greenhouse gas was first analyzed by chemist and physicist Svante Arrhenius. CBM production also entails leaks of fugitive methane into the atmosphere. Methane is rated as having 72 times the effect on global warming per unit of mass than CO_2. over 20 years, reducing to 25 times over 100 years and 7.5 times over 500 years. Analysis of life-cycle greenhouse gas emissions of energy sources indicates that generating electricity from CBM, as with conventional natural gas, has less than half the greenhouse gas effect of coal.

Multiple Australian studies have indicated the long term negative environmental effects of coal seam gas extraction, both locally and globally.

In the United States, methane escaping from coal during mining amounts to seven percent of total methane emissions. Recovery of coal mine methane in advance of mining is seen as a major opportunity to reduce methane emissions. Companies like CNX Resources have methane abatement programs to reduce greenhouse gas emissions from active and closed mines.

Infrastructure

CBM wells are connected by a network of roads, pipelines, and compressor stations. Over time, wells may be spaced more closely in order to extract the remaining methane.

Produced water

The produced water brought to the surface as a byproduct of gas extraction varies greatly in quality from area to area, but may contain undesirable concentrations of dissolved substances such as salts, naturally present chemicals, heavy metals and radionuclides. In many producing regions the water is treated, such as through a Reverse Osmosis plant and used beneficially for irrigation, water for livestock, urban and industrial uses, or dust suppression.

Pilliga Scrub

In 2012 Eastern Star Gas was fined for "discharging polluting water containing high levels of salt into Bohena Creek" in the Pilliga Scrub. There were "16 spills or leaks of contaminated water" including "serious spills of saline water into woodland and a creek." In 2012, a NSW Legislative Council inquiry criticised the use of open holding ponds, recommending that "the NSW Government ban the open storage of produced water."

Powder River Basin

Not all coalbed methane produced water is saline or otherwise undesirable. Water from coalbed methane wells in the Powder River Basin of Wyoming, US, commonly meets federal drinking water standards, and is widely used in the area to water livestock. Its use for irrigation is limited by its relatively high sodium adsorption ratio.

Groundwater

Depending on aquifer connectivity, water withdrawal may depress aquifers over a large area and affect groundwater flows. In Australia, the CBM industry estimates extraction of 126,000 million litres (3.3×10¹⁰ US gallons) to 280,000 million litres (7.4×10¹⁰ US gallons) of groundwater per year; while the National Water Commission estimates extraction above 300,000 million litres (7.9×10¹⁰ US gallons) a year.

Power generation

In 2012, the Aspen Skiing Company built a 3-megawatt methane-to-electricity plant in Somerset, Colorado at Oxbow Carbon's Elk Creek Mine.

Coalbed methane producing areas

Australia

See also: Coal in Australia

Coal Seam Gas resources are in the major coal basins in Queensland and New South Wales, with further potential resources in South Australia. Commercial recovery of coal seam gas (CSG) began in Australia in 1996. As of 2014, coal seam gas, from Queensland and New South Wales, made up about ten percent of Australia's gas production. Demonstrated reserves were estimated to be 33 trillion cubic feet (35 905 petajoules) as of January 2014.

• Bowen Basin
• Surat Basin
• Sydney Basin

Canada

In Canada, British Columbia is estimated to have approximately 90 trillion cubic feet (2.5 trillion cubic metres) of coalbed gas. Alberta, in 2013, was the only province with commercial coalbed methane wells and is estimated to have approximately 170 trillion cubic feet (4.8 trillion cubic metres) of economically recoverable coalbed methane, with overall reserves totaling up to 500 trillion cubic feet (14 trillion cubic metres).

Coalbed methane is considered a non-renewable resource, although the Alberta Research Council, Alberta Geological Survey and others have argued coalbed methane is a renewable resource because the bacterial action that formed the methane is ongoing. This is subject to debate since it has also been shown that the dewatering that accompanies CBM production destroys the conditions needed for the bacteria to produce methane and the rate of formation of additional methane is undetermined. This debate is currently causing a right of ownership issue in the Canadian province of Alberta, as only non-renewable resources can legally be owned by the province.

United Kingdom

Although gas in place in Britain's coal fields has been estimated to be 2,900 billion cubic meters, it may be that as little as one percent might be economically recoverable. Britain's CBM potential is largely untested. Some methane is extracted by coal mine venting operations, and burned to generate electricity. Assessment by private industry of coalbed methane wells independent of mining began in 2008, when 55 onshore exploration licences were issued, covering 7,000 square kilometers of potential coalbed methane areas. IGas Energy became the first in the UK to commercially extract coalbed methane separate from mine venting; as of 2012, the Igas coalbed methane wells at Doe Green, extracting gas for electrical generation, were the only commercial CBM wells in the UK. The use of CBM (in GWh) for electricity generation in the UK is as shown.

United States

Main article: Coalbed methane in the United States

United States coalbed methane production in 2017 was 1.76 trillion cubic feet (TCF), 3.6 percent of all US dry gas production that year. The 2017 production was down from the peak of 1.97 TCF in 2008. Most CBM production came from the Rocky Mountain states of Colorado, Wyoming, and New Mexico.

Kazakhstan

Kazakhstan could witness the development of a large coalbed methane (CBM) sector over the coming decades, according to industry professionals. Preliminary research suggests there may be as much as 900 billion m3 of gas in Kazakhstan's main coalfields – 85% of all reserves in Kazakhstan.

India

Great Eastern Energy (GEECL) was the first company with a field development plan approved. With the completion of the drilling of 23 vertical production wells by GEECL, coalbed methane became commercially available in India on 14 July 2007 with CNG priced at ₹30 per kg. Initially 90% of the CBM would be distributed as CNG gas to fuel vehicles.

GEECL is responsible for Southeast Asia's first CBM station and is also locating one in the West Bengal city of Asansol.

Prashant Modi, President and Chief Operating Officer of GEECL, said, "With the nation requiring higher energy sources to sustain its development pace, we are confident that CBM will play an important role as one of the prime energy sources for the future generations."

Essar Group's Essar Oil and Gas Exploration and Production Ltd.'s CBM portfolio includes 5 blocks. Currently, only one of them, Raniganj East, is currently operational. The others include Rajmahal in Jharkhand, Talcher and Ib Valley in Odisha and Sohagpur in Madhya Pradesh. The 5 blocks possess an estimated 10 trillion cubic feet (CBF) of CBM reserves.

Ocean deoxygenation

From Wikipedia, the free encyclopedia

 

Global map of low and declining oxygen levels in the open ocean and coastal waters. The map indicates coastal sites where anthropogenic nutrients have resulted in oxygen declines to less than 2 mg/L (red dots), as well as ocean oxygen minimum zones at 300 metres (blue shaded regions).

Ocean deoxygenation is the reduction of the oxygen content of the global oceans and coastal zones due to human activities as a consequence of anthropogenic emissions of carbon dioxide and eutrophication-driven excess production. It is manifest in the increasing number of coastal and estuarine hypoxic areas, or dead zones, and the expansion of oxygen minimum zones (OMZs) in the world's oceans. The decrease in oxygen content of the oceans has been fairly rapid and poses a threat to all aerobic marine life, as well as to people who depend on marine life for nutrition or livelihood.

Oceanographers and others have discussed what phrase best describes the phenomenon to non-specialists. Among the options considered have been ocean suffocation (which was used in a news report from May 2008), "ocean oxygen deprivation", "decline in ocean oxygen", "marine deoxygenation", "ocean oxygen depletion" and "ocean hypoxia". The term “Ocean Deoxygenation” has been used increasingly by international scientific bodies because it captures the decreasing trend of the world ocean's oxygen inventory.

Overview

Oxygen is input into the ocean at the surface, through the processes of photosynthesis by phytoplankton and mixing with the atmosphere. However, organisms, both microbial and multicellular, use oxygen in respiration throughout the entire depth of the ocean, so when the supply of oxygen from the surface is less than the utilization of oxygen in deep water, oxygen loss occurs. This phenomenon is natural, but is exacerbated with increased stratification and/or temperature. Stratification occurs when water masses with different properties, primarily temperature and salinity, are layered, with lower density water on top of higher density water. The larger the differences in the properties between layers, the less mixing occurs between the layers. Stratification is increased when the temperature of the surface ocean or the amount of freshwater input into the ocean from rivers and ice melt increases, enhancing ocean deoxygenation by limiting supply. Another factor that can limit supply is the solubility of oxygen. As temperature and salinity increase, the solubility of oxygen decreases, meaning that less oxygen can be dissolved into water as it warms and becomes more salty.

Ocean deoxygenation is an additional stressor on marine life. Ocean deoxygenation results in the expansion of oxygen minimum zones in the oceans as a consequence of burning fossil fuels. Along with this ocean deoxygenation is caused by an imbalance of sources and sinks of oxygen in dissolved water. Burning fossil fuels consumes oxygen, but because of the large atmospheric oxygen inventory, the associated relative decline of atmospheric oxygen (0.001% per year) is about two orders of magnitude smaller than the current rate of ocean deoxygenation. The change has been fairly rapid and poses a threat to fish and other types of marine life, as well as to people who depend on marine life for nutrition or livelihood. Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats. Total ocean oxygen content has decreased by 1-2% since 1960.

Ocean warming exacerbates ocean deoxygenation and further stresses marine organisms, limiting nutrient availability by increasing ocean stratification through density and solubility effects while at the same time increasing metabolic demand. The rising temperatures in the oceans cause a reduced solubility of oxygen in the water, which can explain about 50% of oxygen loss in the upper level of the ocean (>1000 m). According to the IPCC 2019 Special Report on the Ocean and Cryosphere in a Changing Climate, the viability of species is being disrupted throughout the ocean food web due to changes in ocean chemistry. As the ocean warms, mixing between water layers decreases, resulting in less oxygen and nutrients being available for marine life. Warmer ocean water holds less oxygen and is more buoyant than cooler water. This leads to reduced mixing of oxygenated water near the surface with deeper water, which naturally contains less oxygen. Warmer water also raises oxygen demand from living organisms; as a result, less oxygen is available for marine life.

Melting of gas hydrates in bottom layers of water may result in the release of more methane from sediments and subsequent consumption of oxygen by aerobic respiration of methane to carbon dioxide. Another effect of climate change on oceans that causes ocean deoxygenation is circulation changes. As the ocean warms from the surface, stratification is expected to increase, which shows a tendency for slowing down ocean circulation, which then increases ocean deoxygenation. Coastal regions, such as the Baltic Sea, the northern Gulf of Mexico, and the Chesapeake Bay, as well as large enclosed water bodies like Lake Erie, have been affected by deoxygenation due to eutrophication. Excess nutrients are input into these systems by rivers, ultimately from urban and agricultural runoff and exacerbated by deforestation. These nutrients lead to high productivity that produces organic material that sinks to the bottom and is respired. The respiration of that organic material uses up the oxygen and causes hypoxia or anoxia.

Oceanic oxygen minimum zones (OMZ) generally occur in the middle depths of the ocean, from 100 – 1000 m deep, and are natural phenomena that result from respiration of sinking organic material produced in the surface ocean. However, as the oxygen content of the ocean decreases, oxygen minimum zones are expanding both vertically and horizontally.

As low oxygen zones expand vertically nearer to the surface, they can affect coastal upwelling systems such as the California Current on the coast of Oregon (US). These upwelling systems are driven by seasonal winds that force the surface waters near the coast to move offshore, which pulls deeper water up along the continental shelf. As the depth of the deoxygenated deeper water becomes shallower, more of the deoxygenated water can reach the continental shelf, causing coastal hypoxia and fish kills. Impacts of massive fish kills on the aquaculture industry are projected to be profound.

Overall, local Harmful Algal Blooms (HAB), regional dead zones, and the oceanic phenomena of oxygen minimum zones are contributing to ocean-wide deoxygenation.

Global extent

Ocean deoxygenation has led to suboxic, hypoxic, and anoxic conditions in both coastal waters and the open ocean. Since 1950, more than 500 sites in coastal waters have reported oxygen concentrations below 2 mg liter⁻¹, which is generally accepted as the threshold of hypoxic conditions. Several areas of the open ocean have naturally low oxygen concentration due to biological oxygen consumption that cannot be supported by the rate of oxygen input to the area from physical transport, air-sea mixing, or photosynthesis. These areas are called oxygen minimum zones (OMZs), and there is a wide variety of open ocean systems that experience these naturally low oxygen conditions, such as upwelling zones, deep basins of enclosed seas, and the cores of some mode-water eddies. Oxygen-poor waters of coastal and open ocean systems have largely been studied in isolation of each other, with researchers focusing on eutrophication-induced hypoxia in coastal waters and naturally occurring (without apparent direct input of anthropogenic nutrients) open ocean OMZs. However, coastal and open ocean oxygen-poor waters are highly interconnected and therefore both have seen an increase in the intensity, spatial extent, and temporal extent of deoxygenated conditions.

Drivers of hypoxia and ocean acidification intensification in upwelling shelf systems. Equatorward winds drive the upwelling of low dissolved oxygen (DO), high nutrient, and high dissolved inorganic carbon (DIC) water from above the oxygen minimum zone. Cross-shelf gradients in productivity and bottom water residence times drive the strength of DO (DIC) decrease (increase) as water transits across a productive continental shelf.

The spatial extent of deoxygenated conditions can vary widely. In coastal waters, regions with deoxygenated conditions can extend from less than one to many thousands of square kilometers. Open ocean OMZs exist in all ocean basins and have similar variation in spatial extent; an estimated 8% of global ocean volume is within OMZs. The largest OMZ is in the eastern tropical north Pacific and comprises 41% of this global volume, and the smallest OMZ is found in the eastern tropical North Atlantic and makes up only 5% of the global OMZ volume. The vertical extent of low oxygen conditions is also variable, and areas of persistent low oxygen have annual variation in the upper and lower limits of oxygen-poor waters. Typically, OMZs are expected to occur at depths of about 200 to 1,000 meters. The upper limit of OMZs is characterized by a strong and rapid gradient in oxygenation, called the oxycline. The depth of the oxycline varies between OMZs, and is mainly affected by physical processes such as air-sea fluxes and vertical movement in the thermocline depth. The lower limit of OMZs is associated with the reduction in biological oxygen consumption, as the majority of organic matter is consumed and respired in the top 1,000 m of the vertical water column. Shallower coastal systems may see oxygen-poor waters extend to bottom waters, leading to negative effects on benthic communities. The temporal duration of oxygen-poor conditions can vary on seasonal, annual, or multi-decadal scales. Hypoxic conditions in coastal systems like the Gulf of Mexico are usually tied to discharges of rivers, thermohaline stratification of the water column, wind-driven forcing, and continental shelf circulation patterns. As such, there are seasonal and annual patterns in the initiation, persistence, and break down of intensely hypoxic conditions. Oxygen concentrations in open oceans and the margins between coastal areas and the open ocean may see variation in intensity, spatial extent, and temporal extent from multi-decadal oscillations in climatic conditions.

Measurement of dissolved oxygen in coastal and open ocean waters for the past 50 years has revealed a marked decline in oxygen content. This decline is associated with expanding spatial extent, expanding vertical extent, and prolonged duration of oxygen-poor conditions in all regions of the global oceans. Examinations of the spatial extent of OMZs in the past through paleoceanographical methods clearly shows that the spatial extent of OMZs has expanded through time, and this expansion is coupled to ocean warming and reduced ventilation of thermocline waters. Many persistent OMZs have increased in thickness over the last five decades through both shoaling of the upper limit and downward expansion of the lower limit. Coastal regions have also seen expanded spatial extent and temporal duration due to increased anthropogenic nutrient input and changes in regional circulation. Areas that have not previously experienced low oxygen conditions, like the coastal shelf of Oregon on the West coast of the United States, have recently and abruptly developed seasonal hypoxia.

The global decrease in oceanic oxygen content is statistically significant and emerging beyond the envelope of natural fluctuations. This trend of oxygen loss is accelerating, with widespread and obvious losses occurring after the 1980s. The rate and total content of oxygen loss varies by region, with the North Pacific emerging as a particular hotspot of deoxygenation due to the increased amount of time since its deep waters were last ventilated (see thermohaline circulation) and related high apparent oxygen utilization (AOU). Estimates of total oxygen loss in the global ocean range from 119 to 680 T mol decade⁻¹ since the 1950s. These estimates represent 2% of the global ocean oxygen inventory. Modeling efforts show that global ocean oxygen loss rates will continue to accelerate up to 125 T mol year⁻¹ by 2100 due to persistent warming, a reduction in ventilation of deeper waters, increased biological oxygen demand, and the associated expansion and shoaling of OMZs.

Climate change

Further information: Effects of climate change on oceans

Most of the excess heat from CO₂ and other greenhouse gas emissions is absorbed by the oceans. Warmer oceans cause deoxygenation both because oxygen is less soluble in warmer water, and through temperature driven stratification of the ocean which inhibits the production of oxygen from photosynthesis.

The ocean surface stratifies as the atmosphere and ocean warms causing ice melt and glacial runoff. This results in a less salty and therefore a less dense layer that floats on top. Also the warmer waters themselves are less dense. This stratification inhibits the upwelling of nutrients (the ocean constantly recycles its nutrients) into the upper layer of the ocean. This is where the majority of oceanic photosynthesis (such as by phytoplankton) occurs. This decrease in nutrient supply is likely to decrease rates of photosynthesis in the surface ocean, which is responsible for approximately half of the oxygen produced globally. Increased stratification can also decrease the supply of oxygen to the interior of the ocean. Warmer waters also increase the metabolism of marine organisms, leading to increased respiration rates. In the surface ocean, increased respiration will likely lead to lower net oxygen production, and thus less oxygen transferred to the atmosphere. In the interior ocean, the combination of increased respiration and decreased oxygen supply from surface waters can draw oxygen down to hypoxic or anoxic levels. Not only are low levels of oxygen lethal to fish and other upper trophic level species, they can change the microbially mediated cycling of globally important elements (see microbiology of oxygen minimum zones such as nitrogen; nitrate replaces oxygen as the primary microbial electron acceptor at very low oxygen concentrations. All this, increased demand on herbivores, decreased nutrient supply, decreased dissolved oxygen, etc., result in food web mismatches.

Implications

Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats. Studies have shown that oceans have already lost 1-2% of their oxygen since the middle of the 20th century, and model simulations predict a decline of up to 7% in the global ocean O₂ content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more.

Effects on bioavailability

Diagram of oxygen partial pressure in the environment vs. the inside of a fish gill

Bioavailability is a measure of how readily a substance in the environment (generally a molecular substance) can be obtained by an organism. For the aquatic sciences, bioavailability of oxygen is particularly important since it describes the amount of oxygen (supply) relative to the requirements (demand) of a specific organism whereas concentrations describe just the amount of oxygen in the water.

Oxygen supply

The supply of oxygen available to aquatic organisms is determined by oxygen solubility in water. Oxygen is less soluble in warm, salty water and more soluble in cold, fresh water. The amount of oxygen dissolved in water can be measured as a concentration, percent saturation, or partial pressure. While all these measures tell us how much oxygen is present, partial pressure is the most accurate measure when considering aquatic organisms since the partial pressure of a gas determines how readily it will diffuse across a membrane. If the partial pressure of oxygen is higher on the external, water side of a gill membrane than on the internal, bloodstream side, oxygen will more readily diffuse across the membrane and into the fish.

Oxygen demand

An organism's demand for oxygen is dependent on its metabolic rate. Metabolic rates can be affected by external factors such as the temperature of the water, and internal factors such as the species, life stage, size, and activity level of the organism. The body temperature of ectotherms (such as fishes and invertebrates) fluctuates with the temperature of the water. As the external temperature increases, ectotherm metabolisms increase as well, increasing their demand for oxygen. Different species have different basal metabolic rates and therefore different oxygen demands.

Life stages of organisms also have different metabolic demands. In general, younger stages tend to grow in size and advance in developmental complexity quickly. As the organism reaches maturity, metabolic demands switch from growth and development to maintenance, which requires far fewer resources. Smaller organisms have higher metabolisms per unit of mass, so smaller organisms will require more oxygen per unit mass, while larger organisms generally require more total oxygen. Higher activity levels also require more oxygen.

This is why bioavailability is important in deoxygenated systems: an oxygen quantity which is dangerously low for one species might be more than enough for another species.

Indices and calculations

Several indices to measure bioavailability have been suggested: Respiration Index, Oxygen Supply Index, and the Metabolic Index. The Respiration Index describes oxygen availability based on the free energy available in the reactants and products of the stoichiometric equation for respiration. However, organisms have ways of altering their oxygen intake and carbon dioxide release, so the strict stoichiometric equation is not necessarily accurate. The Oxygen Supply Index accounts for oxygen solubility and partial pressure, along with the Q₁₀ of the organism, but does not account for behavioral or physiological changes in organisms to compensate for reduced oxygen availability. The Metabolic Index accounts for the supply of oxygen in terms of solubility, partial pressure, and diffusivity of oxygen in water, and the organism's metabolic rate. The metabolic index is generally viewed as a closer approximation of oxygen bioavailability than the other indices.

There are two thresholds of oxygen required by organisms:

Respiration- Pcrit and Pleth

• P_crit (critical partial pressure)- the oxygen level below which an organism cannot support a normal respiration rate
• P_leth (lethal partial pressure)- the oxygen level below which an organism cannot support the minimum respiration rate necessary for survival.

Since bioavailability is specific to each organism and temperature, calculation of these thresholds is done experimentally by measuring activity and respiration rates under different temperature and oxygen conditions, or by collecting data from separate studies.

Impacts of climate change

In the context of ocean deoxygenation, species experience the impacts of low oxygen at different levels and rates of oxygen loss, as driven by their thresholds of oxygen tolerance. Generally, we can expect mobile species to move away from areas of lethally low oxygen, but they also experience non-lethal effects of exposure to low oxygen. Further examples are included in the body of this page.

Bioavailability will continue to change in a changing climate. Warming water temperatures mean low solubility of oxygen and increasing deoxygenation, while also increasing the oxygen demands of ectothermic organisms by driving their metabolic rates higher. This positive feedback loop compounds the effects of reduced oxygen concentrations.

Effects on marine taxa

See also: Human impact on marine life

Microbes

In OMZs oxygen concentration drops to levels <10 nM at the base of the oxycline and can remain anoxic for over 700 m depth. This lack of oxygen can be reinforced or increased due to physical processes changing oxygen supply such as eddy-driven advection, sluggish ventilation, increases in ocean stratification, and increases in ocean temperature which reduces oxygen solubility. At a microscopic scale the processes causing ocean deoxygenation rely on microbial aerobic respiration. Aerobic respiration is a metabolic process that microorganisms like bacteria or archaea use to obtain energy by degrading organic matter, consuming oxygen, producing CO₂ and obtaining energy in the form of ATP. In the ocean surface photosynthetic microorganisms called phytoplankton use solar energy and CO₂ to build organic molecules (organic matter) releasing oxygen in the process. A large fraction of the organic matter from photosynthesis becomes dissolved organic matter (DOM) that is consumed by bacteria during aerobic respiration in sunlit waters. Another fraction of organic matter sinks to the deep ocean forming aggregates called marine snow. These sinking aggregates are consumed via degradation of organic matter and respiration at depth. At depths in the ocean where no light can reach, aerobic respiration is the dominant process. When the oxygen in a parcel of water is consumed, the oxygen cannot be replaced without the water reaching the surface ocean. When oxygen concentrations drop to below <10 nM, microbial processes that are normally inhibit by oxygen can take place like denitrification and anammox. Both processes extract elemental nitrogen from nitrogen compounds and that elemental nitrogen which does not stay in solution escapes as a gas, resulting in a net loss of nitrogen from the ocean.

Zooplankton

Decreased oxygen availability results in decreases in many zooplankton species’ egg production, food intake, respiration, and metabolic rates. Temperature and salinity in areas of decreased oxygen concentrations also affect oxygen availability. Higher temperatures and salinity lower oxygen solubility decrease the partial pressure of oxygen. This decreased partial pressure increases organisms’ respiration rates, causing the oxygen demand of the organism to increase.

In addition to affecting their vital functions, zooplankton alter their distribution in response to hypoxic or anoxic zones. Many species actively avoid low oxygen zones, while others take advantage of their predators’ low tolerance for hypoxia and use these areas as a refuge. Zooplankton that exhibit daily vertical migrations to avoid predation and low oxygen conditions also excrete ammonium near the oxycline and contribute to increased anaerobic ammonium oxidation (anammox, which produces N₂ gas. As hypoxic regions expand vertically and horizontally, the habitable ranges for phytoplankton, zooplankton, and nekton increasingly overlap, increasing their susceptibility to predation and human exploitation.

The relationship between zooplankton and low oxygen zones is complex and varies by species and life stage. Some gelatinous zooplankton reduce their growth rates when exposed to hypoxia while others utilize this habitat to forage on high prey concentrations with their growth rates unaffected. The ability of some gelatinous zooplankton to tolerate hypoxia may be attributed to the ability to store oxygen in intragel regions. The movements of zooplankton as a result of ocean deoxygenation can affect fisheries, global nitrogen cycling, and trophic relationships. These changes have the potential to have large economic and environmental consequences through overfishing or collapsed food webs.

Fish

Further information: Hypoxia in fish

A fish's behavior in response to ocean deoxygenation is based upon their tolerance to oxygen poor conditions. Species with low anoxic tolerance tend to undergo habitat compression in response to the expansion of OMZs. Low tolerance fish start to habitate near the surface of the water column and ventilate at the top layer of the water where it contains higher levels of dissolved oxygen, a behavior called aquatic surface respiration. Biological responses to habitat compression can be varied. Some species of billfish, predatory pelagic predators such as sailfish and marlin, that have undergone habitat compression actually have increased growth since their prey, smaller pelagic fish, experienced the same habitat compression, resulting in increased prey vulnerability to billfishes. Fish with tolerance to anoxic conditions, such as jumbo squid and lanternfish, can remain active in anoxic environments at a reduced level, which can improve their survival by increasing avoidance of anoxia intolerant predators and have increased access to resources that their anoxia intolerant competitors cannot.

Effects on coastal habitats

In many places, coral reefs are experiencing worse hypoxia which can lead to bleaching and mass coral die-offs.

 

See also: Marine habitats

Coral reefs

There has been a severe increase in mass mortality events associated with low oxygen causing mass hypoxia with the majority having been in the last 2 decades. The rise in water temperature leads to an increase in oxygen demand and the increase for ocean deoxygenation which causes these large coral reef dead zones. For many coral reefs, the response to this hypoxia is very dependent on the magnitude and duration of the deoxygenation. The symptoms can be anywhere from reduced photosynthesis and calcification to bleaching. Hypoxia can have indirect effects like the abundance of algae and spread of coral diseases in the ecosystems. While coral is unable to handle such low levels of oxygen, algae is quite tolerant. Because of this, in interaction zones between algae and coral, increased hypoxia will cause more coral death and higher spread of algae. The increase mass coral dead zones is reinforced by the spread of coral diseases. Coral diseases can spread easily when there are high concentrations of sulfide and hypoxic conditions. Due to the loop of hypoxia and coral reef mortality, the fish and other marine life that inhabit the coral reefs have a change in behavioral in response to the hypoxia. Some fish will go upwards to find more oxygenated water, and some enter a phase of metabolic and ventilatory depression. Invertebrates migrate out of their homes to the surface of substratum or move to the tips of arborescent coral colonies.

Around 6 million people, the majority who live in developing countries, depend on coral reef fisheries. These mass die-offs due to extreme hypoxic events can have severe impacts on reef fish populations. Coral reef ecosystems offer a variety of essential ecosystem services including shoreline protection, nitrogen fixation, and waste assimilation, and tourism opportunities. The continued decline of oxygen in oceans on coral reefs is concerning because it takes many years (decades) to repair and regrow corals.

Seagrass beds

Globally, seagrass has been declining rapidly. It is estimated that 21% of the 71 known seagrass species have decreasing population trends and 11% of those species have been designated as threatened on the ICUN Red List. Hypoxia that leads to eutrophication caused form ocean deoxygenation is one of the main underlying factors of these die-offs. Eutrophication causes enhanced nutrient enrichment which can result in seagrass productivity, but with continual nutrient enrichment in seagrass meadows, it can cause excessive growth of microalgae, epiphytes and phytoplankton resulting in hypoxic conditions.

Seagrass is both a source and a sink for oxygen in the surrounding water column and sediments. At night, the inner part of seagrass oxygen pressure is linearly related to the oxygen concentration in the water column, so low water column oxygen concentrations often result in hypoxic seagrass tissues, which can eventually kill off the seagrass. Normally, seagrass sediments must supply oxygen to the below-ground tissue through either photosynthesis or by diffusing oxygen from the water column through leaves to rhizomes and roots. However, with the change in seagrass oxygen balances, it can often result in hypoxic seagrass tissues. Seagrass exposed to this hypoxic water column show increased respiration, reduced rates of photosynthesis, smaller leaves, and reduced number of leaves per shoot. This causes insufficient supply of oxygen to the belowground tissues for aerobic respiration, so seagrass must rely on the less-efficient anaerobic respiration. Seagrass die-offs create a positive feedback loop in which the mortality events cause more death as higher oxygen demands are created when dead plant material decomposes.

Because hypoxia increases the invasion of sulfides in seagrass, this negatively affects seagrass through photosynthesis, metabolism and growth. Generally, seagrass is able to combat the sulfides by supplying enough oxygen to the roots. However, deoxygenation causes the seagrass to be unable to supply this oxygen, thus killing it off.

Deoxygenation reduces the diversity of organisms inhabiting seagrass beds by eliminating species that cannot tolerate the low oxygen conditions. Indirectly, the loss and degradation of seagrass threatens numerous species that rely on seagrass for either shelter or food. The loss of seagrass also effects the physical characteristics and resilience of seagrass ecosystems. Seagrass beds provide nursery grounds and habitat to many harvested commercial, recreational, and subsistence fish and shellfish. In many tropical regions, local people are dependent on seagrass associated fisheries as a source of food and income.

Seagrass also provides many ecosystem services including water purification, coastal protection, erosion control, sequestration and delivery of trophic subsidies to adjacent marine and terrestrial habitats. Continued deoxygenation causes the effects of hypoxia to be compounded by climate change which will increase the decline in seagrass populations.

Shrimp ponds in mangrove forests like these leave massive amounts of water pollution and compounds the negative effects of deoxygenation in mangrove forests.

Mangrove forests

Compared to seagrass beds and coral reefs, hypoxia is more common on a regular basis in mangrove ecosystems, through ocean deoxygenation is compounding the negative effects by anthropogenic nutrient inputs and land use modification.

Like seagrass, mangrove trees transport oxygen to roots of rhizomes, reduce sulfide concentrations, and alter microbial communities. Dissolved oxygen is more readily consumed in the interior of the mangrove forest. Anthropogenic inputs may push the limits of survival in many mangrove microhabitats. For example, shrimp ponds constructed in mangrove forests are considered the greatest anthropogenic threat to mangrove ecosystems. These shrimp ponds reduce estuary circulation and water quality which leads to the promotion of diel-cycling hypoxia. When the quality of the water degrades, the shrimp ponds are quickly abandoned leaving massive amounts of wastewater. This is a major source of water pollution that promotes ocean deoxygenation in the adjacent habitats.

Due to these frequent hypoxic conditions, the water does not provide habitats to fish. When exposed to extreme hypoxia, ecosystem function can completely collapse. Extreme deoxygenation will affect the local fish populations, which are an essential food source. The environmental costs of shrimp farms in the mangrove forests grossly outweigh their economic benefits. Cessation of shrimp production and restoration of these areas and reduce eutrophication and anthropogenic hypoxia.