Composition and sources of natural gas in the water
Natural gas is closely related to crude oil. Both substances are thought to have formed in the earth's crust as a result of transformation of organic matter due to the heat and pressure of the overlying rock. All oil deposits contain natural gas, although natural gas is often found without oil. Gas hydrocarbons can also be produced as a result of microbial decomposition of organic substances and, less often, due to reduction of mineral salts. Many of these gases are released into the atmosphere or hydrosphere, or they accumulate in the upper layers of the earth's crust.
The composition of natural gas varies. It depends on the origin, type, genesis, and location of the deposit, geological structure of the region, and other factors. Natural gas chiefly consists of saturated aliphatic hydrocarbons, i.e., methane and its homologues. The deeper the location of gas deposit, the higher the number of methane homologues. In gas condensate fields, the content of methane homologues is usually considerably higher than the level of methane. In gases associated with oil, the content of methane homologues is comparable with the content of methane. Large amount of gases associated with oil is dissolved in this oil. During oil extraction, as the pressure goes down, gases come to the surface of the oil. They are released in the environment in volumes of 30-300 m3 for every ton of extracted oil. These gases give about 30% of the gross total production of combustible gases in the world. However, over 25% of this amount are flared off because of the absence of the needed capacities and equipment for gas collection and processing.
Other components commonly found in natural gas are carbon dioxide, hydrogen sulfide, nitrogen, and helium. Usually, they constitute an insignificant proportion of natural gas composition. However, in some areas, their concentrations can be considerably higher.
Besides the previously mentioned sources of natural gas (transformations of organic matter in the earth's crust, microbial decomposition of organic substances, and reduction of mineral salts), gas hydrates are another extremely promising source of gas hydrocarbons on the sea bottom. According to some estimates [Zubova et al., 1990; Kellard, 1994], the reserves of gas hydrates are an order of magnitude higher than potential recoverable gas resources of all conventional fields in the world.
From the physicochemical point of view, gas hydrates can be considered as a modification of ice that has a high content of gas. They are solid crystallized substances that look like compressed snow. Hydrates form during the interaction of many components of natural gas (methane, ethane, propane, isobutane, carbon dioxide, and hydrogen sulfide) with water under certain combinations of high pressure and relatively low temperature.
Hydrate formation usually accompanies and complicates gas and oil extraction and transportation because hydrates can accumulate on the sides of wells and pipelines and thus plug them. The methods used to overcome these difficulties include pumping different inhibitors (methanol, glycol, and solutions of potassium chloride) into the wells and pipelines, dehydrating the gas, and heating it up to temperatures higher than the temperature of hydrate formation.
Similar to oil, gas enters the environment due to both natural and anthropogenic processes. Among the major mechanisms of methane natural production in the biosphere, the decomposition of organic matter by methane-producing bacteria (e.g., Methanococcus, Methanosarica) deserves a special mention. These bacteria are able to get the energy by reducing carbon dioxide in accordance with CO2 + 4H2 = CH4 + 2H2O reaction. These processes are typical for the silt deposits of lakes and marshes and for marine sediments that are lacking in oxygen and rich in organic matter.
Microbial methane formation in the oceans is usually accompanied by sulfur reduction and the release of hydrogen sulfide. These take place inside the upper part of sediments from the seafloor surface to tens and even hundreds of meters deep. In regions with a cold and moderate climate at depths of over 500 m, methane can accumulate in a form of crystal gas hydrates. In areas with a warmer climate, some methane from shallow formations is often released from the sediments into the water column and then into the atmosphere.
Methane can appear in the marine environment not only due to microbial and biochemical decomposition of the organic substance in bottom sediments. It can also occur as a result of the natural bottom seepage of combustible gases from shallow oil- and gas-bearing structures. Such seeping has been found in the Gulf of Mexico, North Sea, Black Sea, Sea of Okhotsk, and other marine areas. This process can lead to intensive vertical flows of hydrocarbon gases from the bottom to the sea surface. Sometimes it is accompanied by gas hydrate decomposition.
Over the last 100 years, the natural processes of biogeochemical production and distribution of methane in the biosphere are under large-scale anthropogenic impact. According to some estimates, anthropogenic sources contribute as much as 40-70% of methane into the global atmospheric flow of this gas [Novozhevnikova, 1995]. Large quantities of hydrocarbon gases are released during many kinds of anthropogenic activity. These include oil, gas, and coal production and transportation, burning of fossil fuels, intensive rice cultivation, animal farming, and garbage dumping. Lately, the increased levels of methane have been found even in areas of intensive aquaculture in the coastal waters. In these areas, methane could be formed as a result of decomposition of food residuals and metabolites of cultivated water organisms.
The global consequence of all these anthropogenic impacts is the gradual increase of methane concentration in the atmosphere over the last 100 years - from 0.7x10-4% to 1.7x10-4% (in volume). Many scientists believe that gases released due to human activities have already begun to affect the earth's overall temperature and the methane anthropogenic emission is responsible for about 30% of the total warming effect. If the concentrations of methane and other greenhouse gases in the atmosphere keep increasing, global changes in climatic conditions on the earth will be noticeable in the near future.
Another component of natural gas - hydrogen sulfide - is water soluble in contrast with methane. It can cause hazardous pollution situations in both the atmosphere and the water environment. Its proportion in the composition of natural gas and gas condensate, as previously mentioned, sometimes reaches more than 20%. Pollution by hydrogen sulfide can lead to disturbances in the chemical composition of surface waters. This gas belongs to the group of poisons with acute effects. Its appearance in the atmosphere and hydrosphere can cause serious economic damage and medical problems among local population. Unfortunately, in Russia, air, soil, and water pollution by hydrogen sulfide and sulfur dioxide has been reported in a number of regions. Especially severe consequences for human health and biota have been observed in the basin of the low Volga River in the zone of development of the Astrakhanskoe gas condensate field [Ecology and impact of natural gas on organisms, 1989].
The sources of atmospheric pollution also include flaring of natural gas on the offshore platforms and onland terminals. Some estimates [Cairns, 1992] show that about 10% of total gas production and up to 30% of associated gases are burned here. The behavior and distribution of the products of natural gas flaring in the atmosphere, their removal by precipitation, and the impact on the water environment have not been studied. The same situation is true regarding gas emissions at different stages of its production, transportation, and processing.
An important anthropogenic source of gas hydrocarbons in the water environment is the offshore drilling accidents. Their environmental consequences can be very hazardous. Especially dramatic situations developed in the Sea of Asov as a result of two large accidents on drilling rigs in the summer-autumn of 1982 and 1985. These accidents caused long-term releases of large amounts of natural gas into the water accompanied by self-inflaming of the gas. During these events, the levels of methane in surface waters exceeded the background concentrations up to 10-100 times. The air samples also showed very high concentrations of methane. These accidents drastically disturbed the composition and biomass of the water fauna and caused mass mortality of many organisms, including fish and benthic mollusks. Similar incidents probably took place in other regions of the world as well. However, there are no publications on this topic available.
Another potential source of gas in the hydrosphere is damaged gas pipelines, both on the seafloor and on land where they cross over rivers and other water bodies. The causes of such damage can vary from corrosion processes to natural disasters (severe ice conditions, seismic activity, and earthquakes). It should be noted that hydrocarbon gases are piped over great distances totaling many thousands of kilometers. These pipelines cross hundreds of water bodies. Possible pipeline damages can lead to hazardous impacts on water ecosystems. The negative fisheries consequences in such cases may go beyond the limits of local scale. Regional problems can emerge if, for example, an accidental gas blowout or leakage blocks the spawning migration of anadromous fish.
Methane impact on water organisms and communities
Water toxicology of saturated aliphatic hydrocarbons of the methane series has not been developed thus far. This gap cannot be filled by available materials on the toxicity of other gaseous poisons (e.g., carbon oxide, hydrogen sulfide, and ammonia) for fish. Clear behavioral specifics of each of these gases in the water environment do not allow us to extrapolate these data to predict the biological effects of methane and other saturated hydrocarbons. However, the toxicity data on different gaseous poisons can help to reveal some general features of interaction between gaseous traces and marine organisms [Patin, 1993].
The first important feature is the quick fish response to a toxic gas as compared with fish response to other dissolved or suspended toxicants. Gas rapidly penetrates into the organism (especially through the gills) and disturbs the main functional systems (respiration, nervous system, blood formation, enzyme activity, and others). External evidence of these disturbances includes a number of common symptoms mainly of behavioral nature (e.g., fish excitement, increased activity, scattering in the water). The interval between the moment of fish contact with the gas and the first symptoms of poisoning (latent period) is relatively short.
Further exposure leads to chronic poisoning. At this stage, cumulative effects at the biochemical and physiological levels occur. These effects depend on the nature of the toxicant, exposure time, and environmental conditions. A general effect typical for all fish is gas emboli. These emerge when different gases (including the inert ones) oversaturate water. The symptoms of gas emboli include the rupture of tissues (especially in fins and eyes), enlarging of swim bladder, disturbances of circulatory system, and a number of other pathological changes.
These general features of fish response observed in the presence of any gas in the water environment are likely to be found for saturated gas hydrocarbons as well. Available materials derived from the medical toxicology of methane and its homologues support this suggestion.
Medical toxicology distinguishes between three main types of intoxication by methane:
- light, results in reversible, quickly disappearing effects on the functions of central nervous and cardiovascular systems;
- medium, manifests itself in deeper functional changes in the central nervous and cardiovascular systems and increase in the number of leukocytes in the peripheral blood; and
- heavy, results in irreversible disturbances of the cerebrum, heart tissues, and alimentary canal as well as acute form of leukocytosis.
These types most likely adequately describe the general patterns of methane effects in vertebrates. However, its features in respect to ichthyofauna remain to be studied. Fish resistance to the presence of gas at different life stages is of special interest. With most toxicants, the most vulnerable periods are the early life stages. The question of whether this general pattern is typical for saturated hydrocarbons still remains open. The importance of this issue in assessing biological effects of natural gas in the water environment is quite obvious.
During toxicological studies of different gases, including methane and its derivatives, one must take into consideration the influence of other factors (especially temperature and oxygen regime) that can radically change the direction and symptoms of the effect. In particular, increasing temperature usually intensifies the toxic effect of practically all substances on fish because of the direct correlation between the level of fish metabolism and water temperature. From the physiological perspective, this can be explained not only by the general intensification of fish metabolism but also by the increased permeability of the tissues for the poisons and increased oxygen consumption under high temperatures. Thus, toxicant concentrations that do not cause any effect under low temperatures can become lethal with increasing water temperature. This circumstance should be taken into consideration during ecotoxicological assessment of the potential impact of natural gas and other toxicants, especially when studies are conducted in high latitudes. In such regions, methane hydrates may be accumulated during the winter and dissociate during the increased temperatures in the summer. This may be followed by the releasing of free methane with corresponding environmental consequences.
Another critical environmental factor that directly influences the gas impact on water organisms is the concentration of dissolved oxygen. Numerous studies show that the oxygen deficit directly controls the rate of fish metabolism and decreases their resistance to many organic and inorganic poisons. This decrease sometimes depends more on the species characteristics and the rate of their gas metabolism rather than on the nature of the poison. From the physiological perspective, such a phenomenon is explained by the fact that the level of hemoglobin in fish blood and the rate of blood circulation through the gills increase under oxygen deficit. Clearly, such effects are of special interest when interpreting the data on fish response to natural gas in situations of significant change in the oxygen regime (e.g., during eutrophication of water bodies or seasonal and weather variations of the oxygen content).