Sources, types, and volumes of waste discharges
Practically all stages and operations of offshore hydrocarbon production are accompanied by undesirable discharges of liquid, solid, and gaseous wastes.
The proportions and amounts of discharged wastes can change considerably during production. For example, the amount of solid drilling cuttings usually decreases as the well gets deeper and the hole diameter becomes correspondingly smaller. The volumes of produced waters increase as the hydrocarbon resources are being depleted and production moves from the first stages toward its completion. Drilling in the upper layers of bottom sediments (up to approximately 100 m) can be done without using complex drilling fluids. In such cases, seawater with additives of special clay suspensions can be used instead.
The discharges of produced waters considerably dominate over other wastes. Produced waters include formation water, brine, injection water, and other technological waters. Formation water and brine are extracted along with oil and gas. Injection water is pumped into the injection wells in hundreds of thousands of tons for maintaining the pressure in the system and pushing the hydrocarbons toward the producing wells. All of these waters are usually polluted by oil, natural low-molecular-weight hydrocarbons, inorganic salts, and technological chemicals. These waters need to be cleaned before they are discharged into the sea. Such cleaning under marine conditions is a complicated technical task. Special separation units on the platforms are used for oil separation. Depending on its quality, the produced water is either discharged into the sea or injected into the disposal well. Sometimes the oil-water mixtures are transported along the pipelines to onshore separation units.
Produced waters, including injection waters and solutions of chemicals used to intensify hydrocarbon extraction and the separation of the oil-water mixtures, are one of the main sources of oil pollution in the areas of offshore oil and gas production. It is significant that, as a hydrocarbon reservoir is being depleted, the ratio between the water and oil fraction in the extracted product increases, and water becomes the prevailing phase. At the same time, both the volumes of discharged waters and the difficulties of their treatment increase.
Inevitably, all kinds of drilling are associated with drilling wastes, including drilling muds and cuttings. Drilling cuttings are removed from drilling muds and cleaned in special separators. The amount of oil left on cuttings after cleaning is much higher when using oil-based fluids. Separated drilling muds and cleaning fluids used to treat cuttings are partially returned to the circulating system. Drilling cuttings and the rest of the drilling muds are either dumped overboard or transported to the shore for further treatment and disposal, depending on the situation and ecological requirements. The first variant is the most usual and is practiced almost everywhere, while the second one still remains an unrealized (or seldom realized) ecological requirement.
Recently, a technology was developed to remove the drilling wastes, especially cuttings, by reinjecting their slurry into a geological formation. This gives some hope to achieving zero discharge of oil-containing wastes during offshore oil and gas production. Some other measures (such as slim-hole drilling) to reduce discharges, particularly in environmentally sensitive locations, are being investigated by the industry.
The environmental hazard of drilling muds is connected, in particular, with the presence of lubricating materials in their composition. These lubricating substances usually have a hydrocarbon base. They are needed for effective drilling, especially in case of slant holes or drilling through solid rock. The lubricants are added into the drilling fluids either from the very beginning as a part of the original formulations or in the process of drilling when the operational need emerges. In both cases, the discharges of spent drilling muds and cuttings coated by these muds contain considerable amounts of relatively stable and toxic hydrocarbon compounds and a wide spectrum of many other substances.
One of the potential sources of oil pollution is produced sand extracted with oil. The amount of produced sand coated by oil can vary a lot in different areas and even during production in the same area. In some cases, it constitutes a considerable part of the extracted product. Most often, this sand is cleaned of the oil and dumped overboard at the well site. Sometimes, it is baked or calcified and transported to the shore.
The other discharges into the marine environment (deck drainage, sanitary and domestic wastes, and so on), do not play essential roles in the environmental situation in the areas of oil and gas developments. They are treated and disposed in accordance with the norms regulating discharges from the ships.
Chemical composition of discharged wastes
As noted earlier, the spectrum of chemicals entering the marine environment at different stages of oil and gas production is very wide. They include many hundreds of individual compounds and their combinations. Broadly speaking, all can be divided into two large groups. The first group consists of the extracted oil and gas hydrocarbons, which the following chapters will discuss in detail. The second group, which this section will review, unites the rest of the natural and technological components used at different technological stages.
Drilling fluids and cuttings. Drilling wastes deserve special attention. The volume of drilling wastes usually ranges from 1,000 to 5,000 m3 for each well. Such wells can number into dozens for one production platform and many hundreds for a large field.
Drilling cuttings separated from drilling muds have a complex and extremely changeable composition. This composition depends on the type of rock, drilling regime, formulation of the drilling fluid, technology to separate and clean cuttings, and other factors. However, in all cases, drilling fluids (muds) play the leading role in forming the composition of drilling cuttings.
No precise, standard formulation exists for drilling fluids. Their composition depends on the needs of the particular situations. These differ considerably in different regions and may even radically change during each drilling process while drilling rocks of very different structure (from solid granite formations to salt and slate strata). At present, two main types of drilling fluids are used in offshore drilling. They are based either on crude oil, oil products, and other mixtures of organic substances (diesel, paraffin oils, and so on) or on water (freshwater or seawater with bentonite, barite, and other components added). During the last 10 years, the preference is given to using the less-toxic water-based drilling fluids. However, in some cases, for example during drilling of deviated wells through hard rock, using oil-based fluids is still inevitable. The oil-based fluids, in contrast with the water-based ones, are usually not discharged overboard after a single application. Instead, they are regenerated and included in the technological circle again.
Originally, the oil-based drilling muds included diesel fuel as their base component due to its availability and low cost. However, starting in the 1980s, especially after many countries prohibited the use of diesel in drilling muds, the oil companies started to develop new formulations that replaced diesel oil with less hazardous substances. Alternative drilling fluids are composed mainly from low-molecular-weight, less toxic and more water-soluble, aromatic compounds and substances of paraffin structure. Research in this direction continues at present. Products of animal, vegetable, or synthetic origin are tested in order to find the optimal base for drilling fluids.
Recently, a new generation of drilling fluids based on the products of chemical synthesis with ethers, esters, olefins, and polyalphaolefins has been developed [Burke, Veil, 1995]. Such drilling fluids allow highly deviational or horizontal drillings to be conducted. From the environmental perspective, the most important fact is that they have low toxicity as compared with other drilling formulations. In spite of the relatively high cost of the synthetic-based drilling fluids, their technological and environmental advantages open wide possibilities for their effective use in oil and gas production.
Each component of a drilling fluid has one or several chemical and technological functions. For example, barite (BaSO4) is used to control and regulate hydrostatic pressure in the well. Emulsifiers (alkyl-acrylate sulfonate, alkylacryl sulfate, and others) form and maintain emulsions. Sodium and calcium chlorides create conditions for maintaining an isotonic osmotic balance between the water phase of the emulsion and surrounding formation water. Organophilic clays (such as amine treated bentonite clay) as well as organic polymers and polyacrylates ensure the optimal fluid viscosity necessary for drilling under different geological conditions. Sodium sulfite, ammonium bisulfite, zinc carbonate, and other oxygen scavengers are pumped into the well to prevent the corrosion of drilling equipment in the oxidizing environment. Lime is added to increase the pH of drilling fluids, which helps to reduce corrosion and stabilize the emulsions in the muds.
As a result of many technological operations and procedures, drilling muds and cuttings are saturated with hundreds of very different substances and compounds. It is their discharges into the sea that pose one of the main ecological threats during offshore oil production. In particular, many countries express concern regarding biocides, which are used to suppress microflora in the drilling and other circulating fluids. The list of such compounds includes over one hundred names. The most widespread biocides used in the oil and gas production practice include sodium salts of hypochlorite, formalin releasers, and glutaraldehyde as well as biguanidine and quaternary ammonium, and a number of other compounds. The composition of some compounds is not always known. Some biocides are highly toxic. Many countries either discourage (for example, in case of carbamates and thiocarbamates) or prohibit (for example, in case of dichlorophenols and pentachlorophenates) their use by the offshore oil and gas industry.
Drilling discharges also contain many heavy metals (mercury, lead, cadmium, zinc, chromium, copper, and others) that come from components of both drilling fluids and drilling cuttings. Chapter 6 gives the ecotoxicological assessments and comparison of different drilling fluids and drilling cuttings.
Produced waters. Produced waters usually include dissolved salts and organic compounds, oil hydrocarbons, trace metals, suspensions, and many other substances that are components of formation water from the reservoir or are used during drilling and other production operations. Besides, produced waters can mix with the extracted oil, gas, and injection waters from the wells. All of the above make the composition of the discharged produced waters very complex and changeable. It is practically impossible to speak about some average parameters of this composition, especially because reliable and complete analytical studies of these wastes are very rare.
Petroleum hydrocarbons are always present in produced waters, especially when the latter are mixed with other technological waters and solutions. However, the levels of oil in discharges vary extremely. They depend not only on the specific technological situation but on the fractional composition of the oil and the effectiveness of the oil/water separation methods as well. The oil separators mainly remove particulate and dispersed oil, while dissolved hydrocarbons in concentrations from 20 mg/l to over 50 mg/l go overboard as part of the discharged waters [Somerville et al., 1987; GESAMP, 1993]. The volumes of such discharges reach thousands of tons of oil a year.
Another characteristic of the chemical composition of most produced waters is their very high mineralization. It is usually higher than the seawater's salinity reaching up to 300 g/l. Such mineralization is caused by the presence of dissolved ions of sodium, potassium, magnesium, chloride, and sulfate in produced waters. Besides, produced waters often have elevated levels of some heavy metals [Neff et al., 1987] as well as corrosion inhibitors, descalers, biocides, dispersants, emulsion breakers, and other chemicals.
Recent studies have revealed that produced waters frequently contain naturally occurring radioactive elements and their daughter products, such as radium-226 and radium-228. They are leached from the reservoir by formation waters and are carried to the surface with produced waters, oil, and gas. During contact with seawater, these radionuclides interact with sulfates, precipitate, and form a radioactive scale. In spite of a relatively low level of radioactivity, concern exists that this process can create centers of increased radioactive risk. This phenomenon has become a focus of attention in a number of countries. Applying the regulations defined by some international agreements, such as the London Dumping Convention (1972), that do not allow discharges of radioactive material into the marine environment are considered to be justified in this case [GESAMP, 1993].
Other wastes. Large quantities of produced waters, drilling muds, and drilling cuttings, discussed above, as well as discharges of storage displacement and ballast waters are the source of regular and long-term impacts of the offshore industry on the marine environment. Besides these discharges, sometimes the need arises to conduct a one-time discharge of short duration. Such situations include, in particular, chemical discharges during construction, hydrostatic testing, commissioning, pigging, and maintenance of the pipeline systems. The pipeline discharges usually contain corrosion and scale inhibitors, biocides, oxygen scavengers, and other agents. The volumes of these wastes can be rather considerable. In the North Sea, they reach up to 300,000 m3 of treated water discharged over a short period (hours to days) [GESAMP, 1993]. The discharge regime usually ensures that the dilution decreases the concentration and toxicity of the wastes to safe levels beyond a 500-meter radius from the place of discharge [Davies, Kingston, 1992].
Similar situations emerge during other technological and maintenance activities. Examples include cleaning and anticorrosion procedures, discharging the ballast waters from the hydrocarbon storage tanks, well repairing, well workover operations, replacing the equipment, and others. These discharges often contain surface-active substances, such as lignosulfonates, lignites, sulfo-methylated tannins, and many other chemicals with about a hundred names.
Although the atmospheric emissions accompany most of the oil and gas operations, this factor has not gained any special attention in the context of offshore developments. The available information is very limited and controversial. At the same time, in some areas of onland production, for example in Western Siberia and near Astrakhan in Russia, this source of pollution poses a serious threat to the water and onland ecosystems and to human health. For example, in the Nizhnevartovki region (Tumen area), the atmospheric emission of hazardous substances from the Samotlorskoe oil field development in 1989-1992 varied from 0.38 million to 1.1 million tons a year [Krupinin, 1995]. The high content of hydrogen sulfide (6-30%) and other toxic substances in the natural gas and atmospheric emissions on the Orenburgskoe and Astrakhanskoe gas condensate fields created situations close to ecological catastrophes [Karamova, 1989].
Atmospheric emissions take place at all stages of oil and gas industry's activities. The main sources of these emissions include:
- constant or periodical burning of associated gas and excessive amounts of hydrocarbons during well testing and development as well as continuous flaring to eliminate gas from the storage tanks and pressure-controlling systems;
- combustion of gaseous and liquid fuel in the energetic units (diesel-powered generators and pumps, gas turbines, internal combustion engines) on the platforms, ships, and onshore facilities; and
- evaporation or venting of hydrocarbons during different operations of their production, treatment, transportation, and storage.
In spite of the fact that some countries now prohibit flaring of oil-associated gases, it remains one of the major sources of atmospheric emissions in the world. These gases are dissolved in the crude produced oil. As the pressure goes down, they bubble out in amounts up to 300 m3 for each ton of extracted oil. The associated gases give about 30% of the gross world production of gaseous hydrocarbons. However, because of the undeveloped technology and lack of required capacities and equipment on many field developments, up to 25% of all associated gases are flared. In Russia alone, the volumes of annually burned (flared) oil-associated gases reach up to 10-17 billion cubic meters [VNIIP, 1994]. Astronauts have witnessed that the view of the gas-burning torches, for example above Western Siberia or the Persian Gulf, is an impressive proof of the large scale of human economic activity and, we would add, of its bad management as well.
Components of atmospheric pollution caused by oil and gas development include gaseous products of hydrocarbon evaporation and burning as well as aerosol particles of the unburned fuel. From the ecological perspective, the most hazardous components are nitrogen and sulfur oxides, carbon monoxide, and the products of the incomplete burning of hydrocarbons. These interact with atmospheric moisture, transform under the influence of solar radiation, and precipitate onto the land and sea surfaces to form fields of local and regional pollution.
Clear evidence of the impact of atmospheric emissions on the marine environment from the offshore flaring was found, in particular, during well testing in the Canadian zone of the Beaufort Sea. Here, the ice surface around the test site where intensive flaring of combustible wastes occurred was polluted by atmospheric fallout of heavy oily residue. The chemical composition of the residue was similar to one of the higher-molecular-weight fractions of produced oil [GESAMP, 1993].
According to some estimates [Kingston, 1991], up to 30% of the hydrocarbons emitted into the atmosphere during well testing precipitate onto the sea surface and create distinctive and relatively unstable slicks around the offshore installations. The results of the aircraft observations in the North Sea indicate that such slicks are found with an average frequency of 1-2 cases per every hour of flight [ICES, 1995].
Technical means to rectify and prevent atmospheric pollution during offshore oil and gas production are practically identical to the analogous methods that are widely and often effectively used on land and in other industries. However, offshore atmospheric emissions thus far have not gotten the deserved attention, probably due to the remoteness of these developments from densely populated places.