Surface processing involves all processes intended to produce the oil or gas from its sub-surface rock/sediment columns, separate the various components, purify them and present them in the standard, which is acceptable in the petroleum market. The method used to bring to the surface and to collect the oil/gas from under surface depends on the depth of the substrate rock, well pressure gradients ( natural flow, gas lifting and well stimulation methods arise) , stage of production ( whether primary, secondary or tertiary) among other factors. Most wells produce, in addition to oil and gas, mixtures rich in water and sometimes sand. While each of these components have varying density and initial separation may therefore be done through density defined procedures, this criteria alone is unable to obtain pure components, and is successful only as a matter of proportions. Additional separation requires dedicated separators able to obtain components with 99% purity.
Once the oil/gas/water/sand components have been separated and purification began, the gas is channel to the Gas Processing Plant while the oil is passed through a heat exchanger in preparation of fractionation into various solids, liquids and gases .For instance, gasoline, a commodity sold independently of other liquid hydro-carbons, is not obtainable directly from the initial surface production. It must be obtained through fractionation of the oil that comes from the wells. Not only that, the various markets require gasoline with different octane ratings and additional refining must be done to meet market specifications. The mode of transportation of different petroleum products depend on distinct considerations such as economies of scale, as well as safety regulations and corrosion management.
The surface/refining processing section will focus in the processing (both surface and refinery) procedures applicable in the Prudhoe Bay, including the types and number of wells to be sank, production methods, separation methods and finally, refining. The fractionating column as well as finished product channeling will be highlighted.
Processing (Surface and Refining)
Once initial prospecting is finalized and necessary equipment laid out in preparation for field production, well sinking will be carried out and the various products obtained. Recovery of under-surface fluids usually is a complicated process where the wellhead fluids are impure, existing in complex mixtures with different densities, pressures, flow rates and chemical and physical properties. The flow, called the well stream, is initially at high pressure and temperature as it exits the reservoir formation, cooling as it nears the surface. In addition, evaporation of the gaseous components of the stream upon reaching the surface causes significant physical and chemical changes to the products mix. Once the mixture has been extracted, additional processing is carried out in order to minimize corrosion as well as prevent equipment damage. Corrosion is mainly attributed to presence of carbon dioxide in the oil/gas substrate. It’s corrosive properties are magnified in under-surface wells where temperatures are higher and water vapour plenty, leading to formation of corrosive carbonic acids. The internal bore corrosion in the bay is expected to be from a carbon dioxide action. The conditions of high temperatures (about 200 degrees F) coupled with the relatively high CO2 presence in the reservoir gas of 12% may accelerate pipes corrosion. To mitigate this, chemical treatments will be added, including 13Cr tubing which has an excellent resistance (Jones 1971). Once the mixture is retrieved, the next step will involve separation.
The Prudhoe Bay Specifics
The Prudhoe Bay is an area covering 600 square miles with an oil/gas rich sedimentary rock basin at an average depth of 4000 feet and with varying thickness. The initially recoverable wellhead comprises of a gas/oil/water matrix trapped under sea-sloping shale and sitting on a water aquifer under higher pressure. The initial productions will utilize natural surface flow as the components are under high internal pressure. In later production stages, deeper seated oil/gas substrates maybe exploited through well stimulation and gas lifting techniques.
The initial operation will include sinking 600 wells of natural flow, 400 wells of gas lift as well as 300 wells with injectors. The injected wells will use the Water Alternating Gas (WAG) type equipment, with both water and gas being recycled from the natural well products. The injection will be up to 200 MMCFD, with the expected pressure build-ups up to 2000 psi. The natural flow wells will be maintained at 2400 psi, with corrosion resistance enhancements. The injection pipe tubing will have the diameters ranging from 3.5 to 7 inches to tolerate the high pressure amounts. The extended reach drilling (ERD) and Ultra Extended reach Drilling (UERD) will be used to sink wells as deep as 10000 feet and horizontal departures as much as 4000 feet from a main down-hole area.
The reservoir terrain may usually contain up to four materials: oil, gas, water and sand. The surface retrieved mixture must, therefore, be separated before the further processing. The water normally in oil basins is salinated and is usually 200-300 kg/ tone of production. In addition, dissolved minerals of 10 -15 kg per cubic meter may be present. Also, natural gas present in the production is in the range of 50 -80 M3/tone. These materials negatively affect the production as well as the transportation of oil, thus, they must be separated from the mixture. Sand, present in a suspended form in the mixture, is also separated before the further processing (British Petroleum 2006). These minerals largely contribute to corrosion of extraction, channelling and processing equipment. Sand, a common mixture component in development field where oil/gas lies in a sandstone basin, leads to severe damage in the transportation equipment.
Mixtures are very hard to transport, requiring special conditions in which safety and economic considerations are critical. For this reason, raw mixtures cannot be transported far from the wells before preliminary processing commences. Major operations conducted in preliminary processing include de-salting, de-sanding, dehydrating, and degassing the product stream. The section below focuses on the separation process.
The first stage of surface processing involves separation of wellhead components (Rucker and Strieter, 1992). Specific designed equipment called a separator does the separation work. Separators assume different shapes and can be oriented in different positions in order to effectively perform their work. The different shapes and orientations is mainly determined by the nature of the mixture they handle. The major types of separators as classified by shape are the spherical and the cylindrical. With respect to orientation the main types are horizontal and vertical. Horizontal separators are mainly preferable in production where the ratio of gas to liquid is high. This allows bigger likelihoods of gas to rise to surface, due to the small depth of rise as opposed to vertical separators where gas may have to rise a larger distance to float on the surface. Vertical separators are favoured where the gas to liquid ratio is smaller. Where the ratios are intermediate, a spherical separator is usually used to allow for greater mobility and enhance ease of separation. In addition, a series of separators maybe used in conditions where the initial wellhead pressure is high, in order to reduce the pressure in each successive separator.
The inter-relationship of the major forces in a separator is critical to the equipment’s efficiency. Gravity and inertia are the most effective forces in the separator environment. In addition hydro cyclone force, created by the centrifugal movement of some separators, is significant. Separators are classified according to operating pressure. The low category is usually 0.6MPascal, the median range is between 0.6Mpascal and 2.5Mpascal, while the high pressure category is above 2.5MPascal. The gas separation phases are also a differentiating factor between different separators. A two phase separation process involves separating the gas from the liquid components. A three phase process, however, includes additional dehydration of liquid hydrocarbons that may escape the first two separation stages. Tube Performance gas/oil separators, a comparatively recent development in comparison to the separators mentioned above, are a promising new equipment of use in surface processing due to ease of manufacture and flexible applicability in field situations.
Gas and Oil separators have varying capacity, with a median range between 500 and 20,000 cubic Meters daily, depending on their size, structure and the nature of incoming mixtures in terms of physical and chemical attributes. Separation takes place in four sections. The first stage involves obtaining free floating gas from oil, usually through application of density differentiating techniques. The second section involves precipitation, where partially dissolved gas as well as suspended gas bubbles is obtained from the gas/liquid matrix. The collection section retains the liquid coming from the separating section, while the final section, called the capture section, collects any tiny liquid droplets held in the separated gas. The efficiency of separators is measured by the amount of liquid droplets retained in the extracted gas, as well as the amount of gas suspended in the collected liquids. In order to enhance efficiency, oil is heated to increase fluidity and reduce density, while also allowing trapped gas to rise to surface. In addition, separation pressure is reduced in order to allow easier surface movement of lighter components. A typical separation should attain oil drops in gas mass of 0.05Kg/cubic meter of gas and entrapped gas volume of 0.5 cubic meters per tonne of oil. In circumstances where the separator contains a single section, there is the need for additional to enhance the purity of a product. The section below will discuss in detail separation process for the four main components of the wellhead including oil, gas, salinated water and sand.
Water is an expected by-product in the Prudhoe Bay oil production. The estimated volumes per metric ton of raw produce is between 200-300 Kg/t. water will be separated from the mixture in the oil dehydration sump. To do this, heating will take place in the sump followed by pumping of mixture and mixing with the 30-60 grams of a demulsifying agent. This will happen at the modular manifold before the emulsion has been sent to the heat exchange. Due to the infrastructural orientation of the Prudhoe Oil facility, a demulsifier injection will take place 100 meters before the mixture reaches the separator, where its effect will ease the dumping of water and surfactant to leave the oil behind. The water-oil separation requires heat in order to aid in demulsification. The mixture is pre-heated to 60 -70 degrees centigrade before entering a separator. The separated water exiting the oil sump will be re-used for injection purposes (Abdel-Aal 2004)
Prudhoe Bay will be using waterflooding as a deelopment technique. This procedure will result in large volumes of water running in the production basin and getting mixed with other wellhead components. The bulk of the waste water maybe recycled in future flooding events, this water is treated in the extraction site through tanks, pits, brine-receptors or compound separators. Skimming is done to recover additional water contained in the waste water before being reused for wellflooding or other purposes.
Gas separation will take place in two stages. The first stage will involve a temporary separation for the purposes of a component volumetric analysis in the Gas Metering station (GMS) just after the produce extraction. The impure components will then be recombined and sent through the collector to the booster pumping station for the gas extraction. The remaining components will mainly be partly the degassed oil and saline water mixture. Final degassing will be done at the Oil Treatment Facility (OTF) to obtain gas free oil. A modular manifold pressure will be sustained at between 1 – 1.5 Mega Pascal, high enough to allow gas to exit from the separator and flow to the Gas processing Plant (GPP). The separated gas will undergo dehydration through the exposure to aromatic hydrocarbons such as ethylene to absorb the water in commercial gas. In the later phases, commercial gas fractionation will be carried out to obtain different gas products (British Petroleum 2006).
From the separator, oil will be in the major part water free, but the complete dehydration will still be necessary. In addition, it is estimated that this oil will have 500-1000 mg/litre of salt, which needs to be extracted (Wilkirson et Al. 1988). Therefore, fresh water will be re-introduced in the oil/salt mixture to dissolve the mineral deposits in oil. This oil will then proceed to an electric dehydrator for a complete water removal. This final product will be ready for grading and packaging in the readiness for transit.
The Prudhoe Basin is based on sandstone as the oil rock. Sand will, therefore, be a component of produced mixture, mainly in minute fragments. Sand particles, being of the higher density and larger granular attributes than any other component, may be separated by a mass filtration and sedimentation on a small scale. The huge amount of filtrate obtainable per day in the production will require more modern and robust as well as faster means of separation. The proposed method is to run the oil in a stir loop and to add the Toluene and Imidazolium IL in order to form separation layers with sand and clay particles remaining at the bottom. The distillate will then be guided away under a gravitational free flow to leave the mineralized, but mainly sediment free oil ready for the electric de-mineralization and dehydration in the next stages.
The heat exchange is used to indirectly transfer heat from one oil column to another without any physical contact of the two media. In the oil refinery operation, oil is pre-heated in the heat exchanger area before being transported to the separator. This pre-heating helps in demulsification and prepares the oil/water/surfactant mixer for a next stage of separation. In the Prudhoe oil production, this installation will precede the separator and oil sump units and the pipeline joining the heat exchanger to the main separator will be about 100M long to allow the sufficient demulsification of extract components. To mitigate effects of fouling in the heat exchange, a regular chemical treatment of exchanger will be scheduled in order to eliminate or reduce the energy related losses associated with the product fouling in the exchanger (British Petroleum 2006).
The initial phases of the Prudhoe project may not involve commercial sales of gas. The factory installation will, however, be constructed to accommodate the fractional distillation of gas into various products of different octane rating, boiling points and densities. The heat exchange will be used to pre-heat the oil to temperatures in the range of 260-380 degrees in the preparation for channelling into the distillation column (Jones 1971). The intended distillation products will include heavy oil products such as lubricating oil, wax and asphalt (collected at the temperatures between 380-400 degrees), fuel oil (370 –320 degrees), diesel oils (less than 300 degrees), kerosene (collected at 200 degrees), gasoline (150 degrees) and lighter gases collected at less than 100 degrees. A detailed discussion of the refining section will be included below.
Oil and Gas Dehydration
Dehydration of oil is a complicated process especially because water and oil, though immiscible, can exist as a rather stable emulsion under certain conditions such as high pressure and temperature. This happens in oil wells, so that oil does not readily float in water even though they have a clear density difference. In addition, the water- oil emulsion does not contain pure water, but salinated water with mineral contents of values averaging 30Kg/ 1000Kg of water. Purification of oil is therefore a twofold process involving dehydration and desaltation. Water and minerals removal from oil is done in the oil treatment plants or in the refineries. This process typically involves breaking down the emulsion of oil-water and minerals components. The emulsions maybe of two types namely natural and artificial. The natural emulsions take place under the wells and are present during initial product extraction. These emulsions are easily broken in the surface processing equipment. Synthetic emulsions are those that result in the processing itself due to existence of conducive situations such as high pressure and temperature in the transportation channels, separators and treatment plants. Synthetic emulsions are typically broken through repeated ablution. During this process, pure water is passed through the oil repeatedly in order to dissolve any minerals still trapped in the oil. Typically, oil does not dissolve most of the minerals trapped within it. It is important to remove minerals from the oil in order to ensure the final product attains the specified purity, and protect the machines using the product from the corrosive effect of minerals contained in impure oil. After dehydration, the water percentage in the oil drops to 0.5 – 1.0%, while metal chlorides, the major residual mineral impurities in oil, drops to 100 – 800mg per litre in the initial stage of dehydration and demineralization. In the second phase, water drops to 0.05 – 0.1 %, while chlorides drop to 3.5mg per litre (OGMP).
Prudhoe Bay Oil Dehydration
A large number of older oil production facilities use the process of hot settling since it is cheaper and faster than other modern methods. In this process, the oil/water matrix will be pre-heated to 70 degrees C to allow quicker settling of heavier water drops. Other methods include the electric, chemical and thermo-chemical methods. Prudhoe Bay processes will use the chemical method. In this method, demulsifiers will be added in the oil to break the emulsion. Non ionic surfactants will be deployed into the emulsion in very small quantities typically 50g per tonne. These surfactants will dissolve the natural emulsifiers in the matrix. Action of these demulsifiers will lead the water drops to coalesce and, becoming larger, will easily drop to the bottom. In the final dehydration process, the high purity oil will be passed through electric dehydrator electrodes at high voltages of up to 30,000 volts. This high voltage will lead to separation of molecules based on polarity, where any charged cations / anions will deflect to one side. Desalting is an important process as it prolongs the life of processing plants and reduces frequency of equipment overhauls, as well as reduces losses associated with the process of oil-recycling. Natural gas will also be dried during transportation by passage through unsaturated hydrocarbon streams such as ethylene and glycol which dissolve any residual water particles in the gas. Hydrated glycol is re-dried and reused as a drying agent. Methanol will be added in small quantities in the natural gas line to act as an anti-freeze during transportation. It also will suppress the hydration point of natural gas in order to retain a stable product phase for easy transportation through pipelines as well as other methods such as tankers and containers.
Process Flow of Surface facilities
The Prudhoe Bay surface and refining processing will flow in a systematic way from the extraction to final transportation either by ship or pipeline. The process flow will start with the extraction machinery (well machinery) from where the product will flow to the separator. The separator section will have three sections for gas/water/oil/impurities separation. After the separator, different components will be channel differently. The gas will be passed on to the Gas Processing Plant for additional dehydration and additives injection. The water will be routed to the Water Treatment Plant (WTP) for skimming before re-use. The oil from the separator section will leave the manifold and enter the heat exchange section where its temperature will be raised to above 350 degrees in preparation for the fractionating column. Once it leaves the heat exchange, it will be admitted into the fractionation plant where it will undergo fractional distillation to recover the various distillation components. The final products will be ready for transportation and maybe channelled through the installed pipeline of other alternative travel methods.
After completion of water/gas/oil/sand separation, the non gas liquid (NGL) from the extraction are separated into chemically distinct products for different applications. This process happens in the fractionizer. However, oil must be passed through the heat exchange before being admitted into the fractionating equipment.Fractionation involves breaking down the non gas hydrocarbons (NGL) found during production into their base components. The underlying principle in hydrocarbon fractionation involves their different boiling points. In the fractionating column, different hydrocarbons boil off at different temperatures, and can be collected in separate chambers. Various types of fractionators exist, and they are usually named according to the major component they target. The fractionators are arranged as follows; deethenizer (extracts ethane), depropanizer (extracts propane), debutinizer (boils off butanes). The fractionation process separates the hydrocarbons starting with the lightest and proceeds to the highest.
Prudhoe Bay Fractionation
The proposed fractionation equipment will involve a pre-heat exchange system and full range fractionation column, extracting hydrocarbons within a four carbon range for each fraction. The base fraction will collect the heavy, high boiling point hydrocarbons with a carbon number higher than 20 such as waxes and asphalt at temperatures of 280-350 degrees C. High density oils with C numbers between 16 and 20 will be collected at temperatures between 350- 380 degrees, while fuel oils will be collected at ranges nearing 400 degrees. Kerosene range fuels have boiling points between 175 – 275 degrees, and have molecular carbons between 12 and 16. These will be collected before the petrol range of products, which have boiling points 20 and 200 degrees. This range of hydrocarbons has between 5 and 12 molecular carbons, being lighter than any previous fraction. The various fraction products in Prudhoe Bay will not be mixed again, but maybe transported independently either in separate containers, or, in the case of pipeline transport, in separate pipe chambers within the parent pipe into intended destinations.