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Oil drilling industry has been constantly on the front page of the international media for the last couple of years. Two of the main reasons were the price fluctuation of the market due to the economic crises and the Gulf of Mexico incident. A series of public debates have initiated with the main concern of stopping the exploration of oil in favour of other sources of energy, especially renewable energy. In addition, oil companies gained an unattractive reputation and the general public reached the false conclusion that the main responsibility for the current problems is held by the governments and the companies. The hypocrisy of the situation derives from the fact that the ultimate link in the consumption chain derives from day to day human needs and is dictated by the majority.
Therefore, the increase or decrease of exploitation due to environmental/public concern will have a major change on one of our most valuable assets: the energy. We can argue if we are or not prepared to make the switch, but although conservation and innovation can reduce the amount of oil used, and the peak of world oil production has been exceeded, oil will not fall from the top of the energy resources at any time soon. Offshore drilling remains as necessary as ever to the world's energy needs.
The search for oil, in one form or another has started hundred of years ago. The growth of the industry and the continuously growing demand of the population have pushed the 'oil rush' out in the oceans. For the last 40 or 50 years humankind has developed new methods to extract oil efficiently from the continental shelves and at large depths. Offshore drilling for oil and natural gas is "one of the greatest technological breakthroughs in recent decades" (1). Although onshore oil extraction has never been an easy task, providing oil from beneath the sea is even more difficult and risky.
Offshore drilling rigs must be installed in order for the drilling to be possible. There are more types of rigs, depending on the circumstances such as distance to the seabed and to the shore, location and size of the field and weather conditions. These are most commonly assembled at the site with the help of a barge equipped with heavy lift cranes. It is vital to choose or construct a platform as to get the most of the oil reservoir with both great concerns to the safety of the personnel and the environment.
"Types of offshore oil and gas structures include: 1, 2) conventional fixed platforms; 3) compliant tower; 4, 5) vertically moored tension leg and mini-tension leg platform; 6) SparÂ ; 7,8) Semi-submersiblesÂ ; 9) Floating production, storage, and offloading facility; 10) sub-sea completion and tie-back to host facility". (3)
There is no significant difference between offshore and onshore wells, except for the offshore environment concern due to the discharge of wastes. The main way by which offshore drilling is undertaken is by using rotary drilling to get to the crude oil reservoir. The link between the rig, situated at the water surface and the seabed is made through a series of steel pipes connected together. These pipes form a conduit called a "raiser". Throughout the riser, drilling fluids find their way between the rig and the well. The connectivity of the riser is made in a way that allows up and down movement and it can also "bend slightly with the wave-induced movement of the rig" (6).
As interpreted by the Earth Science Australia website (2), which adapted technical papers from the Australian Institute of Petroleum, the starting of the drilling process is considered the moment when the drill bit reaches the seabed. This stage is also referred to as "spudding". There are two types of drill bit types used in the petroleum industry: "a roller cone or rock bit which usually has three cones armed with steel or tungsten carbide teeth or buttons" (2) and "a diamond bit, imbedded with small industrial diamonds" (2). Drill collars, made out of heavy sections of pipe, are put in place so that the drill bit stability increases.
Furthermore, the rock formations are then cut or crushed by the drill bit which is attached to a drill string. The drill string is composed out of the slender steel pipes and a series of other tools. The drill string (drill pipe) is controlled from the surface and rotated by a turntable. As the drill cuts deeper, growing its way to the oil reservoir, the drilling crew from the rig adds extra lengths of drill pipes. The pipes are modular and one joint (module) consists of a piece of 30 feet long and about 600 pounds with the diameter between 36 inches down to 8.5 inches. The pulling of the drill string in or out of the hole is called "tripping". In an obvious manner, the diameter of the pipe decreases as the hole deepens.
In order to minimize the risk of a possible blowout a blowout preventer is put in place so that the drill string passes through it. As presented in the nearby figure and in the Schlumberger Oilfield Glossary (6), the blowout preventer (BOP) is a system of large specialized vales situated at the top of the well with the ability to close and control the wells. The BOP is operated hydraulic using hydraulic actuators and is considered the most important mechanism in controlling the well, as stated in the Occupational Safety & Health Administration website (10), with regards to the oil and gas industry. While drilling, formation fluids (gas, oil, water under pressure from beneath the surface) are present in the well and their pressure is opposed by that of the drilling fluid (mud). In case that the formation fluids have a greater pressure than the drilling fluids, the BOP is activated so that the underground pressure is kept under control and a blowout prospect is avoided. The drilling crew can reopened the BOP if the mud density is increased to a level that the operators regain "pressure control of the formation" (6).
As stated above, the first role of the drilling fluids is to control the hydrostatic pressure of the well. In addition, the mud removes cuttings from the well, keeps the drill assembly cool, clean and lubricated by the flow of the mud, strengthens the well hole with a layer of clay maintaining wellbore stability and seals permeable formations. There are different types of drilling fluids, but most of them consist of a combination of water, clay and other chemicals. From the platform, the mud is pumped under pressure through the drill string reaching the drill bit and cooling it down. On its way up, the fluid pumps to the surface crushed or cut rock in the space between the drill string and the walls of the hole. The fluid is then recycled and directed to the mud pits, but only after a filtering process has removed the cuttings. In the mud pits, after proper testing, the fluid is chemically treated in order to be reused in the drilling process. Impurities, natural gases or other flammable materials which can be brought to the surface together with the mud are disposed using shale shaker and conveyor technology (11).
Completion of the well
In order for the well to be productive, it must be completed. According to the Encyclopaedia Britannica, after the well has been bored to its target depth, casings are set in place. Cemented is then used to seal the space between the casing and borehole wall with the role "to prevent fluid movement between formations"(7). The production tubing is then set in place and extended until it reaches the oil reservoir. Perforations are then executed upon the casing to allow the flow of the formation fluid through the openings (4). A perforator tool with bullets or explosive charges is introduced in the well to create these perforations. This will allow the fluid to push its way to the rig, due to the natural pressure of the reservoir. This process ensures a good flow because of the good control of the perforation numbers and intervals. For the most efficient reserves perforations 30 centimetres apart are enough to create a flow of formation fluid into the well. If the formation is not as productive as wanted, the rock around the well is then fractured. This will ensure a higher flow of the fluid into the well. To crack the rock, an inert fluid is injected at high pressure into the well. Another method which provides the same results is the use of an acid to dissolve parts of the rock (7).
A complex subsea equipment is installed so that the well can be functional. It is called the 'Christmas tree' and it consists of a network of valves situated at the top of the well. Their role is to control and "regulate flow from the well and allow tools for subsurface work to be lowered through the tubing on a wire line" (7).
Primary, secondary and enhanced oil recovery
As a mean to support carbon dioxide oil recovery in the Europe, Tzimas underlines in his document the standard methods by which oil is recovered at the current time (8). Natural mechanisms are the drivers which facilitate primary recovery of the oil. At the start of the extraction, the pressure is high enough to push the formation fluid into the well and even to the surface. But moreover the pressure of the reservoir is not constant and therefore production can not rely only on natural pressure. To increase the productivity, oil companies have developed means by which they can enhance the primary recovery. A most common installation is a motor operated pump placed at the bottom of the production tubing and "a walking beam, an arm that rises and falls like a seesaw" (7) installed on the surface. The walking beam is connected to the piston of the pump. This method of artificial lifting is mainly useful when the formation fluid pressure is high enough to ensure that the oil reaches the well, but lower than needed to push the oil to the rig. A second method for helping the fluid flow to the platform is based on lowering the density of the oil and therefore allowing the pressure of the reservoir to be enough to reach the surface. This is done by injecting gas bubbles through a special valve situated at the bottom of the tubing. A third method uses produced oil. This is pushed down the well under "high pressure to operate a pump at the bottom" (7). The percent of oil extracted by using only primary recovery is below one third, even with the help of the lifting methods described. There are also situations when this type of extraction is not economically viable and secondary recovery is then needed to extract the crude oil out of the reservoir.
"Expected sequence of oil recovery methods in a typical oilfield." (8)
Secondary recovery implies gas or water under injection in order for the formation fluids to be substituted and for the pressure to be increased. Gas reinjection, different from gas lift mentioned above, is based on pushing natural gas in the upper part of the reservoir, where gases naturally occur. The recycling of the gas causes a rise in the pressure and also lowers the viscosity of the oil making it easier to pump. In this way the output of the well is increased. This method is applicable where the rock formation allows the oil to flow to the bottom of the reservoir by gravity. When this flow does not occur, other methods of extraction must be used. Recovery through water injection is also used to stabilize the pressure of the reservoir. A very common practice for the offshore industry, water injection uses seawater, which is first filtered and deoxygenated before being used. Water flooding is done in order to replace the oil which has been pumped out of the well and to direct the remaining oil towards the production wells. The production is increased two times "than expected from primary means alone" (7).
Enhanced recovery, also known as tertiary, is based on a series of procedures that are used after water injection reached its limits or where the density of the oil doesn't allow it to flow using primary and secondary recovery methods. According to the article from Encyclopaedia Britannica on petroleum production, there are two types of methods used in offshore extraction to continue adequate production: miscible and thermal.
The first category includes injection of natural gas. The difference from the first two types of recovery comes from the fact that the pressure of the gas is so high that it makes the gas and oil miscible. However, there are a few downsides in reservoir where the homogeneity of the oil is not constant, because of the risk that large areas of oil are bypassed. Another miscible method is done by "putting a band of soap-like surfactant material ahead of the water" (7). As a result the rock is cleaned because of the surface tension created between the injected material and the formation fluid. The water used is mixed with a polymer "in order to prevent the water from breaking through and bypassing the surfactant" (7). Because this method works better in non-carbonate rock, for the carbonate rock the use of carbon dioxide is recommended. The only issue with CO2 injection lies in the transport of the gas. Although it is a heavy debated subject because of its green aspect, too little progress has been done in this direction.
A thermal method wide spread is by heating the reservoirs using steam. This is mainly possible in onshore production and it consists of an injection of steam though a well in order for the oil to reduce its viscosity. In situ combustion is an alternate thermal method used in the industry. Through this process a part of the oil is burnt in order to heat and allow the flow of the oil. Compressed air is needed to support the combustion.
The extracted fluid contains water. First of all oil and gas reservoirs have a natural water layer which is extracted to the surface no matter the method used, and secondly, the use of injected water in the extraction process is another source. The formation fluid collected to the platform must be separated into three: oil, gas and water. After the separation of gas, an API, oil water separator, is used for the other two compounds. The API uses specific gravity difference between oil and water. So, the crude oil rises on the top of the separator, while the water and the solids will settle in the middle and on the bottom, as seen in the figure above. Before the water will return to the ocean it is cleared from almost all oil and chemicals with the help of mechanical separation and chemical treatment. An example of such a process is the dissolved air flotation. The clean crude oil is then directed to storage from where it can be transported either by pipelines or by ships.
"Conventional & Parallel Plate Oil/Water Separators" (9)
The transport of the offshore crude oil from the platforms is done mainly by tanker ships and subsea pipelines. The tankers capacity is limited and therefore, where long term extraction is expected, a network of pipelines is put into place. This is the "safest and most economical method to transport petroleum" (7) and it is widely spread in the North Sea.
The technologically processes described in this paper are not intended to take part in the political and social debate, but only to present the situation from an engineering point of view. The extraction process of offshore oil, alongside with onshore production, was one of the principal energetic tools of the 20th century. As for the present century, the world targets expected for 2020 and 2050 may radically change the energetic balance. But no matter of the future directions taken and of the environmental concerns, the offshore oil industry was one turning point in the progress of the world as we know it.