How is oil extracted under water?
As you know, man-made disasters do not happen on their own. They are satisfied with people. In the oil and gas industry, the consequences of incompetence are dire. The tragedy of the Deepwater Horizon platform at the Macondo field and the release of oil on the Montara shelf well in the Timor Sea in 2009 clearly demonstrated the diabolical potential of the “human factor”. There are almost no places where oil oozing from sand can be scooped up in buckets. But there are still plenty of technologically complex hydrocarbons in the thickness of the geosphere. Some 30 years ago, drilling on the ocean floor, in eternal darkness and cold, under pressure, crushing the titanium hulls of submarines, like beer cans, was fantastic. However, it is extremely dangerous today. And therefore prohibitively expensive.
For example, the first 15 wells of the Tupi deepwater field in the Santos Basin “flew $ 1 billion to the Petrobras and BP consortium. In order to get to this oil reservoir with recoverable reserves of 8 billion barrels, drillers had to overcome 2 km of water, a hundred meters of salt-attacking metal and another 5 km of “puff pie” of rock with large reservoir pressure drops.
Equally difficult geophysical conditions are also off the coast of Angola, where drilling is carried out at depths from 1.5 to 2.5 km, and in the Gulf of Mexico, where frequent hurricanes complicate the operation of offshore platforms and drill-drilling ships. In the western regions of the North Sea, where the North Uist (depth 1.3 km) and Rosebank (1.1 km) deposits were recently discovered, as well as on the East Coast of Canada, violent storms with a five-meter wave rage for more than 250 days a year. In the Sea of Okhotsk, and especially in the Arctic, heavy ice, frosts and temperature differences in the working zone from -1 ° C at the wellhead to 130 ° C in the face are opposed to oil workers.
At the bottom
Before drilling a deep-water well, the drilling vessel (in the professional jargon “drillship”) “hangs” above the bottom point set by geophysicists, continuously adjusting its position by the thrust of the propellers of the GPS-based dynamic positioning system. After that, the first link of the future well, the conductor, descends through the through-hole shaft in the ship’s hull on the drill string. This is a thick-walled steel pipe foundation weighing 200 tons or more and up to 27.5 m high with a flange for connection with wellhead fittings.
Under the attentive gaze of the cameras of the underwater vehicles, the hydraulic monitor bit located inside the conductor blurs the well at the bottom with powerful jets, and the gigantic structure slides into it under the pressure of water. The conductor is tightly concreted in the well with cement paste, which is fed through the drill string and squeezed out into the annulus through a special head.
The test is the mass formed by the contact of astringent minerals with sea water. It turns into an artificial stone in no more than 18 hours. Immediately after this, a bit descends into the well, rotating under the pressure of sea water, like a turbine, and drillers walk about another hundred meters to install the first section of the casing.
To isolate from aquifers and to counter rock pressure, the well is again filled with cement mortar. Tamponage - as the pros call this process - a critical procedure in drilling. The low quality of the “armor”, which resists the enormous pressure of the reservoirs (up to 1000 atm), can lead to the loss of a well worth about $ 100 million and even to an environmental disaster (as happened in Macondo).
Then, a block of blowout preventers (PVP) weighing about 100 tons is lowered from the platform’s platform mouth. It is these powerful automatic gates designed to save the water area from oil pollution in the event of an accident. A vertical pipeline, or riser, is attached to the PVP from above.
A riser, consisting of dozens and sometimes hundreds of separate sections, connects the rig to the wellbore. On the riser, as on the way of life, everything necessary is delivered to the well - a drill string with a hydraulic bit, drilling mud, casing, cement paste, measuring equipment and special tools. On it the spent drilling fluid carries up the rock fragments.
After installing the riser, a routine drilling process begins, lasting several months: sinking a section, lowering another section of the casing, grouting, crimping, leak tests, bit change, sinking again, etc. But as you approach the oil reservoir, the situation is literally the word is heating up: at a depth of more than 5 km, the temperature jumps to 130 ° C, and the pressure reaches 900-1000 atm.
Line of defense
According to James Watson, director of the US Bureau of Safety and Environmental Protection (BSEE), only toughening the reliability requirements for downhole equipment can compensate for the catastrophic manifestations of the human factor. But drilling engineers working in the field are sure that the elements can be kept under reliable control and without much innovation.
The first line of defense of the well is competent cementing adequate to the geophysical properties of the formation. The second line is the suppression of excess pressure erupted into the wellbore fluid by supplying clay drilling mud with a specific gravity of 2.5-3.5 t / m3. As a rule, such a plug effectively clogs oil and gas rushing towards the mouth.
But if the drilling fluid is not able to restrain the onslaught of the fountain, as well as in the event of a sudden demolition of the platform from the drilling point and the drill string is torn off the pump, the operator must shut off the well through a blowout preventer block. The standard deep-sea PVP unit is a multi-storey structure of two or more ring and at least three shear ram preventers.
The PVP unit can be controlled by applying an electric or encoded sonar signal, mechanically using underwater drones and in automatic emergency mode powered by a bottom accumulator in case of damage to the hydraulic system on the riser. In this case, pipe dies first fix the drill string in the channel (if there is one), and shear ones finally plug the well.
In 2010, on the Deepwater Horizon, the first two lines of defense fell due to the incompetence of personnel, and none of the five preventers worked in the PVP unit. However, something like this could have happened much earlier. Back in 2004, the US Subsoil Use Service published shocking data on assessing the reliability of preventers in deep-water wells in the Gulf of Mexico. It turned out that 50% of the tested PVP blocks were not able to plug the well at the moment when it had a drill string or casing, due to insufficient shear dies. Then the scandal was released on the brakes, and after six years ...
Immediately after eliminating the release, the leading companies in the oil and gas sector began feverish development of similar devices, special tools for clearing the mouth of deep-water wells from blockages, developing the technology for their application and delivery to the accident site. One of the most sophisticated systems - the Global Deepwater Well Cap (GDWC) worth $ 50 million - was announced by British Petroleum and Cameron engineers in May this year.
The basis of the GDWC, the mass of which, together with additional equipment is 500 tons, is a 12-meter 100-ton steel plug. In the event of an accident, it will be installed directly from the vessel onto the preventer unit, and two hydraulic wedge gate valves will provide the jamming process. The dispersal spray system (substances that break the oil into tiny droplets) and the methanol feed system for dissolving methane ice, which can be useful in cases where it is necessary to bleed oil from the plug to tankers, are integrated into the plug body.
GDWC is equipped with 28 transitional fittings for adaptation to drilling rigs of all 15 types operating in BP fields and withstands pressure up to 1055 atm. A similar plug is expected soon with a working range of up to 1406 atm. The maximum deployment depth of the GDWC is 4, 000 m.
The GDWC kit includes a mobile accumulator and manipulators for Oceaneering underwater robots: cameras, sonars, searchlights, hydraulic monitors, pipe grippers and a set of pipe claws that can bite steel bars 1.5 m thick. According to BP Vice President Richard Morrison, the system unassembled, packed in 20-foot containers and located at the company’s base in Houston. But if trouble happens, within a week it will be delivered anywhere in the World Ocean. This will require 35 trailers and seven AN-124 or Boeing 747 aircraft. Upon arrival at the destination, the containers will be moored to the cargo helicopters and transferred to the drilling platform, where after assembly with the help of a crane the plug will be sent to the bottom.The article “Case of Macondo” was published in the journal Popular Mechanics (No. 10, October 2012).