Blowout prevention equipment was developed for use in drilling deep wells for the recovery of oil and natural gas. Later this unique oil and gas industry equipment was adapted for use in drilling deep geothermal wells. Natural deposits of oil and gas exist in porous rock formations deep in the earth's crust. These deposits were created by millions of years of sediment burial and confinement by geologic structures. Over time, increased sedimentary burial created high pressures and temperatures in these deposits. Most newly discovered oil and natural gas deposits have static pressures up to about 8,000 psi and temperatures of about 300°F. There are a few abnormally pressured natural gas deposits that have static pressures as high as 16,000 psi. These pressures, although found in deposits at depths of 10,000 ft or greater, are quite dangerous to drilling rig personnel and the environment. Blowout prevention equipment (or the BOP stack) were developed to provide protection of surface from these high pressured deposits.
A blowout occurs when oil and/or natural gas deposits are allowed to flow uncontrollably to the atmosphere at the surface. The first line of defense against the dangers of these high pressure deposits is weighted drilling mud. Water based and oil based drilling muds can be designed so that their specific weights are sufficiently high to provide bottomhole pressures that are slightly higher than the static pressure of the deposits when the drill bit penetrates the host rock formation. When drilling exploratory wells it is not possible to precisely know the static pressure in target oil or natural gas deposits. Therefore, geologic and engineering judgment must be used to estimate the static pressures that might be encountered. These estimates are used to design the weighted drilling mud. But even after the first exploratory wells have been successfully drilled and the oil or gas field is being developed with follow-on development wells, surprises in deposit pressures can occur. When too light a drilling mud is used and a high pressure deposit is drilled, the well will receive a liquid or gas "kick." A kick is a slug of formation liquid and/or gas that has flowed from the formation into the annulus of the well bore. The kick is composed of fluids that have lower specific weights than the heavily weighted drilling muds. Therefore, the kick will "float" in the drilling mud and rise rapidly to the surface. If the kick is mostly natural gas, the gas will expand as it moves up the drill string annulus to the surface. The surface wellhead equipment is the second line of defense against a blowout. The wellhead equipment in the form of the BOP stack must be engineered so that it is capable of containing the high pressure of this gas when it reaches the top of the annulus. This BOP stack must contain this gas pressure while the slug is circulated under control to the surface and expelled from the annulus via a flow line to a remote burn area where the slug can be safely burned off.
The BOP stack can be composed of two types of preventers; 1) the ram-type blowout preventer and, 2) the annular-type preventer. The ram-type preventer can be a blind (shear) ram and or a pipe ram. The blind ram is capable of sealing the well completely by compressing the drill pipe from two sides and failing the pipe steel structure in a manner to prevent the well fluids from escaping to the surface through either the inside of the drill pipe or around the outside of the drill pipe. This vise like action of the two rams essentially forces the pipe to deform between the two rams. The pipe ram acts in a somewhat similar manner as the blind ram. Except the pipe ram has a geometric shape on the end of the rams that conform to the outside surface of the drill pipe. Thus the pipe ram seals against the outside of the drill pipe and prevents well fluids from escaping to the surface around the outside of the drill pipe. The pipe ram does not fail the pipe structure, therefore, drilling mud can be circulated down the inside of the drill pipe to safely allow the kick to be circulated to the surface.
Figure 2-11 shows a cut-a-way view of a twin ram-type blowout preventer. A typical twin preventer will have a pipe ram on the top and a blind ram on the bottom. The cut-a-way shows the bottom blind ram. In the event of a blowout, the pipe ram would be used to seal the well and allow the slug in the annulus to be safely circulated to the surface. In the event that the pipe ram cannot seal the well for the safe circulation of the slug, the blind ram can be actuated to seal the well. These rams can be actuated manually or hydraulically. Figure 2-11 shows the blind rams on the bottom are set up to be actuated manually.
This twin ram-type blowout preventer is flange connected (made up) to the top of the well casing. The bottom of the casing spool is threaded (or welded) to the top of the casing. The top of the spool is flange connected to the bottom flange fitting of the twin ram-type blowout preventer.
Figure 2-7 shows a Type 1 BOP stack which utilizes only the twin ram-type blowout preventer for well control. This BOP stack is fitted with a rotating head flange connected to the top of the twin ram blowout preventer. This is the standard well control set up for air or gas drilling operations directed toward the recovery of oil and natural gas deposits with static bottomhole pressures of the order of 3,000 psi or less. Figure 2-7 shows a Type 2 BOP stack which utilizes three ram-type blowout preventers for well control. This BOP stack is configured with two pipe rams on the top and a blind ram on the bottom. The two pipe rams allow some flexibility in carrying out well control when drilling deep wells with a tapered drill string and when placing a liner string in an air or gas drilled well. This BOP stack is fitted with a rotating head flange connected to the top pipe ram blowout preventer. This stack can be configured for the recovery of oil and natural gas deposits with static bottomhole pressures of up to 5,000 psi.
The annular-type blowout preventer can also be used in a BOP stack (see Type 3 in Figure 2-7). An annular preventer is hydraulically operated. Figure 2-12 shows a cut-a-way view of a typical annular preventer. The closing of the preventer is actuated by hydraulic pressure. This hydraulic pressure forces the operating piston upward against a pusher plate (see Figure 2-12). The pusher plate in turn displaces (compresses) an elastomer donut inward to close and seal on the outer surface of drill pipe, drill collar, casing, or liner. Utilizing an annular preventer in conjunction with ram blowout preventers greatly increases well control flexibility and general rig safety when drilling with air and gas drilling fluids. The Type 3 BOP stack in Figure 2-7 is configured with twin ram-type blowout preventer on the bottom (pipe ram on top and blind ram on the bottom), an annular preventer flange connected to the top of the twin ram preventer, and a rotating head flange connected to the top of the annular preventer. This BOP stack can be configured for the recovery of oil and natural gas deposits with static bottomhole pressures of up to 10,000 psi.
Figure 2-13 shows a schematic of a more recent innovation in BOP stack design. This configuration is a variation of the standard Type 3 shown in Figure 27. Figure 2-13 shows the addition of a pipe ram below the drilling spool. This BOP configuration has evolved for use in underbalanced drilling and completion operations. Underbalanced drilling operations allow the oil and natural gas fluids to continue to be produced by the reservoir formation as the rock is penetrated by the advance of the drill bit. In order for underbalanced drilling operations to be successful, the oil and natural gas formations must be allowed to flow even when connections are being made, during liner operations, or during well completion operations (after drilling operations). The addition of the pipe ram below the drilling spool increases BOP flexibility to accommodate these operations. With the drill string or tubing string in the well and with the upper pipe ram closed, drilling on completions fluids with entrained formation fluids can be safely circulated to the surface through the choke line (attached to the drilling spool). The bottom pipe ram provides a back-up well control device during these operations [7 and 8].
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