The Well Control System or the Blowout Prevention System on a drilling rig is the system that prevents the uncontrolled, catastrophic release of high-pressure fluids (oil, gas, or salt water) from subsurface formations. These uncontrolled releases of formation fluids are referred to as Blowouts. Due to the explosive nature of oil and gas, any spark on the surface can result in the ignition of the fluids and an explosion on the rig. An explosive blowout and the failure of the Well Control System were the causes of the Mocondo Well disaster that killed eleven of the rig crew on the Deep Water Horizon Rig on April 20, 2010 and resulted in 35,000 to 60,000 bbl/day of crude oil to spill into the Gulf of Mexico. We will discuss this later in the lesson.
In the detailed rig schematic (Figure 9.02e), the well control system is comprised of:
A picture of a Blowout Preventer (BOP), pronounced “B-O-P” not “bop”, is shown in Figure 9.11.
The blowout preventers are the principal piece of equipment in the well control system and are operated hydraulically; pressurized fluids are used to operate pistons and cylinders to open or close the valves on the BOP. The Accumulators (Item 18 in Figure 9.02) are used to store pressurized, non-explosive gas and pressurized hydraulic fluid to run the hydraulics systems on the rig. The accumulators store enough compressed energy to operate the blowout preventers even if the Power System of the rig is not operating.
The blowout preventer is a large system of valves each of which is capable of isolating the subsurface of the well from the rig to provide control over the well. These valves are typically stacked as shown in the Figure 9.11 and sit below the rig floor on land wells or some offshore wells; or they may sit on the seabed on other offshore wells.
A schematic diagram of a blowout preventer is shown in Figure 9.12.
Figure 9.12 shows three type of valves (there are others) – an Annular Preventer, Blind Rams, and Shear Rams. The Annular preventer is the ring-shaped piece of equipment on the top of the BOP in Figure 9.11. As the name implies, the annular preventer is used to prevent flow through the annular space between the drill string or casing and the annular preventer. The annular preventer can also be used for non-cylindrical pipe, such as the kelly, or open hole. The annular preventer consists of a doughnut shaped bladder that when in the open position allows the drill pipe to rotate but in the closed position seals the annulus. Figure 9.13 provides a schematic of the annular preventer.
In Figure 9.13, the blue area represents the doughnut-shaped bladder. As mentioned earlier, in the open position, (A), the drill pipe can be rotated or can be run up or down; while in the closed position, (B), the bladder pushes out, closing off the drill pipe, kelly, or open hole. The bladder based sealing element is not as effective as the ram type sealing elements; however, almost all blowout preventer stacks include at least one annular preventer.
Schematics of the ram-type preventers: the blind rams, the shear rams, and the pipe rams (pipe rams are not shown in Figure 9.12) are shown in Figure 9.14.
This figure shows that:
A blowout begins as a Kick (entry of subsurface formation fluids into the wellbore). What distinguishes a kick from a blowout is that a kick can be controlled while a blowout is uncontrollable. We have already discussed two of the defenses against kicks when we discussed drilling fluids when we listed the objectives of the drilling fluid:
In the first objective re-quoted above, if we can keep the pressure exerted by the drilling mud greater than the pore pressure, then we know that fluids will flow in the direction of the mud to the formation. This cannot always be achieved. For example, if we drill through a natural fracture or if our mud density is too great and we inadvertently fracture one formation, then we may lose large quantities of the drilling fluid into the fracture (Lost Circulation). In this situation, instead of having the full weight of the mud column exerting pressure on a second (porous and permeable) formation, we may only have a fraction of the oil column height exerting a lower pressure on that second formation.
In the second objective re-quoted above, if we deposit an impermeable Drill Cake (filter cake) across an otherwise porous and permeable formation, then for a slightly Underbalanced Pressure (drilling fluid pressure lower than the formation pressure) we have created a seal between the wellbore and the formation. Again, this is not a Failsafe System because at greater underbalanced pressures, the higher formation pressures may be able to displace the drill cake.
The two previously discussed methods are used to help prevent a kick from occurring, but as mentioned they are not always successful, and kicks may still occur. The causes of a kick include:
The following are Indicators/Warning Signs of a kick:
When a kick occurs, the Operating Company and Drilling Company always have well-specific plans in-place for all wells to ensure that any controllable kick does not turn into an uncontrollable blowout. I cannot go into the details of a well-specific procedures, but they will include some of the following features if a kick occurs during drilling operations:
Other procedures will be used if the kick occurs while tripping into or out of the well. The details of some aspects of this procedure such as hard or soft shut-ins and the circulation methods, The Driller’s Method and The Weight and Wait Method, will be discussed in detail in your later drilling classes. More importantly, for every well that you are involved with, there will always be Daily Safety Meetings that discuss the current status of the well and the important safety aspects of all drilling activities related to that day’s operations.
So, we have discussed the role of drilling fluid to exert pressure on porous and permeable formations and to coat them with an impermeable filter cake to help prevent kicks from occurring. We have also discussed the role of the blowout preventer and company procedures to control a kick once one occurs. So, how do blowouts happen?
Perhaps you remember the Macondo Blowout (Deep Water Horizon Rig) disaster. The name Macondo was the Prospect name (remember, we discussed prospects and well proposals in a previous lesson) while the Deep Water Horizon was the name of the rig. This was the largest oil spill in the Gulf of Mexico. When the disaster occurred, eleven members of the rig crew were killed by the explosion when the natural gas ignited.
The following YouTube video, "Deepwater Horizon Blowout Animation" (11:22), describes the Deep Water Horizon disaster and what its root causes were:
After learning about offshore drilling rigs, drilling crews, components of the drilling rig, kicks, and blowouts, I would highly recommend watching the movie “Deep Water Horizon” and use your knowledge about oil and gas well drilling to identify some of the technical aspects of the film. Ask yourselves some technical questions:
Links
[1] http://www.oil-gasportal.com/drilling/technologies/
[2] https://gulfofmexicooilspillblog.wordpress.com/2010/12/24/gulf-of-mexico-oil-spill-blog-blowout-preventer-evidence/
[3] https://oges.info/library/145849/BOP-PRESSURE-TESTING-PROCEDURE
[4] https://en.wikipedia.org/wiki/Blowout_preventer
[5] https://commons.wikimedia.org/wiki/User:Egmason
[6] https://creativecommons.org/licenses/by-sa/3.0
[7] https://creativecommons.org/licenses/by-nc-sa/4.0