Patent Application
20190145256
This invention relates to the method of detecting methane or other gases in the bore of a subsea stack before they enter the lower pressure drilling riser and begin to dangerously expand, tending to blow the drilling mud out of the riser.
When drilling an oil or gas well into the seafloor, drilling mud is circulated down a central drill pipe, goes through a drill bit at the bottom where the hole is being drilled, and then circulates back up through the annular area between the central drill pipe and the hole. The hole consists of a lower portion which is the size of the drill bit doing the drilling, up the slightly larger bore of the last casing string landed, through the considerably larger bore of the subsea blowout preventer stack at the seafloor, and finally up the slightly larger drilling riser all the way to the surface. The outer diameter of the central drill pipe is relatively consistent above a short group of oversize drill collars at the bottom. The velocity of the drilling mud slows each time the hole it is in becomes larger, and the pressure is reduced as the depth is decreased. What this generally means is that free gas can travel up quickly to the area of the seafloor blowout preventer slack, and then slowly moves up to the surface.
When a substantial volume of methane type gases enters the wellbore during the drilling process and arrive up to the level of a seafloor blowout preventer stack, they can be so small as to not be noticeable in most cases.
If you take the case of a blowout preventer stack in 10,000 feet depths and presume a volume of gas the size of a ping pong ball (1.57″ diameter) is in the bore. When the gas gets to the ocean surface it's volume will be 20% bigger than two basketballs (9.4″ diameter each), given a typical drilling mud weight.
As the drilling mud travels up the bore of the drilling riser above the seafloor blowout preventer stack, additional gases may come out of solution and both the gas already out of solution and that which comes out of solution expand rapidly. What coming out of solution is can be easily seen by a soft drink bottle or a bottle of champagne. An unopened bottle will show no gas bubbles, and is under some pressure inside. When opened and the pressure is thereby reduced, suddenly some gas bubbles appear.
As if gas expanding from the size of one ping pong bails to more than twice the size of a basketball isn't bad enough, gas coming out of solution basically expands from nothing to potentially a very large size.
This has been a problem as long as oil and gas wells have been drilled. The best detection method presently is watching the level of the drilling mud pits at the surface. During drilling you would expect the mud to circulate down and back up at generally the same volume, considering that you are drilling a deeper hole all the time which takes additional drilling mud to fill. But you know how fast you are drilling and can compensate for that. If your mud pit level is increasing, it will tell you that something is entering the well bore, with a good chance that it is gas. If your mud pit is falling more than what is expected from your deepening the hole, you are “losing circulation”, generally meaning drilling mud is seeping into the formations. As almost every blowout will tell you, including the 2009 Macondo blowout in the Gulf of Mexico, there was not an adequate system for detecting problems. Additionally, when the mud pits have increased enough to detect it, a high volume of gas has already entered the drilling riser. Means for earlier detection have long been needed.
The object of this invention is to provide a method of detecting free gas within the drilling mud at subsea locations such as a seafloor blowout preventer stack before it has a chance to expand and can be directed out the higher pressure choke or kill lines.
A second object of this invention is to provide a method of defecting whether gas in solution the drilling mud at subsea locations such as a seafloor blowout preventer stack will come out of solution at the lower pressures seen as the drilling mud moves up towards the surface of the ocean.
A third object of this invention is to regularly monitor this situation in a why which can be reported in real time to the personnel at the surface and/or on land.
Referring now to
Below the drilling riser 22 is a flex joint 30, lower marine riser package 32, lower blowout preventer stack 34 and wellhead 36 landed on the seafloor 38.
Below the wellhead 36 it can be seen that a hole was drilled for a first casing string 40, that first casing string 40 was landed and cemented in place, a hole drilled through the first string for a second string, the second string 42 cemented in place, and a hole is being drilled for a third casing string by drill bit 44 on drill string 46.
The lower blowout preventer stack 34 generally comprises a lower hydraulic connector 48 for connecting to the subsea wellhead system 36, usually 4 or 5 ram style blowout preventers, an annular preventer, and an upper mandrel for connection by the connector on the lower marine riser package 32, which are not individually shown but are well known in the art.
Below outside high pressure fluid line 26 is a choke and kill (C&K) connector 50 and a pipe 52 which is generally illustrative of a choke or kill line. Pipe 52 goes down to valves 54 and 56 which provide flow to or from the central bore of the blowout preventer stack as may be appropriate from time to time. Typically, a kill line will enter the bore of the blowout preventers below the lowest ram and has the general function of pumping heavy fluid to the well to overburden the pressure in the bore or to “kill” the pressure. The general implication of this is that the heavier mud cannot be circulated into the well bore, but rather must be forced into the formations. A choke line will typically enter the well bore above the lowest ram and is generally intended to allow circulation in order to circulate heavier mud down the drill pipe into the well to region pressure control of the well. Normal circulation is down the drill string 46, through the drill bit 44, up the annular area between the drill pipe and the casing, and up the choke line.
In normal drilling circulation the mud pumps 60 take drilling mud 62 from tank 64. The drilling mud will be pumped up a standpipe 66 and down the upper end 68 of the drill string 46. It will be pumped down the drill string 46, out the drill bit 44, and return up the annular area 70 between the outside of the drill string 46 and the bore of the hole being drilled, up the bore of the casing 42, through the subsea wellhead system 36, the lower blowout preventer stack 34, the lower marine riser package 32, up the drilling riser 22, out a bell nipple 72 and back into the mud tank 64.
During situations in which an abnormally high pressure from the formation has entered the well bore, the thin walled central pipe 24 is typically not able to withstand the pressures involved. Rather than making the wall thickness of the relatively large bore drilling riser thick enough to withstand the pressure, the flow is diverted to a higher pressure choke line or outside fluid line 26. It is more economical to have a relatively thick wall in a small pipe to withstand the higher pressures than to have the proportionately thick wall in the larger riser pipe.
When higher pressures are to be contained, one of the annular or ram Blowout Preventers are closed around the drill pipe and the flow coming up the annular area around the drill pipe is diverted out through choke valve 54 into the pipe 52. The flow passes up through C&K connector 50, up pipe 26 which is attached to the outer diameter of the central pipe 24, through choking means illustrated at 74, and back into the mud tanks 64.
Valve 54 which would be called a choke or kill valve (probably called choke in that position) is shown mounted on the side 90 of lower blowout preventer stack 34 and having a bore 92 communicating with the bore of the blowout preventer stack. On the opposite side of the tower blowout preventer stack another similar communicating bore 94 is provided and the methane sensor assembly 96 is installed in this bore 94. Blowout preventer stack 34 has an internal bore 98.
Referring now to
Filter 106 restricts particle size coming into the methane sensor assembly at arrow 110 and is continually cleaned by the reverse flow at arrow 120. Bore 124 leads to check valve 126, bore 128 leads to check valve 130 which checks the flow in the opposite direction as check valve 126. Check valve 130 leads to vacuum chamber cavity 132 which leads to bore 134, leading to check valve 136, leading to bore 138, leading to bore 140, and back to filter 106. Drilled hole 142 leads to pressure transmitter 144. Piston 146 is part of cylinder 148, with suitable fittings 150 for operation.
Referring now to
The reason for this distinction in the pressures of the water in the water based drilling muds and the invading methane gas is that water boils and starts to put off pressures when it boils at 212 degrees F. or 100 degrees C. Methane boils and starts to put off significant pressures at −257 degrees F. or −162 degrees C. The vapor pressure of methane at 70 degrees F. or 21 degrees C. is 4,641 p.s.i, or 32,000 kPa. This means pure methane will not be a gas at all until the drilling mud is less than that pressure in typical drilling mud that would be (4641 p.s.i.)/(0.465 lb/ft gradient times 1.33 heavier mud)=7487 feet. That means that pure methane will not evaporate before that depth, which is a little deeper than the 6000 feet for the Macondo blowout. Other potential gases can come out at higher or lower pressures and depths.
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By following this process, a fresh batch of drilling mud is drawn into the methane sensor assembly, tested, documented, and expelled as frequently as desired. The data is communicated to the surface for monitoring and analysis, with appropriate alarms sounding and sent when a change occurs.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
