Patent Application
20100096141
This invention relates in general to pumping well fluid from the seabed to the surface, and in particular to a pump assembly located within a caisson and having a jet pump to reduce gas accumulation in the caisson.
Offshore wells are being drilled in increasingly deeper waters. The wells may have adequate pressure to flow the well fluid to the seabed, but lack sufficient pressure to flow the fluid thousands of feet upward to a production vessel. Proposals have been made to install pumps at the seabed to boost the pressure of the well fluid sufficiently to flow it to the floating production vessel.
Often, the well fluid will be a mixture of hydrocarbon liquid, gas and water. Gas presents a problem for pumps, particularly electrically driven centrifugal pumps. Gas detracts from the efficiency of the pump, and can cause the pump to lock and shut down if a large slug of gas enters.
One proposal for dealing with well fluid having an appreciable quantity of gas is to mount the pump in a caisson. The caisson is located in a tubular bore formed into the seabed and cased to seal it from the earth formations. The caisson may be several hundred feet deep. The well fluid flows in the upper end of the caisson, and gravity causes the liquid to separate from the gas and flow downward in the caisson. The gas tends to collect in the upper portion of the caisson. The submersible pump is located within the caisson at a point where its intake is below the liquid level. The pump is enclosed by a shroud with an inlet at the lower end to force liquid to flow upward by the motor to cool the motor. As the gas cap continues to build, portions will escape and flow into the pump along with the liquid to be pumped into the surface. A possibility exists that the gas cap will grow and push the liquid level too low, resulting in a large quantity of the gas entering the pump and causing it to gas lock. Liquid level controllers have been proposed to open and close the inlet to the caisson to try to maintain the liquid at a desired level above the intake of the pump. A large gas slug could nevertheless still enter the pump and cause a gas lock.
In this invention, the pump is located within a shroud inside the caisson. The pump assembly consists of a jet pump combined with at least one centrifugal pump. An extraction tube extends from the jet pump and has an upper end for location within a portion of the caisson that normally will be a gas accumulation area above the liquid level. The extraction tube has a lower end in fluid communication with well fluid in the interior of the caisson. The jet pump has a venturi configuration to cause a reduced pressure. The lower end of the extraction tube joins a point of reduced pressure in the venturi. During operation, the venturi configuration creates a suction to draw in a small continuous quantity of gas through the extraction tube as the pump operates to avoid the gas cap from becoming too large.
Referring to
A shroud 17 is located within caisson 11. Shroud 17 has an inlet 19 at its lower end. Shroud 17 is a tubular member that is smaller in diameter than the inner diameter of caisson 11 so as to create an annular passage surrounding it for downward fluid flow.
An electrical submersible pump assembly (“ESP”) 21 is mounted within shroud 17. ESP 21 has three pumps 23, 24, 26 in this example. Pumps 23, 24 are typically centrifugal pumps. Pump 26 is a jet pump. Each pump 23, 24 is made up of a large number of stages, each having a rotating impeller and a stationary diffuser. Pump 23 has an intake 25 that is located at the lower end of pump 23 within shroud 17. Jet pump 26 is preferably positioned above shroud 17, between pump 23 and pump 24. Shroud 17 has an upper end 27 that seals around a portion of ESP 21 above intake 25. If desired, the entire length of ESP 21 could be enclosed by shroud 17, but the upper end 27 of shroud 17 only needs to be slightly above pump intake 25. A discharge pipe 29 extends upward from pump 24 and out the upper end of caisson 11. Although shown extending through the top of caisson 11, discharge pipe 29 could alternately extend through a sidewall portion of caisson 11. ESP 21 also has an electrical motor 31 that has a shaft 47 (
An extraction tube 35 has an upper end 37 that is exterior of shroud 17. Extraction tube 35 has an inner diameter much smaller than the inner diameter of discharge pipe 29. Extraction tube 35 has a lower end 39 that is above shroud 17 and in fluid communication with well fluid in the interior of caisson 11.
Referring to
Extraction tube upper end 37 is positioned above the liquid level 40 in caisson 11 at all times. Optionally, a liquid level controller (not shown) may be employed for controlling the flow of fluid into caisson 11, if desired, to maintain liquid level 40 fairly constant.
In the operation, ESP 21 is placed in shroud 17 and installed in caisson 11. The valve (not shown) to inlet 13 is opened, causing well fluid to flow through caisson inlet 13. The well fluid is typically a mixture of hydrocarbon liquid, water and gas. Shroud 17 is immersed in liquid in caisson 11, with liquid level 40 being at least above pump intake 25 and preferably above shroud upper end 27. Liquid level 40 will be below caisson inlet 13. A gravity separation occurs as the fluid flows in inlet 13 and downward in caisson 11. This results in gas freeing from the liquid and collecting in the upper portion of caisson 11. The liquid flows down through the annular passage around shroud 17 and into shroud inlet 19. The liquid flows up alongside motor 31 and into pump intake 25. Pumps 23, 24 increase the pressure of the liquid and discharge it through discharge pipe 29 for flowing the liquid to the surface.
As the liquid flows through jet pump 26, the velocity of the fluid increases as it reaches throat region 59. As the fluid velocity increases, venturi 49 causes a reduced pressure in throat region 59. As a result of the pressure drop, a small amount of gas from the gas cap collecting above liquid level 40 will flow through extraction tube 35. The gas leaves extraction tube 35 and mixes with the liquid flowing into jet pump 26 at inlet 55, located within throat region 59. As the fluid and gas mixture passes throat region 59 and continues through divergent outlet 61 the velocity decreases and the pressure increases before passing through outlet 63 and into pump 24. The flow rate of the gas is fairly constant and relatively small compared to the liquid flow rate, thus is readily pumped by pump 24 along with the liquid up discharge pipe 29. The flow area of extraction tube 35 is much smaller than the total flow area of shroud inlet 19 so as to avoid excessive amounts of gas flowing into pump 24.
The invention has significant advantages. By continuously drawing off a small amount of the gas cap, the size of the gas cap is maintained within the caisson at a minimum dimension. Limiting the size of the gas cap prevents the liquid level from dropping so low such that large slugs of gas could enter the shroud and cause gas locking of the pump. By introducing the gas into the fluid stream through the jet pump after a desired number of pump stages, the gas is able to mix with the fluid in a form that the centrifugal pump can handle.
While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, the caisson could comprise a housing located on the sea floor, rather than within a hole in the sea floor. The housing could be oriented horizontally or tilted rather than vertically.
