Patent Application
20090035067
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 an eduction tube 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 coot 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. An eduction tube that extends out of shroud 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 eduction tube has a lower end in fluid communication with an interior portion of the shroud. During operation, the eduction tube creates a suction to draw in a small continuous quantity of gas as the pump operates to avoid the gas cap from becoming too large.
In one embodiment, the lower end of the tube joins the intake of the pump assembly within the shroud. In another embodiment, the eduction tube extends alongside the shroud and has its lower end at the inlet of the shroud. Preferably the inlet of the shroud in that instance has a venturi configuration to cause a reduced pressure. The lower end of the tube joins a point of reduced pressure in the venturi.
In another embodiment, more than one eduction tube is employed. The tubes may have their upper ends spaced at different distances above the shroud for educting gas from different points in the caisson. In another embodiment, an eduction conduit is mounted to the inlet of a caisson. The eduction conduit leads from the upper end of the caisson back to the inlet for recirculating some of the gas cap back into the well fluid flowing into the caisson. In all of the embodiments, the eduction tube or tubes are sized to have a much smaller flow area than the flow area of the inlet of the shroud, so that significant amount liquid will continue to flow into the inlet of the shroud.
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 a pump 23 that is typically a centrifugal pump. Pump 23 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. 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 that extends upward from pump 23 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 that drives pump 23. Motor 31 and pump 23 are conventionally separated by a seal section 33. Seal section 33 equalizes the pressure of lubricant contained in motor 31 with the well fluid on the exterior of motor 31.
An eduction tube 35 has an upper end 37 that is exterior of shroud 17. Eduction tube 35 has an inner diameter much smaller than the inner diameter of discharge pipe 29. Eduction tube 35 has a lower end 39 that is fluid communication with well fluid in the interior of shroud 17. In the first embodiment, lower end 39 extends to a portion of pump intake 25. When pump 23 is operating, a suction exists at intake 25, causing lower end 39 to have a lower pressure than upper end 37. Upper end 37 is positioned above the liquid level 40 in caisson 11 at all times. Optionally, a liquid level controller (not shown) may employed for controlling the flow of fluid into caisson 11, if desired, to maintain liquid level 40 fairly constant.
In the operation of the first embodiment, 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. Pump 23 increases the pressure of the liquid and discharges it through discharge pipe 29 for flowing the liquid to the surface.
At the same time, a small amount of gas from the gas cap collecting above liquid level 40 will flow through eduction tube 35. The gas leaves eduction tube 35 and mixes with the liquid flowing into pump intake 25. The flow rate of the gas is fairly constant and relatively small compared to the liquid flow rate, thus is readily pumped by pump 23 along with the liquid up discharge pipe 29. The flow area of eduction 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 23. Also, the small cross-sectional flow area of eduction tube 35 assures that liquid will continue to flow up around motor 31 for cooling motor 31.
In the embodiment of
The second embodiment operates in the same manner as the first embodiment by drawing a portion of the gas cap continuously down through eduction tube 51 into shroud 41. In this embodiment, the gas mixes with the liquid as it flows upward around motor 31 and into pump intake 25.
In the embodiment of
In the embodiment of
In the operation of the embodiment of
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 that such large slugs of gas could enter the shroud and cause gas locking of the pump.
While the invention has been shown in only a few 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.
