CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/270,761, filed December 22, 2015, which is incorporated herein by reference.

BACKGROUND

[0002] The present disclosure relates to an improved method and apparatus for hydrocarbon recovery from a well beneath a body of water. More particularly, the present disclosure relates to a method and apparatus for maintaining a controlled hydrostatic pressure in a subsea riser.

[0003] In recent years the search for oil and natural gas has extended into deep waters overlying the continental shelves. In deep waters it is common practice to conduct drilling operations from floating vessels or from tall bottom-supported platforms. The floating vessel or platform is stationed over a well site and is equipped with a drill rig and associated equipment. To conduct drilling operations from a floating vessel or platform a large diameter riser pipe is employed which extends from the surface down to a subsea wellhead on the ocean floor. The drill string extends through the riser into blowout preventers positioned atop the wellhead. The riser pipe serves to guide the drill string and to provide a return conduit for circulating drilling fluids as well as production fluids once the drilling operation is complete.

[0004] In order to control pressure and/or enhance production, gas may be injected into the riser in a technique called "gas lift." A conventional gas lift technique is described in U.S. Pat. No. 4,099,583 to Maus. In this technique, gas is introduced into the riser to reduce the average fluid gradient. However, it is believed that the lightened fluid of this technique includes poorly mixed gas and liquid, allowing the gas portion to rise quickly without providing as much lift to the fluid as might be possible using improved apparatus and methods.

[0005] U.S. Pat. No. 7,510,012 to Fernandes et al. proposes one solution to address the problem of bubbles that are deemed too large. In this approach, a bubble breaker assembly is used to decrease bubble size and provide better distribution. However, such assemblies are intrusive and do not allow passage of a pig through the main bore.

[0006] Other gas lift systems have been proposed to reduce hydrostatic head and increase flow velocity to prevent liquid build-up. For example, U.S. Pat. Pub. No. 2015/0000926 to Beg et al. describes the use of a jet pump downstream of the wellhead to reduce wellhead pressure. However, such jet pump is designed to lower the pressure at the surface which in fact can be additive to the reduction in pressure.

SUMMARY

[0007] An apparatus may include a production riser, an eductor, a motive fluid line, a suction line, and a mixed fluid line. The production riser may extend into a wellbore and be configured to convey production fluid from the wellbore to a wellhead. The eductor may be configured to mix a slip stream of the production fluid with high-pressure motive fluid, providing a mixed fluid. The motive fluid line may extend into the wellbore from a compressor to the eductor and be configured to provide the high-pressure motive fluid to the eductor. The suction line may be configured to provide the slip stream from the production riser to the eductor. The mixed fluid line may be configured to convey the mixed fluid from the eductor to the production riser.

[0008] A method for reducing base pressure in a subsea riser may include providing an eductor upstream of a wellhead. The wellhead may be upstream of the subsea riser. The method may also include supplying the eductor with high-pressure motive fluid and supplying the eductor with production fluid. The method may involve, with the eductor, mixing the high-pressure motive fluid and the production fluid to produce a mixed fluid and introducing the mixed fluid upstream of the wellhead.

[0009] A system may include a wellhead and at least one production riser extending into at least one wellbore and configured to convey production fluid from the wellbore to the wellhead. The system may also include at least one eductor configured to mix a slip stream of the production fluid with high-pressure motive fluid, providing a mixed fluid. The system may further include at least one motive fluid line extending into the wellbore from at least one compressor to the eductor and configured to provide the high-pressure motive fluid to the eductor. The system may include at least one suction line configured to provide the slip stream from the production riser to the eductor. The system may also include at least one mixed fluid line configured to convey the mixed fluid from the eductor to the production riser. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a prior art cross sectional side view of an apparatus including an eductor, in accordance with the present invention.

DETAILED DESCRIPTION

[0011] It is proposed that the cause of poor performance of traditional gas lift might be that the gas is poorly distributed as it enters the subsea riser. Specifically, it is thought that the gas enters the riser in a continuous stream forming large gas bubbles which move fast as compared to the liquid velocity, thus failing to contribute effectively to the reduction in average fluid gradient. It is believed that the use of an eductor downstream of the wellhead may allow for improved gas distribution through the subsea riser and lower riser base pressure, enhancing the overall rate of production. More specifically, the eductor may generate small gas bubbles in the production stream prior to the production stream entering the subsea riser. An eductor may provide a well-mixed dispersion of injection gas and fluid drawn from the production riser through a suction line and that dispersion may be introduced back into the production riser to provide lift.

[0012] Referring to FIG. 1, an apparatus may include a production riser 1, an eductor 2, a motive fluid line 3, a suction line 4, and a mixed fluid line 5. Generally, production fluid 6 moves from the pay zone, through the production riser 1, through wellhead 7, through a subsea riser 8 and to a floating vessel or other host facilities for further processing, treatment, or transport. The improvements described herein may reduce average density, reduce back pressure, and enhance production. Such improvements may mitigate flow instabilities which can enhance the processing capacity of the host facilities. Additionally, in other applications (not shown), an alternate to production fluid 6 may be return fluid from drilling or other subsea operations where lightening of such fluid is desirable to assist movement up an alternate conduit to production riser 1.

[0013] In the specific example of FIG. 1, production fluid 6 from the wellhead 7 is split into a slip stream 9 and a main stream 10. The volume of the slip stream 9 will be controlled by suction created by the eductor 2 and/or valve 11. The slip stream 9 may be drawn, via the suction line 4 to the eductor 2 for combination with high-pressure motive fluid 12 (e.g., high-pressure gas).

[0014] The eductor 2 uses the high-pressure motive fluid 12 and the split stream 9 as an inlet fluid to provide a mixed fluid 13. The eductor 2 may be configured to mix the slip stream 9 with the high-pressure motive fluid 12, providing the mixed fluid 13. The eductor 2 may include a nozzle 14 and a venturi diffuser 15. The nozzle 14 might provide high velocity and high mixing. It is important in the mechanical layout to ensure that the location of the slip stream 9 is in a place of maximum liquid hold-up with the nozzle 14 aligned with gravity in order to maximize the liquid portion of slip stream 9.

[0015] The high-pressure motive fluid 12 passes through the nozzle 14 of the eductor 2 and then joins the slip stream 9 to create the mixed fluid 13. The mixed fluid 13 passes through the venturi diffuser 15 before flowing into the mixed fluid line 5, passing by a valve 16 before joining the main stream 10 in the production riser 1. The mixed fluid 13 in combination with the main stream 10 form a lightened fluid 17 that may pass more easily toward a floating vessel or other host facilities.

[0016] The production riser 1 may extend into a wellbore (not shown) which may be cased or uncased. The production riser 1 may be configured to convey production fluid 6 from the wellbore to the wellhead 7 at the sea floor (not shown). The design of the production riser 1 might be substantially the same as conventional production risers with the exception of the provision of ports or openings for fluid communication between the production riser 1 and the eductor 2. For example, the slip stream 9 from the production fluid 6 might pass into the suction line 4, leaving a main stream 10 to pass upward through the production riser 1.

[0017] The slip stream 9 may pass through the suction line 4, optional valve 11, and into the eductor 2. Thus, the suction line 4 may be configured to provide the slip stream 9 from the production riser 1 to the eductor 2. A typical system may have lines 4 and 5 being around half of the main riser size (e.g., an 8" riser may require 4" conduits for lines 4 and 5 to accommodate the slip stream 9 and mixed fluid stream 13). Velocities at the vena contracta at the outlet of the nozzle 14 will approach the speed of sound and pressure at this point will be below the pressure in the production riser 1. This reduction in pressure is a function of the acceleration of the motive fluid through the venturi orifice and is mostly recovered in the venturi diffuser 15 with the reduction in velocity and deceleration. In this case the pressure in motive fluid line 3 must be at minimum 2.5 times the pressure of the production fluid 6.

[0018] The high-pressure motive fluid 12 may be fed to the eductor 2 via the motive fluid line 3. The motive fluid line 3 may be configured to provide the high-pressure motive fluid 12 to the eductor 2. The motive fluid line 3 may extend into the wellbore from a compressor 18 to the eductor 2. The compressor 18 may be provided outside the wellbore (e.g., above the surface), with the motive fluid line 3 extending into the wellbore. The high-pressure motive fluid 12 may be produced gas (i.e., associated gas) or an inert gas. The compressor may be a standard gas compressor found in most oil production facilities. It is also conceivable but unlikely that the compressor would be a subsea compressor, but likely not located in a well stream. In some situations there may be a high pressure gas riser taking produced gas from separate wells to the production host. This is not all that uncommon, many hosts have multiple fields tied-back, some fields may be higher pressure and higher gas content and could be used to provide the high-pressure motive fluid such that motive fluid line is connected to a nozzle on the gas riser Pipeline End Termination (PLET) assembly and is routed to the motive fluid line 3 and eductor 2 on the new PLET. Thus, the high-pressure motive fluid 12 of the illustrated example may be provided, at least in part, from a different riser tied in to the motive fluid line 3 subsea. In some instances, the high-pressure motive fluid 12 may be a multiphase liquid and gas combination with a gas to oil ration substantially higher than the gas to oil ratio of the production fluid upstream of the motive fluid line 3 and the pressure of the high-pressure motive fluid 12 may be at least twice the pressure of the production fluid upstream of the motive fluid line 3.

[0019] The mixed fluid line 5 may be configured to convey the mixed fluid 13 from the eductor 2, optionally through the valve 16, and into the production riser 1. Thereafter, the mixed fluid 13 may combine with the main stream 10, creating the lightened fluid 17. The lightened fluid 17 may have superior flow properties through a subsea riser 8 that conveys production fluid from the wellhead 7 through a body of water to host facilities (not shown). Specifically, it is believed that the lightened fluid 17 has better flow properties as compared with untreated production fluid 6. For example, it is believed that a better gas dispersion may be present in the subsea riser 8, when using the eductor 2 upstream of the wellhead 7, as compared to traditional methods.

[0020] As illustrated, the mixed fluid line 5 is configured to convey the mixed fluid 13 from the eductor 2 to the production riser 1 at a point upstream of the suction line 4. In one optional variation of the illustrated example, the mixed fluid 13 may be introduced at a point upstream of the suction line 4. Alternatively, the mixed fluid 13 may be introduced at multiple points, in which case those points might all be upstream of the suction line 4, all points might be downstream of the suction line 4, or some points may be upstream of the suction line 4 while other points are downstream of the suction line 4. Similarly, multiple slip streams 9 might be provided in various configurations and positions relative to the mixed fluid line 5.

[0021] As illustrated, the suction line 4 and the mixed fluid line 5 are both upstream of the wellhead 7. Some or all such lines may connect to the production riser inside the wellbore. Alternatively, such connections may be at or near a Christmas tree below the surface of the sea but outside the wellbore.

[0022] In other embodiments which are not illustrated for the sake of simplicity, multiple eductors 2 may be provided or the single eductor 2 may be used to balance or lighten production fluid from multiple wellbores. Thus, a system may include the wellhead 7, at least one production riser 1 as described above, at least one eductor as described above, at least one motive fluid line as described above, at least one suction line as described above, and at least one mixed fluid line as described above. The placement of the eductor or eductors 2 will be dependent on the specifics of the wellhead, Christmas tree, and production riser or risers 1. For example, several eductors 2 may be placed in series or parallel at or near the Christmas tree, but still upstream of the subsea riser 8, such that the subsea riser 8 need only convey the lightened fluid 17. Notably, even if only one eductor 2 is used in only one production riser 1, the lightened fluid 17 from that riser 1 may be combined with untreated production fluid 6 from another production riser 1 with a beneficial effect. In other words, when the lightened fluid 17 from one wellbore is combined with "regular" fluid from another wellbore, the combined fluid in the subsea riser 8 may still have comparatively better flow properties as compared to traditional methods. Thus, it is not necessary to include a separate eductor 2 for each wellbore or production riser 1.

[0023] Various methods of reducing base pressure in the subsea riser 8 are thus apparent. For example, one such method may include providing the eductor 2 upstream of the wellhead 7, the wellhead 7 being upstream of the subsea riser 8. The eductor 2 may be in the wellbore, or might be at a location above the wellbore (e.g., where streams from multiple wellbores have been combined). Providing the eductor 2 may include connecting the eductor 2 to the production riser 1 or otherwise placing the eductor 2 in a position to draw fluid from a production stream and replace the drawn fluid with a mixed gas and fluid stream.

[0024] The method may further include supplying the eductor 2 with high-pressure motive fluid 12. For example, this may involve providing the motive fluid line 3 with the compressor 18 in communication therewith. The high-pressure motive fluid 12 may thus be conveyed from the compressor 18 to the eductor 2 via the motive fluid line 3.

[0025] The method may include mixing the high-pressure motive fluid 12 and production fluid to produce the mixed fluid 13. For example, the eductor 2 may include the nozzle 14 and the venturi diffuser 15 to provide thorough mixing of the slip stream 9 and the high- pressure motive fluid 12.

[0026] Finally, the method may include introducing the mixed fluid 13 upstream of the wellhead 7. For example providing the mixed fluid line 5 configured to convey the mixed fluid 13 from the eductor 2 and simply allowing the mixed fluid 13 to flow from the eductor 2 and join the main stream 10 to create the lightened fluid 17.

[0027] Notably, closing the valve 11 may allow system to still function as a conventional system.

[0028] As compared to conventional methods, it is believed that the use of the eductor 2 will provide for better dispersion of high-pressure motive fluid 12 (e.g., high-pressure gas) in fluid (e.g., production fluid 6) such that the corresponding lightened fluid is better infused with gas as compared to similar fluid provided by conventional gas lift technology. Thus, more efficient lift may be achieved. This advantage becomes greater as the riser diameter increases.