The invention relates generally to devices and methods for unloading, or cleaning wellbores.

In the course of production of hydrocarbon production fluids from a wellbore, it is often necessary to unload, or clean, the wellbore by removing solids and liquids from the wellbore. Typically, removal of such solids and liquids is needed following stimulation. Baker Hughes has used a Sand VacTM and Well VacTM system which features a high pressure fluid jet pump for dislodging debris coupled with a debris vacuum for transporting debris solids and liquids to surface.

From EP 852 571 A1 an apparatus for borehole cleaning is known The apparatus comprises a tubular conveyance for extending from the surface into a borehole to a region to be cleaned, a motor mounted at the end of the tubular conveyance that in use is introduced into the borehole and a pump connected to the motor and having a nozzle. The pump is arranged such that, when positioned in the borehole and operated by the motor, the pump withdraws material from the borehole through the nozzle and pumps it into the tubular conveyance to the surface.

The present invention provides an arrangement and method for unloading wellbores using a high pressure fluid jet pump and debris vacuum in conjunction with nitrogen injection. The wellbore unloading arrangement includes a running string and an unloading bottom hole assembly, wherein the running string is configured to dispose the unloading bottom hole assembly into the wellbore, and wherein the unloading bottom hole assembly is carried by the running string. The unloading bottom hole assembly is configured to inject nitrogen and a liquid into the wellbore, and the unloading bottom hole assembly is further configured to draw debris into the running string using suction. The unloading bottom hole assembly incorporates a jet pump which is operable to inject the liquid through nozzles into the wellbore and subsequently to provide suction power to draw debris through vacuum ports into a flowbore of the running string. The running string may be made up of two strings of inner coiled tubing which are disposed within an outer coiled tubing string.

The unloading bottom hole assembly is affixed to the running string and further incorporates a nitrogen injection tool. Nitrogen injection during operation of the fluid jet pump decreases the density of fluids within the wellbore proximate the bottom hole assembly, thereby improving the effectiveness of the debris vacuum. The inventors have also determined that the combination of fluid jetting, vacuum and nitrogen injection allows a user to unload wellbores, at greater depths than with conventional techniques. Nitrogen injection counteracts hydrostatic pressure that could otherwise preclude the removal of debris.

For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein:

• Figure 1 is a side, cross-sectional view of an exemplary wellbore which contains a wellbore unloading arrangement constructed in accordance with the present disclosure
• Figure 2 is an axial cross-section of the running string of the wellbore unloading arrangement.
• Figure 3 is a side view of an exemplary unloading bottom hole assembly in accordance with the present disclosure.

Figure 1 illustrates an exemplary wellbore 10 which has been drilled through the earth 12 from the surface 14 down to a hydrocarbon-bearing formation 16. Debris 18 is located within the wellbore 10. The debris 18 is typically relatively fine and may include sand and cuttings.

A wellbore unloading arrangement 20 is disposed within the wellbore 10 from the surface 14 via a coiled tubing injector (not shown). The wellbore unloading arrangement 20 includes a running string 22 and an unloading bottom hole assembly 24.

Figure 2 depicts an axial cross-section of the running string 22. It can be seen that the running string 22 includes an outer coiled tubing string 26 which defines a flowbore 28. First and second inner coiled tubing strings 30, 32 are located within the flowbore 28 of the outer coiled tubing string 26.

Figure 3 illustrates an exemplary unloading bottom hole assembly 24 in greater detail. The unloading bottom hole assembly 24 includes an outer housing 34 with a nose portion 36 having injection nozzles 38 for injection of fluids into the wellbore 10. The fluids that can be injected via the injection nozzles 38 are provided via the first and second coiled tubing strings 30, 32. In a preferred embodiment, the lower end of the first coiled tubing string 30 is blocked by a plug 39 to cause fluid flowing through the first coiled tubing string 30 to exit through a lateral opening 41. Vacuum ports 40 are located in the housing 34 to permit fluid and solids to enter the flowbore 28 of the outer coiled tubing string 26 from the wellbore 10. Debris 18 will enter the flowbore 28 of the outer coiled tubing string 26 and then to surface 14. Larger debris 18 may be filtered out from entering the flowbore 28 by screening by the vacuum ports 40.

A nitrogen supply 42 and associated fluid pump 44 are located at surface 14 and are operably associated with the wellbore unloading arrangement 20 to flow nitrogen into the first inner coiled tubing string 30. A liquid supply 46 and associated fluid pump 48 are also located at surface 14 and are operably associated with the wellbore unloading arrangement 20 to flow liquid from the liquid supply 46 into the second inner coiled tubing string 32. The liquid of the liquid supply 46 is typically water but may be other fluids as well. A coiled tubing injection assembly (not shown), of a type well known in the art, is also located at surface 14 and is used to inject the wellbore unloading arrangement 20 into the wellbore 10 as well as remove it from the wellbore 10.

In operation, the wellbore unloading arrangement 20 is disposed into the wellbore 10 and lowered so that the unloading bottom hole assembly 24 is in a desired location proximate the debris 18 to be removed. Nitrogen is pumped through the first inner coiled tubing string 30 to the unloading bottom hole assembly 24. Non-nitrogen liquid is pumped down the second inner coiled tubing string 32 to the unloading bottom hole assembly 24. During operation, a fluid jet pump 33 switches between injection of liquid from the second inner coiled tubing string 32 through nozzles 38 and vacuum power to draw debris into the flowbore 28 of the outer coiled tubing string 26 through vacuum ports 40. The liquid that is injected through injection nozzles 38 will help dislodge debris 18 within the wellbore 10. This dislodged debris 18 can then be drawn into the flowbore 28 through vacuum ports 40.

During operation to remove the debris 18, nitrogen can be flowed from the nitrogen source 42 to injection nozzles 38. Nitrogen injection into the wellbore 10 will preferably occur proximate the debris 18 and may be done continuously as injection of liquid and vacuuming occurs. Also, the wellbore unloading arrangement 20 may be moved axially within the wellbore 10 during these operations to help further dislodge debris 18 and aid its removal.

The inventors believe that nitrogen injection during operation of the fluid jet pump 33 decreases the density of fluids within the wellbore 10 proximate the unloading bottom hole assembly 24, thereby improving the effectiveness of the debris vacuum. Further, the inventors have discovered that effective operation of the liquid injection and debris removal is possible at greater wellbore depths than with conventional systems.