Driven rotary steering system having a variable-orifice valve

1. A rotary steering tool having:

a plurality of hydraulically actuated steering pad assemblies;

a fluid outlet;

a valve comprising a valve port having a variable-area orifice positioned within a primary flow channel of the rotary steering tool, downhole from the steering pad assemblies, and uphole from a drill bit, wherein the variable-area orifice is positioned to face a direction of fluid flow through the primary channel,

wherein the variable-area orifice comprises at least one of the following:

a shutter valve;

a butterfly valve;

a first disk and a second disk overlying the first disk, the first disk comprising a first aperture and the second disk comprising a second aperture, wherein the valve is operable to provide unrestricted flow in a first state in which the first aperture is rotated into alignment with the second aperture, and to provide restricted flow in a second state in which the first aperture is at least partially misaligned with the second aperture; and

a valve opening and a flow restrictor, the flow restrictor comprising a piston and a seat, and wherein the valve is operable to provide unrestricted flow in a first state in which the piston is fully retracted from the seat, and to provide restricted flow in a second state in which the piston is at least partially extended toward the seat.

2. The rotary steering tool of claim 1 , wherein the rotary steering tool is operable to transmit fluid flow to a bottom-hole assembly.

3. The rotary steering tool of claim 1 , wherein the first aperture comprises a plurality of first apertures, and wherein the second aperture comprises a plurality of second apertures.

4. The rotary steering tool of claim 1 , further comprising a plurality of steering pad subassemblies positioned uphole from the valve.

5. A method of operating a rotary steering tool, the method comprising:

modifying a flow rate of fluid through a valve, wherein the rotary steering tool comprises a plurality of hydraulically actuated steering pad assemblies, wherein the valve is positioned downhole of a plurality of steering pad assemblies of the rotary steering tool, the valve comprising a valve port having a variable-area orifice; and

modifying an open area of the variable-area orifice to modify the magnitude of an axial force being applied by at least one of the steering pad assemblies,

wherein the variable-area orifice comprises at least one of the following:

a shutter valve;

a butterfly valve;

a first disk and a second disk overlying the first disk, the first disk comprising a first aperture and the second disk comprising a second aperture, wherein the valve is operable to provide unrestricted flow in a first state in which the first aperture is rotated into alignment with the second aperture, and to provide restricted flow in a second state in which the first aperture is at least partially misaligned with the second aperture; and

a valve opening and a flow restrictor, the flow restrictor comprising a piston and a seat, and wherein the valve is operable to provide unrestricted flow in a first state in which the piston is fully retracted from the seat, and to provide restricted flow in a second state in which the piston is at least partially extended toward the seat.

6. The method of claim 5 , further comprising determining a pressure differential across a drill bit of a drill string, the drill string being fluidly coupled to the rotary steering tool, wherein modifying an open area of the variable-area orifice comprises modifying an open area of the variable-area orifice based on the determined pressure differential.

7. The method of claim 5 , wherein the first aperture comprises a plurality of first apertures, and wherein the second aperture comprises a plurality of second apertures.

8. A non-linear wellbore drilling system comprising:

a rotary steering tool having a plurality of steering pad assemblies, a valve positioned downhole from the plurality of steering pad assemblies, the valve having a variable-area orifice;

a bottom-hole assembly having a drill bit;

a controller communicatively coupled to the valve;

a first pressure sensor in fluid communication with a wellbore annulus; and

a second pressure sensor in fluid communication with a bore of the bottom-hole assembly, wherein the first pressure sensor and the second pressure sensor are communicatively coupled to the controller, and

wherein the variable-area orifice comprises at least one of the following:

a shutter valve;

a butterfly valve;

a first disk and a second disk overlying the first disk, the first disk comprising a first aperture and the second disk comprising a second aperture, wherein the valve is operable to provide unrestricted flow in a first state in which the first aperture is rotated into alignment with the second aperture, and to provide restricted flow in a second state in which the first aperture is at least partially misaligned with the second aperture; and

a valve opening and a flow restrictor, the flow restrictor comprising a piston and a seat, and wherein the valve is operable to provide unrestricted flow in a first state in which the piston is fully retracted from the seat, and to provide restricted flow in a second state in which the piston is at least partially extended toward the seat.

9. The system of claim 8 , wherein the controller is operable to receive pressure measurements from the first pressure sensor and the second pressure sensor, determine a pressure drop across the drill bit based on the received pressure measurements, and modify a flow area of the variable-area orifice based on the determined pressure drop.

10. The system of claim 9 , wherein the variable-area orifice comprises a valve opening and a flow restrictor, the flow restrictor comprising a piston and a seat, and wherein the valve is operable to provide unrestricted flow in a first state in which the piston is fully retracted from the seat, and to provide restricted flow in a second state in which the piston is at least partially extended toward the seat.

11. The system of claim 9 , wherein the variable-area orifice comprises a first disk and a second disk overlying the first disk, the first disk comprising a first aperture and the second disk comprising a second aperture, and wherein the valve is operable to provide unrestricted flow in a first state in which the first aperture is rotated into alignment with the second aperture, and to provide restricted flow in a second state in which the first aperture is at least partially misaligned with the second aperture.

12. The system of claim 11 , wherein the first aperture comprises a plurality of first apertures, and wherein the second aperture comprises a plurality of second

apertures.