Patent Description:
In the oil and gas exploration and extraction industries, forming a wellbore conventionally involves using a drill string to bore a hole into a subsurface formation or substrate. The drill string, which generally includes a drill bit attached at a lower end of tubular members, such as drill collars, drill pipe, and optionally drilling motors and other downhole drilling tools, can extend thousands of feet or meters from the surface to the bottom of the well where the drill bit rotates to penetrate the subsurface formation. At times, drillers have found it useful to control the direction of drilling to follow desired non vertical trajectories to drill through or reach target subsurface formations. Thus, directional drilling can be particularly desirable to reach pockets of oil-bearing rock or to direct the well-bore away from other nearby well-bores. Typically, directional drillers initially drill wells vertically, or nearly vertically, until reaching a desired kickoff point or well depth when the driller attempts to deflect the drill bit and rapidly change the direction of drilling to steer drilling in a desired trajectory. The rapid change in the direction of drilling, also known as dog leg, can be expressed in degrees per <NUM> metres (<NUM> feet) of course length. Directional drillers have used various tools and techniques to kick off wells to achieve desired dog leg, and also to more generally steer the progress of the drill bit though subsurface formations. Early methods of directional drilling used a drilling motor with a bent housing located close to the drill bit. However this method could be problematic because for the periods of time when using such a motor to direct the wellbore, the drill string did not rotate, resulting in slow drilling speed and issues with transporting the drilling cutting back to the surface.

The industry subsequently developed rotary steerable drilling tools which allowed the drill string to be continually rotated when both steering in a direction or just drilling ahead. Most rotary steerable tools can be placed into two categories: point-the-bit and push-the-bit. Point-the-bit tools generally have a shaft on the lower end of the tool which is connected to a drill bit and by pointing the shaft in the intended drilling direction, similar to the method described above for mud motors but with the add advantage of always rotating the drill string. Push-the-bit tools generally have pistons attached to pads which push against the side of the well-bore to direct or guide the drill bit into the required direction.

There are two conventional methods of deploying the pistons on 'push-the-bit' tools. The first uses a closed-loop hydraulics system with items such as a pump, fluid control valves, pistons, and a fluid reservoir. These systems can be quite complex and expensive to build and maintain. The second method involves using the fluid within the drill string which is pumped from the drilling rig though the bottom hole assembly and out through the drill bit. By using this method, the hydraulic power required by the pistons is generated by large motors and pumps at the rig site rather than downhole. One disadvantage of using drilling mud is that it can contain abrasive elements such as sand which rapidly wear the rotary steerable tools. Another disadvantage is drilling mud can also include particles specifically added to block up small holes in the rock formations, and these particles can also cause blockages within the rotary steerable tools. Blockages in the passages, channels and fluid galleries within these tools can impair fluid flow into and out of the pistons and degrade rotary steerable tool performance.

Rotary steerable tools generally include valves known as fluid control valves to control the flow of drilling fluid or mud into the tools' pistons. Two methods can conventionally be used for controlling the actuation of pistons. In one method, a rotary steerable tool includes a valve that can be opened to actuate the piston by allowing the flow of fluid pumped through the drill string into the piston's chamber. After a period of time, the valve is closed to trap fluid in the chamber as the drilling tool continues to rotate. Although the valve remains closed, these tools included small fluid passages with bleed nozzles that allowed fluid to continually escape from the piston chamber back into the wellbore. As fluid continues to escape from the piston chamber through a bleed nozzle piston, the force on the pads pushes the piston back into its inner position and the fluid is forced out through a small bleed nozzle. This is a simple system of operation only requiring the fluid control valve to perform one function, which is to control the flow of fluid into the piston chamber. The downside of this solution though is that the bleed nozzle in the piston can become blocked with lost circulation material or foreign debris. Furthermore energy is consumed in forcing the piston back into its inner position which can result in a reduction of piston force for actual steering control. This then results in reducing achievable rotary steerable tool build rates, particularly at the higher drilling string rotational speeds.

An alternative solution has been to use fluid control valves which control both the flow of fluid into the piston and controls the flow of fluid back out of the piston. But even with these alternative solutions, the design of these fluid control valves still require restricting the exhaust flow of drilling fluid from the chamber of a de-energized piston. In addition, several of these alternative solutions are impractical as their designs are unable to accommodate the large pressure differentials between high and low pressure sides of their fluid control valve components and maintain effective fluid tight seals.

<CIT> describes a rotary valve including a seat and a rotary actuator, each with a surface, the rotary actuator rotatably mounted to a housing. The surfaces can form a seal due to their engagement with an engagement force used to maintain the engagement. One biasing device can elevate pressure in a sealed volume in the valve at a constant level above an external pressure. The elevated pressure can produce a pressure differential across the rotary actuator, thereby producing at least a portion of the engagement force. Another biasing device can act between a splined hub and a mated splined shaft, thereby applying at least a portion of the engagement force through the shaft to the rotary actuator. Fluid flowing through a screen can create a pressure drop, thereby causing a pressure differential across the rotary actuator and applying at least a portion of the engagement force to the surfaces.

<CIT> describes a method including conveying a work string with an integrated valve into a wellbore. As the work string with the integrated valve is conveyed into the wellbore, the valve can be in a first operation mode. When the valve is located within the wellbore, the valve can be adjusted to a different operation mode by selectively rotating at least a portion of the valve without longitudinal movement of the valve relative to the wellbore.

<CIT> describes a fully rotating bias unit for directional drilling rotates at bit speed while providing proportional control of directional response and steering force. The unit disposed on a drillstring transfers rotation to a drill bit and has a bore communicating fluid from the drillstring to the drill bit. Directors are disposed on the unit to rotate with it. Each of the directors is independently movable between extended and retracted conditions relative to the unit's housing. Actuators of the unit are in fluid communication between the bore and the borehole or some other low pressure. Each actuator is independently operable to direct communicated fluid from the bore to extend a respective one of the directors toward the extended condition. Meanwhile, venting of the communicated fluid from the directors to the borehole or other low pressure dump allows the respective director to retract toward the retracted condition.

Accordingly, these alternative are still unable to achieve the desired high build rates that can beneficially provide drillers with additional flexibility. Furthermore, these alternatives have limited ability to adjust the relative timing, duration, and intensity of the activation and deactivation phases to control the performance profile according to specific wellbore needs. What is needed, then, is an improved rotary steerable tool that can achieve the desired high build rates particularly at the higher drilling string rotational speeds that can beneficially provide drillers with desired performance flexibility. What is also needed is a rotary steerable tool in which the relative timing and duration of the activation and deactivation phases can be adjusted by altering downhole operation, or by simple replacement of components, to control the performance profile according to specific wellbore needs.

The present invention provides various embodiments that can address and improve upon some of the deficiencies of the prior art. In one embodiment, for example, a fluid control valve for a rotary steerable tool comprises a fluid control valve body having an inner chamber, a piston gallery extending between the inner chamber and a piston port, and an exhaust gallery extending between the inner chamber and an exhaust port, the inner chamber having a drilling fluid inlet port and also comprises a spool in the inner chamber. The spool has a first passage in fluid communication with the drilling fluid inlet port but not the exhaust port, and a second passage in fluid communication with the exhaust port but not the drilling fluid inlet port. The spool is movable to an actuation position in the inner chamber such that the first passage forms a fluid flow path between the piston gallery and the drilling inlet port, and also movable to a discharge position such that the second passage forms a fluid flow path between the piston gallery and the exhaust port.

According to one option, the fluid control valve body of this embodiment can include at least three piston galleries. According to another option the spool can be configured to rotate between the actuation position and the discharge position.

As another option, in the fluid control valve the exhaust gallery can have a flow path that is unrestricted. As yet another option, the fluid control valve the first passage has a length and a first passage minimum flow cross sectional area at some point along its length. The second passage has a length and a second passage minimum flow cross sectional area at some point along its length, wherein the exhaust gallery has a length and an exhaust gallery minimum flow cross sectional area, and wherein both the exhaust gallery minimum flow cross sectional area and the second passage minimum flow cross sectional area are greater than at least half of the first passage minimum flow cross sectional area.

Another embodiment of the present invention relates to a method of controlling a rotary steerable tool using a fluid control valve. The method includes the step of providing a fluid control valve body having an inner chamber, a piston gallery extending between the inner chamber and a piston port, and an exhaust gallery extending between the inner chamber and an exhaust port, the inner chamber having a drilling fluid inlet port. The method also includes the step of providing a spool in the inner chamber, the spool having a first passage in fluid communication with the drilling fluid inlet port but not the exhaust port, and a second passage in fluid communication with the exhaust port but not the drilling fluid inlet port. Additionally the method includes the steps of receiving fluid from the fluid inlet port into the first passage and discharging the fluid into the piston gallery, when the spool is in an actuation position, and receiving fluid from the piston gallery into the second passage and discharging the fluid into the exhaust gallery when the spool is in a discharge position.

According to one option, the method further includes rotating the spool through an angle from the actuation position to the discharge position. In this option, according to some alternatives, the fluid control valve body includes a plurality of piston galleries. According to one alternative, rotating the spool through an angle can additionally include rotating the spool through an intermediate angle wherein neither the first passage nor the second passage is in fluid communication with any of the plurality of piston galleries. According to another alternative, rotating the spool through an angle includes rotating the spool through an intermediate angle where the first passage and the second passage are in fluid communication with different piston galleries.

According to another option, the step of receiving fluid from the piston gallery into the second passage and discharging the fluid into the exhaust gallery when the spool is in a discharge position can also include discharging the fluid into the exhaust gallery with an unrestricted flow into the wellbore annulus. As an alternative in addition to this option, the first passage can have a length and a first passage minimum flow cross sectional area at some point along its length, the second passage can have a length and a second passage minimum flow cross sectional area at some point along its length, and the exhaust gallery can have a length and an exhaust gallery minimum flow cross sectional area, wherein the exhaust gallery minimum flow cross sectional area is greater than at least half of either the first passage minimum flow cross sectional area or the second passage minimum flow cross sectional area. In a further alternative, the exhaust gallery minimum flow cross sectional area can be greater than at least <NUM> percent of either the first passage minimum flow cross sectional area or the second passage minimum flow cross sectional area.

A further embodiment of the present invention is directed to a rotary steerable tool fluid control valve that comprises a fluid control valve body that has an inner chamber, a piston gallery extending between the inner chamber and a piston port, and an exhaust gallery extending between the inner chamber and an exhaust port, the inner chamber having a drilling fluid inlet port. The fluid control valve also comprises a spool in the inner chamber. The spool has a first passage in fluid communication with the drilling fluid inlet port but not the exhaust port and a second passage in fluid communication with the exhaust port but not the drilling fluid inlet port. The first passage has a length and a first passage minimum flow cross sectional area at some point along its length and the second passage has a length and a second passage minimum flow cross sectional area at some point along its length. The exhaust gallery also has a length and an exhaust gallery minimum flow cross sectional area. In this embodiment, the exhaust gallery minimum flow cross sectional area is greater than at least half of either the first passage minimum flow cross sectional area or the second passage minimum flow cross sectional area. Optionally, the exhaust gallery minimum flow cross sectional area of this embodiment can be greater than at least <NUM> percent of either the first passage minimum flow cross sectional area or the second passage minimum flow cross sectional area. The exhaust gallery minimum flow cross sectional area of this embodiment more preferably can be about the same area or greater than either the first passage minimum flow cross sectional area or the second passage minimum flow cross sectional area.

In an alternative aspect of this embodiment, the spool is movable to a first actuation position in the inner chamber such that the first passage forms a fluid flow path between the piston gallery and the drilling inlet port, and also movable to a first discharge position such that the second passage forms a fluid flow path between the piston gallery and the exhaust port. Optionally the fluid control valve body can include at least three piston galleries and a spool that is movable to a plurality of actuation positions in the inner chamber, such that the first passage forms a fluid flow path between each of the at least three piston galleries and the drilling inlet port, and also movable to a plurality of discharge positions such that the second passage forms a fluid flow path between each piston gallery and the exhaust port. According to one alternative, the spool can have an intermediate position wherein neither the first passage nor the second passage is in fluid communication with any of the plurality of piston galleries. According to another alternative, the spool can have an intermediate position wherein the first passage and the second passage are in fluid communication with different piston galleries.

Referring generally to <FIG>, drilling systems such as drilling system <NUM> can utilize rotary steerable tools with fluid control valves to steer a drill as it bores through a subsurface formation. <FIG> illustrates an embodiment of the drilling system <NUM> as having a bottom hole assembly <NUM> which is part of a drill string <NUM> used to form a desired, directionally drilled wellbore <NUM>. The illustrated drilling system <NUM> comprises a rotary steerable tool <NUM> that includes a steering body. The steering body includes at least one laterally movable steering pad <NUM> and is connected to a tool control system <NUM>. Tool control system <NUM> controls an actuating piston in the steering body which is connected to steering pad <NUM>. Under control of the tool control system <NUM>, the actuating piston can extend to actuate steering pad <NUM>. The tool control system <NUM> can include a fluid control valve and an electronic control unit. By way of example, the one or more steering pads <NUM> may be designed to act against a corresponding pivotable component of the rotary steerable tool <NUM> or against the surrounding wellbore wall to provide directional control. In this particular embodiment, the tool control system <NUM> is housed within a drill collar <NUM> of the rotary steerable tool <NUM>. The drill collar <NUM> and the steering body, which together form the rotary steerable tool <NUM>, are coupled with a drill bit <NUM> which is rotated to cut through a surrounding rock formation <NUM> which may be in a hydrocarbon bearing reservoir <NUM>.

Depending on the environment and the operational parameters of the drilling operation, drilling system <NUM> may comprise a variety of other features. For example, drill string <NUM> may include additional drill collars <NUM> which, in turn, may be designed to incorporate desired drilling modules, e.g. logging-while-drilling and/or measurement-while-drilling modules <NUM>. In some applications, stabilizers may be used along the drill string to stabilize the drill string with respect to the surrounding wellbore wall.

Various surface systems also may form a part of the drilling system <NUM>. In the example illustrated, a drilling rig <NUM> is positioned above the wellbore <NUM> and a drilling fluid system <NUM>, e.g. drilling mud system, is used in cooperation with the drilling rig <NUM>. For example, the drilling fluid system <NUM> may be positioned to deliver a drilling fluid <NUM> from a drilling fluid tank <NUM>. The drilling fluid <NUM> is pumped through appropriate tubing <NUM> and delivered down through drilling rig <NUM> and through a central cavity or bore of drill string <NUM>. In many applications, the return flow of drilling fluid flows back up to the surface through an annulus <NUM> between the drill string <NUM> and the surrounding wellbore wall. The return flow may be used to remove drill cuttings resulting from operation of drill bit <NUM>. The drilling fluid <NUM> also may be used as an actuating fluid to control operation of the rotary steerable tool <NUM> and its movable steering pad or pads <NUM>. In this latter embodiment, flow of the drilling/actuating fluid <NUM> to steering pads <NUM> is controlled by tool control system <NUM> in a manner which enables control over the direction of drilling during formation of wellbore <NUM>.

The drilling system <NUM> also may comprise many other components, such as a surface control system <NUM>. The surface control system <NUM> can be used to communicate with rotary steerable tool <NUM>. In some embodiments, the surface control system <NUM> receives data from downhole sensor systems and also communicates commands to the rotary steerable tool <NUM> to control actuation of tool control system <NUM> and thus the direction of drilling during formation of wellbore <NUM>. In other applications, as discussed in greater detail below, control electronics are located downhole in the rotary steerable tool <NUM> and the control electronics cooperate with an orientation sensor to control the direction of drilling. However, the downhole, control electronics may be designed to communicate with surface control system <NUM>, to receive directional commands, and/or to relay drilling related information to the surface control system.

<FIG> illustrates the rotary steerable tool <NUM> that includes steering body <NUM> with steering pad <NUM>, drill collar <NUM> and stabilizer <NUM>. The steering body <NUM> includes at least one piston connected to its associated steering pad <NUM>. In this embodiment, steering body <NUM> includes three pistons and associated pads. The pistons are designed to extend from an inner to outer position, pushing its associated pad into press against the side of the wellbore to push the tool in the opposite direction.

The collar <NUM> is a typical drilling tool collar with a central passage way to allow for the flow of fluid from the drilling rig to pass through and also to house an electronic control unit.

<FIG> shows a side view of steering body <NUM> and a partial cut away view of the collar <NUM> which together form a rotary steerable tool. Although this figure shows the collar <NUM> as connected to steering body <NUM> to form a rotary steerable tool, collar <NUM> can, in other embodiments, be connected to other devices that can benefit from the functions of the tool control system <NUM>, as an alternative to steering body <NUM>. In the cut away view, the exterior wall of the rotary steerable tool collar <NUM> is cut away to show the central cavity <NUM> of the collar <NUM>. The cavity <NUM> is an extension of, and is in fluid communication with, the uphole portions of the bore of the drill string <NUM>. Therefore, drilling fluid <NUM> under pressure from the rig pumps flows through the rotary steerable tool cavity <NUM>. As <FIG> also shows, electronic control unit <NUM>, filter body <NUM> and fluid control valve <NUM> are located inside the rotary steerable tool collar <NUM>. The fluid control valve <NUM> is an assembly of numerous components that will be described in more detail in <FIG>. These components, alternatively, can collectively be referenced as fluid control valve assembly. The fluid control valve <NUM> attaches to the steering body <NUM>, for example via a pin connection on the steering body <NUM>, and diverts a proportion of drilling fluid via piston galleries in the fluid control valve <NUM> into flow galleries in steering body <NUM>. These fluid galleries in steering body <NUM> are connected to steering body pistons that can extend under the pressure of the drilling fluid to actuate steering pads <NUM>. The filter body <NUM> contains a filter screen that has a series of small holes through which some of the pumped drilling fluid <NUM> flows so that only filtered drilling fluid <NUM> enters the fluid control valve <NUM>. Central cavity <NUM> also houses an electronics control unit <NUM> which is encased in a pressure barrel. In some embodiments, the electronics control unit <NUM> can measure the wellbore position and calculate the required steering direction. The electronics control unit <NUM> can also include a motor that actuates a spool of the fluid control valve <NUM>.

<FIG> is a partial perspective view of the tool control system <NUM> showing the external surface and lower end of the fluid control valve <NUM>, the filter body <NUM>, and a partial view of the electronics control unit <NUM>. Filter body <NUM> receives a proportion of the drilling fluid which is pumped from the rig and which is diverted into the fluid control valve through the filter body <NUM>. The filter body <NUM> screens out large particulates from all drilling fluid <NUM> that enters fluid control valve <NUM>. Fluid control valve <NUM> selectively directs drilling fluid <NUM> pumped from the rig through piston gallery outlet ports <NUM> and into fluid galleries of the steering body <NUM> to energize steering body pistons and actuate one or more steering pads <NUM>. Drilling fluid <NUM> returning from a deenergizing piston, exits the fluid control valve <NUM> via exhaust gallery outlet ports <NUM> and the end of the exhaust galleries, and onwards to the low-pressure zone outside of the rotary steerable tool <NUM> which is commonly known as the annulus.

<FIG> is a cross sectional view through the filter body <NUM> and the fluid control valve <NUM> of the tool control system <NUM>. The fluid control valve <NUM> is an assembly of components including a fluid control valve body <NUM> having an inner chamber <NUM> which is a central cavity in the body into which drilling fluid <NUM> can flow. Preferably, the inner chamber <NUM> can be a cavity with cylindrical side walls formed by the fluid control valve body <NUM>, with a longitudinal central axis that is coaxial with the longitudinal axis of collar <NUM> and the rotary steerable tool <NUM>. The inner chamber <NUM> extends to and has an opening at an uphole end of the fluid control valve body <NUM>, identified as drilling fluid inlet port <NUM>, where filter body <NUM> can be attached and through which filtered drilling fluid <NUM> can flow into an uphole chamber portion 528a of inner chamber <NUM>. At least one a piston gallery <NUM> extends from inner chamber <NUM> to an exterior surface of the fluid control valve body <NUM> where it forms a piston gallery outlet port <NUM>. Piston gallery <NUM> is a hollow passage through which drilling fluid <NUM> can flow between inner chamber <NUM> and galleries or passages in an attached actuating device, such as a steering body <NUM>. In the case of an attached steering body <NUM>, piston gallery <NUM> provides fluid communication between inner chamber <NUM> and the actuating pistons of the steering body <NUM> via galleries in the steering body <NUM>. At least one exhaust gallery <NUM> extends from a downhole chamber portion 528b of inner chamber <NUM> to an exterior surface of the fluid control valve body <NUM> where it forms an exhaust gallery outlet port <NUM>. Exhaust gallery <NUM> is a hollow passage through which drilling fluid <NUM> can flow out of the downhole chamber portion 528b of inner chamber <NUM> and ultimately into the annulus.

Fluid control valve <NUM> includes a valve member or spool <NUM> that has a first passage <NUM> through which fluid can flow between spool inlet ports <NUM> and first passage outlet <NUM>, and a second passage <NUM> through which fluid can flow between second passage inlet <NUM> and downhole chamber portion 528b of inner chamber <NUM> (as shown in <FIG>). Spool <NUM> is located within the inner chamber <NUM> and can be moved into various positions to control the flow of drilling fluid <NUM> from the drilling fluid inlet port <NUM> to each of the piston galleries <NUM> and to control the flow of drilling fluid <NUM> from each of the piston galleries <NUM> via the inner chamber <NUM> to the exhaust galleries <NUM>. Spool <NUM> also isolates and maintains a fluid seal between the uphole chamber portion 528a and the downhole chamber portion 528b, preventing drilling fluid <NUM> in the uphole chamber portion 528a from directly communicating with or flowing into the downhole chamber portion 528b and escaping through any exhaust galleries. To isolate the uphole chamber portion 528a from downhole chamber portion 528b, spool <NUM> preferably extends across the entire cavity to seal against the periphery of the wall of inner chamber <NUM>. According to some embodiments, the seal can be formed by tight tolerances between the spool and the periphery of the wall of inner chamber <NUM>. With these tight tolerances, the gap between the spool and the periphery of the wall inner chamber <NUM> should be small enough to reduce leakage of drilling fluid from high fluid pressure areas in the uphole chamber portion 528a to low pressure areas in the downhole chamber portion 528b so that the adequate pressure differentials can be maintained between the chambers. According to other embodiments, instead of or in addition to relying on tight tolerances to form a seal, spool <NUM> can use any type of suitable sealing element to extend in the gap between spool <NUM> and the periphery of the wall of inner chamber <NUM> to form an effective, durable seal while minimizing friction between the spool <NUM> and the wall of inner chamber <NUM>.

When spool <NUM> is positioned so that first passage outlet <NUM> aligns with at least a portion the opening of a piston gallery <NUM>, the spool provides a flow path between uphole chamber portion 528a and the aligned piston gallery. In this position, the spool can receive drilling fluid <NUM> from drilling fluid inlet port <NUM> into the first passage <NUM> through spool inlet ports <NUM> which can flow to first passage outlet <NUM> and into piston gallery <NUM>. Thus, in this position, although the first passage <NUM> is in fluid communication with the uphole chamber portion 528a and the drilling fluid inlet <NUM>, the first passage <NUM> remains isolated from the downhole chamber portion 528b and exhaust gallery <NUM>.

When spool <NUM> is positioned so that second passage inlet <NUM> aligns with at least a portion of the opening of a piston gallery <NUM>, (as shown in <FIG>) spool <NUM> provides a flow path between the aligned piston gallery <NUM> and the downhole chamber portion 528b. In this position, fluid in piston gallery <NUM> can flow through second passage <NUM> into the downhole chamber portion 528b and exit fluid control valve <NUM> through exhaust gallery <NUM>. Thus, in this position, although the second passage <NUM> is in fluid communication with the downhole chamber portion 528b and the exhaust gallery <NUM>, the second passage <NUM> remains isolated from the uphole chamber portion 528a and drilling fluid inlet port <NUM>.

The positioning of the first passage outlet <NUM>, second passage inlet <NUM>, and piston gallery opening at the wall of the inner chamber <NUM>, can determine the positions in which spool <NUM> provides a flow path between an aligned piston gallery <NUM> and either the drilling fluid inlet. The size and shape of the first passage outlet <NUM>, second passage inlet <NUM> and piston gallery opening at the wall of the inner chamber <NUM> can determine the magnitude of the flow path at various positions of spool <NUM> and the ease with which drilling fluid <NUM> can flow into a piston from the drilling fluid inlet port <NUM> and through first passage <NUM> or flow out of a piston to the annulus via second passage <NUM>, downhole chamber portion 528b and exhaust gallery <NUM>.

A suitable motor can actuate the spool <NUM> and move it from one position to another depending on the positions of the outlets of the piston galleries <NUM> and the positions of the first passage outlet <NUM> and second passage inlet <NUM> by, for example, a rotational motion around a central longitudinal axis of the inner chamber and coaxially with the longitudinal axis of the rotary steerable tool, or by a longitudinal translational movement within the inner chamber. For example, if the openings of one or more piston galleries are distributed radially around the wall of the inner chamber <NUM> at a common position along the inner chamber's central axis that coincides with the positions of first passage outlet and second passage outlet, as shown in <FIG>, the motor can rotate spool <NUM> around the inner chamber's central axis so that the first passage outlet and second passage outlet alternately align with the outlets of the piston galleries. For example, the motor can, be an electrical motor housed in electronic control unit <NUM> that can be coupled via drive shaft <NUM> to rotate spool <NUM> around a central longitudinal axis of the rotary steerable tool <NUM>. With such rotational actuation of the spool <NUM>, controlling the speed of rotation and appropriately selecting the size, shape, and angular positioning of the first passage outlet <NUM> and the second passage inlet, <NUM>, the fluid control valve <NUM> can control the timing and duration of piston extension and retraction enabling the rotary steerable tool to adjust tool performance to better achieve rotary steerable tool dogleg and desired rates of rotation based on different wellbore conditions. To facilitate low friction rotation while maintaining an effective fluid seal and also facilitating replacement and maintenance of spool <NUM>, spool <NUM> can optionally be mounted in inner chamber <NUM> on bearings <NUM>, <NUM> within sleeve <NUM>. This arrangement can provide for more tightly controlling clearance and minimizing fluid to leak between spool <NUM>, bearings <NUM>, <NUM> and sleeve <NUM>.

As shown more clearly in <FIG>, in some embodiments, such as the embodiments shown in <FIG>, spool <NUM> of fluid control valve <NUM> can include a first passage <NUM> through which high pressure drilling fluid <NUM> from the uphole chamber portion 528a can enter and flow before exiting through the first passage outlet <NUM> and into piston gallery <NUM>. Spool <NUM> can further include a lower wall or flange <NUM> which extends to the periphery of the wall of inner chamber <NUM> and around spool <NUM> and helps to seal high pressure drilling fluid <NUM> flowing through first passage outlet <NUM> from low pressure drilling fluid <NUM> in the downhole chamber portion 528b. Lower flange <NUM> therefore includes a low-pressure side <NUM> which can be exposed to low fluid pressure during operation. Spool <NUM> can also include an upper wall or flange <NUM> which extends to the periphery of the wall of inner chamber <NUM> and around spool <NUM> and helps to seal high pressure drilling fluid <NUM> flowing through first passage outlet <NUM> from high pressure drilling fluid <NUM> in the uphole chamber portion 528a. Lower flange <NUM> therefore include a high-pressure side <NUM> which can be exposed to high fluid pressure during operation. However, generally in operation, the pressure difference between fluid adjacent high pressure side <NUM> and fluid in or adjacent first passage outlet <NUM> is negligible compared to the pressure difference between fluid adjacent low-pressure side <NUM> and fluid adjacent in first passage outlet <NUM>. The larger pressure differentials between low-pressure side <NUM> and first passage outlet <NUM> can potentially cause much more severe fluid leakage and pressure loss across lower flange <NUM> compared to the fluid leakage that the fluid pressure differential between high-pressure side <NUM> and first passage outlet <NUM> causes across upper flange <NUM>. Thus, in the areas surrounding the first passage outlet <NUM>, efficient operation of fluid control valve <NUM> can require flange <NUM> to provide a more effective and stronger seal than flange <NUM>.

In addition, fluid control valve <NUM> can include a second passage inlet <NUM> and a second passage <NUM> through which low pressure drilling fluid <NUM> can exhaust from piston gallery <NUM> through downhole chamber portion 528b. To isolate and seal the flow of fluid in and adjacent to second passage inlet <NUM>, upper wall or flange <NUM> helps to seal high pressure drilling fluid <NUM> in uphole chamber portion 528a from leaking into low pressure drilling fluid <NUM> in and adjacent to the second passage inlet <NUM>. Similarly, to isolate and seal the flow of fluid in and adjacent to second passage inlet <NUM>, lower wall or flange <NUM> helps to seal drilling fluid <NUM> flowing in and adjacent second passage inlet <NUM> from leaking into downhole chamber portion 528b. However, generally in operation, the pressure difference between fluid adjacent high pressure side <NUM> and fluid in or adjacent second passage inlet <NUM> is much more significant and greater compared to the pressure difference between fluid adjacent low-pressure side <NUM> and fluid adjacent in first passage outlet <NUM>. The larger pressure differentials between high-pressure side <NUM> and second passage inlet <NUM> can potentially cause much more severe fluid leakage and pressure loss across upper flange <NUM> compared to the fluid leakage that the fluid pressure differential between low-pressure side <NUM> and second passage inlet <NUM> causes across lower flange <NUM>. Thus, in the areas surrounding the second passage inlet <NUM>, efficient operation of fluid control valve <NUM> can require flange <NUM> to provide a more effective and stronger seal than flange <NUM>.

A fluid control valve according to an alternative embodiment of a fluid control valve <NUM> can include an alternate spool <NUM>, shown in <FIG>. Spool <NUM> can also include a first passage <NUM> and a first passage outlet <NUM>, through which high pressure drilling fluid <NUM> from the uphole chamber portion 528a can enter and flow before exiting through the first passage outlet <NUM> and into piston gallery <NUM>. In addition, spool <NUM> can also include a second passage and a second passage inlet <NUM> through which fluid can exit and exhaust from piston gallery <NUM> into downhole chamber portion 528b. However, as will be explained further below, because of the low pressure differentials that generally exist in normal operation in drilling fluid <NUM> between fluid in uphole chamber portion 528a and first passage outlet <NUM> can be negligible, spool <NUM> does not require an upper flange that extends to the periphery of the wall of inner chamber <NUM> to provide a seal between uphole chamber portion 528a and first passage outlet <NUM>. Similarly, because of the low pressure differentials that generally exist in normal operation in drilling fluid <NUM> between fluid in downhole chamber portion 528b and second passage inlet <NUM> can be negligible, spool <NUM> does not require a lower flange that extends to the periphery of the wall of inner chamber <NUM> to provide a seal between downhole chamber portion 528b and second passage inlet <NUM>. By avoiding the use of upper and lower flanges in areas where sufficient sealing can be provided by other means, drag and friction between spool <NUM> and the wall of inner chamber <NUM> can be reduced, facilitating easy rotation and movement of spool within the inner chamber <NUM> especially in the instances where drilling mud <NUM> contains high levels of loss circulation material. However, as can be seen in <FIG>, spool <NUM> includes a serpentine flange <NUM> that extends to the periphery of the wall of inner chamber <NUM> to provide a seal between downhole chamber portion 528b and second passage inlet <NUM>, provide a seal between uphole chamber portion 528a and first passage outlet <NUM> and, in addition, provides a seal between the second passage inlet <NUM>, which can contain fluid at low pressure, and first passage outlet <NUM>, which can contain fluid at high pressure, during normal tool operation.

<FIG> shows alternative valve spool <NUM> installed in fluid control valve <NUM> in a first position to admit drilling fluid <NUM> in uphole chamber portion 528a through first passage <NUM>, first passage outlet <NUM>, and into piston gallery <NUM>, and thereby energize a piston. Valve spool <NUM> can be movably mounted in fluid control valve <NUM> on a low friction a journal, bushing, or bearing, such as bearings <NUM> and <NUM>, optionally within sleeve <NUM>, to lower friction and the resistance of moving spool <NUM> as desired to control the flow of drilling fluid <NUM>. Although no upper wall or flange separates uphole chamber portion 528a from first passage outlet <NUM>, or lower chamber portion 528b from second passage inlet <NUM>, bearings <NUM> and <NUM> should preferably be selected to provide a partial barrier to the flow of fluid between uphole chamber portion 528a from first passage outlet <NUM>, and downhole chamber portion 528b from second passage inlet <NUM>, and thereby provide sufficient sealing. Although some fluid may leak through the bearings <NUM>, <NUM> the bearings should be selected to provide acceptably low leakage given the negligible pressure drop that should generally exist between uphole chamber portion 528a and first passage outlet <NUM>, as well as between and downhole chamber portion 528b and second passage inlet <NUM>, in normal tool operation. Meanwhile, serpentine flange <NUM> should be designed with close tolerances or appropriate seals against the periphery of the wall of inner chamber <NUM> to provide a sufficiently fluid tight seal, as previously described, between uphole chamber portion 528a and downhole chamber portion 528b, and also between second passage inlet <NUM> and first passage outlet <NUM>.

<FIG> shows the spool <NUM> in a second position which allows drilling fluid <NUM> to be discharged from the piston gallery <NUM> through second passage inlet <NUM>, through the second passage of spool <NUM>, and into downhole chamber portion 528b.

According to some embodiments in which the fluid control valve body <NUM> includes a plurality of piston galleries <NUM>, spool <NUM> can be configured so that at certain angles of rotation first passage outlet <NUM> at least partially aligns with an opening of first piston gallery <NUM>, while the second passage inlet <NUM> simultaneously at least partially overlaps with the opening of a second piston gallery <NUM> so that the actuation of one piston through the first piston gallery <NUM> overlaps at least in part with the discharge of another piston as drilling fluid simultaneously exits the piston through the second piston gallery <NUM>. According to other embodiments in which the fluid control valve body <NUM> includes a plurality of piston galleries <NUM>, spool <NUM> can be configured so that there are no angles of rotation at which first passage outlet <NUM> aligns with an opening of first piston gallery <NUM> while the second passage inlet <NUM> simultaneously even partially overlaps with the opening of a second piston gallery <NUM>. In such embodiments, there is no rotational position of spool <NUM> where the actuation of one piston through the flow of drilling fluid into a first piston gallery <NUM> overlaps with the discharge of another piston as drilling fluid simultaneously exits the other piston through the second piston gallery <NUM>.

The cross sectional area open to drilling fluid flow in each piston gallery <NUM> and first passage <NUM> along the flow path from the drilling fluid inlet port <NUM> into a piston being energized can also affect the ability of the tool control system <NUM> to actuate a connected device, such as a steering body <NUM>. Additionally, the cross sectional area open to drilling fluid flow in each piston gallery <NUM>, exhaust gallery <NUM>, and second passage <NUM> along the flow path of drilling fluid <NUM> from a piston to the annulus as the piston exhausts drilling fluid <NUM> and de-energizes it can also affect the performance of the tool control system <NUM> in actuating a connected device, such as a steering body <NUM>. Easier, more open flow of drilling fluid <NUM> along its flow path can allow the control system <NUM> to provide increased performance such as increased tool rotation rates (RPM), more dogleg, and the ability to handle larger volumes of lost circulation material when actuating a steering body. Other potential benefits can include reducing back pressure on pistons as they exhaust drilling fluid. Reducing back pressure can result in lower forces on the pistons and reduced piston wear. Accordingly, the drilling fluid's path from a piston, via a piston gallery <NUM>, second passage <NUM>, and inner chamber <NUM>, through exhaust gallery <NUM> and any other galleries or passages that may be located between the exhaust gallery outlet port <NUM> till its exit to the annulus, preferably includes no small restrictions such as bleed nozzles. In this way, the drilling fluid can travel from the piston to the low-pressure zone of the annulus with a minimal pressure drop. To minimize pressure drop, the cross sectional area of the drilling fluid's flow path as it exits from a piston when it is de-energized should not be unduly restricted as compared to the flow path of the drilling fluid that enters the piston during activation. Accordingly, preferably the minimum flow cross sectional area, i.e., the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery <NUM> or along the length of the second passage <NUM> is greater than at least half of the minimum flow cross sectional area at any point along the length of the first passage <NUM>. More preferably, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery <NUM> or along the length of the second passage <NUM> is greater than at least <NUM> percent of the minimum flow cross sectional area at any point along the length of the first passage <NUM>. Even more preferably, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery <NUM> or along the length of the second passage <NUM> is about the same as or greater than the minimum flow cross sectional area at any point along the length of the first passage <NUM>. Put another way, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery <NUM> or along the length of the second passage <NUM> is unrestricted and is at least <NUM> percent of the minimum flow cross sectional area at any point along the length of the first passage <NUM>. Yet more preferably, drilling fluid flow through exhaust gallery <NUM> should not be reduced by downstream restrictions in the drilling fluid flow path beyond exhaust port <NUM> that reduces the flow cross sectional area to <NUM> percent or less of the minimum flow cross sectional area of the first passage <NUM>.

Claim 1:
A fluid control valve (<NUM>) for a rotary steerable tool (<NUM>) comprising:
a fluid control valve body (<NUM>) having an inner chamber (<NUM>), a piston gallery (<NUM>) extending between the inner chamber and a piston port (<NUM>), and an exhaust gallery (<NUM>) extending between the inner chamber and an exhaust port (<NUM>), the inner chamber having a drilling fluid inlet port (<NUM>);
a spool (<NUM>,<NUM>) in the inner chamber, the spool having a first passage (<NUM>,<NUM>) in fluid communication with the drilling fluid inlet port and the piston port but not the exhaust port, and a second passage (<NUM>) in fluid communication with the exhaust port but not the drilling fluid inlet port;
wherein the spool is movable to an actuation position in the inner chamber such that the first passage forms a fluid flow path between the piston gallery and the drilling inlet port, and also movable to a discharge position such that the second passage forms a fluid flow path between the piston gallery and the exhaust port,
characterized in that the inner chamber includes an uphole chamber portion (528a) and a downhole chamber portion (528b);
wherein the drilling fluid inlet port is in the uphole chamber portion, the exhaust gallery extends between the downhole chamber portion of the inner chamber and the exhaust port; and
wherein the spool maintains a fluid seal between the uphole chamber portion and the downhole chamber portion.