Fluid control valve for rotary steerable tool

A downhole tool control system for a drill string rotary steerable tool that includes a 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. A spool in the inner chamber is movable into a plurality of positions to direct and control the timing and duration of the flow of drilling fluid to energize pistons of the rotary steerable tool, and to de-energize the pistons. The spool includes a first passage in fluid communication with a 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 present invention relates generally to a method and apparatus for controlling a rotary steerable tool for drilling a downhole formation. More particularly, but not exclusively, the present disclosure pertains to a fluid control valve and related method for controlling the steering and orientation in a rotary steerable tool for drilling oil and gas wells.

BACKGROUND OF THE INVENTION

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 100 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. 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.

BRIEF SUMMARY OF THE INVENTION

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 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, 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 75 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 75 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.

DETAILED DESCRIPTION OF THE INVENTION

Referring generally toFIG.1, drilling systems such as drilling system100can utilize rotary steerable tools with fluid control valves to steer a drill as it bores through a subsurface formation.FIG.1illustrates an embodiment of the drilling system100as having a bottom hole assembly102which is part of a drill string104used to form a desired, directionally drilled wellbore106. The illustrated drilling system100comprises a rotary steerable tool108that includes a steering body. The steering body includes at least one laterally movable steering pad110and is connected to a tool control system105. Tool control system105controls an actuating piston in the steering body which is connected to steering pad110. Under control of the tool control system105, the actuating piston can extend to actuate steering pad110. The tool control system105can include a fluid control valve and an electronic control unit. By way of example, the one or more steering pads110may be designed to act against a corresponding pivotable component of the rotary steerable tool108or against the surrounding wellbore wall to provide directional control. In this particular embodiment, the tool control system105is housed within a drill collar103of the rotary steerable tool108. The drill collar103and the steering body, which together form the rotary steerable tool108, are coupled with a drill bit112which is rotated to cut through a surrounding rock formation114which may be in a hydrocarbon bearing reservoir136.

Depending on the environment and the operational parameters of the drilling operation, drilling system100may comprise a variety of other features. For example, drill string104may include additional drill collars118which, in turn, may be designed to incorporate desired drilling modules, e.g. logging-while-drilling and/or measurement-while-drilling modules120. 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 system100. In the example illustrated, a drilling rig122is positioned above the wellbore106and a drilling fluid system124, e.g. drilling mud system, is used in cooperation with the drilling rig122. For example, the drilling fluid system124may be positioned to deliver a drilling fluid126from a drilling fluid tank128. The drilling fluid126is pumped through appropriate tubing130and delivered down through drilling rig122and through a central cavity or bore of drill string104. In many applications, the return flow of drilling fluid flows back up to the surface through an annulus132between the drill string104and the surrounding wellbore wall. The return flow may be used to remove drill cuttings resulting from operation of drill bit114. The drilling fluid126also may be used as an actuating fluid to control operation of the rotary steerable tool108and its movable steering pad or pads110. In this latter embodiment, flow of the drilling/actuating fluid126to steering pads110is controlled by tool control system105in a manner which enables control over the direction of drilling during formation of wellbore106.

The drilling system100also may comprise many other components, such as a surface control system134. The surface control system134can be used to communicate with rotary steerable tool108. In some embodiments, the surface control system134receives data from downhole sensor systems and also communicates commands to the rotary steerable tool108to control actuation of tool control system105and thus the direction of drilling during formation of wellbore106. In other applications, as discussed in greater detail below, control electronics are located downhole in the rotary steerable tool108and 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 system134, to receive directional commands, and/or to relay drilling related information to the surface control system.

FIG.2illustrates the rotary steerable tool108that includes steering body202with steering pad110, drill collar103and stabilizer208. The steering body202includes at least one piston connected to its associated steering pad110. In this embodiment, steering body202includes 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 collar206is 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.3shows a side view of steering body202and a partial cut away view of the collar103which together form a rotary steerable tool. Although this figure shows the collar206as connected to steering body202to form a rotary steerable tool, collar103can, in other embodiments, be connected to other devices that can benefit from the functions of the tool control system105, as an alternative to steering body202. In the cut away view, the exterior wall of the rotary steerable tool collar206is cut away to show the central cavity318of the collar103. The cavity318is an extension of, and is in fluid communication with, the uphole portions of the bore of the drill string104. Therefore, drilling fluid126under pressure from the rig pumps flows through the rotary steerable tool cavity318. AsFIG.3also shows, electronic control unit314, filter body312and fluid control valve310are located inside the rotary steerable tool collar206. The fluid control valve310is an assembly of numerous components that will be described in more detail inFIG.5. These components, alternatively, can collectively be referenced as fluid control valve assembly. The fluid control valve310attaches to the steering body202, for example via a pin connection on the steering body202, and diverts a proportion of drilling fluid via piston galleries in the fluid control valve310into flow galleries in steering body202. These fluid galleries in steering body202are connected to steering body pistons that can extend under the pressure of the drilling fluid to actuate steering pads110. The filter body312contains a filter screen that has a series of small holes through which some of the pumped drilling fluid126flows so that only filtered drilling fluid126enters the fluid control valve310. Central cavity318also houses an electronics control unit316which is encased in a pressure barrel. In some embodiments, the electronics control unit316can measure the wellbore position and calculate the required steering direction. The electronics control unit316can also include a motor that actuates a spool of the fluid control valve310.

FIG.4is a partial perspective view of the tool control system105showing the external surface and lower end of the fluid control valve310, the filter body312, and a partial view of the electronics control unit316. Filter body312receives a proportion of the drilling fluid which is pumped from the rig and which is diverted into the fluid control valve through the filter body312. The filter body312screens out large particulates from all drilling fluid126that enters fluid control valve310. Fluid control valve310selectively directs drilling fluid126pumped from the rig through piston gallery outlet ports404and into fluid galleries of the steering body202to energize steering body pistons and actuate one or more steering pads110. Drilling fluid126returning from a deenergizing piston, exits the fluid control valve310via exhaust gallery outlet ports402and the end of the exhaust galleries, and onwards to the low-pressure zone outside of the rotary steerable tool108which is commonly known as the annulus.

FIG.5is a cross sectional view through the filter body312and the fluid control valve310of the tool control system105. The fluid control valve310is an assembly of components including a fluid control valve body510having an inner chamber528which is a central cavity in the body into which drilling fluid126can flow. Preferably, the inner chamber528can be a cavity with cylindrical side walls formed by the fluid control valve body510, with a longitudinal central axis that is coaxial with the longitudinal axis of collar206and the rotary steerable tool108. The inner chamber528extends to and has an opening at an uphole end of the fluid control valve body510, identified as drilling fluid inlet port530, where filter body312can be attached and through which filtered drilling fluid126can flow into an uphole chamber portion528aof inner chamber528. At least one a piston gallery526extends from inner chamber528to an exterior surface of the fluid control valve body510where it forms a piston gallery outlet port404. Piston gallery526is a hollow passage through which drilling fluid126can flow between inner chamber528and galleries or passages in an attached actuating device, such as a steering body202. In the case of an attached steering body202, piston gallery526provides fluid communication between inner chamber528and the actuating pistons of the steering body202via galleries in the steering body202. At least one exhaust gallery522extends from a downhole chamber portion528bof inner chamber528to an exterior surface of the fluid control valve body510where it forms an exhaust gallery outlet port402. Exhaust gallery522is a hollow passage through which drilling fluid126can flow out of the downhole chamber portion528bof inner chamber528and ultimately into the annulus.

Fluid control valve310includes a valve member or spool506that has a first passage514through which fluid can flow between spool inlet ports508and first passage outlet524, and a second passage602through which fluid can flow between second passage inlet604and downhole chamber portion528bof inner chamber528(as shown inFIG.6). Spool506is located within the inner chamber528and can be moved into various positions to control the flow of drilling fluid126from the drilling fluid inlet port530to each of the piston galleries526and to control the flow of drilling fluid126from each of the piston galleries526via the inner chamber528to the exhaust galleries522. Spool506also isolates and maintains a fluid seal between the uphole chamber portion528aand the downhole chamber portion528b, preventing drilling fluid126in the uphole chamber portion528afrom directly communicating with or flowing into the downhole chamber portion528band escaping through any exhaust galleries. To isolate the uphole chamber portion528afrom downhole chamber portion528b, spool506preferably extends across the entire cavity to seal against the periphery of the wall of inner chamber528. According to some embodiments, the seal can be formed by tight tolerances between the spool and the periphery of the wall of inner chamber528. With these tight tolerances, the gap between the spool and the periphery of the wall inner chamber528should be small enough to reduce leakage of drilling fluid from high fluid pressure areas in the uphole chamber portion528ato low pressure areas in the downhole chamber portion528bso 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, spool506can use any type of suitable sealing element to extend in the gap between spool506and the periphery of the wall of inner chamber528to form an effective, durable seal while minimizing friction between the spool506and the wall of inner chamber528.

When spool506is positioned so that first passage outlet524aligns with at least a portion the opening of a piston gallery526, the spool provides a flow path between uphole chamber portion528aand the aligned piston gallery. In this position, the spool can receive drilling fluid126from drilling fluid inlet port530into the first passage514through spool inlet ports508which can flow to first passage outlet524and into piston gallery526. Thus, in this position, although the first passage514is in fluid communication with the uphole chamber portion528aand the drilling fluid inlet530, the first passage514remains isolated from the downhole chamber portion528band exhaust gallery522.

When spool506is positioned so that second passage inlet604aligns with at least a portion of the opening of a piston gallery526, (as shown inFIG.6) spool506provides a flow path between the aligned piston gallery526and the downhole chamber portion528b. In this position, fluid in piston gallery526can flow through second passage602into the downhole chamber portion528band exit fluid control valve310through exhaust gallery522. Thus, in this position, although the second passage602is in fluid communication with the downhole chamber portion528band the exhaust gallery522, the second passage602remains isolated from the uphole chamber portion528aand drilling fluid inlet port530.

The positioning of the first passage outlet524, second passage inlet604, and piston gallery opening at the wall of the inner chamber528, can determine the positions in which spool506provides a flow path between an aligned piston gallery526and either the drilling fluid inlet. The size and shape of the first passage outlet524, second passage inlet604and piston gallery opening at the wall of the inner chamber528can determine the magnitude of the flow path at various positions of spool506and the ease with which drilling fluid126can flow into a piston from the drilling fluid inlet port530and through first passage514or flow out of a piston to the annulus via second passage602, downhole chamber portion528band exhaust gallery522.

A suitable motor can actuate the spool506and move it from one position to another depending on the positions of the outlets of the piston galleries526and the positions of the first passage outlet524and second passage inlet604by, 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 chamber528at 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 inFIGS.5and6, the motor can rotate spool506around 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 unit314that can be coupled via drive shaft534to rotate spool506around a central longitudinal axis of the rotary steerable tool108. With such rotational actuation of the spool506, controlling the speed of rotation and appropriately selecting the size, shape, and angular positioning of the first passage outlet524and the second passage inlet,604, the fluid control valve310can 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 spool506, spool506can optionally be mounted in inner chamber528on bearings516,520within sleeve518. This arrangement can provide for more tightly controlling clearance and minimizing fluid to leak between spool506, bearings516,520and sleeve518.

As shown more clearly inFIGS.7and8, in some embodiments, such as the embodiments shown inFIGS.5and6, spool506of fluid control valve310can include a first passage514through which high pressure drilling fluid126from the uphole chamber portion528acan enter and flow before exiting through the first passage outlet524and into piston gallery526. Spool506can further include a lower wall or flange705which extends to the periphery of the wall of inner chamber528and around spool506and helps to seal high pressure drilling fluid126flowing through first passage outlet524from low pressure drilling fluid126in the downhole chamber portion528b. Lower flange705therefore includes a low-pressure side703which can be exposed to low fluid pressure during operation. Spool506can also include an upper wall or flange704which extends to the periphery of the wall of inner chamber528and around spool506and helps to seal high pressure drilling fluid126flowing through first passage outlet524from high pressure drilling fluid126in the uphole chamber portion528a. Lower flange705therefore include a high-pressure side701which can be exposed to high fluid pressure during operation. However, generally in operation, the pressure difference between fluid adjacent high pressure side701and fluid in or adjacent first passage outlet524is negligible compared to the pressure difference between fluid adjacent low-pressure side703and fluid adjacent in first passage outlet524. The larger pressure differentials between low-pressure side703and first passage outlet524can potentially cause much more severe fluid leakage and pressure loss across lower flange705compared to the fluid leakage that the fluid pressure differential between high-pressure side701and first passage outlet524causes across upper flange704. Thus, in the areas surrounding the first passage outlet524, efficient operation of fluid control valve310can require flange705to provide a more effective and stronger seal than flange704.

In addition, fluid control valve310can include a second passage inlet604and a second passage602through which low pressure drilling fluid126can exhaust from piston gallery526through downhole chamber portion528b. To isolate and seal the flow of fluid in and adjacent to second passage inlet604, upper wall or flange704helps to seal high pressure drilling fluid126in uphole chamber portion528afrom leaking into low pressure drilling fluid126in and adjacent to the second passage inlet604. Similarly, to isolate and seal the flow of fluid in and adjacent to second passage inlet604, lower wall or flange705helps to seal drilling fluid126flowing in and adjacent second passage inlet604from leaking into downhole chamber portion528b. However, generally in operation, the pressure difference between fluid adjacent high pressure side701and fluid in or adjacent second passage inlet604is much more significant and greater compared to the pressure difference between fluid adjacent low-pressure side703and fluid adjacent in first passage outlet604. The larger pressure differentials between high-pressure side701and second passage inlet604can potentially cause much more severe fluid leakage and pressure loss across upper flange704compared to the fluid leakage that the fluid pressure differential between low-pressure side703and second passage inlet604causes across lower flange705. Thus, in the areas surrounding the second passage inlet604, efficient operation of fluid control valve310can require flange704to provide a more effective and stronger seal than flange705.

A fluid control valve according to an alternative embodiment of a fluid control valve310can include an alternate spool900, shown inFIGS.9and10. Spool900can also include a first passage901and a first passage outlet924, through which high pressure drilling fluid126from the uphole chamber portion528acan enter and flow before exiting through the first passage outlet924and into piston gallery526. In addition, spool900can also include a second passage and a second passage inlet922through which fluid can exit and exhaust from piston gallery526into downhole chamber portion528b. However, as will be explained further below, because of the low pressure differentials that generally exist in normal operation in drilling fluid126between fluid in uphole chamber portion528aand first passage outlet924can be negligible, spool900does not require an upper flange that extends to the periphery of the wall of inner chamber528to provide a seal between uphole chamber portion528aand first passage outlet924. Similarly, because of the low pressure differentials that generally exist in normal operation in drilling fluid126between fluid in downhole chamber portion528band second passage inlet922can be negligible, spool900does not require a lower flange that extends to the periphery of the wall of inner chamber528to provide a seal between downhole chamber portion528band second passage inlet922. By avoiding the use of upper and lower flanges in areas where sufficient sealing can be provided by other means, drag and friction between spool900and the wall of inner chamber528can be reduced, facilitating easy rotation and movement of spool within the inner chamber528especially in the instances where drilling mud126contains high levels of loss circulation material. However, as can be seen inFIGS.9and10, spool900includes a serpentine flange905that extends to the periphery of the wall of inner chamber528to provide a seal between downhole chamber portion528band second passage inlet922, provide a seal between uphole chamber portion528aand first passage outlet924and, in addition, provides a seal between the second passage inlet922, which can contain fluid at low pressure, and first passage outlet924, which can contain fluid at high pressure, during normal tool operation.

FIG.11shows alternative valve spool900installed in fluid control valve301in a first position to admit drilling fluid126in uphole chamber portion528athrough first passage901, first passage outlet924, and into piston gallery525, and thereby energize a piston. Valve spool900can be movably mounted in fluid control valve301on a low friction a journal, bushing, or bearing, such as bearings516and520, optionally within sleeve518, to lower friction and the resistance of moving spool900as desired to control the flow of drilling fluid126. Although no upper wall or flange separates uphole chamber portion528afrom first passage outlet924, or lower chamber portion528bfrom second passage inlet922, bearings516and520should preferably be selected to provide a partial barrier to the flow of fluid between uphole chamber portion528afrom first passage outlet924, and downhole chamber portion528bfrom second passage inlet922, and thereby provide sufficient sealing. Although some fluid may leak through the bearings516,520the bearings should be selected to provide acceptably low leakage given the negligible pressure drop that should generally exist between uphole chamber portion528aand first passage outlet924, as well as between and downhole chamber portion528band second passage inlet922, in normal tool operation. Meanwhile, serpentine flange905should be designed with close tolerances or appropriate seals against the periphery of the wall of inner chamber528to provide a sufficiently fluid tight seal, as previously described, between uphole chamber portion528aand downhole chamber portion528b, and also between second passage inlet922and first passage outlet924.

FIG.12shows the spool900in a second position which allows drilling fluid126to be discharged from the piston gallery526through second passage inlet922, through the second passage of spool900, and into downhole chamber portion528b.

According to some embodiments in which the fluid control valve body510includes a plurality of piston galleries526, spool506can be configured so that at certain angles of rotation first passage outlet524at least partially aligns with an opening of first piston gallery526, while the second passage inlet604simultaneously at least partially overlaps with the opening of a second piston gallery526so that the actuation of one piston through the first piston gallery526overlaps at least in part with the discharge of another piston as drilling fluid simultaneously exits the piston through the second piston gallery526. According to other embodiments in which the fluid control valve body510includes a plurality of piston galleries526, spool506can be configured so that there are no angles of rotation at which first passage outlet524aligns with an opening of first piston gallery526while the second passage inlet604simultaneously even partially overlaps with the opening of a second piston gallery526. In such embodiments, there is no rotational position of spool506where the actuation of one piston through the flow of drilling fluid into a first piston gallery526overlaps with the discharge of another piston as drilling fluid simultaneously exits the other piston through the second piston gallery526.

The cross sectional area open to drilling fluid flow in each piston gallery526and first passage524along the flow path from the drilling fluid inlet port530into a piston being energized can also affect the ability of the tool control system105to actuate a connected device, such as a steering body202. Additionally, the cross sectional area open to drilling fluid flow in each piston gallery526, exhaust gallery522, and second passage602along the flow path of drilling fluid126from a piston to the annulus as the piston exhausts drilling fluid126and de-energizes it can also affect the performance of the tool control system105in actuating a connected device, such as a steering body202. Easier, more open flow of drilling fluid126along its flow path can allow the control system105to 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 gallery526, second passage602, and inner chamber528, through exhaust gallery522and any other galleries or passages that may be located between the exhaust gallery outlet port402till 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 gallery522or along the length of the second passage602is greater than at least half of the minimum flow cross sectional area at any point along the length of the first passage514. More preferably, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery522or along the length of the second passage602is greater than at least 75 percent of the minimum flow cross sectional area at any point along the length of the first passage514. Even more preferably, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery522or along the length of the second passage602is about the same as or greater than the minimum flow cross sectional area at any point along the length of the first passage514. Put another way, the minimum cross sectional area open to drilling fluid flow along either the length of the exhaust gallery522or along the length of the second passage602is unrestricted and is at least 95 percent of the minimum flow cross sectional area at any point along the length of the first passage514. Yet more preferably, drilling fluid flow through exhaust gallery522should not be reduced by downstream restrictions in the drilling fluid flow path beyond exhaust port402that reduces the flow cross sectional area to 95 percent or less of the minimum flow cross sectional area of the first passage514.

Thus, although there have been described particular embodiments of the present invention of a new and useful Fluid Control Valve for Rotary Steerable Tool it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.