Patent Publication Number: US-11035174-B2

Title: Strategic flexible section for a rotary steerable system

Description:
The present application is a U.S. National Stage patent application of International Application No. PCT/US2018/033037, filed on May 16, 2018, which claims priority to U.S. Provisional Application No. 62/513,365 filed May 31, 2017 both entitled “Strategic Flexible Section for a Rotary Steerable System,” the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to rotary steerable systems (RSS), e.g., drilling systems employed for directionally drilling wellbores in oil and gas exploration and production. More particularly, embodiments of the disclosure relate to rotary steerable systems having flexible collar therein for achieving a desired steering radii. 
     Directional drilling operations involve controlling the direction of a wellbore as it is being drilled. Usually the goal of directional drilling is to reach a target subterranean destination with a drill string, and often the drill string will need to be turned through a tight radius to reach the target destination. Generally, an RSS changes direction either by pushing against one side of a wellbore wall with steering pads to thereby cause the drill bit to push on the opposite side (in a push-the-bit system), or by bending a main shaft running through a non-rotating housing to point the drill bit in a particular direction with respect to the rest of the tool (in a point-the-bit system). In a push-the-bit system, the wellbore wall is generally in contact with the drill bit, the steering pads and a stabilizer. The steering capability of such a system is predominantly defined by a curve that can be fitted through each of the drill bit, steering pads and the stabilizer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described in detail hereinafter, by way of example only, on the basis of examples represented in the accompanying figures, in which: 
         FIG. 1  is a partial cross-sectional side view of a directional wellbore drilled with a bottom hole assembly including an RSS; 
         FIG. 2  is a schematic view of a bottom hole assembly including a flexible collar coupled to an up-hole end of an RSS; 
         FIG. 3A  is a schematic view of an RSS having a flexible collar coupled between a steering section and a control section thereof; 
         FIG. 3B  is a cross sectional view of the flexible collar of  FIG. 3A   
         FIG. 4  is a schematic view of an RSS having a flexible collar wherein control components are disposed within a flexible collar; 
         FIG. 5A  is schematic illustration of an example flexible collar having a generally cylindrical configuration; 
         FIG. 5B  is table illustrating geometric and stiffness characteristics of two example flexible collars configured as the flexible collar of  FIG. 5A  and constructed of different materials (steel and titanium); 
         FIG. 6  is a graphical view illustrating the dogleg severity achievable with the two example flexible collars of  FIG. 5B  as a function of weight on bit at a variety of inclinations illustrating improved build rate capabilities; 
         FIG. 7  is a graphical view illustrating a the dogleg severity achievable with the two example flexible collars of  FIG. 5B  as a function of weight on bit at a variety of inclinations illustrating improved drop rate capabilities; and 
         FIG. 8  is a flowchart illustrating a process of configuring and constructing a rotary steerable system. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure includes an RSS having a flexible collar coupled therein that permits a desired turning radius to be achieved. The flexible collar may be positioned at an up-hole end of a bottom hole assembly including an RSS, or alternatively, the flexible collar may be positioned between a steering section and the controller of the RSS. The parameters affecting the geometry and stiffness characteristics of the flexible collar may be selected strategically to match the requirements of the particular wellbore being drilled. Also, a drill bit for the rotary steerable system may be selected such that a side cutting efficiency of the drill bit, together with the placement and stiffness characteristics of the flexible collar, may be selected strategically to match the requirements of the particular wellbore being drilled. By selecting these parameters strategically, improvements related to tool length, bending stiffness, bending stress, torsional stiffness, shear stress due to torsion and increased dogleg severity tolerance may be obtained. 
       FIG. 1  is a partial cross-sectional side view of a directional wellbore drilled with a bottom hole assembly (BHA) including an RSS. An exemplary directional drilling system  10  is illustrated including a tower or “derrick”  11  that is buttressed by a derrick floor  12 . The derrick floor  12  supports a rotary table  14  that is driven at a desired rotational speed, for example, via a chain drive system through operation of a prime mover (not shown). The rotary table  14 , in turn, provides the necessary rotational force to a drill string  20 . The drill string  20 , which includes a drill pipe section  24 , extends downwardly from the rotary table  14  into a directional wellbore or borehole  26 . The borehole  26  may exhibit a multi-dimensional path or “trajectory.” The three-dimensional direction of the bottom  54  of the borehole  26  of  FIG. 1  is represented by arrow  52 . 
     A drill bit  50  is attached to the distal, downhole end of the drill string  20 . When rotated, e.g., via the rotary table  14 , the drill bit  50  operates to break up and generally disintegrate the geological formation  46 . The drill string  20  is coupled to a “drawworks” hoisting apparatus  30 , for example, via a kelly joint  21 , swivel  28 , and line  29  through a pulley system (not shown). During a drilling operation, the drawworks  30  can be operated, in some embodiments, to control the weight on drill bit  50  and the rate of penetration of the drill string  20  into the borehole  26 . 
     During drilling operations, a suitable drilling fluid or “mud”  31  can be circulated, under pressure, out from a mud pit  32  and into the borehole  26  through the drill string  20  by a hydraulic “mud pump”  34 . Mud  31  passes from the mud pump  34  into the drill string  20  via a fluid conduit (commonly referred to as a “mud line”)  38  and the kelly joint  21 . Drilling fluid  31  is discharged at the borehole bottom  54  through an opening or nozzle in the drill bit  50 , and circulates in an “uphole” direction towards the surface through an annular space  27  between the drill string  20  and the side  56  of the borehole  26 . As the drilling fluid  31  approaches the rotary table  14 , it is discharged via a return line  35  into the mud pit  32 . A variety of surface sensors  48 , which are appropriately deployed on the surface of the borehole  26 , operate alone or in conjunction with downhole sensors  70 ,  72  deployed within the borehole  26 , to provide information about various drilling-related parameters, such as fluid flow rate, weight on bit, hook load, etc. 
     A surface control unit  40  may receive signals from surface and downhole sensors (e.g., sensors  48 ,  70 ,  72 ) and devices via a sensor or transducer  43 , which can be placed on the fluid line  38 . The surface control unit  40  can be operable to process such signals according to programmed instructions provided to surface control unit  40 . Surface control unit  40  may present to an operator desired drilling parameters and other information via one or more output devices  42 , such as a display, a computer monitor, speakers, lights, etc., which may be used by the operator to control the drilling operations. Surface control unit  40  may contain a computer, memory for storing data, a data recorder, and other known and hereinafter developed peripherals. Surface control unit  40  may also include models and may process data according to programmed instructions, and respond to user commands entered through a suitable input device  44 , which may be in the nature of a keyboard, touchscreen, microphone, mouse, joystick, etc. 
     In some embodiments of the present disclosure, the rotatable drill bit  50  is attached at a distal end of a bottom hole assembly (BHA)  22  comprising a rotary steerable system (RSS)  58 . In the illustrated embodiment, the BHA  22  is coupled between the drill bit  50  and the drill pipe section  24  of the drill string  20 . The BHA  22  and or/the RSS  58  may comprise a Measurement While Drilling (MWD) System, with various sensors, e.g., sensors  70 ,  72 , to provide information about the formation  46  and downhole drilling parameters. The MWD sensors in the BHA  22  may include, but are not limited to, a device for measuring the formation resistivity near the drill bit, a gamma ray device for measuring natural radioactivity of the formation  46 , devices for determining the inclination and azimuth of the drill string  20 , and pressure sensors for measuring drilling fluid pressure downhole. The MWD sensors may also include additional/alternative sensing devices for measuring shock, vibration, torque, telemetry, etc. The above-noted devices may transmit data to a downhole communicator  33 , which in turn transmits the data uphole to the surface control unit  40 . In some embodiments, the BHA  22  may also include a Logging While Drilling (LWD) System. 
     A transducer  43  can be placed in the mud supply line  38  to detect mud pulses responsive to the data transmitted by the downhole communicator  33 . The transducer  43  in turn generates electrical signals, for example, in response to the mud pressure variations and transmits such signals to the surface control unit  40 . Alternatively, other telemetry techniques such as electromagnetic and/or acoustic techniques or any other suitable techniques known or hereinafter developed may be utilized. By way of example, hard wired drill pipe may be used to communicate between the surface and downhole devices. In another example, combinations of the techniques described may be used. A surface transmitter/receiver  80  communicates with downhole tools using, for example, any of the transmission techniques described, such as a mud pulse telemetry technique. This can enable two-way communication between the surface control unit  40  and the downhole communicator  33  and other downhole tools. 
     The BHA  22  and/or RSS  58  can provide some or all of the requisite force for the bit  50  to break through the formation  46  (known as “weight on bit”), and provide the necessary directional control for drilling the borehole  26 . The RSS  58  may include a steering section with steering pads  60  extendable in a lateral direction from a longitudinal axis AO of the RSS  58  to push against the geologic formation  46 . The steering pads  60  may comprise hinged pads, arms, fins, rods, energized stabilizer blades or any other element extendable from the RSS  58  to contact the side  56  of the borehole  26 . The steering pads  60  may be circumferentially spaced around the RSS  58 , and may be individually extended to contact the side  56  of the borehole  26  to alter an angle of the longitudinal axis of the RSS  58  with respect to the borehole  26  while drilling and/or apply a side force to the drill bit  50 . The steering pads  60  may include a set of at least three externally mounted steering pads  60  to exert force in a controlled orientation to deviate the drill bit  50  in the desired direction for steering. In some embodiments, the steering pads  60  are energized by a small percentage of the drilling fluid or mud  31  pumped through the drill string  20  and drill bit  50  for cuttings removal, cooling and well control. The RSS  58  is thereby using the “free” hydraulic energy of the drilling fluid or mud  31  for directional control. For traditional electrical servomotor/solenoid-type drive systems, the power requirement is in the order of 100-300 W. The steering pads  60  may provide an adjustable force or extension to assist in controlling the direction of the borehole  26 . The RSS  58  also includes a stabilizer  62  coupled to a control section thereof. 
       FIG. 2  is a schematic view of a bottom hole assembly  100  including a flexible section or flexible collar  102  coupled to an up-hole end of an RSS  104 . The flexible collar  102  may generally be constructed to exhibit a lower bending stiffness than the RSS  104  and other components of the BHA  100 . The flexible collar  102  may include a structural connector  106  such as threads, latches, etc. at leading or downhole end thereof for selectively coupling to a trailing or uphole end of the RSS  104 . The RSS  104  includes a control section  110 , flow control section  112  and steering section  114 , each of which may be packaged in a single housing with a greater bending stiffness than the flexible collar  102 . Alternatively, structural connectors  116  may be provided between the control section  110 , the flow control section  112  and the steering section  114 . The flexible collar  102  may include a drill string coupler  120  at an uphole end thereof for coupling the BHA  100  to the drill pipe section  24  ( FIG. 1 ) of the drill string  20 . The bottom hole assembly  100  may then exhibit greater flexibility than the RSS  104  alone. 
     In other embodiments, the flexible collar  102  may positioned within the RSS (see  FIG. 3 ) or at other locations within the drill string  20 . When the flexible collar  102  is positioned within the RSS, the flexible collar  102  may include a structural connector  116 , threads, latches, etc., at leading or downhole end thereof for selectively coupling to a trailing or uphole end of the steering section  114  of the RSS  104 . In some embodiments the steering section  114  may contain the flow control section  112  (see  FIG. 3A ), e.g., the steering section  114  and the flow control section  112  may be housed together with no structural connector therebetween. The flexible collar  102  may also include a structural connector  116 , threads, latches, etc. at trailing or uphole end thereof for selectively coupling to a leading or downhole end of the control section  110  of the RSS. 
     The flexible collar  102  may be strategically designed to achieve a desired dogleg severity (DLS) capability from the RSS  104  with a given placement of the flexible collar among the other components of the flexible collar  102 . Geometric sizing, material selection, and the physical construction characteristics of composite or other non-metallic materials for the flexible collar  102  may be selected to enable the RSS  104  to meet specific capability requirements. Generally, sizing of the flexible collar  102  includes selecting an outer diameter (OD), an inner diameter (ID) and a length of the flexible collar. One material property considered for material selection is the Modulus of Elasticity (E). Another material property considered for material selection is the Modulus of Rigidity (G). The strategic sizing and material selection for the flexible collar  102  may be used to increase or maximize the DLS capability when desired, e.g., to drill a high DLS build, curve, drop or turn section of a wellbore  26  ( FIG. 1 ). Similarly, the strategic sizing and material selection may be used to limit or minimize the DLS capability when desired, e.g., to drill a lower DLS build, drop or turn section, or to drill vertical, tangent, lateral, or horizontal sections of a well bore  26  in instances where a lower DLS capability is desired and/or where a high DLS capability may be problematic. Strategic sizing and material selection of the flexible collar  102  enables other attributes of the RSS  104  and flexible collar  102  to be optimized including: tool length, bending moment, bending stress, torsional stiffness, shear stress due to torsion, and increased DLS tolerance. 
     The drill bit  50  is coupled to the downhole end of the steering section  114 , which includes a plurality of steering pads  60  or other pushing devices for steering the drill bit  50 . The steering pads  60  may be constructed as hinged pad pushers, steering pistons or similar pistons such as those found on adjustable gauge stabilizers (not shown). The flow control section  112  is coupled above the steering section  114  (or comprises an uphole portion of the steering section  114 ), and is operable to divert a portion of the total drilling fluid or mud  31  ( FIG. 1 ) pumped through the BHA  100 . Typically, the flow control section  112  may include a valve set  210  ( FIG. 3 ) that deviates about 1-4% from the main mud flow. The diverted portion passes through a filter element before being directed to the respective steering pad  60  or pushing device through flow paths defined in the steering section  114 . The flow deviation is generally achieved using mechanically driven/controlled valve assemblies  210 , but other arrangements are also contemplated such as a single rotating valve that distributes the diverted portion of the flow to the respective steering pad  60  or pushing device through flow paths defined in the steering section  114 . In order to control and drive the mechanical valve assemblies  210 , servo motor, gearbox and/or bearing assemblies are traditionally employed. These gearbox and/or bearing assemblies can require volume compensation systems, if oil filling is required, and sealing solutions to prevent the ingress of drilling fluid or mud  31 . 
     The control section houses an electronics assembly  212  ( FIG. 3A ) including Directional and Inclination (D&amp;I) sensor packages, Gamma Ray (GR) sensor packages, and others types of MWD or LWD sensors. The control section  110  may also include a CPU, power conditioning, and communication device (e.g., the downhole communicator  33  ( FIG. 1 )). Power generation and/or power supply components are also generally located inside the control section  110 . The power generation and/or supply components need to be sufficiently sized to power the electronics assembly  212 , drive the mechanical valve assemblies  210  or single rotating valve and overcome any frictional losses created by seals, bearings, gearboxes, etc., or the valve itself. The stabilizer  62  is coupled to an outer housing  122  of the control section  110 . 
     The theoretical steering capability of the BHA  100  is generally defined by a curve that can be fitted through the stabilizer  62 , steering pads  60  and drill bit  50 . These are the components that generally contact the geologic formation  46  ( FIG. 1 ) when forming the wellbore  26 . Flexing of the control section  110 , flow control section  112  and steering sections  114  can increase the steering response of the BHA  100  in operation, but flexing of these sections  110 ,  112 ,  114  is typically limited in order to prevent damage or disruption of the internal components of these sections  110 ,  112 ,  114 , which could lead to a reduction in directional control accuracy (e.g., toolface control). 
       FIG. 3A  is a schematic view of an RSS  200  having the flexible collar  102  between the steering section  114  and control section  110  of the RSS  200 . This arrangement may be particularly useful when strategic sizing and material selection for the flexible collar  102  are employed to increase or maximize the DLS capability of the RSS  200 . The steering section  114  is housed together with the flow control section  112  in a housing  206 . The valve assemblies  210  or single rotating valve of the flow control section  112  are disposed in a portion of the housing  206  generally up-hole of the steering pads  60 . The control section  110  includes a modular sensor and control electronics assembly  212 . 
     In the arrangement of  FIG. 3 , the valve assemblies  210 , single rotating valve, or other flow control devices in the flow control section  112  may require an electrical connection to the modular sensor and control electronics assembly  212 . Where the valve assemblies  210  include a battery or other power source (not shown) contained in the housing  206  of the steering section  114 , the valve assemblies  210  may only need instructions to be communicated across the flexible collar  102 . The instructions may be received by a communication reception unit  218  of the steering section  114 . Where the valve assemblies  210  do not include a power source, the valve assemblies  210  may need to receive instructions as well as power through the flexible collar  102 . Instructions and data may be transmitted through a multi-conductor communication cable  222 , wire or other electrical conductor extending through the flexible collar  102 . A communication transmission unit  224  may be operatively coupled to the modular electronics assembly  212  to receive instructions therefrom, and may be operatively coupled to the communication cable  222  to transmit the instructions therethrough. Since only an electrical communication cable  222  needs to pass therethrough (e.g., no mechanical drive shaft may be necessary), the flexible collar  102  with reduced bending stiffness may be added very close to the drill bit  50 , i.e., directly above the steering pads  60 . 
     A leading stabilizer  230  may be provided in the steering section  114 , and extends laterally from the housing  206 . The leading stabilizer  230  may prevent a portion of the bending moments applied to a drill string  20  ( FIG. 1 ) extending through a curved borehole from being reacted at the steering pads  60 . These bending moments have been found, in some instances, to cause the steering pads  60  to retract into the housing  206 , thereby preventing effective steering of the drill bit  50 . The leading stabilizer  230  may be disposed adjacent or above the steering pads  60 , and may protrude from the same housing  206  as the steering pads  60 . 
     A power section  232  is provided above the control section  110 . The power section  232  may include turbine blades (not shown) that extract energy from drilling mud  31  ( FIG. 1 ) pumped down the drill string ( FIG. 1 ) to generate electrical power for the electronics assembly  212 , communication transmission unit  224 , communication reception unit  218  and the valve assemblies  210 . The valve assemblies  210  or single rotating valve may rely on an electric motor (not shown) for selectively providing drilling mud to the steering pads  60 . 
     In case flexing is not required, a flex collar  102  could become a possible future upgrade. In some embodiments, the flexible collar  102  could also be used to mount sensors to measure and record drilling parameters such as weight on bit (WOB), torque on bit (TOB), and bending loads; important data that can be used as for directional control. In order to increase the steerability and response of the RSS  200 , a selection of direction and inclination sensors may be placed below the flexible collar  102 , e.g., in the steering section  114  to provide an early indication of directional output. A flexible collar  102  may be designed, constructed and positioned within the RSS  200  to make the RSS  200  highly agile and provide a high DLS capability. Near bit direction; and/or inclination measurement data may be provided by a dynamic measurement package  240  in the steering section  114  or the flexible collar  102  (see  FIG. 4 ) for measurement of the direction and/or inclination of the drill bit  50  and/or other characteristics of a drilling operation. A survey grade sensor package  242  may be provided in the control section  110  for providing MWD and/or LWD capabilities. The near bit measurements can be of a lower quality and will be combined with the higher quality direction and inclination (D&amp;I) data from the control section to make steering decisions. 
     As indicated above, the control section  110  features a modular electronics assembly  212  including sensor packages for D&amp;I, GR, and others as well as CPU, power conditioning, and communication. The power generation/supply module section is also generally located inside the Control Section  110 . In order to allow easy diagnostics and maintenance a high degree of modularity is very desirable combined with onboard diagnostics and memory on each module to allow fault finding, service life tracking and accumulative run history capture. 
     The steering section  114  may include a set of at least three externally mounted actuator assemblies or steering pads  60  that exert force against the wellbore  26  ( FIG. 1 ) in a controlled orientation to deviate the drill bit  50  in the desired direction for steering. The steering pads  60  may be energized by a small percentage of the drilling fluid or mud  31  ( FIG. 1 ) pumped through the drill string  20  ( FIG. 1 ) and drill bit  50  for cuttings removal, cooling and well control. The RSS  200  is thereby using the “free” hydraulic energy of the drilling fluid for directional control. The actuator assemblies or steering pads  60  may be piston assemblies, hinged pads or energized stabilizer blades in various embodiments. By utilizing the “free” hydraulic energy of the drilling fluid pumped through the drill string  20 , only the energy to control the fluid flow needs to be provided. For traditional electrical servomotor/solenoid-type drive systems, the power requirement is in the order of 100-300 W. Electromechanical material based actuators offer a significantly reduced power requirement, low heat generation, a design with no moving parts that require hermetic sealing and oil filling and related compensation systems, low wear rates, high stiffness, a proportional response and a very compact design. Compared to the power requirements of traditional electrical drive systems (100-300 W) the power requirement of an electromechanical material based flow control device  210  could be as low as 10 W, or lower. The low power consumption combined with compact design should allow flow control devices  210  to be mounted externally to the steering section  114  in close proximity to the steering pads  60 . This reduces the need for expensive gun drilling operation to create flow paths in the RSS  200  to port the drilling fluid or mud to the steering pads  60 . Alternately, a single rotating valve can distribute flow to the steering pads  60  through a manifold and gun drilled ports. 
     In another embodiment the flow control devices  210  are located inside the steering pads, e.g., inside a piston assembly operable to drive the steering pads  60 . The compact design offers a key advantage in that it allows the control electronics and sensors  240  used for directional control to move much closer to the drill bit  50 , which allows for a better directional control. In other embodiments, the RSS  200  can be equipped with a compact and self-contained module for a traditional flow-control section  112 , within or attached to the steering section  114 . 
       FIG. 3B  is a cross-sectional view of the flexible collar  102 . The flexible collar  102  generally defines a first outer diameter OD 1  at leading end  240  and a trailing end  242  thereof. The first outer diameter OD 1  may be similar to the outer diameters of the housings  122  ( FIG. 2 ) and  206  ( FIG. 3A ) of the control section  110  and steering section  114 . A necked down portion  246  between the leading and trailing ends  240 ,  242  defines a second outer diameter OD 2  that is less than the first outer diameter OD 1 . The necked down portion  246  provides a reduced bending stiffness to the flexible collar  102 . In other embodiments, the flexible collar  102  can be implemented in forms other than a traditional necked down collar. For example, the flexible collar  102  may be a generally cylindrical tubular member e.g., the first and second outer diameters OD 1 , OD 2  (and a third outer diameter OD 3 ) may all be equal. A hard-faced wear band or stabilizer ( 280 ) may be provided on OD 3  to prevent excessive wear in case of contact with borehole  26 , or to limit lateral deflection of the trailing end  242  of flexible collar  102  within borehole  26 . In other embodiments, the flexible collar may be configured as a fully articulated universal joint. In the case of a fully articulated universal joint, the joint may be defined on an exterior of the housing such that the entire outer diameter of the flexible collar below the joint articulates. The lower the bending stiffness of the flexible collar  102  or flex section, the more the RSS  200  ( FIG. 3A ) behaves like a point-the-bit rotary steerable system with the potential of achieving very high dogleg severities. 
     Data and power transmission through the flexible collar  102  can be achieved in a variety of ways, e.g., a wired extender running through the flexible collar  102 , electrical conductors attached to or integrated with the flexible collar  102  or even wireless power/data transmission over a short distance. As illustrated in  FIG. 3B , the flexible collar  102  includes electrical connectors  250 ,  252  at the leading and trailing ends  240 ,  242  to facilitate coupling the flexible collar  102  to other sections  110 ,  112 ,  114 ,  232  of the RSS  200 . The connectors  250 ,  252  may comprise rotary connectors, e.g., connectors that may engage corresponding connectors in other RSS sections  110 ,  112 ,  114 ,  232  of by relative rotational movement therebetween. In some embodiments, structural connectors  254 ,  256  such as threads may be provided for coupling the flexible collar  102  to other sections  110 ,  112 ,  114 ,  232 , such that the relative rotational motion establishes both structural and electrical connections between the flexible collar  102  and the other sections  110 ,  112 ,  114 ,  232 . In some embodiments, the connectors  250 ,  252  may comprise 8-pin rotational connectors to accommodate the data and power transmission through the flexible collar  102 . Depending on the power requirements of the flow control section, a small battery or compact power generation module, e.g., vibration based, could be included. In that case only data transmission would be required facilitating a wireless flexible collar  102 . 
     The connectors  250 ,  252  may be operably coupled to one another with electrical cable  222  ( FIG. 3A ). In some embodiments, a gun-drilled longitudinal bore  260  may be provided through a wall  262  of the flexible collar  102 . The longitudinal bore  260  may be radially offset from a primary flow passage  264  extending through the flexible collar  102 . 
     The flexible collar  102  could be made replaceable and/or repositionable among the sections  110 ,  112 ,  114 ,  232  of the RSS  200  to configure the RSS  200  based on a required steering response. Detailed modeling may be performed to determine if a particular flexible collar  102  or flex section is necessary to achieve the required dogleg severity for a particular project. For example, the required dogleg severity may be a consideration in selecting a flexible collar from a source of available flexible collars  102 , or a flexible collar  102  may be constructed according to a sizing and material selection based on the required dogleg severity for the project. In some embodiments, a drill bit  50  ( FIG. 3A ) may also be selected and/or constructed to provide a necessary side cutting efficiency to accommodate or complement a particular configuration and arrangement of a flexible collar  102  in an RSS  200 . Side-cutting efficiency of the drill bit refers to the ability of the drill bit to drill laterally as a ratio of the ability of the drill bit to drill axially. Side-cutting efficiency (SCE) may be defined as: 
     
       
         
           
             SCE 
             = 
             
               
                 ( 
                 
                   Rate 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
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                     Penetration 
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                     Side 
                   
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                   Force 
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                   of 
                   ⁢ 
                   
                       
                   
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                   Axial 
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                     Penetration 
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                     Axial 
                   
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                   Weight 
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     The typical range of SCE for a PDC drill bit is 0.01 to 0.50. In some examples, if it is determined that an RSS  200  having a particular arrangement is capable of providing a greater DLS capability than necessary, a drill bit  50  having a relatively low side cutting efficiency may be selected in order limit the DLS capability to improve the durability or reliability of the RSS  200 . For example, a drill bit  50  having a relatively low side cutting efficiency may be selected to ensure that the flexible collar  102  bends only to a predetermined percentage of its capability along the planned path of a wellbore  26  ( FIG. 1 ). Alternately, if it is determined that an RSS  200  having a particular arrangement is not capable of providing the desired DLS capability, a drill bit  50  having a relatively high side cutting efficiency may be selected in order to achieve the drilling objectives. 
       FIG. 4  is a schematic view of an RSS  300  having a flexible collar  302  wherein control components  304  are disposed within a flexible collar  302 . The control components  304  may include any of the equipment described above for the electronics assembly  212  ( FIG. 3A ) and any other modular control assemblies for operating the RSS  300 . Using some of the material selection and strategic sizing techniques discussed below, the inner diameter ID of the flexible collar  302  may be sufficiently increased for some applications to accommodate the modular control assemblies  304  as well as provide sufficient fluid flow therethrough. This arrangement may reduce an overall length OL of the RSS  300  for some applications. As illustrated in  FIG. 4 , the flexible collar  302  is illustrated schematically as including a necked down section, but as described above, generally cylindrical or other configurations are contemplated as well. 
     In some of the embodiments described herein, a push-the-bit rotary steerable concept is described with a flexible collar  102 ,  302  between the steering section  114  and the control section  110  of the RSS to improve the turning radius capability. The strategic sizing and material selection of the flexible collar  102 ,  302  may further improve the turning radius capability, or limit this capability when desired. As the flexible collar  102 ,  302  is made more flexible, the dogleg severity (DLS) capability of the RSS is increased. A high DLS capability is desirable for many oil and/or gas wellbores  26  ( FIG. 1 ). For example, a short curve length on a build section can maximize the amount of reservoir exposure of a subsequent lateral production section. Other applications may require a high DLS capability such as: avoiding other wellbores; achieving a desired DLS capability in a problematic formation by selecting a configuration that normally provides a higher than needed DLS capability to compensate for unconsolidated rock, low rock strength, overgauge borehole, formation trends, formations faults or other formation problems; avoiding or exiting problematic or undesirable geologic formations; or drilling sidetrack sections from an existing wellbore  26 . 
     Many oil and/or gas wells do not require a high DLS capability. In these instances, the flexible collar  102 ,  302  may be made stiffer (and therefore more stable), and the DLS capability of the RSS may be decreased. It may be desirable to run a stiffer RSS with a lower DLS capability to avoid creating or reduce creation of ledges or short segments of locally high DLS that are sometimes generated by the use of high DLS capable tools while trying to drill a low DLS segment, e.g., straight in vertical, tangent, lateral or horizontal sections of a wellbore. In addition, high DLS capable systems are less stable and may generate wellbore oscillations or spiraling, which may be avoided by using a relatively stiff flexible collar  102 ,  302 . 
     The strategic selection of the side-cutting efficiency of drill bit  50  may be used in conjunction with the sizing and material selection of the flexible collar  102 ,  302  to achieve the desired results. In some instances, a drill bit  50  with a relatively high side-cutting efficiency may be selected for use with a particular flexible collar  102 ,  302 . For example, when maximum DLS capability is desired, a maximum flexibility flexible collar  102 ,  302  may be combined with a drill bit  50  having maximum SCE, subject to other constraints such a stress, rate of penetration, etc. In some instances, a drill bit  50  with a relatively low side-cutting efficiency may be selected for use with a particular flexible collar  102 ,  302  arrangement to limit the DLS capability of an RSS  58 ,  200 ,  300 . For example, the selection of a drill bit  50  having a relatively low side-cutting efficiency may be selected to prevent the flexible collar  102 ,  302  from flexing to its capacity in operation. This may improve the stability of an RSS  58 ,  200 ,  300  and limit many of the undesirable features of wellbores. Ledges, local high DLS, and well bore oscillations or spiraling create drag that limits the length of tangent, lateral or horizontal sections of a wellbore. These undesirable features can also make it difficult to run liners, casing, and completions equipment in or out of a wellbore. In some instances, a drill bit  50  with relatively high side-cutting efficiency may be selected to enhance the DLS capability of a relatively stiff flexible collar  102 ,  302 . The relatively stiff flexible collar  102 ,  302  may be desired to limit vibration or torsional oscillations and yet still achieve a desired DLS objective with a higher SCE drill bit  50 . The full range of stiffness of the flexible collar  102 ,  302  along with the full range of SCE of drill bit  50  may be considered together when strategically selecting the OD, ID, length and material of the flexible collar  102 ,  302  and the SCE of drill bit  50  to achieve the desired DLS and other wellbore objectives. 
     For at least the reasons articulated above, it is desirable to strategically select the configuration of an RSS  58 ,  200 ,  300  and SCE of drill bit  50  to match the needs of the wellbore  26  being drilled. By selecting an appropriate combination of the OD, ID, length, material of the flexible collar, the position of the flexible collar  102 ,  302  within the BHA  22 , and/or a side cutting efficiency of a drill bit  50  for use with the BHA  22 , the needs of the wellbore  26  may be accommodated. The selection of these parameters may also provide other benefits including providing a more desirable length, bending stiffness, bending stress, torsional stiffness, shear stress due to torsion, and increased DLS tolerance as discussed below. 
     Referring to  FIG. 5A , an example flexible collar  402  is described having a generally cylindrical configuration. The flexible collar has a length (L), an inner diameter (ID) and an outer diameter (OD). Although flexible collar  402  exhibits a simplified geometry, the principles discussed below with reference to flexible collar  402  also apply the more complex geometries of the flexible collars  102 ,  302  described above. Generally speaking, increasing flexibility of the flexible collar  402  may be achieved by one or more of the following: (1) Decreasing the outer diameter (OD), (2) increasing the inner diameter (ID), (3) increasing the length (L) and (4) decreasing modulus of elasticity (E) of the flexible collar  402 . Conversely, increasing stiffness of the flexible collar  402  may be achieved by one or more of the following: (1) increasing the outer diameter OD, (2) decreasing the inner diameter ID, (3) decreasing the length (L), and (4) increasing the modulus of elasticity (E) of the flexible collar  402 . 
     Creating the desired outer diameter (OD), inner diameter (ID) and length (L) can be achieved by conventional machining, casting or forging techniques when a metallic material is selected. Non-metallic materials such as composites, fiberglass, plastics, etc. can also be produced with the combination desired outer diameter (OD), inner diameter (ID), length (L) and modulus of elasticity (E). Modulus of elasticity (E) is a physical mechanical property of the material, and thus can thus be selected by choice of material. In the case of composites or some other non-metallic materials, the physical construction of the material itself may be manipulated to provide a desired modulus of elasticity (E). 
     Non-limiting examples of conventional metallic materials used in downhole tool applications with representative values of modulus of elasticity include: Steel or Stainless Steel (28-30×10 6  psi); Beryllium Copper (19.5×10 6  psi); Titanium (13.9-19×10 6  psi); and Aluminum (10×10 6  psi), austenitic nickel-chromium-based alloys such as Inconel 718 (29.6×10 6  psi). In some applications, Magnesium materials may be selected. 
       FIG. 5B  is table illustrating geometric and stiffness characteristics of two example flexible collars  402   Ti ,  402   Sd  constructed of different materials (steel and titanium) illustrating how a particular DLS capability of the RSS  58  ( FIG. 1 ) may be provided by appropriately selecting available design parameters. In the example illustrated in  FIG. 5B , the titanium and steel flexible collars  402   Ti ,  402   Stl  have the same length (L). The titanium flexible collar  402   Ti , however, exhibits a much larger (OD) and (ID) than the steel flexible collar  402   Stl , and therefore provides a much larger Area Moment of inertia (I). If the two materials had the same Modulus of Elasticity (E), the Area Moment of Inertia (I) indicates the titanium flexible collar  402   Ti  would be 43% stiffer than the steel flexible collar  402 . However, the Modulus of Elasticity (E) of the titanium collar  402   Ti  is only 48% of the steel flexible collar  402   Stl . Since the length (L) of the flexible collars  402   Ti ,  402   Stl  is the same, the net stiffness may be represented by E×I. The net effect is that the titanium flexible collar  402   Ti  is only 69% as stiff as the steel flexible collar  402   Stl . The steel flexible collar  402   Stl  is much stiffer than the titanium flexible collar  402   Ti  even though the outer diameter (OD) and inner diameter (ID) are much smaller. 
       FIGS. 6 and 7  illustrate the effect of these two flexible collars  402   Ti ,  402   Stl  and the dissimilar associated stiffness on the DLS capability of the RSS  58 . In  FIG. 6 , the Build Rate or dogleg severity (DLS), is shown for the two different stiffness flexible collars as a function of Weight-On-Bit (WOB) at a variety of inclinations (0 degrees, 30 degrees, 60 degrees and 90 degrees). In  FIG. 7  the Drop Rate is shown as a function of WOB. The build rate generally relates to the DLS in the vertical plane as inclination is increasing with depth and the drop rate generally relates to the DLS in the vertical plane as inclination is decreasing with depth. 
     In the example illustrated in  FIG. 6 , the titanium flexible collar  402   Ti  provides about 5 to 11 degrees per 100 ft. greater build rate capability than the steel flexible collar  402   Stl  across the range of WOB and inclination because it is more flexible (it is 69% as stiff as the steel flexible collar  402   Stl ). As illustrated in  FIG. 7 , for this particular example the titanium flexible collar  402   Ti  has a 7 to 18 deg/100 ft greater drop rate over the steel flexible collar  402   Stl  across the range of WOB and inclination because it is more flexible, being only 69% as stiff. 
     From the examples illustrated in  FIGS. 5A-7 , it may be demonstrated that the selection of a material for the flexible collar with a lower modulus of elasticity (E) than steel may provide a greater flexibility to achieve higher DLS capabilities. Materials with a lower modulus of elasticity (E) than steel include, but are not limited to, titanium, beryllium copper, and aluminum. Additional improvements over the steel flexible collar may also be realized from a selection of a titanium material as illustrated in  FIGS. 5-7 . 
     For example, a reduced overall length “OL” (see  FIG. 4 ) of a tool may be realized, or relatively short RSS  58 , may be provided with a titanium flexible collar  402   Ti  or a flexible collar constructed of dissimilar materials with respect to a steering section of the RSS  58 . The titanium flexible collar  402   Ti  in the example of  FIGS. 5-7  enables a larger inner diameter (ID) than the steel flexible collar  402   Stl . With the smaller inner diameter (ID) of the steel flexible collar  402   Stl , it may be impractical to run electronics/control modules e.g., control components  304  (see  FIG. 4 ) in the flexible collar  402   Stl  due to the size required for the modules, the space needed for supports/centralizers between the modules and the inner diameter (ID) of the collar  402   Stl , and the flow area needed between the modules and the inner diameter (ID) of the collar  402   Stl  (and in particular the flow area through the supports/centralizers). Thus, with a steel flexible collar  402   Stl , wires or cables  222  may extend through the length of the flexible collar to electrically connect the control module section and the steering section (see, e.g.,  FIG. 3A , not to scale). With the larger inner diameter (ID) that the titanium flexible collar  402   Ti  enables, it may be practical to run the electronics/control components  304  within the flexible collar  402   Ti  (see, e.g.,  FIG. 4 , not to scale). The length that would be consumed by the wires or cables  222 , can be used by the control components  304  or any other electronics module desired. The overall length “OL” of the RSS  300  can be significantly reduced. 
     By selecting titanium for construction of the flexible collar  402   Ti , a reduced bending stiffness and bending stress may also be realized. Bending moment is proportional to (E×I)/radius of curvature, e.g., the smaller the radius of curvature, the larger the bending moment. Radius of curvature is inversely proportional to DLS, e.g., the larger the DLS the smaller the radius of curvature. Hence, bending moment is proportional to (E×I)×DLS. For a given DLS, a reduction in (E×I) is enabled by the titanium flexible collar  402   Ti , hence a reduction in bending moment. 
     Bending stress is proportional to bending moment×(OD/2)/I. Thus, bending stress is proportional to (E×I)×DLS×(OD/2)/I. Because “I” appears both in the numerator and denominator it divides out and, hence, bending stress is proportional to E×DLS×(OD/2). In the example, the titanium flexible collar  402   Ti  lowers modulus of elasticity (E), but increases the outer diameter (OD). As long as the reduction in the modulus of elasticity (E) is proportionately larger than the increase in outer diameter (OD), bending stress is reduced, as enabled by the titanium flexible collar  402   Ti . Lower bending stress is very desirable in RSS applications. 
     By selecting a titanium flexible collar  402   Ti , decreased torsional stiffness and reduced shear stress due to torsion may also be realized. Torsional stiffness is proportional to (J×G)/Length of the flexible collar  402   Ti , where J represents the polar moment of inertia and G represents the modulus of rigidity. For a given length (L), in this specific example the titanium flexible collar  402   Ti  reduces torsional stiffness (e.g., J×G is lower for the titanium flexible collar  402   Ti ), which is not necessarily desirable in all instances. Some optimization can occur with length (L) by reducing the length (L) of the titanium flexible collar  402   Ti  to increase the torsional stiffness balanced against the increase in bending stiffness and bending stress. 
     However, shear stress due to torsion is proportional to Torque×(OD/2)/J. The titanium flexible collar  402   Ti  enables a larger value of J, hence a lower shear stress due to torsion, even as OD increases because J is a function of OD 4 . Reduced shear stress due to torsion is very desirable in RSS applications. 
     An increased DLS tolerance may also be realized by selecting the titanium flexible collar  402   Ti . As shown in the example of  FIGS. 5-7 , decreasing stiffness using the titanium flexible collar  402   Ti  increases the DLS capability. But because of the lower bending stress at a given DLS, the titanium flexible housing  402   Ti  enables a higher DLS to be tolerated. 
     Referring to  FIG. 8 , a procedure  500  for configuring and constructing a rotary steerable system is described. Although the steps described below may be performed in the order illustrated in  FIG. 8 , at least some of the steps may be performed in a different order without departing from the scope of the disclosure. At step  502 , a maximum DLS required for drilling a wellbore is determined. The required or maximum DLS may include, e.g., the largest build rate or drop rate in a planned wellbore path or trajectory of the wellbore. 
     At step  504 , a selection of a combination of parameters for a flexible collar is made based on the required DLS. For example, the combination of parameters may be selected to provide the rotary steerable system with sufficient flexibility to achieve the maximum dogleg severity. The parameters include geometrical parameters, e.g., an outer diameter (OD), an inner diameter (ID), a length (L) of the flexible collar. The parameters may also include the material parameters, e.g., modulus of elasticity (E). A material is selected for the flexible collar based at least in part on the modulus of elasticity (E) selected (step  506 ). In some embodiments, the material selected for the flexible collar may be dissimilar from a material of other sections in the RSS. For example, housings for a control section  110 , flow control section  112  and a steering section  114  may be constructed of steel, while Titanium or Inconel 718 may be selected for the flexible collar. 
     At step  508 , a placement of the flexible collar within the rotary steerable system is selected. Where the required DLS is relatively high, a placement of the flexible collar between a steering section and a control section may be complemented. Where the required DLS is relatively low, or where stability is a significant concern, a placement of the flexible collar at an up-hole end of a control section  110  may be contemplated. Next, a drill bit may be selected for use with the RSS (step  510 ). A side cutting efficiency to of the drill bit may be a consideration in the selection. Where the required DLS is relatively high, a relatively high side cutting efficiency may be selected, which may permit the flexible collar to reach its flexural capacity in operation. Where the required DLS is relatively low, a drill bit having a relatively low side cutting efficiency may be selected, which may limit the flexing of the flexible collar in operation. The DLS capability with a relatively stiff flexible collar may be enhanced by a relatively high SCE drill bit. The DLS capability with a relatively limber flexible collar may be tempered by a relatively low SCE drill bit. 
     Once the parameters and an arrangement of the RSS are all selected, at step  512  an initial dogleg severity capability of the RSS may be determined based on the selected placement, material, and combination of parameters for the flexible collar. In some embodiments, the initial DLS capability is determined mathematically, e.g., using finite element analysis models and techniques. In other embodiments, the DLS capability is determined empirically by constructing a RSS according to the selected parameters and observing the capability achieved in a test or actual working wellbore. 
     Next, the procedure  500  proceeds to decision  514  where the initial DLS capability is compared to a predetermined tolerance for the DLS capability. If it is determined that the initial DLS capability is sufficiently close to DLS severity required, the procedure  500  may proceed to step  516  where the RSS and/or drill bit are constructed based on the initial selected placement and parameters of the flexible collar, and/or drill bit SCE, and then deploying the RSS into a wellbore (step  518 ) with the selected drill bit. 
     If at the decision  514 , it is determined that the initial DLS capability is not within the predetermined tolerance, the procedure  500  may return to step  504  (or any of steps  506 ,  508 ,  510 ), where adjusted selections may be made. An adjusted placement, material and combination of parameters may be made that yields an adjusted dogleg severity capability that is more proximate the dogleg severity required than the initial dogleg severity capability. In some embodiments, where the DLS capability determined in step  512  is insufficient, an adjusted modulus of elasticity (E) may be selected that is lower than the initial modulus of elasticity (E) selected to yield a more flexible DSS. Conversely, where the DLS capability determined in step  512  is greater than necessary to accommodate the DLS severity required, a drill bit having a lower side cutting efficiency may be selected to improve the stability and/or durability of the RSS. The procedure  500  may repeat iteratively until the DLS capability determined is within tolerance. 
     Thereafter, the RSS and/or drill bit may be constructed based on the adjusted placement, material and combination of parameters (step  516 ) and the RSS may be deployed into the wellbore to achieve the dogleg severity required with the adjusted drill bit. 
     The aspects of the disclosure described below are provided to describe a selection of concepts in a simplified form that are described in greater detail above. This section is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one aspect, the disclosure is directed to a method of configuring a rotary steerable system. The method includes (a) determining a maximum dogleg severity required for drilling a wellbore along a planned wellbore path, (b) determining a combination of parameters for a flexible collar to provide the rotary steerable system with sufficient flexibility to achieve the maximum dogleg severity, the parameters including an outer diameter, an inner diameter, a length and a modulus of elasticity, (c) selecting a material for the flexible collar based on the modulus of elasticity determined, and (d) assembling the rotary steerable system with the flexible collar having to the combination of parameters and selected material. 
     In some embodiments, the method further includes selecting a drill bit having a side cutting efficiency determined to cause the flexible collar to bend a predetermined percentage of a bending capability or capacity of the flexible collar at the maximum dogleg severity along the planned wellbore path, and assembling the rotary steerable system with the drill bit. The side cutting efficiency selected may be determined to limit a DLS capability of the rotary steerable system. 
     In one or more exemplary embodiments, the method may further include selecting a placement of the flexible collar with respect to a steering section and a control section of the rotary steerable system. In some embodiments, the placement of the flexible collar is selected to be between a steering section and a control section of the rotary steerable system. The material selected for the flexible collar may be dissimilar from materials of the steering section and control section. In some embodiments, the material selected includes at least one of the group consisting of titanium, austenitic nickel-chromium-based alloys, and berriluim copper. 
     In some example embodiments, the combination of parameters is determined to provide a desired tool length for the RSS. The combination of parameters may also be determined to provide a bending stiffness or bending stress desired for the flexible collar, a torsional stiffness or shear stress due to torsion desired for the flexible collar, or a DLS tolerance to be achieved. 
     In another aspect, the disclosure is directed to a method of configuring and deploying a rotary steerable system. The method includes (a) determining a maximum dogleg severity required for drilling a wellbore along a planned wellbore path, (b) selecting a combination of parameters for a flexible collar, the parameters including an outer diameter, an inner diameter, a length and a modulus of elasticity, (c) selecting a material for the flexible collar based on the modulus of elasticity selected, (d) selecting a placement of the flexible collar within the rotary steerable system (e) determining an initial dogleg severity capability of the rotary steerable system having the selected placement, material, and combination of parameters for the flexible collar, (f) selecting an adjusted placement, material and combination of parameters determined to yield an adjusted dogleg severity capability that is more proximate the maximum dogleg severity required than the initial dogleg severity capability (g) constructing the rotary steerable system based on the adjusted placement, material and combination of parameters, and (h) deploying the rotary steerable system into a wellbore to achieve the maximum dogleg severity required along the planned wellbore path. 
     In one or more example embodiments, the method further includes selecting a drill bit having a side cutting efficiency determined to cause the flexible collar t bend a predetermined percentage of the adjusted dogleg severity capability at the maximum dogleg severity required along the planned wellbore path. In some embodiments, selecting a drill bit includes selecting a drill bit exhibiting a side cutting efficiency determined to reduce or limit the adjusted dogleg severity capability of the RSS. The method may also include selecting a placement of the flexible collar that is between a steering section and a control section of the rotary steerable system or selecting a placement of the flexible collar at an up-hole end of control section of the rotary steerable system. 
     In some embodiments, an adjusted modulus of elasticity is selected and an adjusted outer diameter is selected, wherein the adjusted modulus of elasticity is lower than an initial modulus of elasticity and the outer diameter is greater than an initial outer diameter such that the adjusted dogleg severity capability is greater than the initial dogleg severity capability. In some embodiments, the inner diameter of the flexible collar is selected to accommodate a modular control and sensor unit therein. In some embodiments, the initial outer diameter of the flexible collar is selected such that the flexible collar exhibits a necked down portion therein. The adjusted placement, material and combination of parameters may be determined to provide a desired tool length for the RSS, a bending stiffness or bending stress desired for the flexible collar, a torsional stiffness or shear stress due to torsion desired for the flexible collar. 
     In another aspect, the disclosure is directed to a rotary steerable system. The rotary steerable system includes a drill bit, and a steering section coupled to an upper end of the drill bit. The steering section includes at least one steering pad extendable in a lateral direction to push against a wellbore wall in operation. A control section includes electronics therein for at least one of sensing parameters of a drilling operation and for transmitting instructions to the steering section. A flexible collar is coupled between the steering section and the control section, flexible collar having a lower bending stiffness than the steering section and constructed of a material selected to be dissimilar with respect to a material selected for the steering section. 
     In some embodiments, the steering section may be constructed of a steel material and the flexible collar may be constructed of an austenitic nickel-chromium-based alloy, titanium, beryllium copper or aluminum material. The control section may include a modular control and sensor unit therein, and wherein the modular control and sensor unit may extend at least partially into the flexible collar. 
     The Abstract of the disclosure is solely for providing the United States Patent and Trademark Office and the public at large with a way by which to determine quickly from a cursory reading the nature and gist of technical disclosure, and it represents solely one or more examples. 
     While various examples have been illustrated in detail, the disclosure is not limited to the examples shown. Modifications and adaptations of the above examples may occur to those skilled in the art. Such modifications and adaptations are in the scope of the disclosure.