Abstract:
A directional drilling system and method are provided for directional drilling a borehole by continuous rotating of the drill string in combination with an arrangement of drilling motor assemblies at the lower end of the drill string to effect drilling along a curved path and a substantially straight path. A first drilling motor assembly is coupled to a drill bit and operable to rotate the drill bit to effect drilling of the borehole. A second drilling motor assembly, positioned on the drill string above the first drilling motor assembly, is operable to rotate the first drilling motor assembly in a direction opposite the direction of rotation imparted to the drill string from the surface and to the drill bit by the first drilling motor assembly. A control system associated with the second drilling motor assembly controls fluid flow through the second drilling motor assembly so that the first drilling motor assembly is substantially rotationally stationary with respect to the rotating drill string when drilling a curved path of the borehole.

Description:
FIELD OF THE INVENTION 
     The present invention relates to directional drilling and more specifically to an arrangement of drilling motor assemblies suitable for use in downhole drilling operations. 
     BACKGROUND 
     Directional drilling can be described as the intentional deviation of a wellbore from the path it would naturally take. This is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components and drill bits. In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically. 
     The most common way to directional drill is through the use of a bend near the bit in a downhole steerable mud motor. Directional drilling is accomplished with the alternating combination of two drilling operations. In the sliding mode the drill string is slowly rotated to orient the bend in the desired direction so that the bend points the bit in a direction different from the axis of the wellbore. Once oriented by pumping mud through the mud motor, the bit turns while the drill string does not rotate but rather slides, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drill string so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. 
     In directional drilling operations the sliding phase of drilling lacks the efficiency associated with rotating the drill string. This inefficiency is a result of the drag of the sliding drill string along the borehole and the sole use of the mud motor for drilling the borehole. 
     In recent years the industry has seen the development of rotary steerable systems for used in directional drilling. These systems employ the use of specialized downhole equipment to replace conventional directional tools such as mud motors. A rotary steerable tool is designed to drill directionally with continuous rotation of the drill string from the surface, eliminating the need to slide a steerable mud motor. Continuous rotation of the drill string allows for improved transportation of drilled cuttings to the surface resulting in better hydraulic performance and reduced well bore tortuosity due to utilizing a steadier steering model. Rotary steerable systems are costly as compared to mud motor systems, so more the traditional mud motor systems are more economically preferable in conventional directional drilling applications. 
     The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY 
     A directional drilling system and method are provided for directional drilling a borehole by continuous rotating of the drill string in combination with an arrangement of drilling motor assemblies at the lower end of the drill string to effect drilling along a curved path and a substantially straight path. A first drilling motor assembly is coupled to a drill bit and operable to rotate the drill bit to effect drilling of the borehole. The first drilling motor assembly is configured to angularly tilt the rotational axis of the drill bit relative to the axis of the section of the borehole being drilled to provide directionality to the borehole. A second drilling motor assembly, positioned on the drill string above the first drilling motor assembly is operable to rotate the first drilling motor assembly in a direction opposite the direction of rotation imparted to the drill string from the surface and to the drill bit by the first drilling motor assembly. The rotational speed of the second drilling motor assembly is controlled by a control assembly. A control assembly associated with the second drilling motor assembly controls fluid flow through the second drilling motor assembly so that the first drilling motor assembly is substantially rotationally stationary with respect to the rotating drill string when drilling a curved path of the borehole. 
     The foregoing summary is not intended to summarize each potential embodiment or every aspect of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of the apparatus in use for directional drilling; 
         FIG. 2  is a diagrammatic view of the second drilling motor assembly illustrating the fluid flow path through the second drilling motor assembly 
         FIG. 3  is a schematic view of the fluid control system for controlling fluid flow through the second drilling motor assembly. 
     
    
    
     DETAILED DESCRIPTION 
     In describing various locations relative to the Figures the term “downhole” refers to the direction along the axis of the wellbore that looks toward the furthest extent of the wellbore. Downhole is also the direction toward the drill bit location. Similarly, the term “lower end” refers to the portion of the assembly located at the downhole end of the respective assembly. The term “uphole” refers to the direction along the axis of the wellbore that leads back to the surface, or away from the drill bit. Similarly, the term “upper end” refers to the portion of the assembly located at the uphole end of the respective assembly. The term “clockwise” refers to rotation to the right as seen looking downhole and the term “counterclockwise” refers to rotation to the left as seen looking downhole. In a situation where the drilling is more or less along a vertical path, downhole is truly in the down direction, and uphole is truly in the up direction. However, in horizontal drilling, the terms up and down are ambiguous, so the terms downhole and uphole are necessary to designate relative positions along the drill string. 
     Referring to  FIG. 1 , the drill string  10  within a borehole  12  is rotatable by a drilling rig  14  located at the earth&#39;s surface  16 . Rotation of the drill string  10  is provided from the surface in a manner known in the art, such as by a rotary table or a top drive system. A bottom hole assembly  18 , commonly referred to as a BHA, is coupled to the downhole end of the drill string  10 . The BHA  18  comprises a drill bit  20  at the downhole end of the BHA  18  which is coupled to a first drilling motor assembly  22 , which may comprise a downhole steerable mud motor. The first drilling motor assembly  22  includes a bent housing member  24 . An MWD assembly  26  is coupled to the uphole end of the first drilling motor assembly  22 . A control assembly  28  is coupled to the uphole end of the MWD assembly  26  and a second drilling motor assembly  30  is coupled to the uphole end of the control assembly  28 . The uphole end of the second drilling motor assembly drilling  30  is connected to drill string  10 . 
     In the illustrated embodiment the first drilling motor assembly  22 , as known in the drilling art, comprises a connecting sub, which connects the first drilling motor assembly  22  to the drill string  10 , a power section, which consists of the rotor and stator; a transmission section, where the eccentric power from the rotor is transmitted as concentric power to rotate the drill bit  20  in a first direction; a bearing assembly which protects from off bottom and on bottom pressures; and a bottom sub which connects the first drilling motor assembly  22  to the drill bit  20 . In the preferred embodiment the drill bit  20  is rotated by the first drilling motor assembly  22  in a first rotational direction for drilling the borehole  12 . In the preferred embodiment, the first rotational direction is clockwise. 
     The bent housing  24  is included in the first drilling motor assembly  22 . The bent housing assembly  24  can be configured to have a bend using different bend angle settings. The bent housing assembly  24  may comprise a fixed bent housing assembly, which has a fixed bend angle, or an adjustable bent housing assembly, which has the ability to pre-set the bend angle before the BHA is placed in the borehole or which has the ability to adjust the bend angle during the drilling operations. Typically, the bent housing assembly  24  can have an angle setting from 0 degrees to 4 degrees. The amount of bend angle is determined by rate of directional change needed to reach the drilling target zone. 
     The MWD assembly  26 , coupled to the uphole end of the first drilling motor assembly  22 , may contain a steering system, incorporating magnetometers and accelerometers to measure and transmit data related to inclination, direction and orientation of the BHA  18  within the borehole  12  to equipment at the surface. An operator can periodically or continuously monitor the tool face orientation of the BHA  18  through periodic data surveys of inclination, direction and orientation to control the drilling process. An example of the process of monitoring tool face is shown in U.S. Pat. No. 6,585,061, which is incorporated herein by reference. 
     The control assembly  28  is coupled to the uphole end of the MWD assembly  26  and the downhole end of the second drilling motor assembly  30  is coupled to the uphole end of the control assembly  28 . The second drilling motor assembly  30 , coupled at the uphole end to the drill string  10 , includes a power section, which consists of the rotor and stator; a transmission section, where the eccentric power from the rotor is transmitted as concentric power which can rotate the first drilling assembly  22  in a second direction; a bearing assembly which protects from pressures; and a bottom sub which connects the second drilling motor assembly  30  to the first drilling motor assembly  22 . In the preferred embodiment, the second drilling motor assembly  30  comprises a low speed, high torque power section, having a rotational speed in the range from approximately 25 rpm to approximately 80 rpm and a torque range from approximately 2,500 ft. lbs. to 28,000 ft. lbs. depending on the motor diameter which can be of a diameter from 2⅞ inches to 11¼ inches, and configured for rotating the first drilling motor assembly  22  in the second direction. In the preferred embodiment, the second direction is the counterclockwise. 
     As the general operation of the second drilling motor assembly  30  is known in the art of drilling, such operation will not be detailed in reference to  FIG. 2 . Rather, in  FIG. 2  there is illustrated in more detail the second drilling motor assembly  30  showing the fluid flow path noted by arrows  32  through the second drilling motor assembly  30 . Fluid is pumped from the surface through the drill pipe  10  into the uphole end of the second drilling motor assembly  30  which is connected to the drill pipe  10 . The fluid flows into the central annulus  34  in the downhole direction where a portion of the fluid flows through the passage  36  through the upper flex shaft  38  and a portion of the fluid is diverted to flow in the annulus  40  between the housing  42  and the upper flex shaft  38 . The fluid flowing in the annulus  40  continues to flow through the second drilling motor assembly  30  passing through the rotor/stator section  44  to provide rotational motion of the stator  47  in the counterclockwise direction. The portion of the fluid flow through the passage  36  through the upper flex shaft  38  continues to flow in the downhole direction through the lower flex shaft  48  which is connected to the downhole end of the rotor  46 . Coupled to the downhole end of the lower flex shaft  48  is the control assembly  28 , which will be described in more detail in reference to  FIG. 3 . 
     The control assembly  28 , may be coupled to the uphole end of the MWD assembly  26  and the downhole end of the second drilling motor assembly  30  or the control assembly  28  may be incorporated into the second drilling motor assembly  30 , as illustrated in  FIG. 2 . 
     Referring to  FIG. 3 , control assembly  28  is illustrated in more detail. In the illustrated configuration the uphole end of control assembly  28  is connected to the downhole end of the second drilling motor assembly  30 . The downhole end portion of the lower flex shaft  48  is supported within the housing of the second drilling motor assembly  30  by radial bearing  50 . The downhole end portion of flow tube  48  cooperates with poppet  54  to form a control valve to control the fluid flow through rotor  46  of the second drilling motor assembly  30 . Control of the fluid flow through rotor  46  of the second drilling motor assembly  30  allows for control of the rotational rate of the second drilling motor assembly  30 , which further allows for control of the direction and rate of rotation of the first drilling motor assembly  22 . 
     In the illustrated embodiment, control assembly  28  further includes a turbine assembly  56  driven by fluid flow for generating electrical power for the electronics  58  located in the control assembly  28 . The electronics  58  controls the operation of poppet  54  as well as other devices, such as pressure sensor  60 , located in control assembly  28 . Pressure sensor  60  detects, by way of port  62 , pressure command signals transmitted from pressure signaling equipment (not illustrated) locate at the surface  16 . It should be recognized that various pressure transmission methods are commonly used in the drilling industry, for example one such system is illustrated in U.S. Pat. No. 5,390,153 which is incorporated herein by reference. In addition, various other methods of transmission are known in the industry, such as wired drill pipe and electromagnetic methods. 
     The drilling system described herein allows for the continuous rotating of the drill string while orienting in a specific drilling direction and rotating while drilling a substantially straight borehole. In a typical drilling operation, drill string  10  rotation at the surface varies from approximately 30 to 120 rpm. In the event that orientation is required to control deviation or direction of the borehole  12 , the drill string  10  rotation from the surface would be slowed preferably to between approximately 35 to 65 rpm. The control assembly  28  will be activated in response to a pressure signal sent from the surface to control fluid bypass through the second motor assembly  30  to regulate the rotational speed of the first drilling motor assembly  22  to a substantially non-rotating position relative to the drill string  10 . As torque from the first drilling motor assembly  22  driving the bit  20  changes, the control assembly  28  will control the fluid bypass through the second motor assembly  30  to maintain the rotation speed of the first drilling motor assembly  22  to a substantially non-rotating position relative to the drill string  10 . After the desired direction or inclination of the borehole has been achieved, rotation of the drill string  10  from the surface will be increased to the normal range and the control assembly  28  would be set for a fluid bypass level, approximately 50% in the preferred embodiment, typical for normal drilling operations. The tool face data for monitoring the relative rotational position of the first drilling motor assembly  22  is be derived from the MWD assembly  26 . 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.