Abstract:
A method of and system for directional drilling reduces the friction between the drill string and the well bore. A downhole drilling motor is connected to the surface by a drill string. The drilling motor is oriented at a selected tool face angle. The drill string is rotated at said surface location in a first direction until a first torque magnitude without changing the tool face angle. The drill string is then rotated in the opposite direction until a second torque magnitude is reached, again without changing the tool face angle. The drill string is rocked back and forth between the first and second torque magnitudes.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the field of oil and gas well drilling. More particularly, the present invention relates to a method and system for directional drilling in which the drill string is rotated back and forth between selected surface measured torque magnitudes without changing the tool face angle, thereby to reduce friction between the drill string and the well bore.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is very expensive to drill bore holes in the earth such as those made in connection with oil and gas wells. Oil and gas bearing formations are typically located thousands of feet below the surface of the earth. Accordingly, thousands of feet of rock must be drilled through in order to reach the producing formations. Additionally, many wells are drilled directionally, wherein the target formations may be spaced laterally thousands of feet from the well&#39;s surface location. Thus, in directional drilling, not only must the depth but also the lateral distance of rock must be penetrated.  
           [0003]    The cost of drilling a well is primarily time dependent. Accordingly, the faster the desired penetration location, both in terms of depth and lateral location, is achieved, the lower the cost in completing  
           [0004]    While many operations are required to drill and complete a well, perhaps the most important is the actual drilling of the bore hole. In order to achieve the optimum time of completion of a well, it is necessary to drill at the optimum rate of penetration and to drill in the minimum practical distance to the target location. Rate of penetration depends on many factors, but a primary factor is weight on bit.  
           [0005]    Directional drilling is typically performed using a bent sub mud motor drilling tool that is connected to the surface by a drill string. During sliding drilling, the drill string is not rotated; rather, the drilling fluid circulated through the drill string cause the bit of the mud motor drilling tool to rotate. The direction of drilling is determined by the azimuth or face angle of the drilling bit. Face angle information is measured downhole by a steering tool. Face angle information is typically conveyed from the steering tool to the surface using relatively low bandwidth mud pulse signaling. The driller attempts to maintain the proper face angle by applying torque or drill string angle corrections to the drill string.  
           [0006]    Several problems in directional drilling are caused by the fact that a substantial length of the drill string is in frictional contact with and supported by the borehole. Since the drill string is not rotating, it is difficult to overcome the friction. The difficulty in overcoming the friction makes it difficult for the driller to apply sufficient weight to the bit to achieve an optimal rate of penetration. The drill string exhibits stick/slip friction such that when a sufficient amount of weight is applied to overcome the friction, the drill the weight on bit tends to overshoot the optimum magnitude.  
           [0007]    Additionally, the reactive torque that would be transmitted from the bit to the surface through drill string, if the hole were straight, is absorbed by the friction between the drill string and the borehole. Thus, during drilling, there is substantially no reactive torque at the surface. Moreover, when the driller applies drill string angle corrections at the surface in an attempt to correct the bit face angle, a substantial amount of the angular change is absorbed by friction without changing the face angle in stick/slip fashion. When enough angular correction is applied to overcome the friction, the face angle may overshoot its target, thereby requiring the driller to apply a reverse angular correction.  
           [0008]    It is known that the frictional engagement between the drill string and the borehole can be reduced by rocking the drill string back and forth between a first angle and a second angle. By rocking the string, the stick/slip friction is reduced, thereby making it easier for the driller to control the weight on bit and make appropriate face angle corrections.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides a method and system for directional drilling that reduces the friction between the drill string and the well bore. According to the present invention, a downhole drilling motor is connected to the surface by a drill string. The drilling motor is oriented at a selected tool face angle. The drill string is rotated at said surface location in a first direction until a first torque magnitude without changing the tool face angle. The drill string is then rotated in the opposite direction until a second torque magnitude is reached, again without changing the tool face angle. The drill string is rocked back and forth between the first and second torque magnitudes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a pictorial view of a directional drilling system.  
         [0011]    [0011]FIG. 2 is a block diagram of a directional driller control system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    Referring now to the drawings and first to FIG. 1, a drilling rig is designated generally by the numeral  11 . Rig  11  in FIG. 1 is depicted as a land rig. However, as will be apparent to those skilled in the art, the method and system of the present invention will find equal application to non-land rigs, such as jack-up rigs, semisubmersibles, drill ships, and the like.  
         [0013]    Rig  11  includes a derrick  13  that is supported on the ground above a rig floor  15 . Rig  11  includes lifting gear, which includes a crown block  17  mounted to derrick  13  and a traveling block  19 . Crown block  17  and traveling block  19  are interconnected by a cable  21  that is driven by draw works  23  to control the upward and downward movement of traveling block  19 . Traveling block  19  carries a hook  25  from which is suspended a top drive  27 . Top drive  27  supports a drill string, designated generally by the numeral  31 , in a well bore  33 . Top drive  27  can be operated to rotate drill string  31  in either direction.  
         [0014]    According to an embodiment of the present invention, drill string  31  is coupled to top drive  27  through an instrumented sub  29 . As will be discussed in detail hereinafter, instrumented top sub  29  includes sensors that provide drill string torque information according to the present invention.  
         [0015]    Drill string  31  includes a plurality of interconnected sections of drill pipe  35  a bottom hole assembly (BHA)  37 , which includes stabilizers, drill collars, and a suite of measurement while drilling (MWD) instruments including a steering tool  51 . As will be explained in detail hereinafter, steering tool  51  provides bit face angle information according to the present invention.  
         [0016]    A bent sub mud motor drilling tool  41  is connected to the bottom of BHA  37 . As is well known to those skilled in the art, the face angle of the bit of drilling tool  41  used to control azimuth and pitch during sliding directional drilling. Drilling fluid is delivered to drill string  31  by mud pumps  43  through a mud hose  45 . During rotary drilling, drill string  31  is rotated within bore hole  33  by top drive  27 . As is well known to those skilled in the art, top drive  27  is slidingly mounted on parallel vertically extending rails (not shown) to resist rotation as torque is applied to drill string  31 . During sliding drilling, drill string  31  is held in place by top drive  27  while the bit is rotated by mud motor  41 , which is supplied with drilling fluid by mud pumps  43 . The driller can operate top drive  27  to change the face angle of the bit of drilling tool  41 . Although a top drive rig is illustrated, those skilled in the art will recognize that the present invention may also be used in connection with systems in which a rotary table and kelly are used to apply torque to the drill string The cuttings produced as the bit drills into the earth are carried out of bore hole  33  by drilling mud supplied by mud pumps  43 .  
         [0017]    Referring now to FIG. 2, there is shown a block diagram of a preferred system of the present invention. The system of the present invention includes a steering tool  51 , which produces a signal indicative of drill bit face angle. Typically, steering tool  51  uses mud pulse telemetry to send signals to a surface receiver (not shown), which outputs a digital face angle signal. However, because of the limited bandwidth of mud pulse telemetry, the face angle signal is produced at a rate of once every several seconds, rather than at the preferred five times per second sampling rate. For example, the sampling rate for the face angle signal may be about once every twenty seconds.  
         [0018]    The system of the present invention also includes a drill string torque sensor  53 , which provides a measure of the torque applied to the drill string at the surface. The drill string torque sensor may implemented as a strain gage in instrumented top sub  29  (illustrated in FIG. 1). The torque sensor  53  may also be implemented as a current measurement device for an electric rotary table or top drive motor, or as pressure sensor for an hydraulically operated top drive. The drill string torque sensor  53  provides a signal that may be sampled at the preferred sampling rate of five times per second.  
         [0019]    In FIG. 2, the outputs of sensors  51  and  53  are received at a processor  55 . Processor  55  is programmed according to the present invention to process data received from sensors  51 - 53 . Processor  55  receives user input from user input devices, such as a keyboard  57 . Other user input devices such as touch screens, keypads, and the like may also be used. Processor  55  provides visual output to a display  59 . Processor  55  also provides output to a drill string rotation controller  61  that operates the top drive ( 27  in FIG. 1) or rotary table to rotate the drill string according to the present invention.  
         [0020]    According to the present invention, drilling, tool  41  is oriented at tool face angle selected to achieve a desired trajectory. As drilling tool  41  is advanced into the hole, processor  55  operates drill string rotation controller  61  to rotate drill string  35  in a first direction while monitoring drill string torque with torque sensor  53  and tool face angle with steering tool  51 . As long as the tool face angle remains constant, rotation controller  61  continues to rotate drill string  35  in the first direction. When the steering tool  51  senses a change in tool face angle, processor  55  notes the torque magnitude measured by torque sensor  53  and actuates drill string rotation controller  61  to reverse the direction of rotation of drill string  31 . Torque is a vector having a magnitude and a direction. When torque sensor  53  senses that the magnitude of the drill string torque has reached the magnitude measured in the first direction, processor  55  actuates rotation controller  61  reverse the direction of rotation of drill string  31 . As drilling progresses, processor  55  continues to monitor drill torque with torque sensor  53  and actuates rotation controller- 61  to rotate drill string  31  back and forth between the first torque magnitude and the second torque magnitude. The back and forth rotation reduces or eliminates stick/slip friction between the drill string and the well bore, thereby making it easier for the driller to control weight on bit and tool face angle.  
         [0021]    Alternatively, the torque magnitude may be preselected by the system operator. When the torque detected by the torque sensor  53  reaches the preselected value, the processor  55  sends a signal to the controller  61  to reverse direction of rotation. The rotation in the reverse direction continues until the preselected torque value is reached again. In some embodiments, the preselected torque value is determined by calculating an expected rotational friction between the drill string ( 35  in FIG. 1) and the wellbore wall, such that the entire drill string above a selected point is rotated. The selected point is preferably a position along the drill string at which reactive torque from the motor  41  is stopped by friction between the drill string and the wellbore wall. The selected point may be calculated using “torque and drag” simulation computer programs well known in the art. Such programs calculate axial force and frictional/lateral force at each position along the drill string for any selected wellbore trajectory. One such program is sold under the trade name WELLPLAN by Landmark Graphics Corp., Houston, Tex.  
         [0022]    While the invention has been disclosed with respect to a limited number of embodiments, those of ordinary skill in the art, having the benefit of this disclosure, will readily appreciate that other embodiments may be devised which do not depart from the scope of the invention. Accordingly, the scope of the invention is intended to be limited only by the attached claims.