Patent Publication Number: US-6698534-B2

Title: Direction control in well drilling

Description:
FIELD OF THE INVENTION 
     The present invention relates to direction control in well drilling and, more specifically to bottom hole assemblies for performing such drilling. 
     BACKGROUND OF THE INVENTION 
     In the drilling industry there is a need to be able to directionally drill a well so that the well trajectory follows a desired path. This may be necessary in order to avoid another obstacle such as another well or in order to accurately aim for a reservoir to be exploited. One of the existing methods of doing this is to use a bottom hole assembly including an orienting device to steer the drill bit in the desired direction. One particular application for this equipment is in short radius gas wells which are drilled in an underbalanced condition (i.e. well flowing). This technique can significantly improve well productivity, and therefore well economics. 
     This type of well and operation has various specific characteristics. One is build-up rates of over 50°/100 ft (a radius of curvature below 30 to 35 m). This causes high bending forces on the tool; the tools need to physically bend around the curve, because the geometry does not allow straight tools to pass. There is therefore a need for short, slim assemblies to help with rig-up and to negotiate the bend. There is also undamped vibration coming from the drill bit and motor, which adversely affects tool life and reliability. The techniques and requirements of this type of application are already known but all existing equipment suffers from reliability and usability problems, resulting from the way in which the tools are designed. 
     OBJECT OF THE INVENTION 
     The general object of the present invention is to provide an improved bottom hole assembly. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a bottom hole assembly for drilling a well, comprising a non-magnetic tubing, an orienter, a motor and bit fed with drilling fluid which passes through the non-magnetic tubing, and a sensor package contained within the non-magnetic tubing. The assembly has a drilling fluid flow tube passing through the non-magnetic tubing adjacent to the sensor package, and means for determining the relative position measurement between the non-magnetic tubing and the motor. The relative position measurement is the difference between the toolface values (orientations) of the non-magnetic tubing and the motor. 
     The orienter is preferably of annular form, with the flow tube passing axially through it to the motor. 
     The present device is suitable for use with coiled tubing and has specific advantages, in terms of reliability and operation, which make it particularly suited to harsh environments 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A bottom hole assembly embodying the invention will now be described, by way of example, without limitation to the scope of the invention, and with reference to the drawings, in which: 
     FIGS. 1A and 1B show known steerable drill strings including pointing orienters; 
     FIGS. 2A and 2B show bottom hole assemblies for the drill strings of FIGS. 1A and 1B; 
     FIG. 3 shows the present bottom hole assembly; 
     FIGS. 4A and 4B illustrate respectively the geometry of the known and the present systems; and 
     FIG. 5 shows the present assembly in more detail. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows a drill string comprising a drill pipe  10 , an orienter  11 , a mud motor  13 , a bent sub  14 , and a drill bit  15 . The orienter  11  introduces a bend or deviation into the axis of the drill string, so that viewed vertically from above, the top part of the drill string is seen end on as a circle and then the lower end (mud motor, bent sub, and drill bit) projects away from that circle. The arrow  12  indicates that the bearing of the lower part of the drill string, as seen in such a vertical view, can be adjusted by the orienter. We will use the term “turning” for this bearing adjustment (which must not be confused with the normal rotation of the motor and drill bit for drilling). Some orienters can be stepped on to turn indefinitely; others can turn only over a finite range (e.g. 420°). 
     FIG. 1B shows a variant of the drill string of FIG. 1A, in which there is a pointing orienter  11 A with torque resistant joint  11 B, and the bent sub is omitted. 
     FIG. 2A shows a typical bottom hole assembly of the FIG. 1 type in more detail. The assembly is suspended from coiled tubing  21  through which a cable  20  passes. The coiled tubing is attached to an orienter  11  located at the top of a non-magnetic tubing  25 . The cable  20  passes through the orienter to a swivel  22  which coupled it to a steering tool  23 . A centralizer  24  locates the steering tool centrally in the non-magnetic tubing  25 . A motor  13  and bent sub  14  are attached to the lower end of the non-magnetic tubing  25 , and the drill bit  15  is mounted on the end of the motor. 
     The steering tool  23  is a sensor package which comprises various sensors such as accelerometers, magnetic sensors which sense the direction of the earth&#39;s magnetic field, etc., so that the position and orientation of the bottom hole assembly is known. The non-magnetic tubing  25  is a casing, flexible enough to permit the bottom hole assembly to be physically bent when a well with a tight curve is being drilled, and is non-magnetic so that the earth&#39;s magnetic field can reach the steering tool. The bent sub introduces a small bend (of up to say 3°), which makes it easier to get a bend in the well hole started. The fluid for the motor flows through the non-magnetic tubing  25  around the steering tool  23 . 
     FIG. 2B shows a variant on this design, where the orienter is in a different position on the bottom hole assembly. 
     In order to directionally guide the drill string, the motor and bit need to be turned (as discussed above), i.e. to have the direction or bearing of the lower part of the bottom hole assembly adjusted to a desired value. This is effected by the orienter, which turns the motor (i.e. adjusts its bearing) in relation to the coiled tubing above. The steering tool is connected mechanically to the motor below and to the electric cable above. The swivel between the cable and the steering tool prevents the cable from becoming twisted. The fluid used to drive the drilling motor passes through the coiled tubing, through the orienter, through the annular space around the steering tool, and then to the motor. The centralizer around the steering tool keeps the steering tool centered in the non-magnetic tubing and prevents it from waving around. (There may be more than one centralizer.) 
     Both of these designs suffer from the same problems. In harsh environments, such as those experienced when nitrogen gas is the drilling fluid, there are very high vibrational forces. These are caused by the poor damping characteristics of gas and the high fluid flow rates. Under these conditions two things happen: the steering tool tends to flap around even when centralized with the spring centralizers, and the fluid flow creates vortices which can cause erosion locally in the flow path. All joints that have any free play tend to fail because of the environment which can cause serious wear. 
     In the present system, a flow tube for the drilling fluid is passed from the top of the bottom hole assembly to the top of the motor. This tube is of a uniform diameter and can be constructed to have no joints that need to be broken in order to work with the tool. This creates a smooth flow path, so creating less fluid turbulence and therefore less vibration and risk of erosion. 
     More specifically, referring to FIG. 3, the present bottom hole assembly is suspended from a coiled tube  21  containing a cable  20 . A non-magnetic tubing  25  has an orienter  11  at its lower end connected to a motor and bent sub  13 / 14  with a drill bit  15  at its end. A steering tool  23  is located in the non-magnetic tubing  25 , and a flowtube  30  also passes through the non-magnetic tubing  25 , alongside the steering tool  23 ; this flowtube forms an extension of the coiled tube  21  and is coupled to the motor/bent sub  13 / 14 . The flowtube  30  can conveniently be of roughly circular section, and it and the steering tool  23  are held side by side in the non-magnetic tubing  25 . 
     However, this arrangement creates a problem which needs to be overcome. In the known designs, the steering tool is rotationally linked to the motor and bent sub such that when the orientation of the motor and bent sub is changed, the steering tool orientation is also changed. In the present design, the mechanical layout prevents this from happening, so the steering tool does not respond in relation to the motor orientation. The orientation of each part is referred to as the toolface reading. 
     The relationship between the steering tool and the motor toolfaces for the known tools of the FIGS. 2A and 2B type can be shown diagrammatically as in FIG.  4 A. Arrow  40  shows how the toolface for the steering tool turns, and arrow  41  shows how the toolface for the motor turns. The steering tool is mechanically coupled to the motor, so these two toolfaces are the same. 
     FIG. 4B shows the corresponding relationship for the present arrangement. Arrow  40  shows how the toolface the steering tool turns, and arrow  41  shows how the toolface for the motor turns. In this arrangement, these two toolfaces are not necessarily the same. The difference in rotational position between the steering tool and the motor is called the relative position measurement. The arrow  42  indicates the difference between these two toolfaces. To set the motor toolface (arrow  41 ) to a desired value, it is necessary to take the value of the steering tool toolface (arrow  40 ), which is obtainable from the sensors in the steering tube and then to adjust the relative position measurement (arrow  42 ) accordingly. (FIG. 4B shows the flowtube as lying centrally in the non-magnetic tubing, but in practice the flowtube will generally be displaced to lie opposite the steering tool.) 
     For this, it is necessary to measure the difference in rotational position between the steering tool and the motor. This value is determined by a sensor inside the orienter. This sensor can be a discrete sensor such as a resolver, or the value can be determined by the signal generated from the sensors (e.g. Hall effect) inside a brushless DC motor such as are typically used in such an application. Depending on the type of sensor used, it may be necessary to have a non-volatile memory to remember the relative position measurement in the event of a power loss. 
     FIG. 5 shows the present assembly in more structural detail. The coiled tubing  21  is connected via a tubing end connector  50  and a cable head  51  to an electric release unit  52 . Below that, and contained within the non-magnetic tubing, there is a sensor section  53  which contains a telemetry unit  54  and a sensor assembly  55 . The sensor assembly can conveniently contain a vibration sensor, a pressure sensor, a weight-on-bit (load) sensor, a natural gamma ray sensor, and so on. The sensor section  53  is generally adjacent to the steering tool  23  and the flowtube. 
     It will of course be realized that there is considerable freedom in the division of sensors between the steering tool and the sensor assembly. The flowtube may be offset in the non-magnetic tubing, leaving a lune-shaped space around it, or substantially central, as shown in FIG. 4B, leaving an annular space around it. The various sensors of the instrumentation may be located as convenient in this space around the flowtube. The instrumentation can be robustly supported along the length of the non-magnetic tubing. 
     The orienter  11  of the present apparatus is preferably of annular form, such that the flow tube passes axially through it to the motor  13 . 
     The present technique is equally applicable to rotating and pointing orienters. (In a conventional rotating orienter, the lower part physically rotates relative to the upper part to turn the orientation; in a pointing orienter, the lower part does not physically rotate.) For a pointing orienter, two position readings need to be taken from the orienter mechanism to define the direction in which the motor/bit is pointing (a bent sub is not required). These are then used to create the relative position measurement, which is used with the steering tool toolface reading to define direction of point. 
     Thus in the present arrangement, a straight through flow path for the drilling fluid is created by the flow tube. The packaging of the instrumentation in the non-magnetic tubing makes it possible to provide anti-vibration mounting, and also contributes to shortening the overall length of the arrangement. Further, the steering tool is mechanically decoupled from the motor movement by using a feedback signal from the orienter; this also means that a pointing orienter can be used. 
     Alternative embodiments using the principles disclosed will suggest themselves to those skilled in the art upon studying the foregoing description and the drawings. It is intended that such alternatives are included within the scope of the invention, the scope of the invention being limited only by the claims.