Abstract:
A method of controlling a downhole operation includes: deploying a work string into a wellbore, the work string comprising a deployment string and a bottomhole assembly (BHA); digitally marking a depth of the BHA; and using the digital mark to perform the downhole operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. provisional patent application Ser. No. 61/496,784, filed Jun. 14, 2011, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention generally relate to a control system for downhole operations. 
         [0004]    2. Description of the Related Art 
         [0005]    In well construction and completion operations, a wellbore is formed to access hydrocarbon-bearing formations (e.g., crude oil and/or natural gas) by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, and/or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annulus is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annulus with cement. The casing string is cemented into the wellbore by circulating cement into the annulus defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons. 
         [0006]    Sidetrack drilling is a process which allows an operator to drill a primary wellbore, and then drill an angled lateral wellbore off of the primary wellbore at a chosen depth. Generally, the primary wellbore is first cased with a string of casing and cemented. Then a tool known as a whipstock is positioned in the casing at the depth where deflection is desired. The whipstock is specially configured to divert milling bits and then a drill bit in a desired direction for forming a lateral borehole. 
       SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the present invention generally relate to a control system for downhole operations. In one embodiment, a method of controlling a downhole operation includes: deploying a work string into a wellbore, the work string comprising a deployment string and a bottomhole assembly (BHA); digitally marking a depth of the BHA; and using the digital mark to perform the downhole operation. 
         [0008]    In another embodiment, a method of performing a downhole operation in a wellbore includes monitoring operational parameters associated with the downhole operation; marking a reference point in a monitoring system; in response to the marking of a reference point, using the monitoring system to provide target values for selected operational parameters for execution of the downhole operation; and controlling the execution of the downhole operation according to the target values. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0010]      FIG. 1  is a diagram of a control system, according to one embodiment of the present invention. 
           [0011]      FIGS. 2A-2C  illustrate a sidetrack milling operation conducted using the control system, according to another embodiment of the present invention.  FIG. 2A  illustrates a pilot bit engaging a top of the whipstock.  FIG. 2B  illustrates the milling operation near the start of the core point.  FIG. 2C  illustrates the milling operation near completion. 
           [0012]      FIG. 3  illustrates a hardware configuration for implementing the control system, according to another embodiment of the present invention. 
           [0013]      FIG. 4  illustrates a reference database of the control system, according to another embodiment of the present invention. 
           [0014]      FIG. 5  is a screen shot of an operator interface of the control system. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a diagram of a control system  1 , according to one embodiment of the present invention. The control system may be part of a milling system. A primary wellbore  3   p  has been drilled using a drilling rig  2 . A casing string  4  has been installed in the primary wellbore  3   p  by being hung from a wellhead  15  and cemented (not shown, see  FIG. 2A ) in place. Once the casing string  4  has been deployed and cemented, a mill string  5   b,d  may be deployed into the primary wellbore  3   p  for a sidetrack milling operation. 
         [0016]    The drilling rig  2  may be deployed on land or offshore. If the primary wellbore  3   p  is subsea, then the drilling rig may be a mobile offshore drilling unit, such as a drillship or semisubmersible. The drilling rig  2  may include a derrick  6 . The drilling rig  2  may further include drawworks  7  for supporting a top drive  8 . The top drive  8  may in turn support and rotate the mill string  5   b,d . Alternatively, a Kelly and rotary table (not shown) may be used to rotate the mill string  5   b,d  instead of the top drive. The drilling rig  2  may further include a mud pump  9  operable to pump milling fluid  10  from of a pit or tank (not shown), through a standpipe and Kelly hose to the top drive  8 . The milling fluid  10  may include a base liquid. The base liquid may be refined oil, water, brine, or a water/oil emulsion. The milling fluid  10  may further include solids dissolved or suspended in the base liquid, such as organophilic clay, lignite, and/or asphalt, thereby forming a mud. 
         [0017]    The drilling rig  2  may further include a control room (aka dog house) (not shown) having a rig controller  11 , such as a server  11   s  ( FIG. 3 ), in communication with an array  12  of sensors for monitoring the milling operation. The array  12  may include one or more of: a mud pump stroke counter (Pump Strokes), a hook load cell (Hook Ld), a hook (and/or drawworks) position sensor (Hook Pos), a standpipe pressure (SPP) sensor, a wellhead pressure (WHP) sensor, a torque sub/cell (Torque), a turns (top drive or rotary table) counter (Turns), and a pipe tally (Tally). From the sensor measurements and values input by an operator, the rig controller  11  may calculate additional operational parameters, such as bit (or BHA) depth (measured and vertical), flow rate, rate of penetration (ROP), rotational speed (RPM) of the deployment string  5   b,d , and weight-on-bit (WOB). Alternatively, one or more of these additional parameters may be measured directly as the other parameters in the array  12  or calculated by any other device or process. The rig controller  11  may also have one or more wellbore parameters stored, such as bottomhole depth (measured and vertical). 
         [0018]    The milling fluid  10  may flow from the standpipe and into the mill string  5   b,d  via a swivel. The milling fluid  10  may be pumped down through the mill string  5   b,d  and exit a lead mill  13   m,p , where the fluid may circulate the cuttings away from the mill and return the cuttings up an annulus formed between an inner surface of the casing  4  and an outer surface of the mill string  5   d,b . The milling fluid  10  and cuttings (collectively, returns) may flow through the annulus to the wellhead  15  and be discharged to a primary returns line (not shown). Alternatively, a variable choke and rotating control head may be used to exert backpressure on the annulus during the milling operation. The returns may then be processed by a shale shaker  16  to separate the cuttings from the milling fluid  10 . One or more blowout preventers (BOP)  17  may also be fastened to the wellhead  15 . The mill string  5   b,d  may include a deployment string  5   d,  such as joints of drill pipe screwed together, and a bottom hole assembly (BHA)  5   b.  Alternatively, the deployment string may be coiled tubing instead of the drill pipe. 
         [0019]      FIGS. 2A-2C  illustrate a sidetrack milling operation conducted using the control system  1 , according to another embodiment of the present invention.  FIG. 2A  illustrates a pilot bit  13   p  engaging  27  a top of the whipstock  18   w.    FIG. 2B  illustrates the milling operation near a start of a core point  24 .  FIG. 2C  illustrates the milling operation near completion. The BHA  5   b  may include the lead mill  13   m,p , drill collars, a trail (i.e., secondary or flex) mill  14 , measurement while drilling (MWD) sensors (not shown), logging while drilling (LWD) sensors (not shown), and a float valve (to prevent backflow of fluid from the annulus). The deployment string  5   d  may also include one or more centralizers (not shown) spaced therealong at regular intervals and/or the BHA  5   b  may include one or more stabilizers. The mills  13   m,p ,  14  may be rotated from the surface by the rotary table or top drive  8  and/or downhole by a drilling motor (not shown). Alternatively, the BHA may include an orienter. 
         [0020]    The lead mill  13   m,p  may include a mill bit  13   m  and a pilot bit  13   p.  The trail mill  14  may include a mill bit. Each bit  13   m,p    14  may include a tubular housing connected to other components of the BHA  5   b  or to the deployment string  5   d,  such as by a threaded connection. Each bit  13   m,p    14  may further include or more blades formed or disposed around an outer surface of the housing. Cutters may be disposed along each of the blades, such as by pressing, bonding, or threading. The cutters may be made from a hard material, such as ceramic or cermet (i.e., tungsten carbide) or any other material(s) suitable for milling a window. 
         [0021]    The milling system may further include a deflector  18   w,a . The deflector  18   w,a  may include a whipstock  18   w  and an anchor  18   a.  The anchor  18   a  may or may not include a packer for sealing. The deflector  18   w,a  may be releasably connected (i.e., by one or more shearable fasteners) to the BHA  5   b  for deployment so that the milling operation may be performed in one trip. The anchor  18   a  may be mechanically and/or hydraulically actuated to engage the casing  4 . The whipstock  18   w  may be releasably connected to the anchor  18   a  such that the whipstock may be retrieved, an extension (not shown) added, and reconnected to the anchor for milling a second window (not shown). Alternatively, the anchor and/or the deflector may be set in a separate trip. 
         [0022]      FIG. 3  illustrates a hardware configuration for implementing the control system  1 , according to another embodiment of the present invention. The control system  1  may include a programmable logic controller (PLC)  20  implemented as software on one or more computers  21 ,  22 , such as a server  21 , laptop  22 , tablet, and/or personal digital assistant (PDA). The software may be loaded on to the computers from a computer readable medium, such as a compact disc or a solid state drive. The computers  21 ,  22  may each include a central processing unit, memory, an operator interface, such as a keyboard, monitor, and a pointing device, such as mouse or trackpad. Alternatively or additionally, the monitor may be a touchscreen. Each computer  21 ,  22  may interface with the rig controller via a router  23  and each computer may be connected to the router, such as by a universal serial bus (USB), Ethernet, or wireless connection. The interface may allow the PLC  20  to receive one or more of the rig sensor measurements, the operational parameters, and the wellbore parameters from the rig controller  11 . Each computer  21 ,  22  may also interface with the Internet or Intranet via the rig controller  11  or have its own connection. Alternatively, the PLC software may be loaded onto the rig controller instead of the computers. 
         [0023]      FIG. 4  illustrates a reference database  25  of the control system  1 , according to another embodiment of the present invention. The control system  1  may further include the window milling reference database  25 . The database  25  may be loaded locally  25   c  on the milling server  21  and/or accessed (or updated) from a master version  25   m  possibly via the Internet and/or Intranet. The database  25  may include locations of known or expected events during a window milling operation, such as one or more of: beginning of cutting for each mill, beginning of cutout for each mill, maximum deflection, start and end of whipstock retrieval slot  19  ( FIG. 2B ) (may also include end of retrieval lug), start, middle, and end of the core point  24 , and kickoff point  26 . The locations may be a distance from a known reference point, such as a top  27  of the whipstock. The events may be used to divide the window milling operation into two or more regions, such as a cutout region, a maximum deflection region, a retrieval slot region, a core point region, and a kickoff region. The database  25  may include a set of locations for each of various casing sizes and/or weights (two different sets shown). 
         [0024]    The database  25  may also include minimum and maximum target values of one or more milling parameters, such as ROP, RPM, and/or WOB, for each region or each event. For example, the database  25  may include a first minimum and maximum ROP for the cutout region, a second minimum and maximum ROP for the maximum deflection region, a third minimum and maximum ROP for the core point region, and a fourth minimum and maximum ROP for the kickoff region. The target values of one or more the milling parameters may be predetermined or may vary depending on values measured during the milling process. The target values of one or more the milling parameters may be constant or may vary based on a particular casing size or weight (only one set of target values shown for each parameter). If the target values of a particular milling parameter vary with casing size and/or weight, then the database may include a set of target values for the parameter for each casing size and/or weight. The database  25  may also include predetermined comments based on previous experience for one or more particular regions or events. Alternatively, the database  25  may only include a target value for one or more of the milling parameters instead of a minimum and maximum. 
         [0025]      FIG. 5  is a screen shot of an operator interface  30  of the control system  1 . In operation, the operator  28  may enter (and/or the PLC  20  may receive from the rig controller) known parameters into the PLC  20 , such as casing parameters (i.e., size and weight), BHA parameters (mill sizes, types, and spacing), and deflector parameters. The mill string  5   b,d  may be run into the primary wellbore  3   p  to a desired depth of the window  3   w.  The whipstock  18   w  may be oriented by rotation of the deployment string  5   d  using the MWD sensors in communication with the rig controller via wireless telemetry, such as mud pulse, acoustic, or electromagnetic (EM). Alternatively, the mill string may be wired or include a pair of conductive paths for transverse EM. The PLC may record the orientation. The anchor  18   a  may be set with the whipstock  18   w  at the desired orientation. The deflector  18   a,w  may be released from the BHA  5   b.    
         [0026]    The BHA  5   b  may then be rotated by rotating the deployment string  5   d  (and/or operating the drilling motor) and milling fluid  10  may be pumped to the BHA  5   b  via the deployment string  5   d.  The mill string  5   b,d  may then be lowered toward the whipstock  18   w.  The PLC  20  may monitor the torque and may calculate and monitor a torque differential with respect to time or depth. The BHA  5   b  may be lowered until the lead mill  13   p,m  (i.e., pilot bit  13   p ) engages  27  the whipstock  18   w  ( FIG. 2A ). The PLC  20  may detect engagement by comparing the torque differential to a predetermined threshold (from the reference database  25 ). The PLC  20  may then alert the operator  28  when engagement is detected and the operator may digitally mark  31  the pipe by clicking on an appropriate icon  32 . The digital mark  31  may represent a reference point for the PLC  20  to monitor and control the downhole operation. Alternatively, the PLC may automatically mark the pipe. Alternatively, the operator may disregard the PLC&#39;s suggestion and mark the pipe based on experience. 
         [0027]    Once the pipe is digitally marked  31 , the PLC  20  may correlate the target values from the database  25  with BHA/bit depth by calculating the depths of the events/regions from the database  25  using the digital mark. The PLC  20  may then display a default set of target windows  33   a - c  for one or more of the operational parameters, such as ROP  33   a,  RPM  33   b,  and WOB  33   c.  If the target values for a particular operational parameter are predetermined, the PLC  20  may display the particular target window for the entire milling operation. If the target values for the particular operational parameter depend on actual measurements of the parameter or other parameters, the PLC  20  may calculate the particular target based on the actual parameter, other actual parameters, or differentials thereof, and criteria from the database  25 . The criteria may vary based on the current event or region of the milling operation. The PLC  20  may then illustrate the calculated window for the current depth  41 . The PLC  20  may also monitor actual values for the operational parameters (from the rig controller  11 ) and display plots of the various parameters for comparison against the respective target windows. The PLC  20  may receive and plot the actual values in real time. The PLC  20  may display the parameters (target and actual) plotted against time or depth (selectable by the operator). The PLC  20  may also monitor actual BHA/bit depth  41 . 
         [0028]    The PLC  20  may also interface with a flow model  34 . The flow model  34  may be executed during the milling operation by the rig controller  11 , the milling server  21 , or an additional computer (not shown). The flow model  34  may calculate a target SPP  34   t  based on sensor measurements received from the rig controller  11 . The PLC  20  may also display a target plot  34   t  for the received target SPP and plot the actual SPP (from the rig controller) for a graphical comparison. Additionally, the flow model  34  may calculate a cuttings removal rate and calculate a flow rate of the milling fluid  10  necessary to remove the cuttings. The flow model  34  may monitor the milling fluid flow rate and compare the actual flow rate to the calculated flow rate and alert the operator if the actual flow rate is less than the calculated flow rate needed for cuttings removal. The PLC  20  may also calculate a maximum flow rate based on a maximum allowable SPP, formation fracture pressure, or equivalent circulation density (ECD) limits and compare the actual flow rate to the maximum. 
         [0029]    Alternatively, an operator may change the default target plots to illustrate target plots for one or more additional parameters, such as rathole depth. 
         [0030]    The PLC  20  may also generate an animation  35  of the BHA  5   b,  whipstock  18   w,  and casing  4  to scale (or not to scale) and update the animation based on actual BHA/bit depth  41 . The animation  35  may allow an operator  28  to view engagement of the mills  13   p,m ,  14  with the casing  4 . The PLC  20  may also offset or adjust the animation  35  based on actual parameters, such as torque and/or drag. The animation  35  may also illustrate rotational speed (or velocity) of the mill string  5   b,d.    
         [0031]    The operator  28  may monitor the parameters displayed by the PLC  20  and make adjustments, such as altering RPM and/or WOB, as necessary to keep the operational parameters within the respective target windows. Alternatively, the rig controller may be capable of autonomous or semi-autonomous control of rig functions and the PLC may make adjustments to keep the operational parameters within the respective target windows. The operator  28  may then only monitor, subject to override of the autonomous control. The PLC  20  may also compare the actual parameters to the target windows and alert the operator  28  if any of the parameters depart from the respective target windows. The PLC  20  may also warn the operator  28  if the actual parameters approach margins of the respective windows. For the calculated windows, the PLC  20  may forecast a portion of the window and display the forecast portion to facilitate control by the operator  28 . This predictive feature may allow the operator to make corrections to the operational parameters in anticipation of the forecasted changes. The PLC  20  may then correct the forecast on the next iteration. The PLC  20  may also warn the operator  28  if a differential of a particular parameter indicates that the parameter will quickly depart from the target window. 
         [0032]    The PLC  20  may iterate in real time during the milling operation. Once the milling operation is complete (including the milling of any required rathole), the mill string  5   b,d  may be removed and the milling BHA  5   b  replaced by a drilling BHA. The drill string may be deployed and the lateral wellbore drilled through the casing window  3   w.  Alternatively, the milling BHA may be used to drill the lateral wellbore. Once drilled, the lateral wellbore may be completed, such as by expandable liner or expandable sand screen. 
         [0033]    The PLC  20  may continue to track the digital mark  31  during the drilling and completion operations so the mark may be reused to retrieve the whipstock  14   w  or assist in passing of future completion BHA(s) through the window  3   w.  As discussed above, an extension may be added to the whipstock  14   w  for use in milling a second window. Additionally, the PLC  20  may allow the operator to make a plurality of digital marks and track the marks for future reference. 
         [0034]    Additionally, the PLC  20  may include a chat (aka instant messaging) feature  36  allowing communication of the operator  28  with one or more remote users, such as engineers  29 , located at a remote support center. The PLC  20  may also communicate with the remote support center such that the engineers  29  may view a display similar to that of the operator  28 . 
         [0035]    Additionally the PLC  20  may include a digital tally book  37 . The digital tally book  37  may include a progress indicator  37   i  and a comments section. The comments section may allow the operator  28  to enter comments  37   e  during the milling operation. The comment entries  37   e  may be time and depth stamped for later evaluation and be represented by an icon  38  on the progress indicator  37   i.  The progress indicator  37   i  may be a depth-line when the depth selector is chosen and a timeline when the time selection is chosen. The digital mark  31  may be illustrated on the progress indicator  37   i.  The PLC may also illustrate one or more events using pointers, such as core point (CP)  39 , kickoff point (KP)  40 , and current depth  41 . The comments from the database  25  may also be illustrated as icons (not shown) on the progress indicator. 
         [0036]    The PLC  20  may save the operational data such and include a playback feature  42  such that the operation may be later evaluated. The operational data may be encoded with time and depth stamps for accurate playback. 
         [0037]    Alternatively, the PLC may monitor actual values and display target values for setting the anchor and orienting the whipstock. The deflection angle of the whipstock may be input by the operator. The values may include azimuth, inclination, and/or tool face angle. The PLC may display the actual and target values to ensure that the correct orientation is obtained. This display may allow the operator to make adjustments based on actual data from the MWD sub to account for wellbore deviation. The PLC or the operator may digitally mark the pipe before, during, and/or after setting anchor and orienting the whipstock. 
         [0038]    Alternatively, the PLC may include a simulator so that the milling operation may be simulated before actual performance. Alternatively, the reference database may be a historical database including the operational parameters for similar previously milled wellbores and the historical operational plots may be used instead of target windows. 
         [0039]    Alternatively, the control system may be used with other downhole operations, such as a fishing operation for freeing and retrieving a stuck portion of a drill string. The digital pipe mark may be made when a fishing tool, such as a spear or overshot, engages the stuck portion of the drill string. The pipe mark may be tracked and reused if the stuck portion must be milled due to failure of the fishing operation. The control system may also be used for drilling out casing shoes, packers, and/or bridge plugs. The control system may also be used for setting liner hangers or packers. The control system may also be used for milling reentry of the parent wellbore (milling through a wall of the liner at the junction of the parent and lateral wellbore) as discussed and illustrated in U.S. Pat. No. 7,487,835, which is herein incorporated by reference in its entirety. 
         [0040]    Additionally, the PLC may include additional threshold parameters for detecting actuation of the deflector. For example, WOB and/or torque differentials may be monitored and compared to thresholds to confirm actuation of the anchor and/or release of the whipstock and anchor from the BHA. Alternatively, the threshold parameters may be used to confirm other operations, such as engagement of a drill bit with a casing shoe, engagement of a liner hanger with a casing; engagement of the fishing tool with the stuck portion; or the engagement of a drill or mill bit with a bridge plug or packer. 
         [0041]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.