Page 338 - Air and Gas Drilling Manual
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Chapter 8: Air, Gas, and Unstable Foam Drilling    8-21
                               pressures up to 1,000 psig and intermittent operation at pressures up to  1,200  psig.
                               These high pressures are usually not  encountered in  air and gas drilling  operations.
                               But  sometimes unforeseen high  back pressures are encountered while drilling  (e.g.,
                               high  influx of formation water or producing fluids) and such  pressures  have  to  be
                               managed in order not to lose the well.  Also, if more volumetric flow rate is required
                               (to lift  formation fluids) during the drilling  operation, a third compressor  unit  can
                               placed in service (all deep drilling  operations have a standby compressor available).
                               The addition of this third compressor will also result in higher injection pressures.
                                 8.3.2 Major and Minor Losses and Injection Pressure
                                   Once  the  compressor  system  units  have  been  selected  for  a  given  drilling
                               operation, the actual volumetric flow rate capability of the compressor system  will
                               be known.   Knowing the actual volumetric flow rate to  be injected into  the top  of
                               the drill string, calculations can be made to determine the injection pressure required
                               by the individual compressors in the system.  It is important to  accurately determine
                               the injection pressure.  Once the injection pressure has been determined, this value is
                               compared to  the  maximum  output  capability  of  the  compressor.    If  the  obtained
                               injection  pressure  value  is  less  than  the  maximum  pressure  capability  of  the
                               compressors,  then  the  compressor  system  is  tentatively  capable  of  providing
                               compressed  air  to  the  drilling  operation.    To  complete  the  assessment  of  the
                               compressor system, the prime mover of the compressor must be checked to  ascertain
                               if it is capable of producing the power needed to produce the required pressure.  The
                               power output  of a prime mover is  sensitive to  its  surface  elevation  location.    The
                               power output of a prime mover will  decrease as the surface elevation location above
                               sea level is  increased.   Therefore, it  is  necessary to  compare the horsepower needed
                               by the compressor to  the  horsepower  available  by  the  prime  mover  at  its  surface
                               elevation location.
                                   The  calculation  procedure  for  determining  the  injection  pressure  requires
                               working  from  the  exit  to  the  blooey  line  (i.e.,  the  known  atmospheric  pressure)
                               upstream  through  the  circulation  system  to  injection  pressure.    During  this
                               calculation procedure, all the pressure positions  in  the annulus and inside  the  drill
                               string  where  there  are  changes  in  annulus  and  drilling  string  geometry  will  be
                               determined.  Of particular interest to the oil and natural gas recovery industry and the
                               geothermal fluids recovery industry is the determination of the bottomhole pressure.
                               In underbalanced drilling  operations it  is  very important  to  design  the  drill  string
                               used to  drill  the well.   A properly designed drill  string will  yield  the  bottomhole
                               pressure  lower  than  the  pore  pressure  in  the  fluids  producing  formation.    Thus,
                               accurate pressure prediction in  the  circulation  system  is  needed  to  assure  that  the
                               drilling operation will not cause formation damage.
                                   In order to calculate accurate circulation pressures (i.e., bottomhole and injection
                               pressures) it  is  necessary to  utilize all possible standard fluid mechanics major and
                               minor friction loss  terms in  carrying out  these calculations.   Descriptions  of  these
                               major and minor losses are detailed in  the  fluid  mechanics  literature  [12].    These
                               losses  will  be  explained  and  utilized  in  the  calculations  used  in  the  illustrative
                               examples that follow.
                                   Illustrative Example 8.3b  describes  the  implementation  of  the  basic  planning
                               step No. 9 given in Section 8.1.
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