Net-pressure

Net-pressure (Pnet) is the pressure in a hydraulically created fracture that holds it open. It isn't the botomhole treating pressure in the wellbore, as perforation and near-wellbore friction have to be subtructed to obtain Pnet.

If pressure measured at the earth surface is used to make the calculation for Pnet the equation for Pnet is as follows:

       Pnet = Ps - Ph - Ppf - Pperfs - Pnwb -Pcl
          where:
            Ps =  Injection Pressure at surface
            Ph = Hydrostatic pressure
            Ppf = Pressure loss in the pipe (tubing, drill pipe or casing) 
            Pprfs = Prssure loss in the perforations 
            Pnwb = Pressure loss in the near-wellbore area
            Pcl =  Pressure at fracture closure

If reflected pressure (casing or annulus side), a.k.a. deadstring pressure is used to make the calculation for Pnet the equation for Pnet is as follows:

       Pnet = Ps - Ph - Pperfs - Pnwb -Pcl
          where:
            Ps =  Injection Pressure at surface
            Ph = Hydrostatic pressure
            Pprfs = Prssure loss in the perforations 
            Pnwb = Pressure loss in the near-wellbore area
            Pcl =  Pressure at fracture closure

Note that there is no pipe friction term, as there is no fluid movement in the side where the pressure measurement is made.

If botomhole pressure is used to make the calculation for Pnet the equation for Pnet is as follows:

       Pnet = Pbh - Pperfs - Pnwb -Pcl
          where:
            Pbh =  Botomhole pressure
            Pprfs = Prssure loss in the perforations 
            Pnwb = Pressure loss in the near-wellbore area
            Pcl =  Pressure at fracture closure

Note that there is no hydrostatic pressure term as this is included in the botomhole pressure measurement. Also there is no pressure loss in the pipe term, as the pressure measurement is made at the end of the pipe. So, the pressure sensor is not sensing pressure loss due to friction in the pipe.

A minifrac test is the most common method for obtaining Pnet directly. Using the Shut-in interval of the minifrac test, one selects the Instantaneous Shut-In Pressure (ISIP) and the Fracture closure pressure (Pcl). Thus:

      Pnet = ISIP - Pcl

If a rate step-up test has been performed, the Fracture Extension Pressure (FEP) is determined, thus a more accurate value for Pnet can be determiined, as follows:

     Pnet = FEP - Pcl

Note that FEP is always less than ISIP.