Page 296 - Well Control for Completions and Interventions
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Well Kill, Kick Detection, and Well Shut-In                  289


                 Example (SI units):
                      3
                 A1 m influx with a reservoir pressure of 3500 kPa would result in
                     3
              34.65 m of gas at the surface:
                                       3500 3 1
                                  V 2 5        5 34:65 m 3
                                         101
                 Note 101 is atmospheric pressure in kPa at standard conditions.

              7.5.1 Gas migration in a closed-in system
              Gas entering a liquid filled well will migrate towards the surface, as it is
              less dense. The speed of gas migration depends on liquid viscosity, with
              gas generally moving more quickly through brine than mud. A rule of
              thumb widely used by drillers assumes a gas migration of 1000 ft./h. For
              clear fluids this is almost certainly an underestimate. Flow loop experi-
                                                               2
              ments have shown that gas can migrate at up 6000 ft./h.
                 If the well remains shut-in at the surface, migrating gas is pre-
              vented from expanding. Pressure in the gas bubble will remain the
              same as it moves towards the surface, and consequently, a pressure
              increase will be observed at the surface. Unless measures are taken to
              reduce gas pressure as it migrates upwards, very high surface and BHP
              will result. There will be a risk of formation fracture, or of even
              greater concern, exceeding pressure limitations on critical completion
              components.
                 The effect migrating gas has on both surface and downhole pressure is
              demonstrated in the following example:
                 Well depth—5000 ft. (TVD).
                 Reservoir pressure 3000 psi.
                 Fluid in well—10 ppg brine.
                 Closed-in wellhead pressure 400 psi.
                 Gas enters the well and migrates towards the surface. By the time the
              gas bubble reaches 4500 ft. (500 ft. above the reservoir) the pressure at the
              surface will have risen to:

                            WHCIP 5 P res  ð0:053 3 TVD gas 3 FDÞ         (7.8)
                 where:
                   WHCIP: wellhead closed-in pressure in psi;
                   P res : reservoir pressure when the influx occurred;
                   TVD gas : the TVD of the gas bubble;
                   FD: fluid density in PPG.
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