24 the surface choke 44 continues to be operated to keep the bottom hole pressure substantially constant note that the foregoing is correct for water based drilling fluid if oil based drilling fluid is used the oil based fluid will be very close to its original density because any gas will be dissolved in the oil based fluid reduction of fluid density will not occur until exsolution of the gas when this actually takes place varies depending on wellbore conditions fig9 shows that while the kick volume at the bottom of the wellbore was ten barrels the kick will expand substantially as the kick moves up the choke line 24 to the surface the choke line 24 unit volume in the present example 0 0197 bbl ft thus in a system in 10 000 feet water depth the total choke line volume is 197 barrels fig1 shows the surface choke 44 being operated to keep bottom hole pressure constant as the kick fluid is discharged through the choke 44 a typical indication that bottom hole pressure is constant is a constant drill string pressure as shown by sensor dpp fig1 shows restarting the mud return pump 38 the valves 34 36 to.