Phase state variations for supercritical carbon dioxide drilling

A phase state prediction model during supercritical carbon dioxide (SC-CO2) drilling is established, considering the enthalpy changes caused by the flow work variations and changes in kinetic energy, as well as the potential energy caused by the fluid velocity along with physical property variations. The results show that variations in the flow work affect the temperature field of the SC-CO2 fluid significantly. Phase state transitions of the fluid in the drill pipes and the annulus exist; in this example, the depth of the phase transition point in the drill pipes is in the range of 600 to 1000 m, while in the annulus, it is in the range of 400 to 700 m. Different results are observed when the phase state prediction is conducted by adjusting the discharge capacities, injecting temperature and pressure. When the discharge capacity is increased, the phase transition points in the drill pipes and the annuluses move downward; the pressure of the bit nozzle upstream increases gradually, while the pressure of the bit nozzle downstream changes slightly. When the wellhead back pressure is increased, the depths of the phase transition points in the drill pipes and the annulus decrease. Moreover, the pressure variations of the bit nozzle upstream and downstream can be divided into two stages: the fast growth stage and the slow growth stage. When the injection temperature is increased, the depths of the phase transition points in the drill pipes and the annulus are reduced; the temperature drop and the pressure drop at the nozzle change slightly. (c) 2015 Society of Chemical Industry and John Wiley & Sons, Ltd