Numerical Modeling of Pressure and Temperature Profiles Including Phase Transitions in Carbon Dioxide Wells

Abstract Geological storage of carbon dioxide will usually be at conditions above the critical temperature and pressure, so the carbon dioxide will exist as a single dense phase. However, conditions in the upper part of a carbon dioxide well with surface temperatures below the critical point of 31 C can lead to boiling and condensation in the well. The consequences of this are most apparent when flow rate changes, for example when a well is shut-in or if there is a well blowout. We have calculated density profiles for wells experiencing different thermal conditions to determine how bottom-hole pressures are related to wellhead pressures. There are two limiting cases, one when the fluid is in thermal equilibrium with the rock at the same horizon, the other when there is no heat exchange with the casing or the rock. We find that in deeper wells static columns can exist in a stable state with liquid to the surface, but for shallower wells or wells in depleted reservoirs that a static column can be initially unstable with two-phase conditions near the surface. In producing wells, as the flow rate increases from static conditions, the pressure and temperature at the wellhead increases until high production rates are reached when the wellhead temperature then decreases, which can be to very low values. For injection wells, bottom-hole conditions are confined between the wellhead and the reservoir temperature. In general, phase change does not prevent carbon dioxide injection. Nevertheless care is needed in shallower or depleted reservoirs for the interpretation of reservoir pressure, the use of pressure for monitoring, and in all reservoirs for the management of blowouts.