2018 Volume 40 Issue 1 Pages 17-31
Geothermal production wells sometimes show oscillations of wellhead pressure and flow rate when completed with multiple feed zones. This instability of the production causes a lack of steam supply and unstable turbine inlet pressure at a power plant. In the worst-case, steam production of the well may cease. However, its mechanism has not been well understood. In this study, we developed a numerical model of transient steam-water two-phase flow in a geothermal production well. This wellbore model is coupled with a reservoir model so we can treat flow rate change with time from the reservoir into the wellbore and deal with a well with multiple feed zones. We use a homogeneous flow model of steam-water mixture in a wellbore. Thus, we developed a numerical simulator coupled with a modified semi-implicit method, and succeeded in suppressing a generation of mass error when phase change occurs. The simulation results show that oscillation of flow rate in a wellbore occurs as temperature difference of the fluids between the feed zones becomes large. The results also show that the oscillation is generated by interactions among pressure, pressure drop, and specific enthalpy of fluids in a wellbore.
