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.

This study proposes an indirect method to estimate the ground effective thermal conductivity in the formation of a depth over several tens to one hundred meters by averaging probability-weights of all soil/rock types. Thermal response test results are used as the in-situ measurements for determining a set of the individual effective thermal conductivities in the least square. The probability values of all soil/rock types are estimated at any location and depth through the indicator kriging interpolation with the adjacent water well data. In this study, 76 test data were used for analysis and the individual effective thermal conductivities of 8 soil/rock types, i.e., clay, sand, gravel, volcanic ash, Quaternary volcanic rock, Neogene fine rock, Neogene coarse rock, and bedrock were determined. For comparison, this study demonstrated a conventional method to estimate the ground effective thermal conductivity as a thickness-weighted average. The conventional method requires the direct geo-information such as geologic columns, which are not often obtained especially in the deep zones several meters deep. Also in this study, only 25 test data included the geologic columns. Thus, the conventional method resulted in the insufficient agreements of ground effective thermal conductivity between the estimates and the measurements. On the other hand, the analysis of the proposed method was performed in all 76 test data by using the probability estimates through the kriging system. As a result, all individual effective thermal conductivity values were reasonably obtained: the values of clay, sand and volcanic ash were almost equal to the reference values in texts. The value of gravel was relatively large probably because of the natural convection flows along the borehole during heating. The values of soft Neogene rocks were smaller than those of hard Quaternary volcanic rock and bedrock. The estimation errors of the ground effective thermal conductivity were close to the possible error by testing itself, except for the results over 3 W/(m･K). The K-fold cross validation (K= 10) also indicated the stability of the analysis results. This means the potential use of the method not only in the study area but also in other regions. This proposed method contributes to various heat transport problems in practice by providing the estimates of the ground effective thermal conductivity at any location. The method is also efficiently used to construct a database of the ground effective thermal conductivity, as shown in a case study of Sapporo City.

The recent geophysical exploration method, such as airborne gravity gradient survey, is progressed tremendously. The measurement over wide area at short intervals in the short period of time leads to hope for the efficient tools of the geothermal resources assessments. The commonly applied methods for the surface-layer density estimation and its application to surfacelayer correction of the acquired gravity gradient data is based on one kind of surface-layer density, which may not always be reflecting the frequently variable surface-layer density especially in the geothermal prospecting areas, therefore, the corrected gravity gradient and gravity would be inadequate. The new approach, namely the horizontal gravity gradient stack (HGGS), is proposed for the assessment of surface-layer density, and the method is applied with the filtering of the horizontal gravity gradient data (Data). The comparison between the HGGS filtered acquired Data and the equivalent calculated response of the DEM based surface-layer is analyzed for estimating the surface-layer density by the moving window correlation (MWC) methods. Its application to the actually acquired Data around the Ogiri geothermal plant in the western Japan is analyzed, and its results fit adequately well with the existing fractured zones and hydrothermal alteration zones as the low surface-layer density area, and the possible prominent distributions of intrusions and lava deposits as the high surface-layer density area. The surface-layer density corrected vertical gravity gradient and vertical gravity is calculated for comparison with the seismic sections. Some discrepancies of both data lead us a suspicion how deep the gravity gradient data is reflecting the geological anomalies.