In order to evaluate the change in flow characteristics of a geothermal production well with increasing the drilling depth, two-phase flow analysis in a wellbore was carried out assuming the well inlet condition. Pressure at the well inlet was given by the vertical pressure profiles of saturated water whose water level was located at 0 m and -100 m, and inlet steam quality of x1=0 was used. The steam and water flow rates of a 3000 m deep well at a constant well head pressure (P2=10 ata) are estimated 194 t/h and 183 t/h respectively.
In order to evaluate the effect of boundary conditions, particularly that of topography, on heat flow in mountainous areas utilization of boundary element method for solving Laplace's equation is proposed. Numerical solution by this method and the actual data of precise tem-perature logging are compared for a well located at the center of a valley in a two dimensional topographic profile, assuming non-uniformness of mean surface temperature due to the slope attitude of the ground surface. In the case of H-3 hole with a depth of 250 m in Sengan Area, observed temperature gradient decreases down to the bottom, which agrees well with the numerical solution with the undisturbed geothermal gradient value of 5.0×10-2°C/m. A comment is also made on the deviation of observed temperature gradient from the numerically generated temperature log at isolated short intervals.
Kurobe Jobu railway tunnel was accomplished in 1940, and the maximum wall temperature between Asobaradani and Sennindani was 165 °C. Since then, the tunnel has been considerably cooled by natural ventilation, but the present maximum wall temperature near small fumaroles or hot springs exceeds 90°C in places, and such a high temperature is supposed to be caused by thermal fluid flowing through fractures. Meteoric water which flows through fractures from the ground surface is assumed to extract and transport heat from the rock matrix above the tunnel to the wall of the tunnel and cause a high temperature near small fumaroles or hot springs. The assumption was evaluated by a simple mathematical model. The model consists of the cylindrical rock mass having a fracture through which water flows downward at the center. The differential equations govering the rock temperature were numerically solved by the finite difference method. Consequently in case many small fractures in the upper part of the rock mass join a single large fracture in the lower part, the temperature near fumaroles or hot springs can exceed 90 °C.
To predict heat extraction from hot dry rock by water circulation through a thin hydraulic circular fracture, a simple methematical scheme is presented. As the scheme does not treat complicated phenomena occurring in the fracture, it has an advantage of calculating the extracting water temperature for a long period easily. Calculations are carried out for the single fracture system made along the middle vertical plane of a rectangular rock body surrounded by thermally insulated planes; the size of the rock is 400m×307m×400m, the initial rock temperature is 250°C, the injection water temperature is 20°C and flow rates are 6 to 6001Jmin. Examples of calculations show that the single fracture system extracts the heat stored near the fracture and that the flow rate may be limited if one expects to get the high temperature water, though the water with the almost constant temperature can be taken out for a rather long period.
Thermal properties of rocks at elevated temperature and high pressure have been highly concerned in geothermal energy development and in many fields of engineering. This paper describes development of a needle probe method -a transient hot wire method for simple and fast laboratory measurements of temperature and pressure dependence of the thermal conductivity of rocks and other poor conductors. A cylindrical sample of high purity fused quartz as a standard material of the thermal conductivity and samples of two granite are heated by a thin needle probe having 2.0 mm 0. D. and 80 mm effective length, the resulting temperature increase at the effective center point of the probe being monitored by a thermo-couple, a digital voltmeter and a pen recorder. The experimental temperature data to yield values of the conductivity is obtained in about fifteen minutes. Conductivity values of fused quartz and the granite specimens are presented in the temperature range 300-800°K. The absolute accuracy better than ±3.0 % verified by measurement of the standard sample for this method. The thermal conductivity of the granite specimens exhibitsaT-1 dependence on temperature, T, as expected for these type of rocks. Sample preparation is simple and not critical. The method described is well suited for measurement of the variation of thermal properties with temperature and pressure.
Hot Dry Rock technology requires a thorough knowledge of the variation of the thermal properties of rocks up to depth of the order of five kilometers. Temperature dependences of the thermal conductivity of 42 core samples from three test wells having 300 m depth at the field test site for the HDR technology in Yakedake geothermal area are presented in the temperature range 20-500°C. They are measured by a needle probe method. The thermal conductivity of the core sample exhibits θ1, θ0 and θ-1 dependences on temperature, θ, macroscopically. However, they are more complicatedly classified into θ1+θ0, θ-1+θ0and θ-1+θ1 dependences on θ in addition to above the three dependences, microscopically. In situ thermal conductivities of the rocks and terrestrial conduction heat flows at depths 110, 200 and 300 m in one of the test wells, HY well, are estimated by the obtained data.