On the basis of a one-dimensional liquid flow along a horizontal path, theoretical tracerconcentration/time behavior at any given locations along the path are calculated: the parameters used are the distance from the tracer release point and the average flow velocity. These calculations show the relationships between the distance and the tracer arrival time, and between the distance and the peak concentration time. These relationships are then plotted on a group of curves. A simple method, based on these theoretical curves, and field tracer test data showing both the tracer initial returns and its peak concentration times, can be used to estimate the flow velocity and the length of the flow path between a tracer release well and an observation well. This method was tested by comparing the theoretical results with actual field data from two geothermal areas. The results of this comparison show that this method can be successfully used when tracer concentration peaks are clearly recognizable.
For Hot Dry Rock (HDR) geothermal energy development, it is necessary to establish techniques for creating artificial fractures and for evaluating a man-made circulation system as geothermal reservoirs. In this study, we have developed the Fluid Flow Tomography (FFT) technique using the Vertical Electric Profiling (VEP/hole-to-surface) method in which a casing pipe of fracturing well can be used for a current electrode. The fluid flow behavior was continuously detected and monitored with 30 stations on the ground surface during massive hydraulic fracturing operations. The VEP method provides lateral distribution of major fractures with time, while the crosshole (hole-to-hole) survey gives informations on fracture depths connected among the injection and production wells. Therefore, we can image the fluid front with time variations and major fractures in three-dimentions by the joint use of VEP (hole-to-surface) and crosshole (hole-to-hole) surveys on the HDR project.
The purpose of this study was to clarify the level of thermal output per DCHE at which power generation using a downhole coaxial heat exchanger (DCHE) becomes economical. Another goal was to clarify the geothermal resources favourable for this system. The following conclusions are drawn in this paper: (1) When heat is transferred only by conduction in the formation as in Hot Dry Rock, the power generation sytem appears to be uneconomical. On the other hand, in the case of Hot Wet Rock where the convective heat transfer mechanism is dominant, there appears to be a greater possibility of the realization of this system. (2) In the case in which the estimated net thermal output after one year of operation was 10.4 MWt/DCHE, the adjusted mean power generation cost for 15 years of plant operation under the current subsidy system was estimated to be 16.4 Yen/kWh. In this case, the power generation system might become economical in the near future. Hence, under conditions similar to those assumed in this study, 10 MWt/DCHE of the net thermal output is though to be a good criterion for judging whether the system is practical or not. In order to investigate the possibility of the realization of this system and to clarify the favourable characteristics of the formation in further detail, it is essential to accumulate in-situ data carrying out heat extraction experiments in high temperature and permeable formation.
Silicic magma-wall rock interactions are poorly understood compared with shallow-depth geothermal phenomena. Knowledge on the surroundings of old intrusive bodies exposed at the surface suggests that such interactions may occur in two types: contact metamorphism or high temperature hydrothermal alteration related to a porphyry-type deposit. Their features are in contrast with each other and indicate that contact metamorphism may result from the thermal conduction from the magma, while hydrothermal alteration of the porphyry-type deposit may result from the juvenile brine released from the magma. Factors controlling these alternatives are considered and two hypotheses are suggested. One is the effect of the permeability of wall rocks, and the other is that of the two-phase region of juvenile magmatic fluids.The latter is probably more essential but the former is also not negligible. Both hypotheses suggest that, if the top of the magma is shallower than ca. 2.5 kilometers, hydrothermal alteration of the porphyry-type deposit occurs, whereas, if it is deeper than ca.2.5 kilometers, contact metamorphism occurs.
Numerical simulations of pressure transients during nonisothermal injection tests in geothermal reservoirs were carried out. The downhole pressure response to a cold water injection test (into a hot aquifer) was shown to consist of three portions when pressure disturbance was plotted as function of the logarithm of time. The first is a straight-line portion of which the slope is dictated by the viscosity of hot water. This initial period lasts until approximately one wellbore volume of water has been injected. Next, when cold water begins to enter the formation, pressure begins to increase very rapidly. The slope of the [P-log (t)] curve becomes very steep. Finally, at later times, another asymptotic straight-line region is approached. In the porousmedium case, or the fracturedmedium case when its fracture spacing is not too large, the slope of this late-time asymptotic region is governed by the viscosity of cold water. For larger fracture separations, the slope of this late-time region is governed by the average of injected and reservoir fluid viscosities. An approximate analytical solution for this problem (“borehole capacity effects”) was also developed for the porous-medim and the fractured-medium of large fracture spacing cases.