Theoretical analysis of water loss and back flow through a prefractured plane intersecting a borehole is made. Firstly we derive the equation of continuity in terms of the water pressure in the prefractured plane and its aperture. By solving the equation by means of the difference method, the influences of the water pressure in the borehole, tectonic stress and topography of the surface of the prefractured plane are clarified. The results are expected to be useful to estimate the topography and aperture of the prefractured plane and the tectonic stress normal to the prefractured plane.
Heat transfer from convecting geothermal fluid to an extremely slender isothermal cylinder struck into a hydrothermal system is the center of concern in the development of thermal energy extraction technique with large heat pipes. A present study focuses on transient be-haviors of heat transport through permeable rock formation. The hydrothermal system is idealized as a water-saturated homogeneous porous medium with either forced or natural convection in the axial direction. The transient heat transfer process for both convection systems is analyzed when a cylinder surface temperature is suddenly lowered. Both numerical and analytical approaches are employed to delineate the heat transfer characteristics. The results reveal the presence of three distinct regimes along the course to achieve a steady state for a given set of parameters. Conductive regime, where the radial heat conduction is the dominant mode of transfer process, appears first. At large time the heat transfer becomes independent of time and steady state preveils, where the radial heat diffusion is balanced by the axially convecting heat. Between the two extremes there is a transitional regime, where the effect of convection is detected by the deviation of heat transfer rate from that predicted by the purely conductive solution. The time required for achieving steady state is found to be proportional to a nondimensional parameter indicating the surface curvature in the individual systems. More specifically, the time is proportional to the square of aspect ratio based on the cylinder radius and inversely proportional to the convective velocity. The time leading to steady state in the naturally converting system is, in general, much greater than that in the forced convection.
The hydraulic properties of the subsurface system created in the course of the Γ-Project, Tohoku University, are presented. Furthermore, for a subsurface system consisting of an artificial crack and two wells for geothermal heat extraction, methods are proposed for predicting the flow impedance and for estimating the area of effective heat exchange surface from tracer test. First, water flow in a penny shaped crack with an inlet and an outlet was analyzed. Based on this analysis, a method was constructed for predicting the flow impedance of this subsurface system just stated. The method was then applied to the subsurface system of the Γ-Project. The predictions agreed well with the field experimental results. The velocity field of the flow in a penny shaped crack was also applied to the simulation of the outlet tracer response profile. The results of the simulation were compared with those of the tracer test performed during the Γ-Project to obtain the effective heat exchange surface.
A one-dimensional (vertical) steady model is treated to investigate the occurrence of the subsurface two-phase flow system for the case of no net fluid transport; specially a close investigation is made for a fractured-type reservoir. When the heat flux lies between specific limits, the vapor-dominated two-phase flow system is possible (the primary condition), which is the similar result obtained by Schubert and Straus (1979). Even if the heat flux exceeds the upper limit of the primary condition, either the vapor-dominated or the water-dominated two-phase flow system is possible under permeabilities larger than a specific value (the secondary condition). In general, the primary condition provides for shallow (low temperature) systems, whereas the secondary condition provides for deep (high temperature) systems. It is also shown that occurrences of the two-phase flow system are classified into three types due to the magnitude of permeability. Based on permeability data, it is thought to be that the case of the large permeability is applicable to real systems. However, whether the deep high tem-perature part becomes the vapor-dominated or the water-dominated cannot be defined only by the thermodynamic condition, but may depend on other factors such as the hydrologic condition in the field.
Geothermal resources discovered in India consist of warm/hot water systems. The main geothermal manifestations are distributed in three areas: (1) a Continental collision zone, Himalayan region, marginal depressions, and Himalayan fore deep, (2) Peninsular shield areas, and (3) Coastal areas. There are three different types of thermal manifestations in India de-pending on meteorological, topographical and geographical conditions. The temperatures of the geothermal springs in the Himalayan regions are medium temperatures (max. temp. of 107°C) and are found in steep remote terrains with cold-dry conditions and seasonal temp. variations from -40 to 20°C. In peninsular and coastal regions, the thermal manifestations have generally low temperatures (55°C-90°C) with seasonal variations from 15°C to 45°C under hot-dry and hot-humid conditions, respectively, with the exception of the Cambay, West coast and Tatapam-Salbardi areas which are of medium temperature (108°C-120°C). Such thermal manifestations may be utilized for non-electrical purposes.
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