We measured changes of longitudinal wave velocity caused by hydraulic fracturing in rock specimens. Three fracturing experiments were performed in cubical granite samples (coarse grained lnada granite) of 296 mm edge length under triaxial stress state generated by means of a twin-oil-jack sytem. The direction of fracturing plane was controlled by the stress state as well as natural weak planes in granite. The measurements of longitudinal velocity were made along several paths in the planes perpendicular to the bore hole before and after fracturing experiments, and along three paths perpendicular both to the maximum stress axis and the bore hole during an injection of pressurized water. In these experiments, heavy differences of velocity that reach more than 20 % were observed between fractured area, which was created by injection of pressurized water, and unfractured area. The velocity differences also depend on the direction of fracture planes. When wave propagates parallel to the fracture plane, very high velocities were observed only in the vicinity of the fracture plane. However, high velocities were observed in the wide area when wave propagates perpendicular to the fracture plane. These results imply strong anisotorpies of velocity in fractured rock, and suggest that these anisotropies must be taken into account when we try to estimate extents of fractured area by the measurements of microseismic hypocenters.
The decrease of permeablity caused by the deposition of silica in a porous column (50 cm×5 cm φ) has been studied using the Otake geothermal water at about 90°C and at pH 8. A cylindrical cell was filled with aluminum beads and rock particles both about 2 mm in diameter to make the column. Silica concentrations in the water supplied were 475, 493, and 522 ppm, and their values were constant during each experiment. The initial flow rates were controlled by changing the total hydraulic potentials which were kept constant during the experiment. The hydraulic potentials in the column were measured by eleven manometers in order to calculate the permeability. The silica was deposited chiefly in the first 10 cm of the column. The amount of silica scale decreased rapidly with distance regardless of the material in the column or the silica concentration in the water. The decrease in permeability was inversely proportional to the amount of silica scale, decreasing to 0.5-5 % of the initial value in the uppermost part of the column, whereas it remained about 80 % of the initial value at more distant places. It can be concluded that, when the secondary porosity of the scale ranges from 0.90 to 0.95, the Kozeny-Stein equation can be used to express the relation between the permeablity and the amount of silica scale.
Heat discharge from a reservoir with a permeable surface should be considered by both mass and heat transfer processes. This paper proposes a new boundary condition on the basis of the so-called film theory; in which a liquid film is assumed to be in a thin water layer on the surface. A liquid heat transfer coefficient can be used to describe a new boundary condition. The proposed boundary condition is applied to Takenoyu geothermal area, Kumamoto, Japan in order to explain its underground thermal structure. The calculated results of heat and mass discharge and underground temperature distribution agree well with the observed ones.
This paper describes effects of heat transfer on tracer response in a naturally vertical fractured geothermal reservoir on the basis of a two-dimensional dispersion model. The theoretical results are as follow; (1) It is necessary to consider effects of heat transfer on the tracer response in a system of Peclet number larger than about unity and with thermal convective flow. (2) The time required to reach the peak of tracer and the accumulation quantity of tracer until that time under non-isothermal state are quite different from those under isothermal state. (3) A tracer response with two peaks can be seen in a reservoir where the inlet is comparatively close to the outlet.
Exact equations which describe unsteady distributions of temperature, pressure and saturation in the two-phase flow system in porous medium are derived. Based on the equations, some hydraulic characteristics of the two-phase flow system are investigated. Effects of change in external loads on geothermal state are actually so small that they can be neglected from the equations. Perturbing a quiescent horizontal reservoir by small perturbations, a diffusion equation concerning propagation of pressure change is deduced. Checking up its coefficients, the followings are clarified. (i) The most significant hydraulic storativity (compressibility) of the two-phase flow system is originated from the phase change. (ii) The value of it depends mainly on temperature; from 101 bar-1 at 100°C to 10-3 bar-1 at 350°C. Consequently, to elucidate the distribution of underground temperature is very important from the hydraulic point of view. (iii) Heat conduction may affect the propagation of pressure change in the two-phase reservoir of which permeability is 10-3 darcy in the magnitude and temperature is below 200-250°C.
A downhole coaxial heat exchanger has as an advantage its simplicity in configuration. Furthermore, it is applicable to a wide variety of geothermal resources such as Hot Dry Rocks, magma bodies and geothermal reservoirs which need artificial lift for production. In order to find an optimum design for the downhole coaxial heat exchanger, we have examined the effect of several design parameters on the performance of the heat exchanger by using a numerical simulator. The parameters that were treated as variable include circulation mode, thermal conductivity of the inner pipe, diameters of the wellbore and the inner pipe. The following results were obtained: 1) Reverse circulation is suitable for attaining maximum thermal output, because the length over which heat loss occurs along the borehole is shorter for reverse circulation than for forward circulation. 2) Much higher outlet water temperature and net thermal output can be obtained by using an insulated inner pipe as opposed to a conventional steel pipe. The improvement ratio of net thermal output amounts to 10.3 for reverse circulation and 8 for forward circulation, assuming thermal conductivity of the inner pipe is 0.01 kcal/mh°C and water flow rate is 0.3 m3/min. 3) Due tq difference in water density, a circulation pressure arises in the downhole coaxial heat exchanger. No pumping work is needed for circulation when temperature differences between inlet water and outlet water are sufficiently large and the design of the heat exchanger is appropriate. 4) Considering the result of the experiments (Pan et al., 1982; Freeston and Pan, 1983), it can be considered that thermal output of a downhole coaxial heat exchanger with an insulated inner pipe and reverse circulation is much greater than that of a U-tube heat exchanger. 5) Thermal output increases with increasing diameter of the wellbore when reverse circulation is adopted, as it was shown for forward circulation by Horne (1980). However, only an 11% increase in thermal output is attained for a diameter increment of 62% with a certain condition specified in this work. Economically, it is advisable not to make the wellbore diameter larger, but to drill more wells of small diameters to increase thermal output. 6) Although the dependence of thermal output on the inner pipe diameter is much less than that on the diameter of the wellbore, friction loss and Pout-Pin are very much sensitive to the inner pipe diameter. 7) There is a value for inner pipe diameter which gives a minimum thermal output, if other parameters are kept the same. At a certain value of inner pipe diameter the total cost per output energy becomes minimum, if we consider the cost of pumping work needed to circulate water and the cost of the inner pipe. 8) Friction loss can be minimized with the inner pipe diameter at 11.7 cm for a case in which the diameter of the wellbore is 21.6 cm and the wall thickness of the inner pipe is 1.9 cm. At this time, the water velocity ratio between the inner pipe and annulus is 1.64:1 and Reynolds Number ratio between them is 3.16:1.
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