Kuju-iwoyama, the most intense surficial activities in central Kyushu, is situated in an explosion crater of Kuju volcano. It exists in central part of Kuju volcano. High heat flow values higher than 100mW/m2 are observed around Kuju volcano. Simulation of the regional high heat flow was conducted in terms of conductive cooling of magma reservoir. As a result, it is shown that the magma is not molten at present but maintains high temperature of 600°C at a depth of 5km. From the depth, magmatic steam is supplied to the two-phase geothermal reservoir beneath Kuju-iwoyama.
A numerical experiment with three-dimensional Virtual Basement Displacement Method based on elasto-plastic finite element method was applied to an analysis of geologic structure and fracture systems in northern part of the Kurikoma geothermal area. In addition, a comparison between results of two- and three-dimensional experiment for the same area was made. The result of two-dimension al experiment is described in Mizugaki (1991). The result of three-dimensional experiment shows that the basement rocks and Miocene pyroclastic rocks are highly strained at major geothermal fields, such as Uenotai, Kawarage, Doroyu and Arayu, whereas small strain values are calculated for strata at minor geothermal fields. Principal stress distribution in this experiment has no dominated direction. In practice strain causes fracturing in rocks, and highly strained zones can be interpreted in highly fractured zones. Therefore this result suggests that the highly fractured zones are formed in rocks at major geothermal fields. This three-dimensional experiment reproduces the fracture systems in geothermal fields better than two-dimensional experiment.
In this study, in order to extract the information on fracture opening, fracture propagation and permeability of the fracture from pressure-time records during hydraulic fracturing, fracturing process is numerically simulated paying attention to the relations between the form of pressure-time curve, fracture opening/propagation behavior and fluid flow in the fracture. Hydraulic fracturing with constant water injection is modelled numerically for a rectangular longitudinal fracture in an impermeable rock mass and intersecting a borehole. Analysis of rock deformation is based on the theory of linear elasticity and also on linear fracture mechanics. Fluid flow is analyzed as laminar flow, and permeability of the mechanically closed fracture is taken into account as one of parameters in the simulations. Then the rock deformation and the fluid flow in the fracture are coupled in the modelling. The results show that both the permeability of the fracture and water injection rate significantly influence the form of pressure-time relations on early stage of fracture opening. When the permeability is sufficiently high, there is no peak in the pressure-time relations. When the permeability is low, on the other hand, the pressure-time relations have a peak, whose value becomes higher with the water injection rate. Furthermore the simulations show that the behavior of borehole presusre with fracture propagation are affected by the pre-existing fracture length.
Since 1985, a research and development on the technique of heat extraction from hot dry rock has been performed by NEDO (the New Energy and Industrial Technology Development Organization) at Hijiori, Yamagata prefecture (NEDO, 1991). Heat extraction tests showed that heated water and steam were continually produced through the reservoir created artificially in a hot dry rock. To evaluate this artificial reservoir, well logging and production tests were also conducted. From information obtained in these tests, it was estimated that the artificial reservoir might consist of the system of rock matrix and fracture-network (including natural and artificial fractures) as shown in Fig. 1. A theoretical base on fluid flow and thermal energy transport for this system should be developed to understand the production behavior. Several works have been done for modeling on fluid and heat flow in the area of frac-tured rocks (Bodvarsson, 1972; Bodvarsson et al., 1982; Chida et al., 1990; Gringarten, et al., 1975; Masuda, Y., et al., 1991; Pruess, K., 1985). Because previous models were developed to analyze fluid and heat flow in the rock mass with a single fracture or some parallel fractures, it is difficult to apply them to the rock matrix/fracture network system mentioned above. This paper deals with the numerical simulation of thermal energy transport in the matrix block/fracture network system. In particular, attension was focused on the validity of the proposed algorithm for energy transport simulations by comparing with that of a fine meshed porous model.
Field tests for the development of Hot Dry Rock have been conducted at Hijiori in Yamagata prefecture. In these injection tests, the flow rates were usually kept constant and the associated wellhead pressures were measured. In this paper, the relations between wellhead pressures and flow rates are summarized for the experiments from 1985 through 1990. Then, the bottomhole pressures were estimated by numerical simulation using the flow rates and wellhead pressures for each experiment. From the relation between bottomhole pressures and flow rates, following results were obtained.(1) Even after the hydraulic fracturing test conducted in 1986, the bottomhole presusre required to inject water at a given flow rate was decreased by repeating injection. (2) An assumption was made that there existed a fractured zone with a thickness of l4m around the bottomhole of the injection well. The permeability of the fractured zone was calculated from the relations between bottomhole pressure and flow rates. In each experiment, permeability was not constant but increasing with increasing bottomhole pressure. The permeability at a given flow rate increased very much after the experiment Exp. 8604, Exp. 8802 and Exp. 8901, in which a large amount of water compared to the former experiments was injected under high pressure. (3) A fracture with constant thickness around the wellbore was then assumed and this thickness was estimated from the relation between bottomhole pressures and flow rates. All the experiments were divided into four terms, i.e. term 1(Exp. 8501-Exp. 8603), term 2 (Exp. 8604-Exp. 8801), term 3(Exp. 8802-Exp. 8901) and term 4 (Exp. 8902 to Exp. 9004). At the beginning of each term, the experiments with large amount of water under high injection pressure were conducted. There existed linear tendencies between bottomhole differential pressure and fracture width for each term. The least square method was applied to obtain the initial fracture width. As a result, the initial fracture width was found to be increasing up to 0.15mm with time.
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