This paper proposes a new mathematical model for heat transfer in water-steam flow under the boiling condition in a porous medium and its comparison with the experimental data already reported by the authors. The conclusions are as follows:(1) The calculation obtained from the new mathematical model almost agrees with the experimental data of the steam ratio.(2) The apparent thermal conductivity defined in the mathematical model is an important factor in estimation of heat transfer of the water-steam flow accompanied by boiling in a porous medium.(3) The apparent thermal conductivity decreases remarkably in the region of less than 0.2 of water saturation.
A numerical experiment with two-dimensional elasto-plastic finite element method was applied to an analysis of geologic structure and fracture systems in the geothermal area. The experiment is based on the Virtual Basement Displacement Method (Kodama et a1., 1985). The virtual basement is preliminarily assumed, and deformation of upper-layer strata caused by its movement is experimentally reproduced for each step of deformation. The location selected as a model area is the northern part of the Kurikoma geothermal area, Akita prefecture, Japan, which has many active geothermal fields. The geology of this area consists of Quaternary volcanic rocks, Sanzugawa Formation (Pliocene lake deposits), Minasegawa Formation (late Miocene volcanic rocks), Doroyu Formation (middle Miocene marine sediments), and basement rocks (granite and schist). The basement rocks broadly subsided from Akinomiya to Minase River. This subsided zone is filled with thick tuff layers of Minasegawa Formation (> 1000m), and it has been considered to be a caldera (Utada and Ito, 1986). The central part of this subsided zone was raised after collapse of the caldera. This experiment was executed in a geologic section trending NE-SW, perpendicular to the general geologic structure trending NW-SE. The development processes of geologic structure are divided into the following five steps:step 1: sedimentation of Doroyu Formationstep 2: sedimantation of Minasegawa Formationstep 3: uplifting of the central area (1)step 4: sedimentation of Sanzugawa Formationstep 5: uplifting of the central area (2) Calculated deformation and fracture distribution in each step shows that the highly fractured vertical zones are formed along edges of the uplifted zone in step 3. It also suggests that radially distributed fracture systems are formed around the uplifted zone in step 5. Deep geothermal reservoirs have been found at Uenotai, located at the eastern edge of the basement uplifted zone. There fracture systems are distributed in the basement rocks and around dikes in Minasegawa Formation (Naka et at, 1987). Kawarage and Arayu are also active geothermal fields along this section. The result of this experiment indicates that underground strata at these geothermal fields have been fractured with basement uplifting in step 3 and 5. The distribution of the fracture system shown by this numerical experiment is considered to represent the actual fracture distribution. This method is useful when the fracture systems are not found through surface mapping, for example, the surface is covered with Quaternary volcanic rocks, such as the Uenotai field.
LINC (Line-Source (Sink) Implanted Networks for Fractured Continua) method based on a double porosity and double permeability model is proposed in order to analyze the heat and fluid flows in fractured rock formation. In the present method the heat and fluid flows in the fracture and those in the less permeable rock are calculated separately, then the interaction between the fractures and the rock formation is taken into account through sink and source terms in the governing equations, describing the transport phenomena in the respective system. A specific problem to which the present numerical method is applied assumes a single fracture that drains heat from neighboring blocks of homogeneous and isotropic imkpermeable rock. The outlet temperature history is monitored to evaluate the amount of heat extraction from the rock. The numerical results by the present method are favorably compared with the available analytical data. And it is observed that the outlet temperatures at early times of heat extraction measurably decrease if the fluid properties are considered to change with thermodynamic condition.
The geothermal energy exploration, which began in 1960, has been implemented by the MTA General Directorate until now. In 30 geothermal fields, 242 gradient, 74 shallow and 35 deep wells have been drilled. The explored geothermal fields are water-dominated and hot water type. Turkey is located on an active tectonic belt. The geothermal areas are found along many grabens, main faults and fracture zones. In geothermal areas, although reservoir developments depend on secondary porosity and permiability due to intense tectonism, they also show different characteris-tics according to lithology type. In surveyed geothermal fields, the main reservoir rock type is marble-limestone (Paleozoic-Mesozoic). However, rocks which are magmatic-metamorphic in origin like granite, diabase, spilite, (Paleozoic-Mesozoic), ignimbrite and andesite (Neogene) type rocks also form reservoirs. The com-mon cap rocks consist of claystone, sandstone, conglomerate, and clayey limestone type sedimentary rocks (Cenozoic). The sandstone, conglomerate, clayey limestone levels in those units form subor-dinate reservoirs. During exploitation some problems due to reservoir lithology were encountered. The major problem was scaling, particularly in carbonate (limestone-marble) type reservoirs. Another problem is sandstone erosion caused by loosely cemented sandstone and conglomerate. Elsewhere, expansion of clay in the cap rock created another problem during drilling. In this paper, reservoir lithology and problems based on lithology type will be presented. Also some areas like Denizli-Kizildere, Aydin-Germencik, Canakkale-Tuzla, Kestanbol, Izmir-Balcova, Ankara-Kizilcahamam, etc. will be given as examples.
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