The Appi area was selected as one of fields to be investigated under NEDO's project “Geothermal Development Promotion Survey”. Drilling of 6 wells, various investigations and integrated analyses were conducted on the Appi area between 2000 and 2003 fiscal years. The geothermal system is located around the stepped faults at the south edge of the Appi collapsed structure. The heat source is originated from a magma chamber at great depth of Mt. Appi-dake which lies to the south of the investigation area. The geothermal fluids ascend the fractures around thefaults at the south edge of the Appi collapsed structure zone. The fracture system is mainly composed of the northward steeply inclined fractures trending east-west, and the southward gently inclined fractures trending east-west corresponding to the bedding plane. The geothermal system is characterized as a liquid dominated reservoir to some extent. On the other hand, a vaporstatic pressure gradient is recognized at the southwestern part, which indicates presence of a vapor phase. The chemistry of the discharge fluid from the Appi investigation wells indicates a steam-heated water with a weak alkaline Na-SO4-HCO3 type. Based on the. numerical simulation, it is estimated that the geothermal reservoir in the Appi area is capable of generating 20 MWe for 30years.
A numerical simulation was conducted to evaluate the Appi reservoir under the Geothermal Promotion Survey project of NEDO between 2000 and 2003 fiscal year. To estimate an electrical generation potential, we constructed an adequate numerical model based on the conceptual model, and predicting a production performance due to field operation using that model, leading to the following findings. 1. Permeability is estimated to be 2.5×10-15m2 for higher formations, 0.25×10-15m2 for lower ones, and 0.02×10-15m2 for the lowest ones based onpressure interference tests. Such low permeabilities were required to reproduce conductive temperature profiles observed at the Appi field. 2. Conductive heat flow of 0.5Wt/m2 was assigned in the southern part of the grid bottom blocks, and upflow of steam with 2, 700kJ/kg internal energy and 0.6kg/s rate in two bottom blocks around Mt. Appi-dake to obtain a good match between observed and calculated temperature profiles. Total recharge rates of energy are 8.125MWt and 1.62MWt from conduction and convection. respectively, which indicates conductive-dominated heat transport at the Appi. 3. Simulations of production performance were carried out for several operation scenarios. It was estimated that the geothermal reservoir in the Appi field is capable of generating 20MWe for 30 years by a single flash power generation system. 4. Although steam discharge without liquid water was recognized under a long term production test, the simulation results predict small amount of water discharge from a few production wells after several years of operation. It is considered that the distribution of two phase region is limited in the southwestern area and decrease of reservoir pressure associated with steam production causes liquid water around the two phase region to migrate into production area.
The Appi area was selected as one of fields to be investigated under NEDO' s project “Geothermal Development Promotion Survey”. Drilling of 6 wells, various investigations and integrated analyses were conducted on the Appi area between 2000 and 2003 fiscal years. The geothermal system is located around the stepped faults at the south edge of the Appi collapsed structure. The heat source is originated from a magma chamber at great depth of Mt. Appi-dake which lies to the south of the investigation area. The geothermal fluids ascend the fractures around the faults at the south edge of the Appi collapsed structure zone. The fracture system is mainly composed of the northward steeply inclined fractures trending east-west, and the southward gently inclined fractures trending east-west corresponding to the bedding plane. The geothermal system is characterized as a liquid dominated reservoir to some extent. On the other hand, a vaporstatic pressure gradient is recognized at the southwestern part, which indicates presence of a vapor phase. The chemistry of the discharge fluid from the Appi investigation wells indicates a steamheated water with a weak alkaline Na-SO4-HCO3 type. Based on the numerical simulation, it is estimated that the geothermal reservoir in the Appi area is capable of generating 20 MWe for 30years.
A long-term circulation test (LTCT) was conducted between 27 November 2000 and 31 August 2002 at a Hot Dry Rock (HDR) test site in Hijiori, Japan. The LTCT wasseparated into two stages. In the first stage, fluid was injected into only a lower reservoir. In the second stage, a dual circulation test was carried out during which fluid was injected into both an upper and a lower reservoir. During the LTCT, we carried out tracer tests every one or two months for monitoring flow in the reservoir. In these tests, tracer reagents including sodium fluorescein were pumped into the lower and upper reservoirs. Fluids from two production wells were sampled and analyzed to obtain tracer response curves. A fiber-optic fluorimeter was also used to obtain real time fluorescein concentration. After fluid analysis, we obtained the tracer response curves and corrected the curves to analyze the change of real reservoir volume with circulation progress. In the first stage of the LTCT, the breakthrough and mode volume of HDR-3 tended to increase. But the volume of HDR-2a tended to decrease and the volume rapidly changed between the 2nd and 3rd tracer tests. And fluid geochemistry and well temperature of HDR-2a rapidly changed between the 3rd and 4th tracer tests. These changes were influenced by an anhydrite scaling process. Firstly, the anhydrite around the injection well dissolved and then precipitated mainly in the lower reservoir and the flow path volume became larger. In the case of HDR-3, this process continued during the circulation. But in the case of HDR-2a, thermal breakthrough occurred in the flow path due to cool injection water. After that, the dissolved anhydrite precipitated in the production well and the flow path volume became smaller. As the fluid pressure from the lower reservoir became higher, the fluid flow from the upper reservoir to the production well stopped and thefluid geochemistry and wellhead temperature rapidly changed. In the dual circulation test, the breakthrough and mode volumes of HDR-2a and HDR-3 via SKG-2 tended to decrease during four months of circulation. In the upper reservoir, thermal breakthrough occurred like in the lower reservoir from HDR-1 to HDR-2a in the first stage. After one month of circulation interruption, thereservoir temperature became higher and the breakthrough and mode volumes increased due to anhydrite precipitation.
The mass flow rate of steam transported into a turbine for geothermal power plant is controlled by the discharge characteristics of production wells and their wellhead pressures, which are, in turn, influenced by not only reservoir properties, but also pressure loss occurred along pipelines during transportation. In order to calculate the pressure loss in the pipeline network a fluid-gathering pipeline simulator named TPGS (Two-Phase Gathering System), which treats the pipeline network, has been developed. In addition, a simulation method to individually couple a wellbore simulator and a fluid-gathering pipeline simulator to a reservoir simulator has been constructed to predict the generating power output. TOUGH2, MULFEWS, and TPGS are the reservoir, wellbore, and fluid-gathering pipelinesimulators, respectively, used for the coupled simulation. This method was applied to predict changes of power output of the Hatchobaru power plant, Japan, overtime. The simulation includes a total length of 4.7km pipeline networks. Predicted changes of power output gave good agreement with the actual power decline trend, meaning the successful coupling of three simulators. Furthermore, the decrease in wellhead pressures and discharge rates of some production wells due to reservoir cooling was predicted. On the other hand, both simulated pressures at wellheads of production wells and at interconnection pipelines were predicted to besmaller by 20% than the actual pressures. This shows that the pressure losses atpipelines calculated by TPGS are underestimated. It is concluded, therefore, that values of surface roughness of the pipelines and correction factors for calculating pressure losses at fittings of pipelines requires some modification to increase the accuracy for TPGS.
Fluid-rock interactions in geothermal fields are controlled by several chemical and physical conditions, such as temperature, hydraulic condition and mass transport properties of dissolved chemical species in geothermal fluids. Entire solution chemistry in the system is affected by mass transport phenomena in solid-liquid interface, not only depending on non-stoichiometric chemical reaction. Dynamic function and formation of leached surface layer in solid and boundary film in liquid were analytically defined using results of hydrothermal flow through experiments. Surface layer corresponds to leached layer of mineral surface where alkali is depleted and Si, Al are enriched. Specifically, the thickness of surface layer was described as a function of Reynolds number, reaction temperature and length from fluid input point. Based on the thickness of surface layer, . The mass transport factors in solid-liquid interface are evaluated to be 10-8 to 10-5 m/s under low temperature and low velocity condition (75°C, 0.33m/hour), and 10-6 to 10-3 m/s under high temperature and high velocity condition (250°C, 2.2 in/hour). These factors for rock forming elements are obviously different from each other and that range evaluated as 103 between alkali ions and Si or Al. Zonal geothermal alteration is regulated by leaching and enrichment of specific ions in hydrothermal fluid characterized by this ion mobility on mineral surface besides chemical reaction.
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