Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.     

Synthetic fluid inclusion logging to measure temperatures and sample fluids in the Kakkonda geothermal field, Japan

Synthetic fluid inclusion logging is a new tool to measure temperatures and sample fluids in high-temperature geothermal wells. Fluid in the microcracks of a crystal can be trapped in inclusions through healing. Fluid inclusions in quartz, for example, can be synthesized easily in geothermal boreholes and can be used as long as the host crystal is stable (e.g. α-quartz is stable up to 573°C). This technique can be applied to high-temperature geothermal wells where conventional temperature measurement methods are not feasible. Cracked crystals of quartz, soaked in silica-saturated solutions in gold or platinum capsules mounted on containers, are placed in a geothermal borehole. Geothermal fluid enters the microcracks in the crystals at the selected sampling depths, and inclusions containing ambient fluid are formed through crack healing. Trapping temperatures of fluid inclusions in quartz are determined by microthermometry using a heating stage with pressure corrections. Other cracked crystals mounted in containers with rupture disks are used for fluid sampling. The first borehole experiment was conducted at WD-1, a deep research hole drilled in the Kakkonda geothermal field, northeast Japan, from September to October 1994 (24 days). Results from the experiment confirmed that temperatures measured from fluid inclusions are consistent with borehole temperatures measured by conventional logging tools.     

Identification of structures within the deep geothermal reservoir of the Kakkonda field, Japan, by a reflection method using acoustic emission as a wave source

Structures within the deep geothermal reservoir in the Kakkonda field have been identified by a newly-developed reflection method using acoustic emission (AE) as a wave source. Reflected waves in three-component AE signals were detected independently of the wave amplitude by examining the linearity of a three-dimensional hodogram.There are many possible reflectors in this field, such as geological boundaries and fractured regions. In this paper are described some deep reflectors beneath the conventionally developed shallow geothermal reservoirs that were revealed by three-dimensional inversion of natural AE waveforms. These identified reflectors agree fairly well with geological constraints based on core samples from the field.     

Genesis of the plutonic-hydrothermal system around quaternary granite in the kakkonda geothermal system, Japan

The Kakkonda plutonic-hydrothermal system has as its heat source the Quaternary Kakkonda granite. The Kakkonda granite has a thick (1.3 km) contact-metamorphic zone, known mainly from the geothermal survey well WD-1a (total depth: 3729 m) drilled by the New Energy and Industrial Technology Development Organization (NEDO). The Kakkonda granite is a stock several tens of square kilometers in area with an upper contact about 1.5–3 km deep. It is a composite pluton varying from tonalite to granite. The early-stage granitic rocks are slightly metamorphosed to biotite grade by late-stage granitic rocks. K-Ar ages of separated minerals from the granitic rocks in both stages show the same cooling ages of 0.24–0.11 Ma for hornblende, 0.21–0.02 Ma for biotite, and 0.14–0.01 Ma for potassium feldspar. These are the youngest ages for granite in the world. The K-Ar ages become almost zero at 580°C for biotite and potassium feldspar, and at 350°C for illite. The Kakkonda granite intruded into a regional stress field in which the minimum principal stress was ENE–WSW and nearly horizontal. The regional stress field coincides with that of a previously recognized F2 fracture system before 0.4–0.3 Ma. Both stages of the Kakkonda granite and the contact aureole are fractured by recent tectonism, resulting in a zone of hydrothermal convection from about 2.5–3.1 km depth up to the surface. The boundary between the zone of hydrothermal convection and the underlying zone of heat conduction occurs 250–550 m below the upper contact of the Kakkonda granite, and has a temperature of 380–400°C.     

Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal field is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>350°C), and a high salinity production fluid. Geological analysis shows that the main reservoir in this field occurs in a fractured zone developed around a dacite intrusion located in the center of the field. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW–SE trending andesite dike swarm which was encountered in some wells.Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium–magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K-feldspar–quartz, sericite–quartz, and albite–chlorite zones.Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this field ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal fluid originated from sea-water and is diluted by ground water during its ascent. Some fluids produced from geothermal wells show low pH (about 4). It is thought that sulfide mineral (PbS, ZnS) precipitation during production produces this acidic fluid.     

High-temperature measurements in well WD-1A and the thermal structure of the kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their Deep Seated Geothermal Resources Survey project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414°C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500°C at 3500 m depth. Temperatures between 500°C and 510°C were also confirmed at 3720 m depth after 129–159 h standing time, using calibrated melting .tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230°C to 260°C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350°C to 360°C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.     

The origin of hypersaline liquid in the quaternary kakkonda granite, sampled from well WD-1a, kakkonda geothermal system, Japan

Hypersaline metal-rich liquid (ca. 40 wt% total chloride species) was obtained from a depth of 3708 m in the Kakkonda geothermal system. Sampling of well WD-1a was conducted by reverse circulation after a standing time of about 196 hours (with temperature recovering to >500°C). Tritium content and the relationship between δD and δ¹⁸O showed that the river water that was circulated in the well had mixed with an isotopically heavy fluid during the standing time. Phase separation occurred during temperature recovery, concentrating the hypersaline liquid in the bottom of the well. This original hypersaline liquid has a salinity of about 55 wt% NaCl eq., consisting of Na–Fe–K–Mn–Ca chloride, rich in Zn and Pb but poor in Cu, Au and Ag. The fluid originates from the Kakkonda granite and mixed with circulating water from the well in a zone of fine fractures induced by thermal stress during drilling.     

Temperature and chemical changes in the fluids of the Obama geothermal field (SW Japan) in response to field utilization

Thermal waters from Quaternary volcanic rocks (predominantly andesites) discharge along faults in the Obama geothermal field of southwestern Japan. The chemistry of more than 100 thermal and ground water samples collected between 1936 and 2005 indicate that the Na–Cl hot spring waters are a mixture of “andesitic” magmatic, sea and meteoric waters. Mixing models and silica and cation geothermometry were used to estimate the SiO₂ and Cl composition and the temperature (∼200 °C) of the reservoir fluids deep in the geothermal system. The isotopic data (¹⁸O and D) are consistent with a mixed origin interpretation of the waters feeding the Obama hot springs, i.e. a large proportion of meteoric and sea waters, and a small magmatic component. Temperatures and chemical concentrations of the thermal waters were affected by the 1944–1959 salt production operations, but have recovered after closure of the salt factories; now they are similar to their pre-1940 values. In the future, the Obama geothermal field may be suitable for electric power generation, although heat and fluid extraction will require careful management to prevent or minimize reservoir cooling.     

Temperature evaluation of the Bugok geothermal system, South Korea

Using a variety of chemical geothermometers and statistical analysis, we estimate the temperature of a possible deeper geothermal reservoir at Bugok, Southern Korea. Shallow thermal aquifers (down to about 400 m depth) are under exploitation in this area; the temperatures (up to 78 °C) of the produced fluids are the highest found in South Korea. Based on hydrochemical data and occurrence, the groundwaters at Bugok can be classified under three groups: Na-SO₄ thermal groundwaters (CTGW) occurring in the central (about 0.24 km²) part of the area; Ca-HCO₃ cold groundwater (SCGW) found in shallow peripheral parts of the CTGW; the intermediate-type groundwater (STGW). The CTGW type is typical of the Bugok thermal waters; they have the highest discharge temperatures and contain very high concentrations of Na (75.1–101.0 mg/L), K (2.9–6.9 mg/L) and SiO₂ (62.0–84.5 mg/L) and are rich in sulfates.The major ion composition of the CTGW suggests that these waters are in partial equilibrium with rocks at depth. The application of various alkali-ion geothermometers yields temperature estimates in the 88–198 °C range for the thermal reservoir. Multiple-mineral equilibrium calculations indicate a similar but narrower temperature range (from about 100 to 155 °C). These estimates for CTGW are significantly higher than the measured discharge temperatures. Considering the heat losses occurring during the ascent of the waters, one can infer the presence of a deeper (around 1.8 km) thermal reservoir in the Bugok area that could be developed for district heating or other direct applications of geothermal heat.     

Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.     

Sustainability of the Hatchobaru geothermal field, Japan

The Hatchobaru power plant Unit No. 1 (55 MW) has been operating since 1977 and Unit No. 2 (55 MW) since 1990. The mean capacity factor of the power plant has reached about 90%. Considering that the long-term operation of the plant, over 30 years for Unit No. 1 and nearly 20 years for Unit No. 2, has been maintained with such a high capacity factor, sustainable development in terms of economic production has been achieved. To maintain a stable operation, systematic reservoir monitoring and reservoir simulation studies have been conducted. The monitoring of changes in reservoir pressure, temperature and gravity indicates that the reservoir is currently approaching a stable state. Results of a simulation study suggest that the sustainable power output of the Hatchobaru reservoir is approximately 120 MW, and each productive fault has the capacity to produce enough steam to generate from 11 to 55 MW. Therefore, it would be possible to maintain the rated power output of 110 MW by optimizing well alignments so that the mass production can be kept within the sustainable productivity of each fault, and the injected water does not cool the production zones.     

Revised hypocenter solutions for microearthquakes in the kakkonda geothermal field, japan

The hypocenters of microearthquakes in the Kakkonda geothermal field have been relocated along the Kakkonda River using a new velocity structure model. Compared to the solution used by the previous velocity model, the depth of the hypocenters is shallower in the relocation. The microearthquakes occurred in a highly fractured region, as suggested by geological and petrological studies based on well data, but did not occur along major tectonic folds and faults. An alternative hydraulic condition might be necessary to trigger the earthquakes. Seismicity in 1995 was lower than in 1988 in Kakkonda. The decrease in the number of events is possibly due to the decrease in the amount of reinjection fluid or the change in the characteristics of the geothermal reservoir.The number of microearthquakes decreases rapidly at 1–2 km below sea-level. Probability density of seismic energy distribution is utilized to indicate the active seismic regions. The model also shows that a contour map of the lower boundary of the high seismic energy region corresponds to the occurrence of cordierite, which was produced by heat from the neo-granitic pluton body, implying that the occurrence of microearthquakes in the Kakkonda geothermal field is controlled by the neo-granitic rocks at depth. The top of the granite can be imaged, using the probability density of seismic energy distribution.     

Characterisation of the Basel 1 enhanced geothermal system

This paper describes the steps that have been undertaken to create an enhanced geothermal system (EGS) at the Deep Heat Mining Project in Basel, Switzerland. Preliminary results from drilling, logging, hydraulic testing and stimulating the Basel 1 well are summarized. The project was suspended following the occurrence of several ‘felt’ microseismic events. Because such events may be an inherent risk with current methods of reservoir stimulation, the paper analyses the possible mechanisms of hydraulically induced shearing processes and suggests methods by which the risk may be reduced. The observations are integrated into a model of the geothermal reservoir where aspects of both the reservoir development and the driving mechanism for the perceptible induced seismic events are considered.     

Shear-wave splitting as a tool for the characterization of geothermal fractured reservoirs: lessons learned

We review our experience with the construction of models of subsurface fracturing in geothermal fields by the inversion of shear-wave splitting (SWS) observations from natural and induced seismic events recorded by local arrays of three-component digital seismometers. SWS is a phenomenon whereby shear seismic waves split into two as a result of the mechanical anisotropy created in an otherwise isotropic rock by aligned micro-fractures. The two split waves travel at different speeds, and the polarization of the faster wave is usually parallel to crack orientation. The time delay between the two split S-waves is proportional to the number of cracks per unit volume.Success in the inversion of SWS data hinges on the assumption that the observed SWS is due solely to the mechanical anisotropy induced by aligned cracks and micro-cracks in an otherwise isotropic matrix. The presence of lithologic anisotropy and/or strong heterogeneity in the reservoir rock limits the resolvability of the method. However, despite the large amount of data and diversity of geologic settings we have studied so far, the above assumption has been found to be reliable. In practice, stability and resolution in the inversion of SWS data are the issues of utmost importance since both are critically dependent on the distribution of the two SWS measured parameters (polarization and time delay) around each seismic sensor.In this paper we discuss a few lessons we have learned as to the value of SWS for geothermal exploration, its limitations and potential extensions, from nearly a decade of practice.     

Hydrology of the Greater Tongonan geothermal system, Philippines, as deduced from geochemical and isotopic data

Fluids in the Greater Tongonan geothermal system exhibit a large positive ¹⁸ O shift from the Leyte meteoric water line. However, there is also a significant shift in ²H. The δ²H−δ¹⁸O plot shows that the geothermal fluids may be derived by the mixing of meteoric water (δ¹⁸O = −7‰ and δ²H = −41‰) with local magmatic water (δ¹⁸O = 10 ± 2‰ and δ²H = −20 ± 10‰). The most enriched water in the Greater Tongonan system, in terms of δ¹⁸O, δ²H and Cl, is comprised of approximately 40% magmatic water. Baseline isotope results support a hydrogeochemical model in which there is increasing meteoric water dilution to the southeast, from Mahiao to Sambaloran and towards Malitbog. The Cl−δ¹⁸O plot confirms that the geothermal fluid in Mahanagdong, further southeast, is distinct from that of the Mahiao-Sambaloran-Malitbog system.     

Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.     

Hyperspectral detection of geothermal system-related soil mineralogy anomalies in Dixie Valley, Nevada: a tool for exploration

Hyperspectral data analysis has been applied to the mapping of soil anomalies that may be related to present or past geothermal systems. Anomalous accumulations of certain soil minerals can indicate buried geologic structures and possible zones of elevated permeability. Hyperspectral data can be used to map these anomalies as part of geothermal exploration activities. The study area for this project was northern Dixie Valley, Nevada, which is host to a structurally-controlled deep-circulation hydrothermal convection system. Advanced visible and Infrared Imaging Spectrometer (AVIRIS) airborne hyperspectral imagery was used. Both supervised and unsupervised spectral unmixing methods were tested to separate minerals from other components in the image. Both methods produced useful spectral end-members leading to the detection of anomalous soil minerals that may be related to the geothermal system and buried geologic structures.     

Physicochemical characteristics and leachability of scale and sludge from Bulalo geothermal system, Philippines

Scale and sludge from Bulalo geothermal field, Philippines, have been characterized by whole rock analysis, radioactivity counting, size analysis, light microscopy, scanning electron microscopy, and X-ray diffraction. Their leachability was assessed by regulatory leaching procedures and by sequential extraction. Both scale and sludge consisted mostly of oxides of Si, Al, and Fe with no radionuclides detected. The scale had 10% S content. Sulfides and silicates were important phases in both samples having size ranges from submicron to 2 mm. Geothermal soils at Bulalo have higher than normal soil levels of As, S, Cu, Cr, Zn, and Pb but regulatory leaching tests indicated that these elements are not released. However, the sequential extraction showed that As, Cu, and Zn were leachable under extreme conditions.     

Field-scale evaluation of the design of borehole heat exchangers for the use of shallow geothermal energy

A numerical model for the simulation of temperature changes in a borehole heat exchanger (BHE) with fluid circulating through U-tubes is developed. The model can calculate the thermal power transferred from heat pumps to BHEs while considering the nonlinear relationship between temperature of the circulating fluid and the thermal power. The use of the developed model enables the design of a geothermal heat pump (GHP) system with the view of pursuing efficiency and financial benefit. The developed model is validated by comparing two measurement datasets with their respective simulation results. The numerical evaluation of a real GHP system with 28 BHEs and 79 heat pumps involved consideration of a base case and modified cases. In all cases, the temperatures of the circulating fluid at the BHE inlet and outlet, heat pump efficiency, and the heating power and electric power of heat pumps were obtained. The estimated cost of electricity in the year 2030 is 0.146 US$/kW. The most cost-effective system in this case is for there to be 4, 6, and 6 BHEs on the first, second, and third floors, respectively.     

Geochemistry of the Rehai and Ruidian geothermal waters, Yunnan Province, China

The geochemical characteristics of the hot spring waters in the Rehai and Ruidian geothermal fields, Tengchong County, Yunnan Province, China, are described and their possible environmental impact assessed. The results show that the alkaline spring waters contain high levels of K, Na, F, Cl, SiO₂, whereas the only acidic spring water in Rehai geothermal field contains high levels of SO₄²⁻, Mn and Fe. As and Sb in the spring waters are in the range 43.6–687 μg/L and 0.38–23.8 μg/L, respectively. As(III) occurs in most spring waters, and constitutes up to 91% of the total As. A fraction of the geothermal As and Sb is trapped in the sinter deposits while most enters the environment. The diffusion of As and Sb into groundwater and downstream crop fields constitutes a threat to the health of the local population.