Fracture system related to geothermal reservoir based on core samples of slim holes. Example from the Uenotai geothermal field,northern Honshu,Japan

In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fractures were located and identified from the records of lost circulation while drilling.Productive fractures are classified into a dominant high-angle (average: 77°) group, and a less common low-angle (average: 28°) group. The high-angle fractures were formed as extensional hydrothermal fractures in response to fault-value behavior triggered by fluid pressure activation under lithostatic conditions. The low-angle fractures formed as extensional and/or extensional shear fractures under hydrostatic-lithostatic transitional conditions, with subhorizontal σ₁ stress. Both kinds of fractures were accompanied by hydrothermal mineral deposition.     

Geothermal slim holes for small off-grid power projects

Economically viable, small (100 kW_e to 1000 kW_e), geothermal power generation units using slim holes are available for the production of electrical power in remote areas and for rural electrification in developing countries. Based on borehole data from geothermal fields in the United States and Japan, slim holes have been proven as adequate fuel sources for small-scale geothermal power plants (SSGPPs) and can deliver enough geothermal fluid to the wellhead in a baseload mode to be of practical interest for off-grid electrification projects. The electrical generating capacity of geothermal fluids which can be produced from typical slim holes (150-mm diameter or less), both by conventional, self-discharge, flash-steam methods for hotter geothermal reservoirs, and by binary-cycle technology with downhole pumps for low- to moderate-temperature reservoirs are estimated using a simplified theoretical approach. Depending mainly on reservoir temperature, the numerical simulations indicate that electrical capacities from a few hundred kilowatts to over one megawatt per slim hole are possible. In addition to the advantage of price per kilowatt-hour in off-grid applications, SSGPPs fueled by slim holes are far more environmentally benign than fossil-burning power plants, which is crucial in view of current worldwide climate-change concerns and burgeoning electricity demand in the less-developed and developing countries.     

Modeling studies for production potential of Chingshui geothermal reservoir

The Chingshui geothermal power plant was decommissioned in 1993 due to a continued decline in production. Although some geothermal exploration and field investigation had been exercised, the production potential of the reservoir is still not well understood. In this paper, numerical modeling approaches for characterization of the geothermal reservoir, investigation of reservoir production performance, and evaluation of exploitation scheme design are presented. At first, a site-scale refined grid numerical model was developed for simulating the natural state of Chingshui geothermal reservoir. Through the model, the production potential of the geothermal reservoir was estimated and the availability of water resources was assessed. We further built production model to simulate the production history during 1981–1993. From the production model, we can conclude that the abnormal drop of the reservoir production capacity is mainly caused by carbonate scaling. Potential production schemes with different reinjection designs were evaluated through the model. Simulation results indicated that a sustainable hot water production capacity of Chingshui geothermal reservoir is about 200 t/h without reinjection, and 300 t/h or even higher with reinjection which is enough for a 3 MWe power plant. The simulation results indicate that reinjection provides an effective approach for maintaining reservoir pressure during hot water/steam production.     

A survey of geothermal reservoir properties

This paper presents results of a literature survey on thermal, hydrological and chemical characteristics of geothermal reservoirs. The data are presented in a table summarizing important fluid and rock parameters. The primary parameters of interest are the permeability, permeability-thickness, porosity, reservoir temperature and concentration of dissolved solids and non-condensible gases. Some preliminary correlations between these parameters are given.     

Galerkin finite-element simulation of a geothermal reservoir

The equations describing fluid flow and energy transport in a porous medium can be used to formulate a mathematical model capable of simulating the transient response of a hot-water geothermal reservoir. The resulting equations can be solved accurately and efficiently using a numerical scheme which combines the finite element approach with the Galerkin method of approximation. Application of this numerical model to the Wairakei geothermal field demonstrates that hot-water geothermal fields can be simulated using numerical techniques currently available and under development.     

The Podhale geothermal reservoir simulation for long-term sustainable production

The Podhale geothermal system, located in the southern, mountainous part of Poland, is the most valuable reservoir of geothermal waters discovered in the country to date and the one with the highest capacities in Central and Eastern Europe. Over 20 years of continuous operation has proved its stable operating parameters – a small drop in pressure and an unnoticeable temperature change. Production of over 500 m³/h of geothermal water with an 86 °C wellhead temperature is current practise, while drilling a new production well and reconstruction of an injection well allows for production that may significantly exceed 600 m³/h. To utilize these vast resources, a binary power cycle for electricity and heat production is considered by group of researchers. The results of numerical modelling of heat extraction from the Podhale reservoir are presented in the article as a preliminary step to the detailed analysis of combined heat and power production through a binary power cycle.     

Coupled reservoir-wellbore simulation of geothermal reservoir behavior

The reservoir simulator TOUGH and the wellbore simulator WFSA have been coupled to model flow of geothermal brine in the reservoir as well as in the wellbore. An outline of the structure of the two computer codes is given, together with the relevant equations. A new module, COUPLE, has been written to serve as an interface between TOUGH and WFSA. Two sample problems are given to illustrate the use of the coupled codes. One of these problems compares the results of the new simulation method to those obtained by using the deliverability option in TOUGH. The coupled computing procedure is shown to simulate more accurately the behavior of a geothermal reservoir under exploitation.     

A new liquid hold-up correlation for geothermal wells

A new liquid hold-up correlation is devised for cased wellbores using high-quality discharge and downhole pressure and temperature data from flowing geothermal wells. The latter dataset encompasses a wide range of wellbore diameters, discharge rates and flowing enthalpies. The measured wellhead pressures for wells in the dataset display excellent agreement with the pressures computed by using the new hold-up correlation. Good agreement between the computed and observed spinner responses provides additional verification of the hold-up correlation. Finally, an example illustrating the use of the hold-up correlation to match downhole pressure and temperature profiles and well characteristic data is given.     

Water content of the Kawah Kamojang geothermal reservoir

A theory is presented for the transient flow of steam in the presence of immobile water, and for transient response of gaseous contaminants in the steam. Simple analyses give the water saturation of the rock, and the porosity-saturation product. For Kawah Kamojang the measured water content is a saturation of 35%.     

State of the art of geothermal reservoir simulation

Computer modeling of geothermal systems has become a mature technology with application to more than 100 fields world-wide. Large complex three-dimensional models having computational meshes with more than 4000 blocks are now used routinely. Researchers continue to carry out fundamental research on modeling techniques and physical processes in geothermal systems. The new advances are adopted quickly by the geothermal industry and have also found application in related areas such as nuclear waste storage, environmental remediation and studies of the vadose (unsaturated) zone. The current state-of-practice, recent advances and emerging trends in geothermal reservoir simulation are reviewed.     

Analysis of the stanford geothermal reservoir model experiments using the LBL reservoir simulator

This paper describes the results of an analysis of data obtained from a series of heat-sweep experiments performed in the Stanford Geothermal Reservoir Model using the Lawrence Berkeley Laboratory reservoir simulator. The physical reservoir model is an experimental system consisting of a pressure vessel which contains a granite rock matrix with production and recharge capabilities to simulate the heat-sweep process in a fractured hydrothermal reservoir under liquid-phase conditions.Arrangements were made with the Lawrence Berkeley Laboratory to test their geothermal reservoir simulator on the physical model data. The objectives were to provide insight into the detailed physical processes occurring in the relatively complex physical system and to provide feedback to LBL on the capability and possible improvements to the LBL reservoir simulator to model a complex physical system.The overall conclusion of this work is that the LBL simulator does an excellent job of predicting the physical processes in the Stanford Geothermal Reservoir Model experiments for extreme thermal gradient conditions and for a system with very complex boundary conditions. The analysis demonstrates the importance of specifying relevant parameters accurately to provide adequate modeling for the important physical processes.     

Acoustic emission analysis of a geothermal reservoir and its application to reservoir control

A detailed analysis has been made of acoustic emissions detected during build-up tests in a geothermal production well in Kakkonda geothermal power plant, Japan, in 1982, 1984 and 1985. The three-dimensional structure of the hydrothermal reservoir in the field and its dynamic behavior have been investigated. The shape and location of the cracked reservoir, the fluid paths and degree of communication between the reservoir and the geothermal wells, have been revealed by this analysis. The dependence of AE (acoustic emission) activity on valve operations has also been studied. The stability of crack-like reservoirs depends on the reduction in flow-rate in the reservoir system, the closing rate of the wellhead valve and on intervals between the tests. Reservoir stability has been successfuly achieved during the 1985 test by valve regulation, according to the results of the AE study.     

Simulation and resistivity modeling of a geothermal reservoir with waters of different salinity

Apparent resistivities measured by means of repetitive dipole-dipole surveys show significant changes within the Cerro Prieto reservoir. The changes are attributed to production and natural recharge. To understand better the observed geophysical phenomena, we performed a simple reservoir simulation study combined with the appropriate DC resistivity calculations to determine the expected magnitude of apparent resistivity change. We consider production from a liquid dominated reservoir with dimensions and parameters of the Cerro Prieto ‘A’ reservoir and assume lateral and vertical recharge of colder and less saline waters. Based on rather schematic one- and two-dimensional reservoir simulations, we calculate changes in formation resistivity which we then transform into changes in apparent resistivity that would be observed at the surface. Simulated changes in apparent resistivities over the production zone show increases of 10 to 20% over a 3 year period at the current rate of fluid extraction. Changes of this magnitude are not only within our ability to discern using proper field techniques, but are consistent in magnitude with some of the observed effects. However, the patterns of apparent resistivity changes in the simulated dipole-dipole pseudosection only partially resemble the observed field data. This is explained by the fact that the actual fluid recharge into the ‘A’ reservoir is more complicated than assumed in our simple, schematic recharge models. DC resistivity monitoring appears capable of providing indirect information on fluid flow processes in a producing geothermal reservoir. Such information is extremely valuable for the development of quantitative predictions of future reservoir performance.     

A generalized non-isothermal tank model for liquid dominated geothermal reservoirs

We present a generalized non-isothermal tank model for predicting the pressure and temperature behaviors of liquid dominated geothermal reservoirs. A geothermal system is represented by a single or multiple tanks. These tanks can represent the reservoir, multiple reservoirs, aquifers or any other component of a geothermal system. The mass and energy balance equations for each tank are solved jointly. One of the main advantages of the model is that only a small number of tanks are used for modeling which avoids over parameterization of a geothermal system and results in faster run times when compared with fully discretized numerical simulators. Synthetic examples are used for studying the effects of heat conduction on reservoir performance, an analysis of the location of injection wells, recovery times of depleted geothermal fields and the benefits of using temperature data for a better characterization of the geothermal system.     

A novel flow-resistor network model for characterizing enhanced geothermal system heat reservoir

The fracture characteristics of a heat reservoir are of critical importance to enhanced geothermal systems, which can be investigated by theoretical modeling. This paper presents the development of a novel flow-resistor network model to describe the hydraulic processes in heat reservoirs. The fractures in the reservoir are simplified by using flow resistors and the typically complicated fracture network of the heat reservoir is converted into a flow-resistor network with a reasonably simple pattern. For heat reservoirs with various fracture configurations, the corresponding flow-resistor networks are identical in terms of framework though the networks may have different section numbers and the flow resistors may have different values. In this paper, numerous cases of different section numbers and resistor values are calculated and the results indicate that the total number of flow resistances between the injection and production wells is primarily determined by the number of fractures in the reservoir. It is also observed that a linear dependence of the total flow resistance on the number of fractures and the relation is obtained by the best fit of the calculation results. Besides, it performs a case study dealing with the Soultz enhanced geothermal system (EGS). In addition, the fracture numbers underneath specific well systems are derived. The results provide insight on the tortuosity of the flow path between different wells.     

Management of the Balcova–Narlidere geothermal reservoir,Turkey

The 2000–2005 management and field monitoring procedures at the Balcova–Narlidere geothermal field, Turkey are described. During that period, fluid production increased from 140 to 300 kg/s and the living space being heated grew from 0.64 to 1.6 million m². The shallow (depth <160 m) injection done between 1996 and 2002 cooled the fluids being produced; the hydraulic connection between shallow production and injection wells was confirmed by tracer tests. Two deep injection wells were drilled to mitigate the problem and to increase injection capacity. Because net fluid extraction was reduced, reservoir pressure drawdown was controlled. Wells drilled after 2000 indicated that the eastern portion of the field had greater potential and yielded higher temperature fluids. After testing and establishing well flow performance, pump capacities were matched to production well capacities. Mineral scaling in wells and surface installations was brought under control reducing the annual cost of inhibitors by about US$100,000. Since all production and injection wells are located near the Agamemnon fault zone and because the capacity of the district heating system is being continuously increased, there is the risk of thermal breakthrough in the production wells.     

Hydrothermal mineral zones in the geothermal reservoir of Cerro Prieto

Detailed petrologic studies completed to date on ditch cuttings and core from 23 wells in the Cerro Prieto field have led to recognition of regularly distributed prograde metamorphic mineral zones. The progressive changes in mineralogy exhibit a systematic relationship with reservoir temperature.The Cerro Prieto reservoir consists of a series of sandstones, siltstones, and shales composing part of the Colorado River delta. The western part of the field contains relatively coarser sediments apparently also derived from the delta and not from the basin margins as formerly thought. The most abundant detrital minerals in the sediments include quartz, feldspar, kaolinite, montmorillonite, illite, chlorite, mixed-layer clays, calcite, dolomite and iron hydroxides. Some of these minerals were also formed diagenetically.The following progressive stages of post-depositional alteration in response to increasing temperature have been observed: (1) diagenetic zone (low temperature), (2) illite-chlorite zone (above ~ 150°C), (3) calc-aluminum silicate zone (above ~ 230°C) and the biotite zone (above ~ 325°C). These zones are transitional to some degree and can be further subdivided based on the appearance or disappearance of various minerals.One immediate application of these studies is the ability, from a study of cuttings obtained during drilling of a well, to predict the temperatures which will be observed when the well is completed.     

Thermodynamic assessment of gas removal systems for single-flash geothermal power plants

Geothermal fluids contain non-condensable gases (NCGs) at various amounts. NCGs flow to a conventional geothermal power plant (GPP) with steam phase and should be withdrawn from the condenser by a gas removal system to prevent increase in condenser pressure and consequently decrease in power generation. Therefore, to remove NCGs from the system is critical especially at high NCG fractions. In this study, the net power output and specific steam consumption of a single-flash GPP is evaluated depending on the separator pressure, NCG fraction and wet bulb temperature of the environment, and three different conventional gas removal options which are two-stage steam jet ejector system, two-stage hybrid system and two-stage compressor system. A simulation code is written in EES to model the plant for each option. The model uses the data of Kizildere Geothermal Power Plant (KGPP) – Turkey, which is a single-flash plant with extremely high NCG fraction, to allow a comparison between the results of the modelling and the operational data of an actual single-flash GPP. Under given conditions, thermodynamic analysis resulted that NCG fraction is the most significant factor on GPP performance and the compressor system is the most efficient and robust option where the influence of the NCG fraction is limited.     

Evolution of the Wairakei geothermal reservoir during 50 years of production

The Wairakei geothermal field has been under production for more than 50 years. Exploration wells show that the high-temperature and very permeable, productive resource extends over about 12 km² within a greater area of about 25 km² that shows various effects of thermal activity. Up to 2006, 3 km³ of fluid and 2750 PJ of energy had been extracted at an average rate of 5250 t/h and enthalpy of 1130 kJ/kg. Significant production started in 1955 and up to 1978 there was no injection of cooled geothermal fluids. During the first decade of operation a pressure drawdown of up to 20 bars (2 MPa) developed and spread evenly across the reservoir, even though fluid extraction was focused within an area of 1 km² close to the northeastern field boundary. This pressure reduction resulted in widespread boiling and formation of segregated steam zones at the top of the reservoir together with inflow of cooler fluids into its northeastern part via the original natural outflow channels. From 1975 to 1997 pressures in the deep liquid reservoir stabilized at 23–25 bars (2.3–2.5 MPa) below the original pressure, with little change up to the time injection commenced in 1998. This natural pressure support indicates that prior to injection there was substantial recharge, 80% of which is assessed as high-temperature deep inflow. Since 1998 about 30% of the extracted fluids have been injected and reservoir pressures have increased by 3–4 bars (0.3–0.4 MPa). To date, significant returns of injected fluids have not been detected in the production areas. Over the 50 years of operation, temperatures in the main production areas have declined from 250 to 220 °C while deeper production zones toward the western boundary of the reservoir have remained at about 250 °C. A series of deeper makeup wells to maintain future production have been drilled in the high-temperature recharge area. An increasing fraction of injection, both in-field and out-field is planned over the next few years.