Modeling studies: The Dachny geothermal reservoir, Kamchatka, Russia

The spatial distribution of pre-exploitation conditions (i.e. temperature and pressure distribution, liquid and vapor saturations, circulation characteristics of high-temperature fluids) in the Dachny field of the Mutnovsky hydrothermal system, obtained using a three-dimensional (3-D) mapping method, are revised on the basis of natural-state simulations performed using the computer code TOUGH2. A 3-D model of the natural-state conditions at the Dachny site was developed. The fine-tuning of the model was achieved by comparing computer results to the observations made in geothermal wells tested during 1983–1988. Also studied was the behavior of the field in response to different exploitation scenarios, assuming production from existing and additional wells needed to supply sufficient steam to a proposed 80 MW_e power plant.     

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.     

Numerical investigation of production behavior of deep geothermal reservoirs at super-critical conditions

Numerical simulations have been performed to predict pressure transients in deep geothermal reservoirs at sub- and super-critical temperatures. First, pressure drawdown and buildup tests of reservoirs with different initial conditions were simulated. The calculated pressure responses are dominated by non-linear changes of fluid kinematic viscosity and compressibility. The pressure of a super-critical zone is shown to cause complex behavior. Short- and long-term production tests in both unbounded and bounded reservoirs were then simulated. Unbounded and bounded reservoirs exhibit very similar short-term production behavior near or above the critical temperature (375–400°C). Unbounded reservoirs of low transmissivity (kh = 1 darcy-meter) exhibit long-term production behavior that depends on whether the reservoir is sub-critical (300–375°C) or super-critical (400°C); substantial increases in flowing enthalpy and declines in feedpoint pressure occur at early times in the super-critical reservoir.     

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.     

Modelling of a european prototype hdr reservoir

The European Hot Dry Rock Geothermal Energy Project, located in the Rhine graben at Soultz-sous-Forets, Alsace, France, is entering a new phase in its development. Over the next few years the existing HDR system will be developed to form an operational Scientific Prototype HDR System. This paper provides an introduction to the collaborative reservoir modelling studies undertaken as part of the European Programme. In particular the paper addresses the general methodology adopted in the reservoir design process and focuses on one of the preliminary objectives of the study, assessment of the minimum HDR doublet separation required to meet the thermal performance objectives during circulation. Two “preliminary reservoir designs” are adopted as starting points for the study, the first based on exploitation of large scale planar fractures, the second on the development of a modular (multi-cell) system based on 3 cells supporting 51/s production each. Estimates were obtained using models based on both idealised geometry and empirical observations of reservoir circulation at the Camborne School of Mines (CSM) HDR project. The results indicate that a wellbore separation of around 400m would be required for the multi-cell system to achieve the required thermal performance of 10% thermal drawdown, or less, during 10 years circulation at 151/s production. Whereas, the wellbore separation required for the single fracture design would be in excess of 650m.     

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.     

The geothermal project Den Haag: 3D numerical models for temperature prediction and reservoir simulation

The proposed Den Haag Zuidwest district heating system of the city of The Hague consists of a deep doublet in a Jurassic sandstone layer that is designed for a production temperature of 75 °C and a reinjection temperature of 40 °C at a flow rate of 150 m³ h⁻¹. The prediction of reservoir temperature and production behavior is crucial for success of the proposed geothermal doublet. This work presents the results of a study of the important geothermal and geohydrological issues for the doublet design. In the first phase of the study, the influences of the three-dimensional (3D) structures of anticlines and synclines on the temperature field were examined. A comprehensive petrophysical investigation was performed to build a large scale 3D-model of the reservoir. Several bottomhole temperatures (BHTs), as well as petrophysical logs were used to calibrate the model using thermal conductivity measurements on 50 samples from boreholes in different lithological units in the study area. Profiles and cross sections extracted from the calculated temperature field were used to study the temperature in the surrounding areas of the planned doublet. In the second phase of the project, a detailed 3D numerical reservoir model was set up, with the aim of predicting the evolution of the producer and injector temperatures, and the extent of the cooled area around the injector. The temperature model from the first phase provided the boundary conditions for the reservoir model. Hydraulic parameters for the target horizons, such as porosity and permeability, were taken from data available from the nearby exploration wells. The simulation results are encouraging as no significant thermal breakthrough is predicted. For the originally planned location of the producer, the extracted water temperature is predicted to be around 79 °C, with an almost negligible cooling in the first 50 years of production. When the producer is located shallower parts of the reservoir, the yield water temperatures is lower, starting at ≈76 °C and decreasing to ≈74 °C after 50 years of operation. This comparatively larger decrease in temperature with time is caused by the structural feature of the reservoir, namely a higher dip causes the cooler water to easily move downward. In view of the poor reservoir data, the reservoir simulation model is constructed to allow iterative updates using data assimilation during planned drilling, testing, and production phases. Measurements during an 8 h pumping test carried out in late 2010 suggest that a flow rate of 150 m³ h⁻¹ is achievable. Fluid temperatures of 76.5 °C were measured, which is very close to the predicted value.     

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.     

Effective fracture network permeability of geothermal reservoirs

This paper presents a new, simple, computationally efficient and practical method to accurately calculate effective fracture network permeabilities (EFNPs) for fracture dominated reservoirs. A set of fracture patterns from the outcrops of geothermal reservoirs in southwestern Turkey were chosen and their EFNP values were predicted using the new method; the computed permeabilities are comparable to those obtained with a commercial software package. The proposed method is based on 2-D fracture outcrop data, and is therefore limited to 2-D fracture networks.     

低温热管地热利用试验及数值模拟

采用低温热管技术有效利用地热,可解决输送低温流体埋地管道周围土壤的冻胀问题。对冻土层的温度场进行理论分析,并开展初步试验研究和数值模拟计算,模拟结果与试验值基本一致,为解决工程实际问题提供一定参考。     

Prediction of geothermal well pressure and temperature profiles

The existing MIDA data bank is dedicated to experimental data of pressure drops and densities of two-phase mixtures flowing in rectilinear ducts; to this was added a MIDA-G sub-bank to collect experimental pressure and temperature profiles in two-phase mixtures in geothermal wells. To date 1879 items of data of 27 pressure and temperature profiles have been collected in five geothermal wells. The previously developed two-phase pressure drop correlation, CeSNEF-2, was used to predict these pressure and temperature profiles, given the bottom-hole data. Results are good for pressure profiles, but are less satisfactory, although acceptable, for temperature profiles. Possible explanations are: these “in-the-field” data are less accurate than laboratory data; an unpredictable modification of the inside surface; uncertainties in heat losses, both along the well and between the different wells; unreliability of the thermodynamic equilibrium hypothesis and of the Dalton and Henry laws in this environment.     

Present status and potential role of geothermal energy in the world

Geothermal energy has come of age as an energy source. It is found in most parts of the world and is harnessed by conventional technology. Commercial production on the scale of hundreds of MW has been undertaken for over three decades both for electricity generation and direct utilization. Some 80 countries have identified geothermal resources, and about 50 have quantifiable geothermal utilization at present. Electricity is produced from geothermal in 21 countries (total production 38 TWh/a) and direct application is recorded in 35 countries (34 TWh/a). Geothermal electricity production is equally common in industrialized and developing countries, but plays a more important role in the latter. Apart from China, direct use is mainly in the industrialized countries and Central and Eastern Europe. Most of the developing countries as well as Central and Eastern European countries still lack trained manpower, but there is a surplus in many industrialized countries. During 1973–1992, investments in geothermal energy amounted to approximately 22 billion USD. The large share of the private sector in the investments shows its confidence in this energy source. Data presented in the WEC Survey of Energy Resources 1995 on the “new renewables” (geothermal, solar, wind, and tidal energy) shows that geothermal has the largest installed electrical capacity (61%) and electricity production (81%) in the world of these four sources.     

Numerical simulation of snow melting using geothermal energy assisted by heat storage during seasons

Numerical simulation programs were developed for estimating temperature field and snow depth on a snow‐melting system using geothermal energy assisted by heat storage during seasons. The system utilized a group of piles underground as a heat exchanger and heat dissipation pipes near the pavement surface, realizing underground solar heat storage from the surface through the seasons. Verification experiments for this system were conducted not only in a relatively mild snowy region, Fukui, but also in a frigid region, Sapporo. Numerical simulation results demonstrated the existence of an optimum space of a group of piles, where snow melting power becomes maximal. The obtained simulation results showed good agreement with the experimental data of both regions, demonstrating the utility and validity of the programs. Also shown was that the proposed system can melt snow well in a frigid region, Sapporo, without the help of a heat pump. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 42(8): 724–744, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20393     

Prediction of the asphaltene deposition profile along a wellbore during natural production from a reservoir

The potential problem of asphaltene deposition during oil production has motivated both academics and industries to predict the asphaltene deposit profile in wellbores and pipelines. In this work, asphaltene deposition profile along an oil field well with the severe problem of asphaltene deposition was predicted. To do this, a comprehensive simulator for modeling of flow parameters such as pressure, temperature, and composition for a multiphase flow of oil, gas, and asphaltene from the reservoir to the surface was developed and coupled with the deposition model. By applying the simulator to an oil field well, it has been found that 60–70% of the total asphaltene thickness formed after 1 month of production, indicating that the problem of asphaltene deposition is bound to the initial stage of wellbore life. Moreover, the simulator was able to predict the accumulated asphaltene thickness and the time of wellbore plugging properly. This prediction is highly crucial if it is aimed to control the well performance and to optimize the productivity.     

The geothermal exploration of Campanian volcanoes: Historical review and future development

Since Roman time, the heat produced by Neapolitan volcanoes was an appeal for people living in and outside the area, for the fruition of the famous thermal baths. This very large area, which spans from Campi Flegrei and Ischia calderas to Somma-Vesuvius volcano, is characterized by high temperature at shallow depth and intense heat flow, and is yet utilized for the bathing and spa treatment industry, while only in the middle of the 20th century a tentative of geothermal exploitation for energy production was performed. Pioneering researches of geothermal resource were carried out in Campanian region since 1930, until 1985, during which a large amount of geological data were collected. In this paper, we make for the first time a review of the history of geothermal explorations in the active Campanian volcanic area. By the analysis of a great amount of literature data and technical reports we reconstruct the chronology and the main information of the drillings performed since 1930 by the SAFEN Company and successively in the framework of the ENEL-AGIP Joint Venture for geothermal exploration. The available data are utilized to correlate the temperatures measured within the deeper wells with the possible sources of geothermal heat in the shallow crust, down to about 8-10 km of depth. Finally, we assess the geothermal potential of the hottest areas, Ischia Island and Campi Flegrei, which have shown the best data and favorable physical conditions for a reliable, and cost-effective, exploitation for thermal and electric purposes.     

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.     

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.     

Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the palinpinon geothermal system, Philippines

Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of −8.5‰ and −54‰ in ¹⁸O and ²H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as “natural tracers” to determine the origin and mixing of different fluids in the reservoir.     

地热能利用浅谈

对地热的现状及利用技术的发展作了分析研究。特别指明用热泵技术可对广泛存在的低温地热加以利用,但要保证设备的技术先进,确保能源利用的投资能源系数合理。     

On production behavior of enhanced geothermal systems with CO2 as working fluid

Numerical simulation is used to evaluate the mass flow and heat extraction rates from enhanced geothermal injection–production systems that are operated using either CO₂ or water as heat transmission fluid. For a model system patterned after the European hot dry rock experiment at Soultz, we find significantly greater heat extraction rates for CO₂ as compared to water. The strong dependence of CO₂ mobility (=density/viscosity) upon temperature and pressure may lead to unusual production behavior, where heat extraction rates can actually increase for a time, even as the reservoir is subject to thermal depletion. We present the first ever, three-dimensional simulations of CO₂ injection–production systems. These show strong effects of gravity on the mass flow and heat extraction due to the large contrast of CO₂ density between cold injection and hot production conditions. The tendency for preferential flow of cold, dense CO₂ along the reservoir bottom can lead to premature thermal breakthrough. The problem can be avoided by producing from only a limited depth interval at the top of the reservoir.