Geothermal reservoir simulation

This paper is intended to be a state of the art review of geothermal reservoir simulation. Its recent application to the modelling of real geothermal reservoirs is described and put in the context of an emerging general approach to reservoir modelling. The use of computer simulation for geothermal well test analysis is described. One of the main recent uses of reservoir simulators has been for conducting numerical experiments aimed at improving the understanding of geothermal reservoir physics. Such studies on fractured reservoirs, the thermal structure of reservoirs and the effects of non-condensable gases and dissolved salts are outlined.     

The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources

The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5–5 km, and at temperatures of 450–600 °C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550–650 °C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000–5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the “pipe”. Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 °C. A deep well producing 0.67 m³/s steam (2400 m³/h) from a reservoir with a temperature significantly above 450 °C could, under favourable conditions, yield enough high-enthalpy steam to generate 40–50 MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide.     

Delocalized organic pollutant destruction through a self-sustaining supercritical water oxidation process

Supercritical water oxidation (SCWO) is a recent development aiming at the destruction of organic pollutants present with low concentrations in waste waters. The present paper focuses on the process simulation of SCWO with emphasis on the proper modelling of supercritical thermodynamic conditions and on the possibility to make the SCWO process self-sufficient from the energetic viewpoint. Self-sufficiency may be of interest to encourage more delocalization of waste water treatment.The process of SCWO for dilute waste water (no more than 5 wt.%) is modelled through the ASPEN Plus^© process simulator. Studies were made to search for energetic self-sufficiency conditions using various technologies for power production from the heat of reaction, like supercritical water expansion in a turbine, use of a closed Brayton cycle (CBC) and use of an organic Rankine cycle (ORC). The results obtained showed that the process is energetically self-sufficient using either a small supercritical turbine, or an ORC. In less restrictive conditions regarding the component efficiencies, the CBC, in theory, also leads to self-sufficiency, but from the analysis, it appears that this solution is less realistic.     

Comparison of simulators for variable‐speed wind turbine transient analysis

This paper presents a comparison of three variable‐speed wind turbine simulators used for a 2 MW wind turbine short‐term transient behaviour study during a symmetrical network disturbance. The simulator with doubly fed induction generator (DFIG) analytical model, the simulator with a finite element method (FEM) DFIG model and the wind turbine simulator with an analytical model of DFIG are compared. The comparison of the simulation results shows the influence of the different modelling approaches on the short‐term transient simulation accuracy. Copyright © 2005 John Wiley & Sons, Ltd.     

Polygeneration Study of Low-to-Medium Enthalpy Geothermal Reservoirs in Mexico

This study combines the thermodynamic analysis of a polygeneration system along with the numerical modelling of the thermal behavior of geothermal reservoirs in...     

Potential use of particle tracking in the analysis of low-temperature geothermal developments

Projects aimed at the exploitation of low-temperature geothermal systems for direct applications rarely undergo detailed analysis prior to their undertaking, and this is especially true in the case of smaller developments. This may be partly related to a lack of numerical modelling during the analysis and design of these systems, as a result of the relative inaccessibility of advective–conductive heat flow simulators. Particle tracking may provide a cost- and time-efficient alternative method for estimating production well temperatures. The results of this study indicate that particle tracking may be useful in the analysis of heat transport in injection–production doublets if pumping rates are sufficiently high relative to well spacing and aquifer thickness.     

Modelling of organic Rankine cycle system and heat exchanger components

Numerical models of a standard organic Rankine cycle (ORC) system and the heat exchangers comprising the system are developed as a design tool platform for a flexible design. The objective is design of an efficient, cost-effective ORC power plant that can effectively exploit low-grade industrial waste heat or low to medium-temperature geothermal fluid. Typical heat exchanger configurations were modelled, including the circular finned-tube evaporator, air-cooled condenser, and flat-plate preheater. A published ORC configuration and process conditions from experiments are used for the thermodynamic cycle analysis in order to validate of the system model. Heat transfer correlations and friction factors are described for the modelling of the heat exchangers. The simulation results of the ORC system provide the design requirements for the heat exchangers. Geometric specifications and performance of the heat exchangers are determined by iterative simulations.     

Thermodynamic evolution of the Los Azufres,Mexico, geothermal reservoir from 1982 to 2002

An investigation has been made of the response of the Los Azufres geothermal reservoir to 20 years of development, beginning in 1982. The simulator WELFLO was used to characterize the thermodynamic conditions of the reservoir fluids. The first response to exploitation consisted of a decrease in pressure and an increase in enthalpy. Small decreases in reservoir pressure associated with large increases in fluid enthalpy characterize the long-term response in the northern production area. In the southern production area, long-term changes include decreases in pressure and mass flow rate, increases in steam production and, in wells affected by injection, increases in both pressure and total mass flow rate. These changes reflect the effects of boiling, cooling and fluid mixing, processes resulting from large-scale fluid production.     

Changes in thermodynamic conditions of the ahuachapán reservoir due to production and injection

Since large-scale exploitation of the Ahuachapán reservoir began in 1975 large changes in the reservoir thermodynamic conditions have occurred. Drawdown of up to 15 bars and significant temperature changes have been observed in the wellfield. Temperatures have declined due to boiling in the reservoir in response to the pressure drawdown; localized and minor cooling due to reinjection of spent geothermal fluids have also been observed. There are indications of cold fluid influx deep into the reservoir from the west and north. Reservoir temperatures show that a significant amount of hot fluid recharge comes to the wellfield from the southeast, and temperatures also indicate that the recharge rate has increased with time as pressure declines in the reservoir. Chemical analyses of the produced fluids show that most wells are fed by a mixture of geothermal fluids and cooler, less-saline waters. The cold water inflow has increased due to exploitation, as demonstrated by decreased salinity of the produced fluids.     

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.     

Life cycle assessment of geothermal binary power plants using enhanced low-temperature reservoirs

Geothermal binary power plants that use low-temperature heat sources have gained increasing interest in the recent years due to political efforts to reduce greenhouse gas emissions and the consumption of finite energy resources. The construction of such plants requires large amounts of energy and material. Hence, the question arises if geothermal binary power plants are also environmentally promising from a cradle-to-grave point of view. In this context, a comprehensive Life Cycle Analysis (LCA) on geothermal power production from EGS (enhanced geothermal systems) low-temperature reservoirs is performed. The results of the analysis show that the environmental impacts are very much influenced by the geological conditions that can be obtained at a specific site. At sites with (above-) average geological conditions, geothermal binary power generation can significantly contribute to more sustainable power supply. At sites with less favorable conditions, only certain plant designs can make up for the energy and material input to lock up the geothermal reservoir by the provided energy. The main aspects of environmentally sound plants are enhancement of the reservoir productivity, reliable design of the deep wells and an efficient utilization of the geothermal fluid for net power and district heat production.     

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.     

An Approximate Relation of a Regenerative Heat Exchanger Effectiveness in Off-Design Conditions

A regenerative heat exchanger is an important component of a thermal system in power units. It is crucial to know the performance of the regenerative heat exchanger in off-design conditions during its design and operation. Advanced regenerative heat exchanger simulators have been developed for many years to describe the performance in off-design conditions. The simulators involve the use of equations for mass, momentum, and energy balances and criteria relations for heat transfer coefficients; the geometrical data of the heat exchanger are also required. Due to high complexity, the calculations are performed iteratively. For this paper, a different approach was taken: The heat exchanger was considered as a “black box.” Based on the data obtained from the simulator, the effect of input variables on the output ones was investigated, so as to propose a relation describing the regenerative heat exchanger performance. To assess this performance, heat transfer effectiveness was proposed, and its two variants were considered. Since two heat transfer effectiveness definitions were assumed, two approximate relations concerning the regenerative heat exchanger were determined. The relations were verified against data obtained from a simulator of a high-pressure regenerative heat exchanger in a medium-power steam condensing unit. A satisfactory accuracy of the proposed relations was obtained.     

Thermodynamic study of the supercritical water reforming of glycerol

Hydrogen can be produced by steam reforming, partial oxidation, autothermal, or aqueous-phase reforming processes using various noble metal based catalysts, but also by supercritical water (SCW) reforming. Using AspenPlus™, a systematic thermodynamic analysis of glycerol reforming using supercritical water has been carried out by the total Gibbs free energy minimization method, which computes the equilibrium composition of synthesis gas (syngas). The predictive Soave-Redlich-Kwong equation of state (EOS) has been used as thermodynamic method in the simulation of the supercritical region, after evaluating it against other EOS methods. A sensitivity analysis has been conducted on supercritical water reforming of pure and pretreated crude glycerol, as obtained from biodiesel production. The effect of the main operating parameters (temperature, concentration of glycerol feed, glycerol purity in the feed of crude glycerol, and pressure) aimed to the hydrogen production has been investigated in the reforming process, by obtaining the mole fraction and molar flow-rate of components in syngas, as well as the hydrogen yield. Selectivity to the different compounds has been also calculated. By this way, the thermodynamic favorable operating conditions at which glycerol may be converted into hydrogen by SCW reforming have been identified. The simulation results agree well with some few experimental data from the literature. This study is the first of a series addressed to glycerol reforming using SCW.     

Second law analysis of a fossil-geothermal hybrid power plant with thermodynamic optimization of geothermal preheater

There is an urgent and compelling need to develop innovative and more effective ways to integrate sustainable renewable energy solutions into the already existing systems or, better yet, create new systems that all together make use of renewable energy. This study aims to establish the optimum working conditions of a geothermal preheater in a power plant that makes use of both renewable and nonrenewable energy resources, where renewable (geothermal) energy is used to boost the power output in an environmentally sustainable way. Hence, two models, one, a simplified model of a Rankine cycle with single reheat and regeneration, and another, with a geothermal preheater substituting the low-pressure feedwater heater (LPFWH), were compared. The Engineering Equations Solver software was used to perform an analysis of the thermodynamic performance of the two models designed. An analysis was done to evaluate the energetic and exergetic effects of replacing a LPFWH with a geothermal preheater sourcing heat from a low temperature geothermal resource (100°C-160°C). Results from the thermodynamic analysis reveal that the hybridization boosts the power output by approximately 4% and it is superior in terms of the second law. Entropy generation minimization analysis was then employed to establish optimal working conditions of the hybrid system (ie, the geothermal preheater modeled as a downhole coaxial heat exchanger).     

Sustainable geothermal utilization – Case histories; definitions; research issues and modelling

Sustainable development by definition meets the needs of the present without compromising the ability of future generations to meet their own needs. The Earth's enormous geothermal resources have the potential to contribute significantly to sustainable energy use worldwide as well as to help mitigate climate change. Experience from the use of numerous geothermal systems worldwide lasting several decades demonstrates that by maintaining production below a certain limit the systems reach a balance between net energy discharge and recharge that may be maintained for a long time (100–300 years). Modelling studies indicate that the effect of heavy utilization is often reversible on a time-scale comparable to the period of utilization. Thus, geothermal resources can be used in a sustainable manner either through (1) constant production below the sustainable limit, (2) step-wise increase in production, (3) intermittent excessive production with breaks, and (4) reduced production after a shorter period of heavy production. The long production histories that are available for low-temperature as well as high-temperature geothermal systems distributed throughout the world, provide the most valuable data available for studying sustainable management of geothermal resources, and reservoir modelling is the most powerful tool available for this purpose. The paper presents sustainability modelling studies for the Hamar and Nesjavellir geothermal systems in Iceland, the Beijing Urban system in China and the Olkaria system in Kenya as examples. Several relevant research issues have also been identified, such as the relevance of system boundary conditions during long-term utilization, how far reaching interference from utilization is, how effectively geothermal systems recover after heavy utilization and the reliability of long-term (more than 100 years) model predictions.     

Classification of geothermal resources by exergy

Geothermal resources have been classified as low, medium and high enthalpy resources according to their reservoir fluid temperatures. There is no general agreement on the arbitrary temperature ranges used. Classification of a geothermal resource by its reservoir fluid temperature can be ambiguous because two independent properties are required to define the thermodynamic state of a fluid. Geothermal resources should be classified to reflect their ability to do thermodynamic work. In this paper, it is proposed that geothermal resources be classified as low, medium and high quality resources with reference to their specific exergy indices (SExI), SExI<0.05, 0.05SExI<0.5 and SExI0.5, respectively. The demarcation limits for these indices are exergies of saturated water and dry saturated steam at 1 bar absolute. These demarcation lines can be plotted on a Mollier diagram to form a classification map of geothermal resources.     

Thermodynamic and exergo-economic assessments of a new geothermally driven multigeneration plant

In this paper, a new geothermal-based multigeneration system is designed and investigated in both thermodynamic and economic analyses. The reason to select the geothermal source is that geothermal power is a renewable and sustainable power resource, and also it is not weather dependent. The proposed geothermal-based multigeneration plant is able to produce power, heating, cooling, swimming pool heating, and hydrogen. The main idea in this renewable-based multigeneration system is to create valuable products by using waste heat of subsystems. Then, by applying thermodynamic analyses, the energy and exergy performances of proposed multigeneration system are computed. Also, parametric work has been performed in order to see the impacts of the reference temperature, geothermal fluid temperature, and geothermal water mass flow rate. Finally, exergo-economic analysis based on exergy destruction or thermodynamic losses is done to gain more information about the system and to evaluate it better. According to the calculations, the overall plant's energy and exergy performances are 32.28% and 25.39%. Economic analysis indicates that hydrogen production cost can be dropped down to 1.06 $/kg H₂.     

Modeling a Geothermal Field in Japan

Studied is a three-dimensional simulator of single-phase, hot-water flow which consists of fracture-type flow and recharge from the ground surface. Equations as applied to the simulator, and some assumptions and methods of solution are also described. These are applied to modeling of a geothermal field where exploration on geothermal activity is being conducted at present. As a result, it has been possible to infer some phenomena which appear to be related to hot-water flow in the earth's crust.