Stanford University December 3, Geothermal Well Test Analysis: Fundamentals, Applications and Advanced Techniques provides a comprehensive review of the geothermal pressure transient analysis methodology and its similarities and differences with petroleum and groundwater well test analysis.
In addition, the book focuses on pressure transient analysis by numerical simulation and inverse methods, also covering the familiar pressure derivative plot. Finally, non-standard geothermal pressure transient behaviors are analyzed and interpreted by numerical techniques for cases beyond the limit of existing analytical techniques.
Provides a guide on the analysis of well test data in geothermal wells, including pressure transient analysis, completion testing and output testing Presents practical information on how to avoid common issues with data collection in geothermal wells Uses SI units, converting existing equations and models found in literature to this unit system instead of oilfield units.
Skip to content. Geothermal Reservoir Engineering. Geothermal Reservoir Engineering Book Review:. Author : E. Author : Malcolm A.
Grant,Paul F. Author : G. Reservoir Engineering Assessment of Geothermal Systems. Geothermal Reservoir Engineering in Perspective. Geothermal Energy Systems. Geothermal Energy Systems Book Review:. Geothermal Reservoir Engineering in Industry. Geothermal Reservoir Engineering Research. Geothermal Reservoir Engineering Second Edition. Workshop on Geothermal Reservoir Engineering 1. Second Workshop Geothermal Reservoir Engineering.
Geothermal Well Test Analysis. Author : Sadiq J. Perhaps the most significant change has been the increasing importance of environmental impacts of development. Impacts on surface springs have always been an issue in development of the associated deep resource, and this concern has prevented some developments. Geothermal reservoir engineering is now clearly a distinct discipline. Distinctive features of geothermal reservoirs generally include the following: 1.
The primary permeability is in fractured rock. The reservoir extends a great distance vertically. For liquid-dominated reservoirs a caprock is not essential, and usually the high-temperature reservoir has some communication with surrounding cool groundwater. The vertical and lateral extent of the reservoir may not be clear.
Basic to all reservoir engineering is the observation that almost everything that happens is the result of fluid flow. The flow of fluid water, steam, gas, or mixtures of these through rock, fractures, or a wellbore is the unifying feature of all geothermal reservoir analysis. Chapter 1 Geothermal Reservoirs 5 1. The terms have been defined to keep their meanings clear and consistent.
Unfortunately, the limited number of terms commonly used makes for considerable difficulty, since many of these terms have general meanings as well as the particular meanings chosen here. Most areas of geothermal activity are given some geographic name. Provided they are distinct and separate from neighboring areas of activity, they have been described as geothermal fields.
The term is intended to be purely a convenient geographic description and makes no presumption about the greater geothermal system that has created and maintains the field activity. The many fields in the world that have double names Mak-Ban, Karaha-Bodas, Bacon-Manito illustrate that exploration has shown that surface activity originally thought to be associated with separate fields is later found to be part of a single, larger field. The total subsurface hydrologic system associated with a geothermal field is here termed a geothermal system.
This includes all parts of the flow path, from the original cold source water, its path down to a heat source, and finally its path back up to the surface. Finally and most important, there is the geothermal reservoir. This is the section of the geothermal field that is so hot and permeable that it can be economically exploited for the production of fluid or heat. It is only a part of the field and only a part of the hot rock and fluid underground. Rock that is hot but impermeable is not part of the reservoir.
Whether a reservoir exists depends in part on the current technology and energy prices. It is a fairly common experience to drill deeper into an existing field, proving additional reservoir volume at greater depth. In the most extreme contrast, an EGS see Chapter 14 project aims to create a reservoir where none exists by creating permeability in hot, otherwise impermeable rock. After briefly considering conductive heat flow, the main topic is convective geothermal systems.
The need for water circulation to great depth is shown, along with a basic conceptual model of a field driven by an upflow of hot fluid.
The dynamic nature of the natural state is stressed. The boiling point for depth BPD model is introduced as representing conditions in high-temperature upflow areas. Fields with lateral outflow and fields without boiling are treated as being equivalent. Vapor-dominated fields are related to their natural upflow of steam.
Exploitation can introduce the flow of additional hot and cold fluids, the formation of a free surface, and an increase in boiling in the reservoir. Chapter 3 considers some quantitative models and different approaches to simplifying real situations. The two dominant approaches are pressure-transient and lumped-parameter models.
Linking them are the concepts of flow transmission of fluid and heat and storage the ability of the reservoir to store fluid in response to pressure change. After discussing homogeneous porous media, possible differences introduced by fractured media are considered.
Beginning with Chapter 4, the book follows the reservoir engineering work that would be performed as part of a field development in roughly chronological order. Chapter 4 discusses the principles of interpreting measurements in geothermal wells and the sometimes complex relationship between measurements in a well and the physical state in the reservoir. Chapter 5 covers downhole PTS instruments and their limitations.
Chapter 6 covers measurements during drilling, which provide information about pressure and sometimes temperature in formations above the reservoir. Chapter 7 covers the measurements made at well completion, with the objectives being defining the model of the well, the depth and magnitude of its permeable zones, and the reservoir conditions at each zone.
Measurements during warmup confirm or refine these interpretations. Methods for initiating and measuring discharge are discussed. A well may discharge various fluids at the wellhead: liquid water, dry steam, or a twophase mixture, with the last being the most common.
From the variation of mass flow and enthalpy with wellhead pressure, it is possible to make inferences about the reservoir fluid, well permeability, and the state of the well. The calculation of wellhead pressure under flowing conditions is described for single-phase flows, and wellbore simulations are used to model two-phase flows. Chapter 9 is a case study of well BR2 at Ohaaki.
It has a year history, from its first drilling in to its abandonment in During this period it exhibited a wide variety of behaviors. Chapters 10e13 turn from properties of the individual well to reservoir-scale properties, with the end objective being the formulation of a conceptual and numerical model of the reservoir and often simplified models of some aspects of reservoir behaviors. Chapter 10 discusses the quantitative and qualitative inferences that can be made from downhole data.
The mapped distribution of pressure, temperature, or chemical composition supplies information about the reservoir structure. Chapter 11 discusses Chapter 1 Geothermal Reservoirs 7 reservoir simulation and the role of the reservoir engineer in the simulation, from the production of detailed well data to the review of results.
It is stressed that a simulation should be calibrated against as wide a set of data as possible. Additional types of data provide more constraints on possible model structure and improve model quality. All have been exploited long enough to show significant changes. Wairakei shows a field being developed from little initial knowledge about geothermal to long-term operation and a sophisticated simulation. Awibengkok shows successful development of a new resource.
Svartsengi shows development and exploration. Balcova-Narlidere is a well-documented low-temperature field. Palinpinon is a field with initially severe injection returns that was converted into a long-term operation.
Mak-Ban is a very high-quality resource successfully developed without major problems. The unexploited Patuha field, a distinctive hybrid between vapor-dominated and liquid dominated, is also summarised. Chapter 13 covers field management with simple lumped-parameter models or decline models, deposition, tracer testing, and injection management.
Chapter 14 covers well stimulation and EGS. The first stimulation method, which is often overlooked, is thermal stimulation by injecting cold water and is the most cost-effective. Acid stimulation provides a means of mitigating deposition problems. Well stimulation of deep sedimentary aquifers is now practiced in many places. Four appendices review the pressure transient theory, gas corrections for output testing, the equations of state and flow in porous media, and provide a glossary of field names referred to in the book.
Examples have been chosen and field experience quoted where it has given a useful illustration, but these examples are simply those that were appropriate and conveniently available. Similarly the References are the works that have been cited in the text, but they are not meant to be a comprehensive coverage of the literature.
The text does not pretend to be a representative survey of the current literature. Where possible, references have been used that are readily available on the Web and, for preference, those that are available at no cost. Equations are written in SI units.
Pressures are absolute unless gauge is specified. Casing and wellbore sizes are conventionally given in inches, and inches are used in the text, but calculations use meters.
The SI unit for permeability is the meter squared. The terms water and steam are used to refer to the liquid and vapor phases of water substance. Where gas content is significant, liquid and vapor are used for the liquid and vapor phases of the water-gas mixture, and water and steam refer to that part of the respective phase that is water substance. Introduction 2. Conductive Systems 2.
Convective Systems: Liquid Dominated 2. Convective Systems: Vapor Dominated 9 10 12 2. Concepts of Changes Under 24 Exploitation 2. Conclusions 27 21 2. These concepts provide a background for the following chapters, which describe the testing of wells and reservoirs, the construction of a conceptual model of the resource, and then a quantitative model. Much of the following discussion relates to unexploited fields in which the thermodynamic state of the field is determined by the natural processes of heat and fluid transport.
This is because the results of testing done early in a new field development are the most challenging to interpret and because the natural state of the field influences its subsequent response to exploitation.
As a geothermal field is developed and information from more wells, along with the system response to production and injection, becomes available, the conceptual model and the numerical simulation model can be further refined.
Anyone who has ever watched a geyser in action or a hot pool bubbling and wondered where the water and heat came from has some sort of mental picture of this small part of a geothermal system.
Each of Geothermal Reservoir Engineering. In the scientific arena, such mental models, which are based on a range of data from various disciplines, together with experience in related research, form the basis for the development of conceptual models that should bring together all the available information into a single coherent model.
However, since a model should be consistent with all the observable aspects of the system, they should all give very similar basic results. These models will vary among individuals, depending on their need.
For example, the fine detail that is essential in a model of flow around a particular well will become insignificant as the scale increases to include the complete geothermal resource. The Thermal Regime of the Earth Over almost the entire surface of the Earth is a flux of heat through the crust upward to the ground surface.
This heat is transported to the surface by conduction through the crustal rocks. A region of potential geothermal resource may be associated with high heat flow. The near surface temperature gradients can be extrapolated through impermeable strata to obtain deep temperatures.
Warm Groundwater Basins One source of water at temperatures above mean surface is from aquifers that are so deep that their temperature is raised by the normal geothermal gradient. The mechanism heating the water in such systems is then simply the vertical conduction of heat through the crust. The fluid flow in the aquifer must be sufficiently slow that there is time for the water to be heated by this conductive Chapter 2 Concepts of Geothermal Systems 11 heat flow.
The general reduction of permeability with depth implies that successful production from greater than a few kilometers requires anomalously high permeability, and the chance of such anomaly decreases with increasing depth.
In some large groundwater basins, a fraction of the heated water circulates back up to the surface where there is suitable permeability and structure. Otherwise the heated fluids may be confined within a particular stratum. Deep Sedimentary Aquifers Deep sedimentary aquifers heated by the normal thermal gradient are found in many continental environments.
These aquifers are usually not part of a currently active circulation system. Figure 2. This simple two-well system in an aquifer is very common in groundwater or petroleum engineering. The only difference is that the fluid is warm. Other examples are found in most larger-scale basin areas where elevated temperatures are encountered around the world.
Warm Springs and Fracture and Fault Systems Many warm springs are found along major fault and fracture lineations throughout the world, suggesting that these major fault systems provide the channels for the flows of warm water that feed the springs.
Such channels provide the means for circulation of cold meteoric water to depths where it is heated by the normal geothermal temperature gradient and then returned to the surface to form warm springs. These are a form of convective system, with convection along the plane of the fault being heated by conductive heat transfer into the fault zone.
The driving force for the circulation is the density difference between the cool descending water column and the hotter rising column. This mechanism differs from a full convective systems discussed later in this chapter in that it is confined to a narrow fault plane with no extensive reservoir and is FIGURE 2.
An example of a faultcontrolled spring system is the hot spring area around Banff, Canada. Geopressured Systems Geopressured geothermal reservoirs are closely analogous to geopressured oil and gas reservoirs. Fluid caught in a permeable stratigraphic trap may, by crustal motion over millions of years, be raised to lithostatic pressure.
A number of such reservoirs have been found in petroleum exploration. Where these reservoirs are found associated with petroleum, the water is generally saturated with methane, and the methane may be a more important energy source than the heat in the water. Reservoir engineering of such a system is more like a petroleum reservoir than a hydrothermal reservoir or groundwater aquifer. Experiments were conducted on existing wells, originally drilled for petroleum exploration, in the s in the U.
No further studies have been done since that time Griggs, EGS exploration in the Cooper Basin see Chapter 14 has revealed similar abnormally high pressures in the ancient granite terrain that is overlaid by about 4 km of more recent sediments. Successful development of the deep geothermal resource would operate at similar high pressures. Hot, Dry Rock or Engineered Geothermal Systems In some locations, rock of low permeability that has been heated to useful temperatures can be found.
The heat source may be from volcanism or an abnormally high geothermal gradient, or there may be impermeable rock on the flanks of a hydrothermal system. Compared with the other systems, these do not have sufficient intrinsic permeability, but they do contain heat. Exploitation of such a system depends on creating permeability by controlled fracturing such that fluid can be circulated through the rock and heat can be extracted.
The fracturing creates a reservoir that did not previously exist. This subject is discussed further in Chapter Introduction: The Dominance of Convection Hydrothermal convective systems are geothermal systems with high temperatures and usually with surface activity. At the present time, all major geothermal power stations operate on such systems. Chapter 2 Concepts of Geothermal Systems 13 In contrast to conductive systems, it is the flow of hot fluid through the system that determines the temperature and fluid distribution.
The natural state of the geothermal reservoir in a convective system is thus dynamic, and knowledge of the natural fluid flow is needed to understand how this natural state was formed. Surface features such as geysers, springs, fumaroles, cold gas vents, and mudpools may be associated with this type of reservoir, and they are the end points for some part of the natural thermal flow. Such natural flows play a dominant role in establishing the state of the fluid within the reservoir, and an understanding of them provides information about reservoir parameters such as vertical permeability, which cannot be determined by other means.
Because new flow patterns created by exploitation will usually overwhelm the natural flows in the reservoir, it is important that appropriate information and data be collected early in the exploration program of a new resource. In low-temperature systems the reservoir fluid is always liquid water, while in higher-temperature systems, steam can also be present.
All geothermal reservoirs located to date can be divided into two typesdliquid dominated and vapor dominatedddepending on whether liquid or steam is the mobile phase. A few reservoirs have separate regions of both types. The majority of reservoirs are liquid dominated and have a vertical pressure distribution that is close to hydrostatic. In vapor-dominated reservoirs the vertical pressure distribution is close to steam-static.
In each case the dominant mobile phase, either liquid or steam, controls the pressure distribution, although the other phase may be present in significant amounts. The remainder of this section considers only liquid-dominated reservoirs.
Vapor-dominated reservoirs are discussed later in this chapter. Deep Circulation and Magmatic Heat Conductive geothermal systems do not require a great deal of heat at depth and can occur anywhere in the world. High-temperature convective systems demand some additional heat above the normal conductive gradient. For his description of the hot spring system in west Iceland, Einarsson visualized something akin to a deep groundwater basin.
His geothermal flux had to be higher than normal to produce the higher-temperature spring discharge, and his aquifers were factures and fissures in the otherwise impermeable basalts. Source: White, D. White produced the model shown in Figure 2. In Figure 2. White suggested a possible range of 2 to 6 km. A significant number of geothermal fields have now been drilled beyond 3 km without finding a bottom to the system, so the fluid circulation depths would appear to be larger rather than smaller.
The buoyancy imbalance between the hot and cold columns drives this fluid back up to the surface through other permeable channels. These systems, which require large amounts of heat compared to the normal crustal heat flux, are generally found in regions of relatively recent volcanism.
This accounts for the large number of geothermal fields associated with volcanic arc and crustal rifting. The fractures or flow paths for the water circulation appear to be associated with structures such as regional rift zones or calderas. The total amount of heat transported out of convective geothermal systems over their lifetimes is largedso large, in fact, that not only must circulating water make close contact, but this magma itself must be convecting or replenished by mechanisms such as crustal spreading.
Some geothermal fields are long-lived. Browne imputes a lifetime of hundreds of thousands of years for Kawerau. At Coso a lifespan of , years has been suggested, at least intermittently Adams et al. Silberman and Chapter 2 Concepts of Geothermal Systems 15 colleagues suggest that Steamboat Springs may have existed for 3 million years, and Villa and Puxeddu suggest that Larderello may be as much as 4 million years old.
In contrast, some fields have much shorter lifetimes. The Salton Sea geothermal field has an estimated lifetime of 3, to 20, years Kasameyer et al. Long lifetimes of geothermal systems cannot be sustained by a single emplacement of magma. For long-lived fields, even if the magma was extensive, many cubic kilometers would be required to supply the cumulative heat discharged White, ; Lachenbruch et al. Similarly Banwell estimated that over its lifetime Wairakei required at least 10, km3 of magma for its heat supply.
For both Wairakei and Larderello the magma volume is too large to be stored under the field. This suggests that the magma source itself must be convecting, so molten rock remains near the zone where heat is exchanged between the magma and the fluid in the geothermal system. Exploitation and System Circulation The deep circulation feature of geothermal systems implies that in general the changes induced by exploitation will not greatly affect the natural upflow from depth.
Assuming a depth of 5 km to the base of the geothermal system, this flow is driven by the pressure difference due to the fluid density between the hot and cold columns, which in this case is about bar.
Thus, the natural state of the reservoir is dynamic, and the fluid distribution is controlled by a dynamic balance of mass and heat flow. Once exploitation occurs, fluid flow to and from wells is generally much greater than the natural flow. This may create a significant flow from parts of the reservoir beyond the depth or areal extent of the wells. With large-scale production and reinjection, the primary induced flow is from the injection wells to the production wells, and induced flow changes outside this area are significantly smaller.
The Vertical Upflow Model and Boiling Point for Depth Models Having considered the processes in the geothermal system as a whole, this section focuses on the smaller, relatively shallow part of the system that contains the reservoir: the area where fluid rises within an exploitable depth from the surface. The simplest case is when the upflow rises vertically from 16 Geothermal Reservoir Engineering greater depth to ground surface. The upflow at great depth consists of water or supercritical fluid.
As this fluid ascends, the pressure decreases, and at some point, depending on the temperature and fluid chemistry, the ascending fluid forms two phases, gas and liquid, which both rise to the surface. The upflow continues toward the surface as gas and liquid. From conservation of mass and energy it is possible to estimate the form of this upflow. The following assumes that the fluid at depth is liquid and boils when it reaches its saturation pressure, as illustrated in Figure 2. For most purposes, conduction can be ignored as a means of heat transport.
Assuming that the pressure at the boiling level depends on the temperature in the upflowing liquid zone, as boiling commences, saturation conditions must apply. In a field where the mass flux density upflow per unit area is low, the excess dynamic gradient is correspondingly small. This excess gradient is present only in the area of upflow, and toward the margins where there is lateral flow, the pressure gradient will be close to hydrostatic.
The BPD approximation implies that steam saturation is close to residual see Appendix 2. The BPD approximation thus not only approximates the pressure and temperature in the reservoir, but it also specifies something about the reservoir fluid: that little mobile steam is present in the boiling zone. Matrix denoted by pale shading. In the liquid-dominated reservoir in Figure 2. The matrix is water-saturated. In the vapordominated reservoir in Figure 2.
As a model, BPDdthat is, a static fluid column everywhere at boiling point until the constant-temperature liquid-water section is reached dis for many purposes a good approximation of the initial state of the upflowing core of the reservoir. Note that this is only an approximation; pressures and temperatures can be higher or lower, and it is incorrect to regard BPD as any sort of theoretical maximum temperature.
The BPD profile is naturally of less value where flows are relevant and actual reservoir pressure gradients are necessary, as in comparing pressures between different wells to determine a lateral pressure gradient. In reality, the rising fluid is cooled by dilution and conduction as much as it would be by boiling, which reduces the amount and extent of boiling. The BPD model is still valid as long as there is some boiling. Typically, boiling conditions will be found in the core of the upflow, with cooler conditions toward the margins.
In all high-temperature geothermal systems, noncondensable gases NCGs are present in the reservoir fluids. The presence of these gases together with dissolved salts changes the saturation relation for the reservoir fluid from that for pure water, with the effect that the pressure at which the liquid phase first boils is greater than that for pure water; in other words, boiling starts deeper.
A modified boiling curve can be computed by adding conservation of gas to conservation of mass and energy. Systems with Lateral Outflow The assumption that the natural flow is entirely vertical is an idealization. Structural control by permeability variation and topographic effects will usually impose some degree of lateral flow. The BPD approximation requires a component of upflow, since boiling conditions can only be maintained if there is some continued upflow of fluid.
Most two-phase geothermal fields have associated nonboiling lateral flows away from the boiling upflow region. If the natural flow is horizontal or turns downward, boiling ceases and liquid reservoir conditions are encountered. Thus, the BPD profile may be applicable only in the upflow region of this type of reservoir.
The lateral outflows from this region will be liquid water although there will usually be some shallow boiling in places along the top of the outflow tongue. Exploration wells will be expected to encounter different regimes within the reservoir, depending on their location.
In the upflow region, boiling conditions should be expected, and in outflow regions, high-temperature liquid conditions with temperature inversions declining temperatures with depth can be expected.
Usually the outflow region of a field is initially explored, and with further drilling, the flow is traced toward the high-temperature source area. Some examples follow.
Tongonan The Tongonan field is located on the island of Leyte in the Philippines. It is a large field with an installed capacity of MW in the greater Tongonan area, which includes the adjacent Mahanagdong field. An overview of the development history is given by Gonzalez and colleagues Source: Seastres et al. The Tongonan upflow rises under Mahiao toward one end of the reservoir.
Here, steam and water rise, creating a limited region of two-phase conditions in the natural state. Water flowed laterally away from the upflow area to ultimately discharge at the Bao Springs and possibly elsewhere.
This combination of upflow and horizontal outflow is very common. The surface activity in a field of this type is a guide to reservoir fluid distribution. The principal springs are at Bao, but the highest temperatures are beneath the steam-heated activity at Mahiao, and it is such steam-heated activity that indicates the area of upflow.
There is an impermeable region separating Tongonan proper from the separate upflow at Mahanagdong. Under production, large pressure drawdown has occurred in the Tongonan area, and the upper part of the reservoir here has become vapor dominated. As is typical of many Basin and Range fields, it is associated with permeability developed along a fault zone that provides the primary source of deep recharge.
The reservoir is on the fault zone rather than an outflow from it. The reservoir is a region around the fault zone. Source: Blackwell et al. Inferences from Pressure Distribution The discussion of systems with outflow introduced the effects of permeability contrasts on the pattern of the natural flow.
Field Management Well Stimulation and Engineered Geothermal Systems Pressure Transient Analysis A1. Gas Correction for Flow Measurements A2.
Equations of Motion and State A3. Malcolm Grant Dr. Grant holds a doctoral degree in applied mathematics from MIT, has participated in various research and management projects, and has worked as a private consultant since He has been involved in assessment and development of 76 geothermal fields in 14 countries. Grant is among the most prestigious scientists in the field of geothermal reservoir engineering and has published many papers in that field in leading journals such as Geothermics.
It is a fairly common experience to drill deeper into an existing field, proving additional reservoir volume at greater depth. In the most extreme contrast, an EGS see Chapter 14 project aims to create a reservoir where none exists by creating permeability in hot, otherwise impermeable rock. Chapter 2 covers the concepts of geothermal reservoirs. After briefly considering conductive heat flow, the main topic is convective geothermal systems.
The need for water circulation to great depth is shown, along with a basic conceptual model of a field driven by an upflow of hot fluid.
The dynamic nature of the natural state is stressed. The boiling point for depth BPD model is introduced as representing conditions in high-temperature upflow areas. Fields with lateral outflow and fields without boiling are treated as being equivalent. Vapor-dominated fields are related to their natural upflow of steam. Exploitation can introduce the flow of additional hot and cold fluids, the formation of a free surface, and an increase in boiling in the reservoir.
Conceptual models form the basis of quantitative modeling, but some qualitative inferences can be made directly. Chapter 3 considers some quantitative models and different approaches to simplifying real situations. The two dominant approaches are pressure-transient and lumped-parameter models. Linking them are the concepts of flow transmission of fluid and heat and storage the ability of the reservoir to store fluid in response to pressure change.
After discussing homogeneous porous media, possible differences introduced by fractured media are considered. Beginning with Chapter 4, the book follows the reservoir engineering work that would be performed as part of a field development in roughly chronological order. Chapter 4 discusses the principles of interpreting measurements in geothermal wells and the sometimes complex relationship between measurements in a well and the physical state in the reservoir.
Chapter 5 covers downhole PTS instruments and their limitations. Chapter 6 covers measurements during drilling, which provide information about pressure and sometimes temperature in formations above the reservoir. Chapter 7 covers the measurements made at well completion, with the objectives being defining the model of the well, the depth and magnitude of its permeable zones, and the reservoir conditions at each zone. Measurements during warmup confirm or refine these interpretations.
Methods for initiating and measuring discharge are discussed. A well may discharge various fluids at the wellhead: liquid water, dry steam, or a two-phase mixture, with the last being the most common. Single-phase flows can be measured by orifice plates or weirs, and two-phase flows are measured by separator, lip pressure and weir, or tracer dilution.
From the variation of mass flow and enthalpy with wellhead pressure, it is possible to make inferences about the reservoir fluid, well permeability, and the state of the well. The calculation of wellhead pressure under flowing conditions is described for single-phase flows, and wellbore simulations are used to model two-phase flows. Chapter 9 is a case study of well BR2 at Ohaaki.
It has a year history, from its first drilling in to its abandonment in During this period it exhibited a wide variety of behaviors. Chapters 10—13 turn from properties of the individual well to reservoir-scale properties, with the end objective being the formulation of a conceptual and numerical model of the reservoir and often simplified models of some aspects of reservoir behaviors. Chapter 10 discusses the quantitative and qualitative inferences that can be made from downhole data.
The mapped distribution of pressure, temperature, or chemical composition supplies information about the reservoir structure. Chapter 11 discusses reservoir simulation and the role of the reservoir engineer in the simulation, from the production of detailed well data to the review of results.
It is stressed that a simulation should be calibrated against as wide a set of data as possible. Additional types of data provide more constraints on possible model structure and improve model quality. All have been exploited long enough to show significant changes. Wairakei shows a field being developed from little initial knowledge about geothermal to long-term operation and a sophisticated simulation. Awibengkok shows successful development of a new resource.
Svartsengi shows development and exploration. Balcova-Narlidere is a well-documented low-temperature field. Palinpinon is a field with initially severe injection returns that was converted into a long-term operation.
Mak-Ban is a very high-quality resource successfully developed without major problems. The unexploited Patuha field, a distinctive hybrid between vapor-dominated and liquid dominated, is also summarised. Chapter 13 covers field management with simple lumped-parameter models or decline models, deposition, tracer testing, and injection management.
Chapter 14 covers well stimulation and EGS. The first stimulation method, which is often overlooked, is thermal stimulation by injecting cold water and is the most cost-effective. Acid stimulation provides a means of mitigating deposition problems.
Well stimulation of deep sedimentary aquifers is now practiced in many places. Finally, a true EGS project is outlined: the creation of permeability and a reservoir in hot rock with little permeability, and the results to date are reviewed. Four appendices review the pressure transient theory, gas corrections for output testing, the equations of state and flow in porous media, and provide a glossary of field names referred to in the book.
The geothermal literature is now so extensive that a comprehensive survey has not been attempted. Examples have been chosen and field experience quoted where it has given a useful illustration, but these examples are simply those that were appropriate and conveniently available.
Similarly the References are the works that have been cited in the text, but they are not meant to be a comprehensive coverage of the literature. The text does not pretend to be a representative survey of the current literature. Where possible, references have been used that are readily available on the Web and, for preference, those that are available at no cost.
Equations are written in SI units. Pressures are absolute unless gauge is specified. Casing and wellbore sizes are conventionally given in inches, and inches are used in the text, but calculations use meters. The SI unit for permeability is the meter squared. The terms water and steam are used to refer to the liquid and vapor phases of water substance. Where gas content is significant, liquid and vapor are used for the liquid and vapor stages of the water-gas mixture, and water and steam refer to that part of the respective phase that is water substance.
This chapter discusses fluid distribution, pressure, and temperatures that can occur in geothermal reservoirs. These concepts provide a background for the following chapters, which describe the testing of wells and reservoirs, the construction of a conceptual model of the resource, and then a quantitative model. Much of the following discussion relates to unexploited fields in which the thermodynamic state of the field is determined by the natural processes of heat and fluid transport.
This is because the results of testing done early in a new field development are the most challenging to interpret and because the natural state of the field influences its subsequent response to exploitation.
As a geothermal field is developed and information from more wells, along with the system response to production and injection, becomes available, the conceptual model and the numerical simulation model can be further refined. Anyone who has ever watched a geyser in action or a hot pool bubbling and wondered where the water and heat came from has some sort of mental picture of this small part of a geothermal system.
In the scientific arena, such mental models, which are based on a range of data from various disciplines, together with experience in related research, form the basis for the development of conceptual models that should bring together all the available information into a single coherent model.
However, since a model should be consistent with all the observable aspects of the system, they should all give very similar basic results. These models will vary among individuals, depending on their need.
For example, the fine detail that is essential in a model of flow around a particular well will become insignificant as the scale increases to include the complete geothermal resource. Over almost the entire surface of the Earth is a flux of heat through the crust upward to the ground surface.
This heat is transported to the surface by conduction through the crustal rocks. One means of prospecting for geothermal reservoirs that is not evident from surface discharge of geothermal fluids blind systems is identifying areas of anomalous heat flow by measuring temperature gradient in wells—either shallow temperature gradient wells or existing deep oil, gas, or groundwater wells.
A region of potential geothermal resource may be associated with high heat flow. The near surface temperature gradients can be extrapolated through impermeable strata to obtain deep temperatures.
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