Geothermal Reservoir Engineering

Malcolm Alister Grant, Paul F Bixley

• Publisher: Academic Press
• Year: 2011
• ISBN:
  9780123838810
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

As nations alike struggle to diversify and secure their power portfolios, geothermal energy, the essentially limitless heat emanating from the earth itself, is being harnessed at an unprecedented rate.

For the last 25 years, engineers around the world tasked with taming this raw power have used Geothermal Reservoir Engineering as both a training manual and a professional reference.

This long-awaited second edition of Geothermal Reservoir Engineering is a practical guide to the issues and tasks geothermal engineers encounter in the course of their daily jobs. The book focuses particularly on the evaluation of potential sites and provides detailed guidance on the field management of the power plants built on them.

With over 100 pages of new material informed by the breakthroughs of the last 25 years, Geothermal Reservoir Engineering remains the only training tool and professional reference dedicated to advising both new and experienced geothermal reservoir engineers.

• The only resource available to help geothermal professionals make smart choices in field site selection and reservoir management
• Practical focus eschews theory and basics- getting right to the heart of the important issues encountered in the field
• Updates include coverage of advances in EGS (enhanced geothermal systems), well stimulation, well modeling, extensive field histories and preparing data for reservoir simulation
• Case studies provide cautionary tales and best practices that can only be imparted by a seasoned expert

Chapter 1

Geothermal Reservoirs

Chapter Outline

1.1. Introduction

1.2. The Development of Geothermal Reservoir Engineering

1.3. Definitions

1.4. Organization of this Book

1.5. References and Units

1.1 Introduction

This book examines the flow of fluid underground, how geothermal reservoirs came to exist, the reservoirs’ characteristics, and how they change with development. In essence, the historic flow of fluid created reservoirs, and the modification of this flow by exploitation is the basis for the science of geothermal reservoir engineering.

Geothermal resources have been used for cultural purposes and mineral extraction for the last 2000 years. The first modern “deep” drilling (>100 meters) to investigate the deeper resource commenced at Larderello in 1856, and the first power generation began almost 50 years later in 1904 (Cataldi et al., 1999). Relative to petroleum or groundwater resources, the development of geothermal resources that followed was slow. The first “real” power development, of 250 kWe at Larderello commenced in 1913, building up to more than 100 MWe by the 1940s. This was done with a steam-dominated resource. It was not until 1958 that electricity was first generated in significant amounts from a high-temperature liquid geothermal resource at Wairakei. Since that time, the development of steam- and liquid-dominated resources for power production has begun in many countries worldwide, with a total installed capacity exceeding 10,000 MWe in 2010.

In both groundwater and petroleum resources, scientific research went along with expanding exploitation. At first this research was primarily directed toward prospecting to find and extract the fluid. As the effects of fluid extraction on the underground resource became apparent, research into the behaviors of wells and underground reservoirs to understand these changes and learn about the underground resource expanded. The first text on the flow of fluids through porous media was published in 1937 (Muskat, 1937), and by the 1940s, groundwater hydrology and petroleum reservoir engineering were scientific disciplines in their own right. Since then, the continuing rapid development in exploiting these resources has led to a significant increase in field experience and research that has resulted in the development of a large and sophisticated reservoir engineering industry focused on the analysis and prediction of subsurface reservoir and well performance.

In comparison, geothermal reservoir engineering is a much smaller industry, with a correspondingly smaller professional workforce, but it too has now accumulated experience to form a specialized profession.

1.2 The Development of Geothermal Reservoir Engineering

Research efforts related to geothermal systems and their exploitation have followed a pattern similar to that for groundwater and petroleum reservoirs. The exception, of course, is that because of its later start, the high-precision tools and computers that were not available in the early days of groundwater and petroleum exploration have naturally played a larger role in the development of geothermal reservoir engineering. Without the sophisticated exploration methods now available to locate potential underground geothermal resources, during the early geothermal developments of the 1950s and 1960s, most exploration wells were drilled into areas defined by the discharge of steam and/or hot water and associated surface thermal activity. The initial geothermal developments were relatively small, with the result that the resource was not stressed, and a reliable understanding of reservoir behavior or for geothermal reservoir engineering was not required to predict future behavior.

This does not mean that no scientific work was carried out. Early ideas of subsurface flow associated with geothermal discharges in Iceland were put forward by Bunsen in the 1840s (Björnsson, 2005), Von Knebel (1906), and Thorkelsson (1910) (Einarsson, 1942). Conduction of heat away from an isolated magmatic intrusion was discussed at about the same time by Ingersoll and Zobel (1913). Research in this area was limited, and publications relating directly to geothermal phenomena were intermittent.

Hot springs have been used for millennia around the world for bathing, cooking, and hydrotherapy, and mineral deposits associated with the surface discharges have been exploited at least since the nineteenth century (Cataldi et al., 1999). The earliest exploitation for electrical energy was the use of geothermal steam at Larderello, Italy, starting in 1904. The progressive development of Larderello during the first half of the twentieth century gave practical experience in handling geothermal steam for power generation but produced little in the way of subsurface reservoir engineering technology. In Iceland, Einarsson (1942) developed the idea of deep circulation as the mechanism supplying surface discharges of geothermal fluid, and Bodvarsson (1951) began defining the heat transfer problems associated with geothermal exploitation. With the initiation of drilling in Wairakei, New Zealand, in the early 1950s, the first substantial amount of subsurface data from a liquid-dominated reservoir became available.

Two approaches to geothermal reservoir assessment developed. The first was to map the reservoir, collecting as much information as possible and using this to define the physical properties of the subsurface object that was being explored. The second approach was to investigate the processes that might be occurring underground in order to see what roles they might play in the reservoir being exploited. In practice these two approaches have continued together in many geothermal fields that have been developed since that time.

During the 1950s at Wairakei, the first approach led to the mapping of subsurface temperatures across the field (Banwell, 1957) and, from these maps, to inferences about the pattern of fluid flow in the reservoir. The second approach suggested the presence of thermal convection due to heat at depth and to theoretical studies of large-scale convection systems in porous media (Wooding, 1957, 1963) and the first numerical modeling (Donaldson, 1962). In a similar vein, studies at Steamboat Springs, Nevada (White, 1957), and Iceland (Bödvarsson, 1964) led to an improved understanding of how cold meteoric water may circulate to a certain depth and flow up to the surface to charge a geothermal field.

More detailed analyses of the form now prominent in geothermal reservoir engineering slowly developed. Pressure transient analyses were applied sporadically in most areas of early geothermal exploration (see, for example, de Anda et al., 1961). In the 1960s, systematic analyses were made of fields in Iceland and Kamchatka (Thorsteinsson & Eliasson 1970; Sugrobov, 1970). Later in the decade, the first attempts to apply petroleum reservoir engineering were made (Whiting & Ramey, 1969; Ramey, 1970).

By the mid-1960s, there was considerable geothermal exploration and development in progress around the world. The first power plants were commissioned in The Geysers in California, and extensive study programs were started in Mexico, Chile, Turkey, El Salvador, Japan, and various fields in the Imperial Valley of California. Data from a range of different fields were being produced and some of it published, and more problems were encountered and analyzed.

The phrase “geothermal reservoir engineering” first appeared in the 1970s, and the area emerged as a distinct discipline. During this decade, scientific effort moved away from theoretical studies of what processes might possibly be important in the reservoir to practical analyses driven by the data now becoming available and the actual problems experienced in development. Coherent conceptual models of reservoirs were developed, consistent with both the large-scale system hosting the reservoir and the local detail determined from well testing. Field developments were normally sized on the basis of volumetric reserve estimates of some form or sometimes on the available well flow alone.

At the beginning of the 1980s, the first numerical simulation codes were developed, and a trial in which several codes were used to simulate a set of test problems demonstrated their consistency (Sorey, 1980). During the 1980s and 1990s, reservoir simulators became more capable, and increasing computing power meant that by the 1990s, it was reasonable to simulate a reservoir with enough blocks to be able to represent known geological structures and varying rock properties within the reservoir. As a result, by the 1990s, larger new developments were being sized on the basis of simulation results (see the discussion of Awibengkok in Chapter 12).

At the same time, downhole instruments were steadily improving in capability and resolution, making detailed temperature-pressure-spinner profiles possible. More important than either simulation or better instruments was the accumulation of collective experience within the profession. Although the basic concepts were all developed by the 1970s, and the fact that many recent technical papers are superficially similar to some from that time, the weight of experience means that these concepts are now being applied more rigorously and more consistently with observation. In some aspects of the geothermal reservoir engineer’s work, it is now possible to refer to normal practice to define the procedures and expected results.

In the first decade of the twenty-first century, experience, simulation, and instrumentation have all continued to improve. Perhaps the most significant change has been the increasing importance of environmental impacts of development. Seismic effects have become a limiting factor on some Engineered Geothermal System (EGS) projects (Glanz, 2010). 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.

2. The reservoir extends a great distance vertically.

3. For liquid-dominated reservoirs a caprock is not essential, and usually the high-temperature reservoir has some communication with surrounding cool groundwater.

4. 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.

1.3 Definitions

Because many different terms are used when discussing geothermal systems (or sections or groupings of such systems), a nomenclature has been selected here that is followed throughout the book. 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.

1.4 Organization of this Book

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 th...