30079_50.pdf

CHAPTER 50

GEOTHERMAL RESOURCES:

AN INTRODUCTION

Peter D. Blair

Sigma Xi

The Scientific Research Society

Research Triangle Park, North Carolina

50.1 INTRODUCTON

15 83

50.4 GEOPRESSURED

RESOURCES

15 85

50.2 HYDROTHERMAL

RESOURCES

15 84

50.5 GEOTHERMAL ENERGY

CONVERSION 15 87

50.5.1 Direct Steam Conversion 1587

50.5.2 Flashed Steam Conversion 1588

50.5.3 Binary Cycle Conversion 1588

50.5.4 Hybrid Fossil/Geothermal

Plants

50.2.1 Vapor-Dominated

Resources

15 85

50.2.2 Liquid-Dominated

Resources

15 85

15 90

50.3 HOT DRY ROCK

RESOURCES

15 85

50.1 INTRODUCTON

Geothermal energy is the internal heat of the Earth. For centuries, geothermal energy was apparent

only through anomalies in the Earth's crust that allow the heat from the Earth's molten core to

venture close to the Earth's surface. Volcanoes, geysers, fumaroles, and hot springs are the most

visible surface manifestations of these anomalies.

Geothermal energy has been used for centuries where it is available for aquaculture, greenhousing,

industrial process heat, and space heating. It was first used for electricity production in 1904 in

Lardarello, Italy.

Geothermal resources are traditionally divided into three basic classes:

1. Hydrothermal convection systems, including both vapor-dominated and liquid-dominated

systems

2. Hot igneous resources, including hot dry rock and magma systems

3. Conduction-dominated resources, including geopressured and radiogenic resources

The three basic resource categories are distinguished by geologic characteristics and the manner

in which heat is transferred to the Earth's surface (see Table 50.1). The following includes a discussion

of the characteristics and location of these resource categories in the United States. Only the first of

these resource types, hydrothermal resources, is commercially exploited today in the United States.

In 1975, the U.S. Geological Survey completed a national assessment of geothermal resources in

the United States and published the results in USGS Circular 726 (subsequently updated in 1978 as

Circular 790). This assessment defined a "geothermal resource base" for the United States based on

geological estimates of all stored heat in the earth above 15°C and within six miles of the surface,

ignoring recoverability. In addition, these resources were catalogued according to the classes given

in Table 50.1. The end result is a set of 108 known geothermal resource areas (KGRAs) encompassing

over three million acres in the 11 western states. Since the 1970s, many of these KGRAs have been

explored extensively and some developed commercially for electric power production.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Table 50.1 Geothermal Resource Classification

Temperature

Resource Type

Characteristics

1. Hydrothermal convection resource (heat carried upward

from depth by convection of water or steam)

a. Vapor-dominated

About 240°C (464°F)

b. Hot-water dominated

1. High temperature

150-350°O (300-660°F)

2. Intermediate temperature

90-150°C (290-300°F)

3. Low temperature

Less than 90°C (290°F)

2. Hot igneous resources (rock intruded in molten form from

depth)

a. Part still molten—"magma systems"

Higher than 650°C (1200°F)

b. Not molten—"hot dry rock" systems

90-650°F (190-1200°F)

3. Conduction-dominated resources (heat carried upward by

conduction through rock)

a. Radiogenic (heat generated by radioactive decay)

30-150°C (86-300°F)

b. Sedimentary basins (hot fluid in sedimentary rocks)

30-150°C (86-300°F)

c. Geopressured (hot fluid under high pressure)

150-200°C (300-390°F)

50.2 HYDROTHERMAL RESOURCES

Hydrothermal convection systems are formed when underground reservoirs carry the Earth's heat

toward the surface by convective circulation of water (liquid-dominated resources) or steam (vapor-

dominated resources). There are only seven known vapor-dominated resources in the world today,

three of which are located in the United States: The Geysers and Mount Lassen in California and

the Mud Volcano system in Yellowstone National Park. The remaining U.S. resources are liquid-

dominated (see Fig. 50.1).

VAPOR-DOMINATED

Mud Volcano Area

Lassen

The Geysers

LIQUID-DOMINATED

Raft River

Cove Fort-Sulphurdale

Roosevelt

Valles Caldera

Long Valley

Coso Hot Springs

Salton Sea

Niland

Heber

Brawley

East Mesa

Fig. 50.1 Known major U.S. geothermal resource areas.

50.2.1 Vapor-Dominated Resources

In a vapor-dominated hydrothermal system, boiling of deep subsurface water produces water vapor,

which is also often superheated by the hot surrounding rock. Geologists speculate that as the vapor

moves toward the surface, a level of cooler near-surface rock may induce condensation, which, along

with the cooler groundwater from the margins of the reservoir, serves to recharge the reservoir. Since

fluid convection is taking place constantly, the temperature in the vapor-filled area of the reservoir is

relatively uniform and a well drilled in this region will yield high-quality superheated steam.

The most developed geothermal resource in the world is The Geysers in northern California,

which is a high-quality, vapor-dominated hydrothermal convection system. Currently, steam is pro-

duced from this resource at a depth of 5,000-10,000 feet and piped directly into turbine-generators

to produce electricity. Geothermal power production capacity at The Geysers peaked in 1987 at over

2,000 mW, but since then has declined to about 1,800 mW.

Commercially produced vapor-dominated systems at The Geysers, Lardarello (Italy), and Mat-

sukawa (Japan) are all characterized by reservoir temperatures in excess of 450°F. Accompanying

the water vapor are small concentrations (i.e., less than 5%) of noncondensible gases (mostly carbon

dioxide, hydrogen sulfide, and ammonia). The Mont Amiata field (Italy) is a different type of vapor-

dominated resource, which is characterized by lower temperatures than The Geysers-type resource

and by much higher gas content (hydrogen sulfide and carbon dioxide). The geology of this category

of vapor-dominated resource is not yet well understood, but may turn out to be more common than

The Geysers-type resource because its existence is much more difficult to detect.

50.2.2 Liquid-Dominated Resources

Hot-water or wet-steam hydrothermal resources are much more commonly found than dry-steam

deposits. Hot-water systems are often associated with a hot spring that discharges at the surface.

When wet steam deposits occur at considerable depths, the resource temperature is often well above

the normal boiling point of water at atmospheric pressures. These temperatures are known to range

from 100-700°F at pressures of 50-150 psig. When such resources penetrate to the surface, either

through wells or through natural geologic anomalies, the water often flashes into steam.

The types of impurities found in wet-steam deposits vary dramatically. Commonly found dissolved

salts and minerals include sodium, potassium, lithium, chlorides, sulfates, borates, bicarbonates, and

silica. Salinity concentrations can vary from thousands to hundreds of thousands of parts per million.

The Wairakei (New Zealand) and Cerro Prieto (Mexico) fields are examples of currently exploited

liquid-dominated fields. In the United States, many of the liquid-dominated systems that have been

identified are either being developed or are being considered for development (see Fig. 50.1).

50.3 HOT DRY ROCK RESOURCES

In some areas of the western United States, geologic anomalies such as tectonic plate movement and

volcanic activity have created pockets of impermeable rocks covering a magma chamber within six

miles of the surface. The temperature in these pockets increases with depth and proximity to the

magma chamber, but, because of their impermeable nature, they lack a water aquifer. They are often

referred to as hot dry rock (HDR) deposits. Several schemes for useful energy production from HDR

resources have been proposed, but all basically involve creation of an artificial aquifer will be used

to bring heat to the surface. The concept is being tested by the U.S. Department of Energy at Fenton

Hill near Los Alamos, New Mexico, and is also being studied in England. The research so far

indicates that it is technologically feasible to fracture a hot impermeable system though hydraulic

fracturing from a deep well.

A typical two-well HDR system is shown in Fig. 50.2. Water is injected at high pressure through

the first well to the reservoir and returns to the surface through the second well at approximately the

temperature of the reservoir. The water (steam) is used to generate electric power and is then recir-

culated through the first well. The critical parameters affecting the ultimate commercial feasibility of

HDR resources are the geothermal gradient and the achievable well flow rate.

50.4 GEOPRESSURED RESOURCES

Near the Gulf Coast of the United States are a number of deep sedimentary basins that are geolog-

ically very young, that is, less than 60 million years. In such regions, fluid located in subsurface rock

formations carry a part of the overburden load, thereby increasing the pressure within the formation.

Such formations are referred to as geopressured and are judged by some geologists to be promising

sources of energy in the coming decades.

Geopressured basins exist in several areas within the United States, but those of current interest

are located in the Texas—Louisiana coast. These are of particular interest because they are very large

in terms of both areal extent and thickness, and the geopressured liquids appear to have a great deal

of dissolved methane. In past investigations of the Gulf Coast, a number of "geopressured fairways"

were identified; these are thick sandstone bodies expected to contain geopressured fluids of at least

Fig. 50.2 Hot dry rock power plant configuration.

300°F. Detailed studies of the fairways of the Frio Formation in East Texas were carried out in 1979,

although only one, Brazoria (see Fig. 50.3), met the requirements for further well testing. Within this

fairway, a particularly promising site known as the Austin Bayou Prospect (ABP) was identified as

an excellent candidate for a test well. This identification was based on the productive history of the

neighboring oil and gas wells, and on the gradient of increasing temperature, permeability, and other

resource characteristics.

If the water in these geopressured formations is also contained in insulating clay beds, as is the

case in the Gulf Coast, the normal heat flow of the Earth can raise the temperature of this water to

nearly 300°C. This water is typically of lower salinity than normal formations and, in many cases,

is saturated with large amounts of recoverable natural methane gas. Hence, recoverable energy exists

in geopressured formations in three forms: hydraulic pressure, heated water, and methane gas.

Fig. 50.3 Geopressured zones: Gulf of Mexico Basin.

The initial motivation for developing geopressured resources was to recover the entrained methane.

Hence, a critical resource parameter affecting the commercial potential of a given prospect is the

methane solubility, which is, in turn, a function of the geopressured reservoir's pressure, temperature,

and brine salinity. The commercial potential of a prospect is also a function of the estimated volume

of the reservoir, which dictates the amount of recoverable entrained methane; the "areal extent,"

which dictates how much methane can ultimately be recovered from the prospect site; and the "pay

thickness," which dictates the initial production rate and the rate of production decline over time.

50.5 GEOTHERMAL ENERGY CONVERSION

The appropriate technology for converting geothermal energy to electricity depends on the nature of

the resource. For vapor-dominated resources, it is possible to use direct steam conversion; for high-

quality liquid-dominated resources, flashed steam or binary cycle technologies can be employed; and

for lower quality liquid-dominated resources, a mixture of fossil and geothermal sources can be

employed.

50.5.1 Direct Steam Conversion

The geothermal resources of central Italy and The Geysers are, as noted earlier, "vapor-dominated"

resources, for which conversion of geothermal energy into electric energy is a straightforward process.

The naturally pressurized steam is piped from wells to a power plant, where it expands through a

turbine-generator to produce electric energy. The geothermal steam is supplied to the turbine directly,

save for the relatively simple removal of entrained solids in gravity separators or the removal of

noncondensible gases in degassing vessels. From the turbine, steam is exhausted to a condenser,

condensed to its liquid state, and pumped from the plant. Usually this condensate is reinjected to the

subterranean aquifer. Unfortunately, vapor-dominated geothermal resources occur infrequently in na-

ture. To date, electric power from natural dry steam occurs at only one area, Matsukawa in Japan,

other than central Italy and The Geysers.

A simplified flow diagram illustrating the direct steam conversion process is shown in Fig. 50.4.

The major components of such systems are the steam turbine-generator, condenser, and cooling

towers. Dry steam from the geothermal production well is expanded through the turbine, which drives

an electric generator. The wet steam exhausting from the turbine is condensed and the condensate is

piped from the plant for reinjection or other disposal. The cooling towers reject the waste heat released

by condensation to the atmosphere. Additional plant systems not shown in Fig. 50.4 remove entrained

solids from the steam prior to expansion and remove noncondensible gases from the condenser. The

most recent power plants at The Geysers also include systems to control the release of hydrogen

sulfide (a noncondensible gas contained in the steam) to the atmosphere.

Direct steam conversion is the most efficient type of geothermal electric power generation. One

measure of plant efficiency is the level of electricity generated per unit of geothermal fluid used. The

plants at The Geysers produce 50-55 Whr of electric energy per pound of 350° steam consumed. A

second measure of efficiency used for geothermal power plants is the geothermal resource utilization

efficiency, defined as the ratio of the net plant power output to the difference in thermodynamic

availability of the geothermal fluid entering the plant and that of the fluid at ambient conditions.

Plants at The Geysers operate at utilization efficiencies of 50-56%.

Release of hydrogen sulfide into the atmosphere is recognized as the most important environ-

mental issue associated with direct steam conversion plants at The Geysers. Control measures are

Fig. 50.4 Direct steam conversion.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: