GEOTHERMAL ENERGY RESOURCES

GEOTHERMAL ENERGY RESOURCES The energy beneath our feet…

The Earth stores a vast amount of energy in form of heat generated majorly by the decay of

radioactive isotopes and partly by the hot core of the Earth. Geothermal energy resources

consists of this thermal energy from the Earth’s interior stored in hot dry rocks as well as

rocks with trapped steam or liquid water. This work treats geothermal energy resources as

renewable, alternative and sustainable energy resources with capabilities of generating

electricity and heat for direct use applications. It concludes with a brief discussion on the

potentials of geothermal energy resources in Nigeria especially in hot regions with surface

manifestations of geothermal energy resources such as the Wikki and Ikogosi warm springs

in Ekiti and Bauchi state respectively.

Aja, Anthony U.

2013

Geothermal Energy Resources Page 1

Aja Anthony Ugochukwu

DEDICATION

To the almighty God the giver of all knowledge: for his guidance throughout this work.


ACKNOWLEDGEMENT

I must sincerely thank my family for their support during the course of this work, the Head of

Department of Geology and Exploration Geophysics, Dr. P.N. Nnabo for supervising this

work, as well as friends and co-students for their contributions and support. I am grateful also

to all authors whose books contributed to the success of this work. God bless you all.


ABSTRACT

The Earth stores a vast amount of energy in form of heat generated majorly by the decay of

radioactive isotopes and partly by the hot core of the Earth. Geothermal energy resources

consists of this thermal energy from the Earth’s interior stored in hot dry rocks as well as

rocks with trapped steam or liquid water. Energy is obtained by drilling wells into geothermal

reservoirs to tap hot water or steam using similar technologies as used in oil and gas

exploiting industries. Geothermal energy resource definitions and applied technology as

described in this context are primarily from resource assessments and characterization studies

conducted in the development of the resources. Classification systems used for classifying

resources are based mainly on their temperatures, geologic settings and intrinsic properties of

reservoirs. This work treats geothermal energy resources as renewable, alternative and

sustainable energy resources with capabilities of generating electricity and heat for direct use

applications. It concludes with a brief discussion on the potentials of geothermal energy

resources in Nigeria especially in hot regions with surface manifestations of geothermal

energy resources such as the Wikki and Ikogosi warm springs in Ekiti and Bauchi state

respectively. The Benue Trough, though, without such surficial manifestations of geothermal

energy resources also has potential thermal regions for the production of medium to high

geothermal energy resources.


Contents CHAPTER ONE

...................................................................................................................................... 8 1.1 Basics

............................................................................................................... 9 1.2 Geothermal Gradient

......................................................................................................... 10 1.3 Source of The Earth’s Heat

........................................................................................................................................ 10 1.4 History

................................................................................... 11 1.5 Characteristics of Geothermal Resources

CHAPTER TWO

......................................................................................................................... 12 2.1 OVERVIEW

................. 12 2.2 CURRENT GEOTHERMAL RESOURCE ASSESSMENT TERMINOLOGY

........................................................................................... 14 2.3 Geothermal Resource Classification

CHAPTER THREE

....................................................................................................... 19 3.1 Exploration and Drilling

.......................................................................................................... 20 3.2 Geothermal Reservoirs

........................................................................................... 20 3.2.1 Types of Geothermal Reservoirs

........................................................................................................... 21 3.2.2 Reservoir Engineering

.......................................................................................................................... 21 3.3 Power Plants

....................................................................................... 24 3.4 Enhanced Geothermal Systems (EGS)

.......................................................................................................... 25 3.5 Direct Use Application

CHAPTER FOUR

......................................................................................... 28 4.1 Direct Greenhouse Gas Emission

............................................................................................... 28 4.2 Local Environmental Impacts

CHAPTER FIVE

........................................................................................................... 30 5.1 Occurrence in Nigeria

....................................................................................................... 31 5.2 Importance in Nigeria

............................................................... 31 5.3 Limitations to resource development in Nigeria

CHAPTER SIX

............................................................................................................................... 32 6.1 Summary

........................................................................................................................... 32 6.2 Conclusions

................................................................................................................. 33 6.3 Recommendations

REFERENCES ……………………………………………………………………………………………………………………..35


LIST OF FIGURES

Figure 1.1: The Earth’s interior - - - - - - 1

Figure 1.2: Temperatures in the Earth- - - - - - 3

Figure 2.1: Geothermal resource and reserve terminologies - - 6

Figure 2.2: An amagmatic geothermal system - - - - 8

Figure 2.3: A magmatic geothermal system - - - - - 9

Figure 3.1: Formation of a conventional geothermal reservoir - - 13

Figure 3.2: Diagram of a flash power plant - - - - 15

Figure 3.3: The Geysers California dry steam plant - - - 16

Figure 3.4: Diagram of a dry steam plant - - - - - 16

Figure 3.5: Diagram of a Flash/Binary power plant - - - 17

Figure 3.6: Enhanced geothermal system - - - - - 18

Figure 3.7: Direct use geothermal heating system configuration - - 19

Figure 3.8: Geothermal heat pumps - - - - - - 20


LIST OF TABLES

Table 2.1: Classification of Geothermal Resources by Temperature - - 9

Table: 2.2: Types of Geothermal Resources and Their Uses- - - 13


CHAPTER ONE

INTRODUCTION

1.1 Basics

The Earth possesses vast underground stores of energy supply in form of heat, whose

potentials are yet to be fully realized. The Earth is divided into shells; the crust, the mantle

and the core. Its interiors where this heat energy occurs, reach temperatures greater than

4,000oC (>7,232

oF). This heat generated and stored in the Earth is known as Geothermal

Energy.

Fig 1.1: Diagram showing the Earth’s interior (Blodgett and Slack, 2009)

The word geothermal comes from the Greek words geo (Earth) and therme (heat).

Geothermal energy is generated from the Earth’s core, almost 6,400km beneath the Earth’s

surface. The double-layered core is made up of very hot melted rock (magma) surrounding a

solid iron center. Very high temperatures are continuously produced inside the Earth by the

slow decay of radioactive materials, a natural process in all rocks. Surrounding the outer core

is the mantle, which is about 2,900km thick and made up of magma and rock. The outermost

layer of the Earth, the land forming the continents and ocean floors, is called the crust. The

crust is not a solid plate; it is divided or broken into pieces called plates. Magma from deep

within the Earth gets to the Earth’s surface through openings at the edges of these plates.

Volcanic processes are very common at these edges. Deep underground, rocks and water

trapped in the Earth absorb heat from this magma.


Geothermal Energy Resources consists of this thermal energy from the Earth’s

interior stored in both rocks and trapped steam or liquid water. They are typically

underground reservoirs of hot water or steam created by heat from the Earth, but also

includes the subsurface areas of dry rock.

A geothermal system should include fundamental elements and processes, such as

fluid and heat sources, fluid flow pathways, and a cap rock or seal, which are necessary for

the formation of a geothermal resource. In some cases, the heated water or steam produced

get to the surface of the Earth as hot water bodies, mostly through fractures. In instances

where the water remains trapped beneath the Earth, wells are dug and the heated underground

water is pumped to the surface. Geothermal energy, the energy produced from this heated

water is a clean, sustainable energy. It is also a renewable energy as depleted heat and water

can be replenished from Earth’s processes.

Geothermal systems are currently exploited in a number of geological environments

where the temperatures and depths vary accordingly. Energy produced from them can be

used in a range of applications; from electric power generation to commercial, industrial and

residential direct heating purposes, and for efficient home heating and cooling through

geothermal heat pumps (GHPs).

1.2 Geothermal Gradient

The temperature of the Earth increases as we go deeper into the Earth’s interior. This

rate of increase in temperature with respect to increase in depth is termed the Geothermal

Gradient of the Earth. Away from tectonic plate boundaries and regions with thermal

anomalies, it is about 25oC per km depth (1

oF per 70 feet depth) in most parts of the world.

The geothermal gradient is a very important factor in the search for geothermal resources.

Resources are usually explored for in areas with unusually higher geothermal gradients (such

as regions around plate boundaries or with active volcanoes) as they have potentials of

producing high-temperature resources.


Fig 1.2: Diagram showing the temperatures within the Earth (Blodgett and Slack, 2009).

1.3 Source of The Earth’s Heat

The Earth’s internal heat comes from a combination of residual heat from planetary

accretion and heat produced through the radioactive decay of particles. The major heat-

producing isotopes in the Earth are Potassium-40, Uranium-238, Uranium-235, and Thorium-

232. It is widely agreed that much of the heat produced in the Earth is provided by

radioactive decay. Generally, geothermal heating is caused by this continuous decaying of

radioactive particles, and the convection and conduction of heat from the very hot inner core

and mantle.

1.4 History

Geothermal energy is not really a new energy resource as is thought. In fact,

geothermal energy has been in use at least since the Paleolithic times. An example is the

oldest known spa, a stone pool in China’s Lisan Mountain built in the Qin dynasty in the 3rd

century BC. Recent technologies only expanded the range of use of geothermal energy in

applications such as electric power generation, space heating, cooling and heating through

heat pumps etc.

The earliest industrial exploitation began in 1827 with the use of geyser steam to

extract boric acid from volcanic mud in Lardello, Italy. In the 20th

century, the demand for

electricity led to the consideration of geothermal power as an electric generating source. The


first geothermal power generator was tested by Prince Piero Ginori on the 4th

of July 1904, at

the same Lardello dry steam field where geothermal acid extraction began (Bertani, 2008).

Further researches in the subject led to the development of new technologies in the

use of this resource. The most recent is the production of electricity from a record low fluid

temperature of 57oC (135

oF). A continuous research on this field as well as development of

new technologies, can further expand the range of applications than what we have today, and

reduce the harmful effects of utilizing this energy resource.

1.5 Characteristics of Geothermal Resources

Some visible features of geothermal energy resources are volcanoes, hot springs,

geysers, and fumaroles. However, most geothermal energy resources cannot be seen as they

are deep underground. The surface manifestation of a geothermal resource existing

underground gives the geologists clues when searching for the resources. When there are no

clues above ground that a geothermal resource is present below, wells are drilled and their

temperatures tested to be sure there is a reservoir. The most active resources are found along

major plate boundaries where Earthquakes and volcanoes are concentrated. This is practically

true, as it is observed that most geothermal activities in the world occur in a region that

borders the Pacific Ocean, known as the Ring of Fire.


CHAPTER TWO

CLASSIFICATION OF GEOTHERMAL RESOURCES

2.1 OVERVIEW

Resource classification is a key element in characterization, assessment and

development of energy resources (Colin et al, 2011). Classifying energy resources is then

important to be able to use and understand consistent terminologies in addressing geothermal

issues such as location, quality and feasibility of development and potential impacts.

For this terminology to be accepted, it must encompass both the fundamental

geological nature of geothermal resources and the practical technological and economic

aspects of resource exploitation, while remaining understandable to the broad community of

non-specialists (Colin et al, 2011).

Geothermal energy resources have been characterized thus, by geologic settings,

intrinsic properties, and viability for commercial utilization. The diversity of both the nature

and exploitation of geothermal resources pose a great challenge in the context of resource

classification.

Classification of this resource has been the feature of many investigations, and until now, no

one approach for classification has been accepted as a basic framework for classification.

However, with technical changes, a recent classification based on more detailed

understanding of geologic and tectonic processes and the potential development of new types

of resources have been adopted by major geothermal industries. This classification is the

basic framework for characterization developed by Muffler and Cataldi in 1978. It is

foundational to resource assessments.

2.2 CURRENT GEOTHERMAL RESOURCE ASSESSMENT TERMINOLOGY

This follows studies by Muffler and Cataldi (1978) for the subdivision of geothermal

resource base. The subdivisions are easily illustrated through a modified McKelvey diagram

(Fig 2.1).


Fig 2.1: Geothermal Resource and Reserve Terminologies, (O’sullivan et al, 2008).

From the diagram, the criteria for classification are; the degree of geologic assurance

regarding resources and the economic/ technological feasibility (often equivalent to depth).

The criteria are also used by geologists or geothermal industries that consider both identified

and unidentified resources.

The geothermal resource base is all of the thermal energy existing beneath the ground in a

specific area, measured from the local mean annual temperature.

The geothermal resource is that fraction of the geothermal resource base at depths shallow

enough to be tapped by drilling, that can be recovered as useful heat economically and legally

at some reasonable future time.

The geothermal reserve is the identified portion of the resource that can be recovered

economically and legally at the present time using existing technology.


2.3 Geothermal Resource Classification

In this context, geothermal resources will be classified by the intrinsic properties of

the resources, the geologic settings where the resources are found, and the technology applied

for resource exploitation.

The three bases for classification are discussed below.

1. Temperature.

Geothermal energy has a lot to do with temperature. The exploration, identification and

utilization of geothermal energy all depends on the temperature of the resources. It is the

fundamental measure of the quality of the resources, and therefore the primary element of

most classification systems.

Different temperature classes exist; some dividing resources into two (high and low), and

others into three (high, low and intermediate) classes. In each case, the temperature

boundaries are set at temperatures significant in either a thermodynamic or an economic

utilization context. The temperature classes are then observed to define a progression of

resources from low to high temperature (or enthalpy) geothermal resources.

The classification approach proposed by Sanyal (2005), which focuses on thermal boundaries

of significance to the geothermal developer, is given below.

Table 2.1: Showing the classification of geothermal resources by temperature.

(Sanyal, 2005)

Temperature

(oC)

Resource Class Resource

Utilization

350

Ultra High

Power generation

Direct uses

300 High Power generation

Direct uses

250 Moderate Power generation

200 Low* Direct uses

Direct uses 150 Very low

100 – 50 Non-electrical


(Low*: with recent technology, geothermal resources with temperatures even lower than

180oC have been used to produce electricity).

2. Geologic Setting

A geothermal system requires basically; water (geothermal fluids), a permeable host rock,

and a heat source. Geologic controls on geothermal resources are therefore considered when

classifying the resources, as most or all the features of a geothermal system are controlled by

geologic processes. The geologic setting of a geothermal system has a fundamental influence

on the potential temperature, fluid composition and reservoir characteristics. Resources have

been classified into two in this context. They are the Amagmatic and Magmatic geothermal

resources.

i. Amagmatic Geothermal Resources:

This refers to resources obtained from environments that are not associated with

magmatic activities or processes. Heat is derived from amagmatic systems solely by deep

circulation. Temperatures of resources obtained from these systems range from 100oC to

260oC.

Fig 2.2: An Amagmatic Geothermal System showing a fault-hosted deep circulation,

(Sanyal, 2005).


ii. Magmatic Geothermal Resources

Magmatic geothermal resources are those related to or associated with magmatic activities.

These systems are observed in volcanic environments associated with recent magmatism.

This is illustrated in the diagram below, showing geothermal manifestations within an island-

arc volcano.

Fig 2.3: A Geothermal System Formed Within A Volcanic Arc Environment, (Sanyal,

2005).

Given the high temperature involved in such settings, it is expected to find heat

sources at much shallow depth in magmatic systems, compared to amagmatic systems.

Generally, resources obtained from magmatic geothermal systems are higher in temperature

and larger in volume than the deep circulation amagmatic systems. It is observed in this

category that geothermal resources are characterized in a geologic/ tectonic context.

3. Conductive Geothermal Resources and Enhanced Geothermal Systems

This classification is based on the characteristics of the reservoir, and the technology

utilized in exploiting the resources.

Resource classifications for sedimentary geothermal resources have been focused on

the methods of exploitation. Though there are diverse utilization scenarios for the resources,

there is consistency in the geologic and thermal environment as the resources are associated

with the predominantly conductive settings of sedimentary basins. Conductive systems


include hot rock and magma over a wide range of temperatures. Most processes, features and

production in such reservoirs are naturally-occurring.

By contrast, Enhanced Geothermal Systems cover essentially the entire range of

geothermal environments from reservoir creation in low permeability and porosity crystalline

rocks at depth, through high porosity-low permeability sedimentary rocks to augmented

production in a producing convective geothermal reservoir. They comprise that portion of a

geothermal resource for which a measurable increase in production over its natural state can

be attained through mechanical, thermal, and/or chemical stimulation of reservoir rock.

Enhanced Geothermal Systems would produce geothermal fluids of higher temperatures at

shallow depths when compared to Conductive Geothermal Resources.

Table 2.2: A general classification of the geothermal resource types and their uses,

(Goldstein et al, 2011).

Type In-situ

fluids Subtype

Temperature

Range

Utilization

Current Future

Convective

systems

(hydrothermal)

Yes Continental

High,

intermediate,

low.

Power, direct use

Submarine High None Power

Conductive

systems No

Shallow(<400m) Low Direct use (GHP)

Hot rock High, low prototypes Power, direct

use

Magma bodies High None Power, direct

use

Deep aquifer

systems Yes

Hydrostatic aquifer High,

intermediate &

low

Direct use Power, direct

use

Geo-pressured Direct use Power, direct

use

Convective (Hydrothermal Systems) include liquid-vapour-dominated types.


Deep aquifers contain circulating fluids in permeable media or fracture zones at

depths typically greater that 3km, but lack a localized magmatic source.

Geo-pressured resources are resources produced at systems with pressure higher than

hydrostatic pressure.


CHAPTER THREE

TECHNOLOGY AND APPLICATION OF GEOTHERMAL ENERGY RESOURCES

3.1 Exploration and Drilling

Geothermal exploration involves the examination of the subsurface in search of viable

active geothermal regions with the goal of building a geothermal power plant, where hot

fluids drive turbines to generate electricity, or for other thermal applications. Since

geothermal resources occur underground, exploration methods include a wide range of

disciplines including Geology, Geophysics, Geochemistry and Engineering. The objectives of

geothermal explorations are to identify and rank prospective geothermal reservoirs prior to

drilling, and to provide methods of characterizing reservoirs that enable the estimates of

geothermal reservoir performance and lifetime. Exploration of prospective geothermal

reservoirs involves estimation of its location, lateral extent and depth with geophysical

methods and then drilling exploration wells to test properties.

Hot spots are characterized by the following during geothermal explorations;

Heat source: shallow magmatic body, decaying radioactive elements or ambient heat

from higher pressures.

Reservoir: a collection of hot rocks from which heat can be drawn.

Geothermal fluid: gas, vapour and water found within the reservoir.

Recharge: an area surrounding the reservoir that rehydrates the geothermal system.

Geothermal wells are drilled over a range of depths down to 5km using methods similar

to those used for oil and gas. Advances in drilling technology have enabled high-temperature

operation and provide directional drilling capability. Wells are drilled from the same pad,

heading in different directions to access larger resource volumes, targeting permeable

structures and minimizing the surface impacts. For other geothermal applications such as

Geothermal Heat Pumps and direct use applications, smaller and more flexible rigs have been

developed to overcome accessibility limitations.


3.2 Geothermal Reservoirs

A geothermal system requires heat, permeability and water. The heat from the Earth’s

core continuously flows outward. Sometimes the heat, as magma, reaches the surface as lava,

but it usually remains below the Earth’s crust, heating nearby rocks and water. When water is

heated by the heat of the Earth, hot water or steam can be trapped in permeable and porous

rocks over a layer of impermeable rock and a geothermal reservoir can form. A geothermal

reservoir is a collection of heated water or steam trapped in permeable and porous rocks

(including fractured crystalline rocks) underlain by impermeable rock layers.

The reservoir system may be formed entirely by natural geologic processes or may be

adjusted artificially to meet this definition.

3.2.1 Types of Geothermal Reservoirs

There are two basic types of geothermal reservoirs. They are the conventional and

unconventional geothermal reservoirs.

1. Conventional Geothermal Reservoirs: They are hot, wet, porous, permeable and often

fractured. They are exploited by producing hot water or steam from the reservoir and

disposing off the depleted steam to the atmosphere or condensing and injecting it back

to the reservoir. Typical oilfield practices such as hydraulic fracturing are used to

enhance production, provided the temperature does not exceed the limits of available

technology.

Fig 3.1: Formation of a conventional geothermal reservoir, (O’sullivan,

2008)

2. Unconventional Geothermal Reservoirs: they are hot, dry, no porosity or

permeability, and no fractures. They are often called Enhanced (Or Engineered)


Geothermal Systems (EGS) or “hot-dry rock” reservoirs. They require hydraulic

fracturing and horizontal wells to obtain a flow path through which water can be

circulated in a closed loop.

3.2.2 Reservoir Engineering

Reservoir engineering efforts are focused on two main goals:

i. To determine the volume of geothermal resources and the optimal plant size.

ii. To ensure safe and efficient operation during the lifetime of the project.

According to O’Sullivan et al (2008), the modern method of estimating reserves and

sizing power plants is to apply reservoir simulation technology. First a conceptual model

using available data is built, and is then translated into a numerical representation, and

calibrated to the unexploited, initial thermodynamic state of the reservoir. Future behavior

can be forecast under selected load conditions using a heat and mass transfer algorithm, and

the optimum plant size is selected. Injection management is an important aspect of

geothermal development, where the use of isotropic and chemical tracers is common.

Depletion of production zones by injected water that has had insufficient contact with hot

reservoir rock can result in production declines.

Given sufficient and accurate calibration with field data, geothermal reservoir evolution

can be adequately modeled and proactively managed. Field operators monitor

thermodynamic and chemical properties of geothermal fluids, and map their flow and

movement in the reservoir. This information, combined with other geophysical data is fed

back to recalibrate models for better predictions of future production.

3.3 Power Plants

The basic types of geothermal power plants in use today are Steam condensing

turbines and binary cycle units.

Steam condensing turbines can be used in flash or dry-steam plants operating at sites

with temperature resources ≥ 150oC. Steam condensing turbines utilizing flash systems, flash

or separate fluids in a series of vessels at successively lower pressures, to maximize the

extraction of energy from the geothermal fluids. The only difference between a flash plant

and a dry-steam plant is that dry-steam plants do not require brine separation, resulting in a

simpler and cheaper design.


Binary cycle plants are commonly installed to extract heat from geothermal fluids

(generally from 70 to 170oC) from hydrothermal and Enhanced Geothermal System-type

reservoirs. Binary plants are more complex than condensing units since the geothermal fluids

pass through a heat exchanger heating another working fluid.

There are also combined or hybrid power plants, which comprises two or more of the basic

types, to improve efficiency and cover a wide resource temperature range.

The power plants in the binary and condensed unit-types are described below.

1. Flash Power Plant: Geothermally heated water under pressure is flashed or

separated in a surface vessel called a separator, into steam and hot water. The

steam is delivered to the turbines, which in turn powers a generator. The liquid is

injected back into the reservoir. An example is the Dixie Valley flash power plant

in Nevada.

Fig 3.2: Diagram of a flash power plant, (GEA Environmental Guide Report, 2009).

2. Dry Steam Power Plant: This plant is installed in areas where steam can be

produced directly from the geothermal reservoir. No separation is needed in this

system. Steam from geothermal reservoirs is delivered directly to run turbines that

power the generator. The used steam is condensed and injected back into the

reservoir as water.


The process involves a closed loop system to work properly. Used up steam is given

off into the atmosphere in some steam power plants, while they are reused in other to

maximize the benefits of the resources.

Steam power plants are usually installed around surface manifestations of underground

geothermal activities, such as geysers without affecting them.

Fig 3.3: The Geysers, California dry steam plant Fig 3.4: Diagram of a dry steam

plant (GEA Environmental Guide Report, 2009).

3. Binary Power Plant: Binary plants use an Organic Rankine Cycle system (ORC).

In this plant type, organic fluids such as Isobutane work together with the

geothermal fluid, but as a secondary fluid. The organic fluid (the secondary)

should boil at a lower temperature than the heated water (the geothermal fluid).

Geothermal water produced from the reservoir transfers its heat energy to the

organic fluid, while kept completely separate from it through the use of a heat

exchanger, expanding the organic fluid into gaseous vapour. It is the force of this

expanding vapour, like steam, that drives the turbines which power the generators.

All of the produced geothermal fluid is injected back into the reservoir. This technology has

made it possible for electricity generation from geothermal resources lower than 150oC


(320oF). Binary geothermal plants also reduce geothermal energy’s already low emission rate

to zero

4. Flash/Binary Combined Cycle: This plant combines the flash and binary

technology to take advantage of the benefits of both technologies. In this plant

type, the geothermal fluid that flashes into steam under reduced pressure is first

converted into electricity, and the low pressure steam exiting the turbine is

condensed in a binary system. There, the fluid heats an organic fluid which still

turns turbines to produce electricity before returning to the geothermal reservoir.

An example is the Puna, Hawaii Flash/Binary combined cycle.

Fig 3.5: Diagram of a Flash/Binary power plant (GEA Environmental Guide Report,

2009).

3.4 Enhanced Geothermal Systems (EGS)

Any geothermal reservoir system which requires stimulation of subsurface regions for

effective utilization is termed as an Enhanced (or Engineered) Geothermal System. A

reservoir consisting of a fracture network is created or enhanced to provide well-connected

fluid pathways between injection and production wells. Heat is extracted by circulating water

through the reservoir in a closed loop and can be used in electricity generation with binary-

cycle plants and for industrial or residential heating.


Fig 3.6: Enhanced Geothermal System. (Goldstein et al, 2011)

The challenge for EGS is to stimulate and maintain multiple reservoirs with sufficient

volumes to sustain long-term production at acceptable rates, and flow impedances, while

managing water losses and risks from induced seismicity. The technology is currently in

some countries, but in a demonstration and experimental stage.

3.5 Direct Use Application

This is the use of geothermal energy directly from reservoirs without having to

convert them for use.

Direct use provides heating and cooling for buildings including district heating, fish ponds,

bathing, wellness and swimming pools, water purification/desalination, industrial and process

heat for agricultural products, and mineral extraction and drying and geothermal heat pumps.


An example of the geothermal direct use application today is at the Idaho Capitol Building in

Boise.

Fig 3.7: Typical Direct Use Geothermal Heating System Configuration

(Blodgett and Slack, 2009)

Geothermal Heat Pumps (GHPs):

Geothermal heat pump technology is based on the relatively constant ground or

groundwater temperature ranging from 4oC to 30

oC at depths of about 10 to 300ft to provide

heating, cooling and domestic hot water for all types of buildings.

To supply heat, the system draws heat from the Earth through the loop and distributes it

through a conventional duct system. For cooling, the process is reversed; the system extracts

heat from the building and moves it back into the Earth loop.

GHPs only collect, concentrate and deliver heat, not producing it, thereby reducing electricity

use; compared to traditional heating and cooling systems.


Fig 3.8: Diagrams of Geothermal Heat Pumps (Blodgett and Slack, 2009)


CHAPTER FOUR

ENVIRONMENTAL IMPACTS OF GEOTHERMAL ENERGY RESOURCES

4.1 Direct Greenhouse Gas Emission

In general, negative environmental impacts associated with geothermal energy

utilization are minor, but hot fluids can emit varying quantities of Greenhouse Gases (GHGs)

which are usually small to negligible in quantity relative to most conventional resources.

Geothermal fluids contain minerals leached from the reservoir rocks and gases, mainly CO2

and a smaller amount of Hydrogen Sulphide, which may be given off during operations.

Composition and amount of gases given off depends on the geological conditions

encountered on the field;emissions given off during operations are managed through process

designs (Goldstein et al, 2011).

4.2 Local Environmental Impacts

There are several potential local environmental impacts from any geothermal power

development. These include:

• Water pollution: Liquid streams from well drilling, stimulation, and production may contain

a variety of dissolved minerals, especially for high-temperature reservoirs (>230°C). The

amount of dissolved solids increases significantly with temperature. Some of these dissolved

minerals (e.g., boron and arsenic) could poison surface or ground waters and also harm local

vegetation. Liquid streams may enter the environment through surface runoff or through

breaks in the well casing.

• Land subsidence: If geothermal fluid production rates are much greater than recharge rates,

the formation may experience consolidation, which will manifest itself as a lowering of the

surface elevation, i.e., this may lead to surface subsidence.

• Induced seismicity: Reinjection of geothermal fluids into reservoirs can sometimes lead to

induced seismicity, especially in Enhanced Geothermal Systems (EGS) where fluids are

forced in under high pressure. The process of opening fractures can occur in a sliding manner

by shear failure or in extensional manner by tensile failure. In either case, acoustic noise

referred to as micro Earthquakes are generated during this process.

• Induced landslides: There have been instances of landslides at geothermal fields. The cause

of the landslides is often unclear. Many geothermal fields are in rugged terrain that is prone


to natural landslides, and some fields actually have been developed atop ancient landslides.

Some landslides can be triggered by large Earthquakes, but it is highly unlikely that

geothermal production and injection could lead to such a massive event. Badly sited wells,

particularly shallow injection wells, may interact with faults and cause slippage similar to

what occurs in the case of induced seismicity.

Other local environmental impacts include:

• Solids emissions:

• Noise pollution:

• Land use:

• Water use

• Disturbance of natural hydrothermal manifestations

• Disturbance of wildlife habitat and vegetation

• Altering natural vistas

• Catastrophic event


CHAPTER FIVE

POTENTIALS IN NIGERIA

5.1 Occurrence in Nigeria

Nigeria is a country endowed with so many energy resources (renewable and

conventional), enough to provide for her energy needs. However, the country still struggles

with meeting the energy demands of the masses for both domestic and industrial use. The

imbalance in the energy mix of the country, majorly because of its total dependency on

petroleum resources for energy production, almost abandoning other energy resources, is one

of the reasons for this.

Countries all over the world today are trying to develop sustainable energy with lesser

environmental impacts, mostly through renewable energy resources such as geothermal

resources. Countries like the U.S, Iceland, Australia, Kenya, have successfully utilized

geothermal energy for the production of energy as well as other industrial, commercial or

domestic applications, thereby, making a step in mitigating harmful environmental effects

caused by the use of conventional fossil fuel plants for power generation.

Geothermal energy can be successfully exploited in Nigeria too, as surficial

manifestations of these resources suggest they occur in the country. There are two Known

Geothermal Resources Areas (KGRAs) in Nigeria; the Ikogosi warm springs of Ondo state

and the Wikki warm springs of Bauchi state. These surficial effusions result from the

circulation of water to great depths through faults in the basement complex of the area.

Within sedimentary areas, high potential gradient trends are identified in the Lagos sub basin,

the Okitipupa ridge, the Auchi-agbede of the Benin flank/ hinge line, and the Abakaliki

anticlinorium. Geothermal gradient indicates that steam would be encountered at a depth of

about 6,000ft (1,800m) in Lagos subbasin and Auchi-agbede areas and at about 4,250ft

(1,300m) in the Abakaliki area (Babalola, 1997). Geothermal resources occur also in the

Niger-Delta, but of a different geothermal process from those listed above. The geothermal

resources occurring in the deeper cretaceous sequences of the Niger-Delta are known as geo-

pressured geothermal resources, produced at systems with pressure higher than hydrostatic

pressure.

The Benue Trough may be one of the major geothermal exploiting geothermal energy

resources areas, should the resource be developed in the country. This is mostly true because


of the magmatic activity which occurred in the Trough in the past. In the Benue fold belt,

extending from the Abakaliki anticlinorium to the Keane anticline and the Zambuk Ridge,

several magmatic intrusions emplaced during the Late Cretaceous line the axis of the Benue

Trough. These magmatic intrusions are surficial manifestations to claims of magmatic

occurrence in the Trough. It is possible that some magma may still be trapped underground,

heating nearby rocks water existing there. Also, it is known that igneous rocks contain

radioactive materials, and these particles produce heat as they decay. It is possible then, that

the crystalline basement rocks with decaying radioactive particles generate heat which heat

overlying rocks and the water trapped in them, forming a geothermal reservoir. Positive

Bouguer activity anomalies also parallel this Trough and are interpreted to indicate shallow

mantle. Apart from the Benue fold belt, the Ikom, the Jos plateau, Bauchi plateau and the

Adamawa areas which also experienced the Cenozoic volcanism and magmatism could be

geothermal energy producing areas.

The potential estimates for geothermal energy resources in Nigeria are not yet known.

The cost involved, and the high need for skilled expertise, may be obstacles towards

developing geothermal as part of the energy mix in Nigeria.

5.2 Importance in Nigeria

Although the potentials of geothermal energy resources in Nigeria are not being

harnessed today, they could be of importance in the following sectors:

Distribution of power in the country.

Serve as a source of employment to the unemployed, and as a source of revenue in

regions where power plants could be installed.

Reducing the country’s dependency on fossil fuels and increase focus on such

renewable energy resources, mitigating negative climatic impacts.

Domestic and industrial uses, such as in direct applications like heating, cooking etc.

5.3 Limitations to resource development in Nigeria

Geothermal energy resources are not being utilized in Nigeria today, mostly due to the

country’s dependence on the oil sector, low funding for research and development of this

energy resource, inadequate skill and technology involved in exploring, exploiting,

developing and managing geothermal energy resources and very low public awareness on the

resources.


CHAPTER SIX

SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

6.1 Summary

Geothermal energy resources are renewable energy resources obtained from the heat

of the Earth, with potentials of providing long-term, secure base-load energy and greenhouse

gas emissions reductions. Heat replenishment from Earth processes and modern reservoir

management techniques enable the sustainable use of geothermal energy as a low-emission,

renewable resource. With appropriate resource management, the tapped heat from an active

reservoir is continuously restored by natural heat production, conduction and convection from

surrounding hotter regions, and the extracted geothermal fluids are replenished by natural

recharge and by injection of the depleted fluids. Geothermal energy resources have served

man both commercially and for his domestic needs for over a century; developing techniques

are expanding ways to which we can fully utilize this energy resources. Geothermal resources

are of different types, depending on the temperature of each resource, geologic settings where

they occur and the characteristics of reservoirs where they can be found. The resource type is

relevant to the development of the resources for utilization. Geothermal energy, the thermal

energy obtained from these resources are converted into electricity by steam condensing and

binary cycle units, or applied directly for direct use applications. Utilization of geothermal

energy has is very beneficial, but it has its potential negative impacts, but they are very

negligible, especially when compared to operating conventional energy resources. There are

potentials of geothermal energy resources in Nigeria, especially in the two known geothermal

resources areas; the Ikogosi warm springs of Ekiti and the Wikki warm springs of Bauchi.

The Benue trough and the Niger-Delta regions also have these potentials. Geothermal

resources are still yet to be exploited in the country.

6.2 Conclusions

Heat given off from the Earth’s interior produces thermal energy by heating

surrounding rocks to produce hot water or steam, which are used for generating thermal

energy or electricity. From the discussions above, the occurrence of geothermal resources in a


region depends almost totally on the geologic setting of that region. Also the nature of the

rock, the availability of water, reservoir characteristics, and in general, the longevity of the

resources all depend on the geologic settings where they are formed. These resources can be

found around the world, but emphasis lies on the temperature of the available resource,

because that factor alone tells on the viability of the resources. That said, geothermal wells

can also be mechanically stimulated or engineered to increase the production capabilities of a

geothermal reservoir, where natural productions are low. Earth processes such as tectonism

and magmatism are important processes which results to very productive geothermal energy

resources. Geothermal energy resources are readily available, but technology and capital

involved in production of energy and maintenance of the resources, have made them scarce.

6.3 Recommendations

Researches on these resources are still very low. The world we have today is one that

has become energy-demanding, and with a growing consciousness of the climatic state of the

environment we live in. producing renewable and sustainable can help in solving such

problems. Governments should create public awareness and fund researches concerning the

geothermal energy resources where they occur, as means of mitigating climatic and energy

problems. Industrializing countries like Nigeria with geothermal energy potentials should

also focus on developing technologies for utilizing these resources, to meet the energy needs

and add a source of income for the nation.

Technologies used in enhanced geothermal systems should also be developed to

reduce hazards caused by hydraulic fracturing. Technologies which maximize the use of

geothermal energy resources from low-temperature resources should also be developed, to

reduce the environmental limitations of utilizing geothermal energy.


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