Geothermal SystemsKelompok IV:

Hasrianti SiregarRifqi Rafif Aly

Dimas Ahmad Syafi’IFadhil Muddasir

Dyas Asri Muthia

Outline• Recource Location and Types• Heat Flow in the Earth• Heat Content and Heat Flow of Magmatic Instrusions• Conceptual Models of Geothermal Systems• Geothermal Fluid Characteristics and Deposits• Geothermal Development• Environmental and Safety Issues• Summary

Sistem Geothermal• Heat Source• Permeable rock• Heat Carrier

I. Resource Location and Types

Type Reservoir Temperature (°C)

Reservoir Depth (km)

Young Igneous Systems ≤370 ≤1.5

Tectonic Systems ≤250 ≥1.5

Geopressured Systems 50 – 190 1.5 – 3

Hot Dry Rock Systems 120 – 225 2 – 4

Magma Tap Systems ≤1200 Shallow crust

II. Heat Flow In The Earth

Heat transfe

r

Radiasi

Konveksi

Konduksi

• The basic equation of conductive heat flow:

q = k(dT/dz)

k = thermal conductivity of rockT = temperaturez = depthdT/dz = geothermal gradient (C/km)

• Thermal conductivity of rock is highly variable (typically 3.5 W/m) and also depends on temperature

Konduksi

Most of the heat in the Earth is generated bygravitational force flows through the mantle and crust to the

surface. (+)decay of radioactive elements, particularly

U, Th, and K.

• Thermal conductivity is also affected by permeability, fracture density, and hydrothermal alteration.

q varies from high values at or near zones of magmatism, tectonism, and thin crust to low values in most older continental crust

Konveksi

• Upwardmovement of water or magma transports heat to the Earth’s surface more effectively than conduction.

• surface manifestations such as geysers, fumaroles, and hot springs.

• Information on porosity and permeability of pathways along fractures and strata boundaries is required to determine flow characteristics and storage capacity of the reservoir.

III. Heat Content and Heat Flow of Magmatic Intrusions

• Heat content of a magma chamber is dependent mostly on magma volume and temperature. • Heat flow surrounding a magma chamber depends on many variables,

especially emplacement temperature, depth, and time. • For large magma bodies, surface heat flow q as a function of time, t,

after instantaneous intrusion is given by:

Z = depth to top magma bodyTe = Instrusions temperature minus ambient temperature

k = thermal conductivity of overlying rocksK= termal diffusitivity of magma

Heat Content and Heat Flow of Magmatic Intrusions

• The calculation just described above rely on several assumptions:

• Convecting hydrothermal fluids can produce geysers, hot springs, fumaroles, and subsurface flows of hot waters that remove heat from a magmatic system much faster than heat loss by conduction alone.

1. all heat transfer around the magma body is by conduction2. preliminary heating stages of the magma body and the rocks it has consumed or displaced are ignored3. replenishment of the magma body by new magma batches, a characteristic of plutons and stocks that form

volcanic fields or volcanic complexes, is ignored4. removal of heat by circulating groundwater is ignored. These processes have contrasting effects on q and

dT/dz within volcanoes

IV. CONCEPTUAL MODELS OF GEOTHERMAL SYSTEM

A. YOUNG IGNEOUS MODEL

B. TECTONIC MODEL

C. VAPOR-DOMINATED MODEL

D. LIQUID-DOMINATED MODELHr = enthalpy of the reservoir fluidHv = enthalpy of separated steam at separation temperatureHl = enthalpy of separated liquid at separation temperature.

VI. GEOTHERMAL DEVELOPMENT

EXPLORATION DECLINE

PRODUCTION NON ELECTRICAL USES

POWER GENERATION MONITORING

EXPLORATION

1. regional exploration strategies initially focus on known areas of hot spring and fumarole activity or known areas of hot aquifers.

2. First efforts are to evaluate young volcanic centers, delineate major geologic struc- tures, assess regional heat flow, and calculate subsurface reservoir temperatures from the geochemistry of fluid samples

3. Once prospective targets are chosen, exploration involves a more de- tailed approach to determine the age and structure of the reservoir rocks, a possible depth to the reservoir, fluid characteristics and pathways, and a conceptual model of the system.

4. The model is then tested by drilling, which primarily confirms the presence (or absence) of a reservoir and its temperature.

5. *Geophysical logs provide temperature–depth profiles, water levels, formation pressures, production zones, and other information.

*Cores provide details on stratigraphy, alteration mineralogy and history, fracture patterns & porosity of the rocks.6. Fluid analyses establish background conditions, assess potential environmental impacts, and evaluate corrosion and precipitation properties.

PRODUCTION

Geothermal systems that enter the development stage are called geothermal fields.

Governments or private companies acquire ownership or development rights to the land necessary for drilling wells, creating infrastructure, and building power plants.

As each well is completed, it is tested to determine fluid enthalpy, mass-flow rate, pressure declines, permeability thickness, and other parameters.

Of about 80 drilled geothermal fields, the mean installed capacity is roughly 100 MWe and the mean reservoir temperature is 250±40◦C. The present cost of a 55MWe geothermal project including exploration, wells, infrastructure, and plant is about US$110 million or about US$2 million/MWe with an expected lifetime of 25 yr. Some fields have existed for >30 yr. Mean development time after exploration is 6 to 7 yr.

POWER GENERATION

Conversion efficiency, available power, P, is approximately

where ΔH is the enthalpy difference between production steam and ambient steam.

About 20% of the fluid mass is injected back into the reservoir after passing through the condensers. The remaining fluid enters the atmosphere from the condensers.

DECLINE NON ELECTRICAL USES

Geothermal resources are not limitless, although they are often classified as ‘‘renewable’’ resources.

Long-term (>30 yr) sustainable power production requires a balance between fluid production rates and natural recharge of the reservoir. Injection of fluids from power plants and other sources, usually around the margins of the field, helps maintain this balance.

Reservoirs with temperatures less than 130◦C are usually not practical for electrical generation, even at high flow rates.

The most common direct-use applications for intermediate- to low-temperature geothermal fluids are spas and health resorts, space heating, green housing, and fish/reptile farming.

Fluids above 60◦C can be used in many industrial applications (preheating, spraying, steaming, pasteurizing, deicing, and sterilizing).

More specialized applications include fruit/vegetable drying, refrigeration, fermentation and distillation of alcohol, mushroom growing, advanced oil recovery, and heap-leach mining.

MONITORING

These may include seismic monitoring, measurements of emissions to the atmosphere or groundwater, temporal changes in fluid chemistry and heat content of production fluids, and any changes in the behavior of thermal features, faults, landforms, streams, and nearby volcanoes.

VII. Environmental and Safety Issues

• H2S pollution of atmosphereH2S is a noxious gas that is immediately dangerous to life and health (IDLH) at levels above 142 mgm 3 of air. As a result, air quality atsome power plants is obnoxious if not hazardous.• Brine pollution of groundwaterMany brines are considered toxic wastes in most developed countries and must be treated or disposed of to render them harmless.Because of obvious economic advantages, most producing fields inject the brine to help maintain reservoir fluid volume and pressure.

• Hydrothermal explosionsIn some geothermal systems, production of reservoir fluids has caused subsurface pressures to decrease, thereby inducing boiling and increasing fumarolic activity. Commonly, hydrothermal explosion craters form where fumaroles and acid springs already exist• LandslidesMany geothermal fields are located in volcanic and tectonic terrains having high relief and rainfall. and tectonic terrains having high relief and rainfall. Because thermal fluids react with the host rocksto produce clays, silica residues, and other alteration products, much of the ground on the slopes above some geothermal systems is unstable.

Environmental and Safety Issues

• Reservoir interference, depletion, subsidence, and induced seismicity

-Long-term production of geothermal fluids can affect some production wells by drawing cooler water in from the margins of the system.

-Thermal features such as hot springs, fumaroles, and geysers may cool down or cease as reservoir pressures and overlying water levels decrease and as cold groundwater mix with reservoir fluids.

-Extensive removal of reservoir fluids may initiate ground subsidence.

-Major subsidence can eventually produce microseismicity and local faulting.

-Fluid injection often induces seismic activity within and near the margins of the reservoir.

Environmental and Safety Issues

Environmental and Safety Issues

• Earthquakes and volcanic hazards.-Most high-temperature geothermal systems are found in tectonicallyactive regions where historic earthquakes and fault displacements

have occurred (Dixie Valley).-Buildings must be constructed to withstand reasonable levels of

ground shaking in worst-case scenarios. Some geothermal fields are located on the flanks of historically active volcanoes (Kilauea, USA). Eruptive style and eruption frequency of nearby volcanoes must be evaluated before power plants and infrastructure are constructed in hazardous locations.