This list contains terms related to geothermal energy and associated technologies.
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Abandonment
Final plugging of wells, and/or permanent dismantling of a well, production platform or other installation.
Acidising
The pumping of acid down the wellbore thereby enlarging pore spaces & fractures in rocks to increase fluid flow in the rocks.
Accoustic Log
A record of the time taken by an acoustic (sound) wave to travel over a certain distance through the geological formations. Also called a sonic log.
Ambient
Natural condition of the environment at any given time.
Annulus
The space between the drill-string or casing and the well wall, or between casing strings, or between the casing and the production tubing.
Appraisal Well
A well drilled to confirm the size or quality (heat content & commercial potential) of a discovery of geothermal resource before commencement of commercial development.
Aquifer
Water-bearing stratum of permeable sand, rock, or gravel.
Back Off
In drilling, to pull the drill-string out of, or partly out of, the borehole. unscrew a joint of drill-pipe, slacken off a line or block.
Barrel of Oil Equivalent
The quantity/volume of hot geothermal fluid that contains the same amount of heat as a barrel of oil.
Baseload Plants
Electricity-generating power plants that are operated around the clock to meet the constant or minimum load on the system. The cost of energy from such plants is usually the lowest available to the system.
Binary-Cycle Plant
A geothermal electricity generating plant suited to geothermal resources say below 220ºC and employing a closed-loop heat exchange system in which the heat of the geothermal fluid (the "primary fluid") is transferred to a lower-boiling-point fluid (the "secondary" or "working" fluid), which is thereby vaporized and used to drive a turbine-generator set.
Bit
A drilling bit. Bits chiefly in use are the steel roller-cutter, and the diamond-insert bit and PDC bit for hard formations, which penetrates by scratching or abrading the rock rather than by crushing or pulverising like the roller bit. There is also the annular diamond-insert core bit, for cutting and retrieving rock samples (in conjunction with a core barrel) usually called "diamond-drilling".
Blowout
Uncontrolled or uncontrollable release of down-hole pressure upward through the well-bore or casing.
Blowout Preventer (BOP)
An emergency shut-off valve installed on the wellhead which incorporates hydraulic pipe rams capable of closing the space around the drillpipe against very high pressures. The will stop any flow into or out of the well.
Bottoms Up
Circulation of drilling fluid in a well, until the bottom hole mud and cuttings reach the surface.
Bridge Plug
A down hole packer assembly used in a well to seal off or isolate a particular formation for testing, acidising, cementing, fraccing etc. Also, a type of plug used to seal off a well temporarily while the wellhead is removed.
Brine
A geothermal solution containing appreciable amounts of sodium chloride or other salts.
BTU
British thermal unit. The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit at standard conditions. (Equal to 252 calories.)
Caliper
A tool for checking casing in a well for deformation before e.g. running drilling tools, which might become stuck, or packers which might leak.
Cap Rocks
Rocks of low permeability that overlie a geothermal reservoir.
Capacity factor
The amount of energy that the system produces at a particular site as a percentage of the total amount that it would produce if it operated at rated capacity during the entire year.
Cascading Heat
A process that uses a stream of geothermal hot water or steam to perform successive tasks requiring lower and lower temperatures.
Casing
The steel pipes with which a well is lined, for protection against collapse of the borehole, and unwanted leakage into or from rock formation, or at the surface. "Joints" of casing are around 10metres long and are normally screwed together as they are run into the well. It is normal to set large diameter casing, (such as 20") called the Conductor (or surface) Pipe after the well has penetrated the layers nearest the surface, and cement it into place, after which the drilling continues with a smaller diameter bit.
The next string of casing, the "Surface String" is cemented inside the previous string and down to the new Casing Point and forms the base for the wellhead. This is followed by one or more intermediate strings depending on the target depth and expected conditions in the well. Finally the Inner/Production String is set and cemented through the reservoir zone, and perforated to allow hydrocarbons to enter the well.
Casing Point
The depth of the lower end of a string of casing.
Casing Shoe
A reinforced section of casing run into a well at the lower end of a string, to protect against bucking or deformation.
Centigrade (ºC)
A unit of measurement of temperature.
Central power
The generation of electricity in large power plants with distribution through a network of transmission lines (grid) for sale to a number of users. Opposite of distributed power.
Cement
Cement is used to "set" casing in the well bore and seal off unproductive resevoirs and apertures.
Choke
An aperture restricting flow in a well or flowline.
Chistmas Tree
The manifold, or arrangement of pipework connections and valves which is installed on the wellhead prior to production. As well as outlets for production, the tree will provide for the injection of mud to "kill" the well, and for the insertion of down-hole maintenance tools and wirelines.
Circulation (Drilling)
The passage of fluids, primarily drilling mud, down the interior of the drill-stem and back to surface viz the annulus.
Commercial Well
A well capable of producing profitably.
Completion
Installation in a well of production tubing and equipment, wellhead and Christmas Tree.
Condensate
Water formed by condensation of steam.
Condenser
Equipment that condenses turbine exhaust steam into condensate.
Cooling Tower
A structure in which heat is removed from hot condensate.
Core/Core Barrel
A vertical section of reservoir or other rock taken in drilling a well, for detailed study and analysis. In order to retrieve the core as intact as possible, it is cut from the rock by an annular core bit. The central column of rock passes through the centre of the bit and, as the bit cuts deeper, is received by a hollow cylindrical Core Barrel above the bit, where it is retained and protected by a series of rubber baffles. When the bit has cut deep enough to fill the core barrel, it is withdrawn from the hole and the core extracted. In this way the actual sequence of rock strata is preserved.
Coring or Core Drilling
Boring into the earth and collecting cores of rock recovered from the centre of the drill string.
Corrosion
Chemical deterioration due to prolonged adverse conditions. Corrosion resistant alloys are generally non-ferrous.
Crust
Earth's outer layer of rock. Also called the lithosphere.
Darcy
The unit of measurement of rock permeability, i.e. the extent to which it will allow a fluid to flow through it.
Development Well
Any well drilled in the course of extraction of geothermal fluids from the resevoir.
Deviated Well
A well whose path has been deliberately diverted from the vertical.
Diamond Drilling
A cored drill hole drilled with a bit which has impregnated diamonds. Diamonds are the hardest known minerals and can cut through all other minerals and rocks.
Differential Pressure
The difference between the pressure in a well due to the mud column and the pressure in the surrounding rock at any point.
Dip
The inclination from the horizontal of the surface of a geological structure.
Dipmeter
A Dip Meter indicates dip relative to a well bore.
Direct Use
Use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation, industrial processes, etc.
Directional Drilling
Drilling in a non-vertical direction.
Distributed power
Generic term for a power supply located near where the power is used. Opposite of central power.
Distributed systems
Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids.
District Heating
A type of direct use in which a utility system supplies multiple users with hot water or steam from a central plant or well field.
Down-Hole
Down a well. The expression covers any equipment for measurement in a well or designed for use in one.
Downtime
A period when any equipment is unserviceable or out of operation for maintenance etc.
Drilling
Boring into the Earth to access geothermal resources, usually with oil and gas drilling equipment that has been modified to meet geothermal requirements.
Drilling Mud
Mixture of water and other additives that circulate within the well. It has the purpose of cooling the bit, removing rock cuttings, preventing cave in and averting a blowout
Drill-pipe
Pipe, which is supplied in "joints" normally of around 33 ft./10m. in length, each being fitted with thicker, or "up-set" reinforced threaded couplings at each end, "male and female" or "pin and box" respectively.
Dry Steam
Very hot steam that occurs without liquid.
Efficiency
The ratio of the useful energy output of a machine or other energy-converting plant to the energy input.
EGS or Enhanced Geothermal Systems
Rock fracturing, water injection, and water circulation technologies to sweep heat from the unproductive areas of existing geothermal fields or new fields lacking sufficient production capacity.
Exploration Well
Exploration is the process of identifying a prospective geothermal region, mainly by reference to regional, and specific, geochemical, geological and geophysical surveys. An Exploration Well is a well drilled to test a potential but unproven geothermal resource.
Fault
A fracture or fracture zone in the Earth's crust along which slippage of adjacent rocks has occurred at some time. A Fault Block is a compartment of a rock formation surrounded or partly surrounded by faults.
Filter-Cake
Build up of mud solids or filtrate on the wall of a well. This helps seal and stabilize the rock face, but too much can cause sticking of the drill string.
Fish/Fishing
Any unwanted object down a well, commonly the lower end of a drill string which has broken off. "Fishing" is trying to recover the Fish, using various attachments to the drill stem or wireline, known as fishing tools.
Flash-Cycle
Power generated from flash steam derived from water in geothermal reservoirs generally above say 220ºC which usually flows to the surface under its own pressure and boils or “flashes” into steam when it is subjected to a controlled pressure drop. The steam is used to drive a turbine-generator.
Flash Steam/Flashing
Steam produced when the pressure on a geothermal liquid is reduced.
Flowing Bottom Hole Pressure
Pressure at the bottom of a well measured at a given flow rate.
Formation Damage
Damage to the reservoir rock around a well due to e.g. plugging with mud, or infiltration by water from the well.
Fracturing/Fraccing
The process of cracking open the rock formation or existing natural fractures in the rock around a well bore to increase production rates from a reservoir. This is normally done by pumping fluid at very high pressures.
Gas Kick
Increase of down hole pressure above that exerted by the column of drilling fluid in a well, allowing gas to escape to the surface. If not controlled this could develop into a blowout.
Geology
Study of the planet Earth, its formation, composition, structure, natural processes, and evolution.
Geothermal
Of or relating to the Earth's interior heat.
Geothermal Energy
The Earth's interior heat made available to man by extracting it from hot water or hot rocks.
Geothermal Gradient
The rate of temperature increase in the Earth with increasing depth below the earth’s surface measured in units of °C/km.
Geothermal Heat Pumps
Devices that take advantage of the relatively constant temperature of the Earth's interior, using it as a source and sink of heat for both heating and cooling. When cooling, heat is extracted from the space and dissipated into the Earth; when heating, heat is extracted from the Earth and pumped into the space.
Gigawatt (GW)
One billion watts = one million kilowatts = one thousand megawatts.
Greenhouse gases
Gases that trap the heat of the sun in the Earth's atmosphere, producing the greenhouse effect; greenhouse gases are water vapour, carbon dioxide, methane, ozone, chlorofluorocarbons, and nitrogen oxides.
HDR or Hot Dry Rock
Subsurface geologic formations of abnormally high heat content that contain little or no water (see also EGS and HFR).
HFR or Hot Fractured Rock
Similar to HDR but tends to be used when subsurface geologic formations of abnormally high heat content contain appreciable amounts of naturally occurring water (See also EGS).
Heat Exchanger
A mechanism or device for transferring thermal energy from one fluid to another.
Heat Flow
Movement of heat from within the Earth or to the surface, where it is dissipated into the atmosphere, surface water, and space by radiation. Heat flow at depth is the product of the geothermal gradient and thermal conductivity and is measured in units of mW/m-2.
Hydrocarbons
Chemical compounds which are primarily comprised of carbon and hydrogen such as petroleum and which have been derived from burial of land or sea plants.
Hydrothermal Resource
Underground systems of natural hot water and/or steam.
Impermeable Rock
A rock with restricted or poorly communicating pore spaces and fractures, such that fluids will not flow through it.
Injection
The process of returning spent geothermal fluids to the subsurface. Sometimes referred to as reinjection.
Insulating Cover Rocks
Rocks having low thermal conductivity which act as thermal insulators and trap the heat in underlying rocks.
Joint
A single length of pipe. Coupled or welded to other lengths, joints become a string. The term joint may also refer to the couplings or joints themselves.
Junk
Any unwanted object 'lost' down a well.
Kilowatt (kW)
1,000 watts—a unit of electric power.
Kilowatt-Hour (kWh)
One thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kilojoule (kJ). 1 kWh is the energy required to power ten 100 watt light globes for 1 hour.
Lithology
The type of rocks and hence the description of different formations encountered by a well.
Load
The simultaneous demand of all customers required at any specified point in an electric power system( see Baseload Plants and Peak Power Plants).
Log/Logging
There are various applications of logs, but chiefly:
- various devices for taking measurements of the character and conditions of the well bore, formations, or fluids encountered by a well, together with the records produced by them. The main types of well logs are electrical, magnetic, mechanical, sonic, nuclear, temperature and well orientation.
- written chronological records such as shift logs, maintenance logs, tour sheets, and mud logs which also record cuttings recovered from a well
Magma
Molten rock within the Earth, from which igneous rock is formed at or near the Earth's surface by cooling.
Make-up/Break Out
To assemble/screw together the sections of joints of a string of pipe. 'Breaking out' is the opposite.
Mantle
The Earth's inner layer of molten rock, lying beneath the Earth's crust and above the Earth's core of liquid iron and nickel.
Marginal
A well, development or project whose commercial profitability borders between profitable and not and is in doubt.
Megawatt (MW)
1,000,000 watts—a unit of electric power. MWe refers to electric output from a generator, MWt refers to gross thermal output from a reactor or heat source (e.g., typically about three times the MWe figure). 1MW=1,000 kilowatts. A 100 MW generator will power one million 100 W light globes simultaniously. A 600 MW generator can service 200,000 domestic customers.
Megawatt hour (MWh)
1MWh is the energy required to power 10,000 100 watt light globes for one hour.
Micro-seismic Survey
Microseismic monitoring is the recording, processing and mapping of small seismic events that are caused inside the reservoir by production of fluids or deliberately induced by hydro-fracturing experiments or surveys. These events have their origin at or near small fractures, most of which are pre-existing. It can by used to monitor the progression of rock fracturing through a resevoir.
Millidarcy
A measure of permeability which is one thousandth of a darcy.
On-Stream
When production is flowing, or plant is in operation.
Open-Flow
Producing a well without chokes. Unrestricted production normally for testing or maintenance purposes.
Open-Hole
An uncased section of well borehole.
Over-pressure
SUB-surface pressure that is abnormally high, exceeding hydrostatic pressure at any given depth. Drilling into over-pressure regions can be hazardous. This can occur when burial of fluid-filled sediments is too rapid to allow some fluids to escape thereby increasing the pore pressures as further sediments are deposited and further fluids accumulate in-situ.
Packer
A seal used to isolate a section of a well, e.g. for testing or production from one of several resevoir zones. Packers are also used in operations such a cementing and acidizing.
Peak Power/Peaking Plants
Electricity generating plants that are operated at a low capacity factor to meet the short lived, variable high demand, peak periods or maximum load on the system. The cost of energy from such plants is usually higher than from base load plants.
Perforation/Perforating Gun
Holes punched in the casing of a well at the resevoir zone to allow fluids to enter the well at a specific zone of the resevoir. A Perforating Gun is a cylindrical tool loaded with explosive charges which are triggered opposite the required production zone, perforating the casing in many places.
Permeability
The capacity of a substance (such as rock) to transmit a fluid. The degree of permeability depends on the number, size, and shape of the pores and/or fractures in the rock and their interconnections. It is measured by the time it takes a fluid of standard viscosity to move a given distance. The unit of permeability is the Darcy.
Penetration/Penetration Rate / ROP
Drilling penetration rate measured in metres. Often referred to as ROP (Rate of Penetration).
Petrology
The study of rocks, their origin, chemical and physical properties and distribution.
Pipe Rack
Where stands of drill pipe are stacked vertically ready for use. Racks or frames are also sometimes used to store tubulars horizontally in yards.
Pipe Rams
Hydraulic rams in a blowout preventer which are shaped to fit around the drill-stem and seal the annulus. Blind Rams are designed in extreme emergency to shear through the drill pipe and seal the well completely.
Plate Tectonics
A theory of global-scale dynamics involving the movement of many rigid plates of the Earth's crust. Tectonic activity is evident along the margins of the plates where buckling, grinding, faulting, and vulcanism occur as the plates are propelled by the forces of deep-seated mantle convection currents. Hydrothermal geothermal resources are often associated with tectonic activity, since it allows groundwater to come in contact with deep subsurface heat sources.
Plug/Plug and Abandon
To seal a well, or part of a well with cement, e.g. before producing from a higher resevoir zone, sidetracking, or leaving the well permanently sealed and abandoned.
Porosity
The ratio of the aggregate volume of pore, fracture spaces or voids in rock or soil to its total volume, usually stated as a percent. Porosity measures the capicity of a rock to hold fluids or gas and hence be a resevoir rock.
Power
Power is the rate at which work is done or work divided by time. Work is the force acting on an object to cause a displacement and does not depend on the amount of time required.
Power Generation Plants
A facility designed to produce electric energy from a form of energy, such as coal, oil, gas, nuclear, hydroelectric, geothermal, solar thermal, and wind.
Production Casing String
The innermost steel lining of a well cemented in place and perforated for production. Note that production tubing is inserted inside this casing.
Production Testing
A production test concerns the capability of a well to produce (productivity) and its effects on the reservoir produced. A production test may continue for several months where extensive data is necessary prior to final commitment to development expenditures etc.
Production Tubing String
The string of pipe installed inside the casing of a production well, to a point just above the reservoir through which the fluids are produced. Its diameter depends on the production flow and pressures anticipated.
Production Well
A development well specifically for the extraction of fluids from the reservoir.
Production Wellhead and Tree
The assembly of casing head, tubing head, connections and well-control valves fitted to a producing well. The "Christmas Tree" is the name given to the complete assembly of valves and connecting flanges.
Reservoir
A porous or fractured rock formation containing liquids or geothermal fluids, such as water or steam.
Reservoir Pressure
The pressure at reservoir depth in a shut-in well.
Reworking a Well
Maintenance work on a well to stimulate production. This may involve cleaning out deposits of silt or scaling, or stimulation techniques such as fracturing or acidizing.
Rig
The drill and other equipment needed for drilling a well.
Roller Bit
A rotary drilling bit which penetrates by pulverising the rock with its toothed wheels.
Rotary Table
The heavy turntable at the centre of a drilling-rig floor, which is rotated by the main rig power supply.
Round Trip
Recovering the drill string from the bottom of the well to the surface and returning it to continue drilling (eg to replace the bit). "Tripping" is slow and arduous and interrupts penetrating rock.
Salinity
A measure of the quantity or concentration of dissolved salts in water.
Scaling
Precipitation of chemicals dissolved in formation waters.
Seismic Surveys
A process of inducing shock waves into the ground and gathering and recording the patterns of shock waves which are reflected from boundaries of underground layers of rock. The patterns are interpreted to create detailed models of the underlying geological formations and structures.
Shale Shakers
Screens for extracting rock cuttings from circulating drilling mud.
Shoe
The strengthened fitting on the lower end of a string of casing to protect the tubulars and help direct the cement to the annulus.
Shut-In Pressure (SIP)
The surface pressure in a shut-in well (static pressure) exerted at the top of a wellbore. The SIP may be zero, indicating that the pressure exerted by any formations open to the wellbore are effectively balanced by the weight or pressure of the hydrostatic column of fluid in the well.
Shut-In Well
A well that it is closed at either the Christmas tree or the BOP.
Side-Track/Side-Tracked Well.
A well re-drilled from an intermediate depth. Wells are re-directed or sidetracked for various reasons, but usually because of technical problems or materials stuck deeper in the original well.
Sidewall Coring
Obtaining rock samples from the sides of a well bore using a special tool.
Spud
To start drilling a new well (or re-start)
Stimulation
A procedure designed to increase the flow rate of fluid through a rock by enhancing its permeability through fracturing or acidizing.
Superheated water
Water between 100ºC and its critical temperature (374ºC) which remains in the liquid state, usually due to a high confining pressure.
Suspended Well
A well which is left temporarily sealed or plugged to be re-entered for further testing or for production purposes.
TDS
Total dissolved solids. Used to describe the amount of solids dissolved in water.
Thermal Conductivity
Thermal conductivity of a rock is the measure of how easily heat is transmitted through the rock. It is power per unit distance per unit temperature and is measured in units of W/m/K.
Thermal Gradient
The rate of increase or decrease in the a rock's temperature with distance measured in units of °C/km (see Geothermal Gradient).
Total Depth (T.D)
- The target depth for a well.
- The achieved (drilled) depth in a well at any one time.
Transmission Line
Structures and electrical conductors that carry bulk supplies of electrical energy from power-generating units.
Trip
The process of taking out or putting back the string of drill pipe.
Turbine
A bladed, engine which is rotaded by a directed current of fluid. In electric power applications, such as geothermal plants, the turbine is attached to and spins a generator to produce electricity.
Twist Off
Breaking the drill pipe due to fatigue or excessive torque.
Up-dip
An area of a structure where the top of the resevoir or rock formation is higher than the point under consideration.
Vapor-Dominated
A geothermal reservoir system in which subsurface pressures are controlled by vapor rather than by liquid. Sometimes referred to as a dry-steam reservoir.
Viscosity/Viscous
The resistance of a fluid to flow, due to the mutual adherence of its molecules.
Watt
The unit of electric power, or amount of work equivalent to one joule(J), done in a second. One ampere of current flowing at a potential or electrical pressure of one volt produces one watt of power. A 100 watt light globe uses 100 watts of electricity to produce light. The wattage of an electrical appliance is the rate at which the appliance converts electricity into heat or light.
Wellhead
"Wellhead" is descriptive of a location or function rather than a specific item of equipment of a well.
Well Logging
Measuring the geological, engineering, and physical properties and characteristics of geothermal reservoirs with instruments placed in the wellbore.
Well Testing
Testing of an exploration or appraisal well is to estimate thermal reserves in communication with the well and well productivity.
Work
See "Power" |
