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Directional Drilling Glossary and Terminology

Directional Drilling Glossary and Terminology

We’ve compiled this extensive glossary of the most common and frequently used directional drilling terms and phrases, to help foster better communication and understanding. Learn the basics and understand horizontal directional drilling terminology. Directional drilling is somewhat new oil and gas industry, and many people find themselves confused with the terms especially when they are very similar to well drilling, but represent different things.

Directional Drilling Definitions



Air Sparging – This method injects air below the water table, removing volatile pollutants from ground water. The method is frequently used in conjunction with soil vapour extraction. To improve biodegradation, engineers sometimes sparge for the purpose of increasing dissolved oxygen in groundwater and soil.

Air Stripping – Another method for removing volatile pollutants from ground water. The process, which is essentially the same as air sparging, is frequently paired with soil vapour extraction.

Annular Seal – A physical barrier that runs the length of a borehole's annulus, often from the surface to a depth of 20 feet or greater. The seal keeps surface water out of the borehole and prevents air sparging or soil vapor extraction devices from short-circuiting into the atmosphere. The annular seal in a horizontal well is often made up of a bentonite plug, bentonite-cement grout, expanding polyurethane foam, an inflatable or other packer, or a mix of these materials.

Annular Space: The annular space is the space between any lining for the pipe and the pipe itself.

Annulus – The space between the components of a well or borehole. The annulus of a borehole, for instance, is the free space between the drill string or well casing and the walls of the bore being drilled. During drilling, drilling mud normally flows through the annulus of the borehole. This is also known as a well annulus after the well materials have been placed, and it is usually sealed near the surface in some way.

Auger Boring: Use of a rotating cutting head to form a bore between a reception pit and a drive pit. Auger flights wound in a helix rotate within a steel casing and remove spoil back to the driveshaft. Auger boring equipment is likely to possess limited steering capacity.


Back Reamer: A cutting head that connects to a drill string’s leading end. Enlarges pilot bores during pull-back operations, enabling the installation of sleeves or carriers.

Bent Sub: A portion of drill stem that is offset behind the drill head. Allows for the drill string to be rotated, orienting the cutting head for steering corrections. Often employed in directional drilling.

Bentonite: Colloquially referred to as directional driller’s mud, this colloidal clay forms a slick, slurry, or gel when water is added to it. Sold under a variety of trade names.

Bioaugmentation – A method for enhancing cleanup via the introduction of microorganisms to break down environmental pollutants. Microorganisms (bacteria or fungus) eat or change the toxic components, making them less harmful or tying them together to prevent migration.

Bioremediation – A technique for remediation that uses local microorganisms to decompose environmental pollutants. In order to promote the activity of natural populations of microorganisms, typical bioremediation procedures augment the available nutrients or oxygen.

Bioventing – Increases the supply of oxygen to in situ native organisms, thereby enhancing bioremediation.

Blind Well – A directionally-drilled, horizontal well that is drilled and completed from a single end. Single-entry wells are another name for them. A pilot bore is bored at the required location and depth utilising steering and a guidance system to establish a blind well. The pilot bit is then taken out of the borehole. The drill string has a forward reamer attached before it is pushed through the pilot bore, as it typically must be reamed to a bigger size before well casing installation. With a large bore diameter, this can be done in phases. Finally, the reamer is removed from the borehole and the well materials (casing and screen) are placed into it.

Bore or Borehole – Drilling operations create an extended cavity called a bore or borehole. This is most commonly a hole filled with drilling mud and cuttings, rather than a void. To complete a well, well casing is pushed or pulled into the borehole.

Boring: (1) The use of a drill string or rotating auger to displace or dislodge spoil, producing a bore (hole). (2) A process for installation of pipelines or conduits via earth-drilling. (3) The obtainment of soil samples for assessment.

Boring Pit: A specified area excavated for placing a machine in line and on grade.

Box – The drill rod portion that has female threading.

Butt Fusion Weld: The joining of two polyethylene pipe ends by rapidly bringing them together under high pressure and forming a homogeneous bond.


Cable Sonde – Instead of internal batteries, this downhole probe or transmitter is powered by energy from the ground surface. A wire running inside the drill rod connects the cable sonde to a drilling rig power source. A length of wire must be threaded through each rod, then attached to the wire extending from the existing string when each rod is added to the drill string. At the drill rig, the surface end of the wire is linked to a swivel that gives electrical contact.

Carrier Pipe: Pipe for carrying product across highway and railroad crossing casings. Carrier pipe is typically made from concrete, clay, steel, plastic, ductile iron, or other materials. Potentially bored directly underneath railroads or highways.

Cased Bore: A bore that is created with a pipe, such as a steel sleeve, being inserted simultaneously during boring, typically pipe jacking or auger boring.

Casing: Liner pipe around bore holes through which carrier pipe(s) are installed. Do not typically carry product.

Chinese Finger – A device made of woven wire that pulls materials into a bore when placed overtop them. It tightens on the material as it is pushed, growing tighter when it is pulled on harder.

Closed Face: Refers to the ability of tunnel boring equipment to seal off its facial opening to prevent soil leakage into the machinery. Also refers to bulkheading in hand-dug tunnels to prevent material inflow.

Compaction Reamer – A reamer that enlarges the diameter of a borehole by compacting the dirt around it. In general, compacting the formation surrounding the well bore is not useful in an environmental well that demands adequate hydraulic contact with the surrounding formation.

Corn Starch – Corn starch is a polymer used to thicken liquids and is a common food component. By removing sperm, endosperm, gluten, and other components from maize grains, starch is concentrated. Corn starch powder is blended with other polymers to make a drilling fluid.

Creep: Time-varying dimensional change in materials after an initial elastic deformation and continued application of stress.

Cured-in-place Pipe (CIPP): A lining technique wherein a thin flexible tube of glass or polymer fiber or fabric is implanted with thermoset resin and inflated into position on the inner wall of a damaged pipeline using fluid pressure before curing the resin to harden the material. With or without the use of a turning belt, the uncured material can be inserted by winch or inverted by water or air pressure.


Deflection – The amount of bend that a drill rod shows. Drill heads are directed by pressing them into formations without rotating them. The rods can only be pushed so far before they begin to excessively deflect.

Deformed Reformed Pipe (DRP): A term that refers to various systems whereby a liner is bent during insertion to reduce its size, then returned to its original shape using heat and/or pressure.

DigiTrak F5: HDD locator manufactured by Digital Control Inc. Digitrak F5 locator was a replacement for Eclipse locator and flagship of Classic F Series product line. The locator featured 5 different frequencies, LWD Data-Logging, and colored display.

DigiTrak Transmitter: Directional drilling transmitter designed and manufactured by Digital Control Inc. Digitrak transmitters are designed to work with DigiTrak brand locators, and range in frequencies, depth and sizes.

Ditch Witch:
One of the biggest U.S. manufacturers of horizontal directional drilling equipment and tooling. Ditch Witch manufactures short to long range HDD rigs.

Drill String: 1) In a drill borehole, the total length of drill pipe or rods, bit, swivel joint, and other components. 2) Rod system for attaching to the drive chuck a cutting or compaction bit.

Drilling Fluid/Mud: A continually pumped mixture of water and usually bentonite and/or polymer to aid cutting, minimize necessary torque, facilitate cutting removal, stabilize the borehole, lower head temperature, and lubricate product pipe. Water alone may be employed in proper soil conditions. Learn more information about drilling fluids - Directional Drilling Fluids.

Double-entry Well – A well, drilled and completed from both ends, that is horizontal and directionally drilled. A pilot bore is bored at the required location and depth utilizing a guiding system and steering to establish a double-entry well. At the well's distal end, the pilot bore is directed to the surface and the drill bit is withdrawn. In most circumstances, prior to well casing installation, the pilot bore must be reamed to a bigger size, therefore the drill string has a reamer attached and is dragged back through the pilot bore. Additional drill rods are inserted behind the reamer as it retracts, ensuring the borehole sees a continuous string of drill steel throughout. For large bore diameters, this can be done in phases. The product line (well casing, conduit, etc.) is linked to the drill string and pulled back after the final diameter is reached.

Dry Hole – This circumstance develops when drilling tools have advanced past the drilling mud. Attempting to advance the borehole too quickly is the most common cause.

Duckbill: A different word for referring to the device for steering that is attached at the front of the directional drilling string.


Elastic Modulus: Stress buildup resulting from a given strain.

Enzyme Breaker – A liquid solution comprising proteins created by biological action that serve as a catalyst to accelerate long chain polymer biodegradation in drilling fluids (that are biodegradable).


Face: The place on the wall of the entrance pit where the bore first enters.

Filter Cake – A cake made up of stacked mineral particles in bentonite-based drilling mud that forms along borehole walls. Filter cake acts as a borehole-formation barrier, reducing the amount of drilling mud required and preventing groundwater intrusion.

Flexural Modulus of Elasticity: Defined mathematically as the stress divided by the material's strain; a measure of a material's stiffness or rigidity. A stiffer material has a higher flexural modulus.

Flexural Strength: The tensile stress of the outermost fibres at the point of failure, given as the bending strength of a material.

Fold and Form Pipe: A pipe rehabilitation process that involves pulling a plastic pipe in a folded shape with a smaller cross-sectional area into an existing conduit and then expanding it with pressure and heat. The reformed plastic pipe fits tightly against the host pipe's ID and conforms to its shape.

Forward Reamer – Reamer that enlarges borehole diameters in single-entry or blind wells.

Frac Out  – Drilling fluid moves from the drill head up the borehole and into a collection pit during routine drilling operations. The fluid pressure inside the borehole can fracture the surrounding formation if the borehole becomes clogged, collapses, or the fluid pressure inside the borehole becomes too high, producing a channel for the fluid to migrate from the well, often upward to the ground surface.

Front Locate Point – The magnetic field formed by the down hole transmitter is used by walkover locating devices to identify the azimuth of the drill head (sonde). When examined in map view, this magnetic field has an hourglass form, with a positive field in front of the sonde and a negative field behind it. The Front Locate is the point in the sonde's magnetic field where the field is both vertically directed and positive.


Gel Strength – A drilling fluid property that determines whether it can be used for suspension and transportation of borehole drill cuttings. It can be defined more particularly as the shear stress at low shear rate after a mud sets quiescently for a given time period (10 seconds and 10 minutes for standard API procedure, though after 30 minutes or 16 hours measurements may also be taken).

Guar – Guar is a liquid-thickening polymer and is a frequent food component. The dehusked seed of the Guar Tree, native to India and Pakistan, is milled to make it. Guar powder is blended with other polymers to make a drilling fluid.

Guided Auger Boring: A phrase used to describe auger boring devices that are comparable to microtunneling but include the steering mechanism actuator in the drive shaft (e.g., a hydraulic wrench turning a steel casing that has an asymmetric face at the cutting head). The word can also refer to auger boring systems that include rudimentary casing articulation at the head that is activated by drive pit rods.


Hydro-lock – During the pullback process, the screens and well casing get "locked" in the borehole. When a borehole collapses, drilling becomes trapped within, either behind or in front of well materials. The drill's ability to pull the casing into the hole is reduced (or eliminated) as pressure rises (or falls).

Hypochlorite – These chlorine and bleach chemicals, which are either sodium- (liquid) or calcium- (powder) based, are commonly used in cleaning and sterilization. To quickly reduce biopolymer drilling fluid viscosity, moderate quantities of hypochlorite are utilized. 


Impact Moling: Boring a hole in a casing with a pneumatic or hydraulic hammer, usually in the shape of a torpedo. The phrase is mainly connected with limited steering or non-steered devices that rely on ground resistance for forward movement and are not rigidly attached to the launch pit. Soil is displaced rather than removed during operation. In suitable ground, a pipe can be dragged or pushed in behind the impact moling tool, or an unsupported bore can be formed. Cables can be drawn in as well.

Inertial Guidance System – A navigation and steering system for an HDD drill, typically in bores that are both long and deep, with exact bore path requirements. In order to find and control the drill head in three dimensions, the system uses sensitive accelerometers and/or gyroscopes coupled to a computing subsystem through a wireline (run inside the drill rod). When depth, surface impediments, and/or radio interference prevent the use of a walkover in environmental drilling, this system is often used. For more information on drilling interference check out Understanding Horizontal Directional Drilling Interference.

Inversion: The technique of using air or water pressure to turn a resin-saturated tube inside out.


Jacking Frame: Housing component for hydraulic cylinders that move pipelines and microtunneling machines. The jacking frame distributes the thrust stress to the pipeline while also distributing the reaction load to the thrust or shaft wall.

Jacking Shield: Excavation is done either by hand or by machine from within this fabricated steel cylinder. Facilities are built into the cylinder that allow for it to be modified to manage line and gradient.


Launch Pit: Another term for Drive Pit, typically used in conjunction with the "launching" of an impact moler.

Liner Plate: Instead of casing, this product lines tunnels and comes in formed steel segments. When fastened together, they form a structural tube that prevents tunnel collapse. The segments have been designed to be fastened together from the inside of the tunnel.

Locator – A steering technician on a drilling crew uses this hand-held electronic instrument in conjunction with a downhole sonde to estimate horizontal placement and drill head depth while drilling. A walkover navigation system employs locators, which normally necessitate the technician having physical access to the ground surface right above the bore path. In a walkover system, the sonde transmits a radio signal that includes the drill head's pitch and rotational direction. A sonde-emitted magnetic field is also interpreted by the locator to identify the depth and horizontal position. The steering technician can record the pitch, depth, and rotational orientation using the locator's displays. Data can also be transmitted by a locator to a remote drill rig display, where a directional driller can interpret it. The most popular HDD locators on the market today are DigiTrak and Subsite.

Because the depth, direction, and location of the drill head are calculated using the field shape and strength of broadcast electromagnetic fields, local interference arising from electronics or ferrous masses can occasionally cause this surveydata to be skewed. A wireline system, replacing the battery with an electrical conductor that pulls power from the drilling rig electrical system, can increase the signal, allowing it to be used in deeper borings or areas where electronic interference is present.

Lubricity – A given material’s characteristics of lubrication. A borehole’s lubricity can be improved by


Microtunneling: A trenchless pipeline installation technology that includes the following features: (1) Remote control – The microtunneling boring machine (MTBM) is controlled from a control panel on the surface. Routine operations do not necessitate personnel entry. (2) Guided - In the MTBM, the guiding system is usually based on a laser beam focused onto a target. (3) Pipe jacking - The technique of installing a pipeline by sequentially pushing the drill pipes and MTBM through the ground with the use of a jacking system. (4) Continuously supported - To balance groundwater and earth pressures, continuous pressure is applied to the excavation face.

Mixed Face: A soil condition in which the bore passes through two or more different types of material.

Modulus of Elasticity (E): The required stress for producing strain that results in a change of length (Young's modulus); a twist or shear (modulus or rigidity); or a change of volume (bulk modulus). Units are dynes per square centimeter.

Mud Motor: Rock drilling technology which derives its power from drilling fluid pressure. Instead of ejecting mud from a bore hole, a mud motor uses flow pressure to spin the drill bit, allowing for rotary cutting and simultaneous steering.


Open Cut: The procedure for gaining access to the necessary level underground for installation, inspection, or maintenance of a cable, pipe, or conduit. Following this, the excavation is backfilled and the surface is restored.

Open Face Shield: A shield at the front of a pipe jack within which manual excavation is performed inside a steel tube.

Ovality: The degree to which the cross-section of a pipe deviates from circularity or perfect roundness.


Packer – A mechanical device that isolates borehole or well segments. Packers can split wells into two or more compartments, each of which can be controlled separately. This also provides a component of an annular seal to prevent surface water infiltration and the exchange of extracted or injected fluids (air, chemical, or other) between the surface and well.

Pin – The drill rod end with male threading.

Pitch – In a horizontal drilling system, the drill head's vertical angle. This is measured in degrees or percent. Wireline or inertial systems typically measure in degrees, while most walkover systems utilize percent. Pitch gives the drill string's dip at or near the drill bit via direct measurement.

Pilot Bore: The making of the first (typically steerable) pass in any boring procedure requiring back-reaming or other enlargement afterwards. Guided boring, 2-pass microtunneling, and directional drilling are the most common examples.

Pipe Eating: A microtunneling-based replacement approach whereby a damaged pipe is dug out, along with the surrounding soil, as though it were a new installation. To function properly, the microtunneling shield machine will almost always require somecrushing capabilities. To improve steering performance, the faulty pipe could be plugged with grout. Some systems, on the other hand, use a proboscis mechanism to cover the pipe in front of the shield, collecting and diverting the existing flow, allowing a sewer, for example, to stay "active."

Pipe Jacking: A method for directly laying pipes behind a shield machine using hydraulic jacking from a drive shaft, resulting in a continuous string of pipes in the ground.

Pipe Ramming: A non-steerable technique for producing a bore in which an open-ended steel casing is driven from a Launch Pit with a percussion hammer. Augering, jetting, or compressed air can all be used to remove soil from the casing.

Pipe Splitting: Slitting an existing pipe longitudinally as a replacement procedure. A new pipe of the same diameter or greater can be pulled in behind the splitting tool at the same time. Pipe Bursting is another term for this.

Polymer Mud – Long-chain organic compounds make up this drilling mud. Polymer mud may be biodegradable as well. Horizontal remediation wells should be constructed with biodegradable polymer mud.

Pothole – A small hole dug from the surface to reach a subterranean utility and provide positive confirmation of its placement.

Pre-pack Casing – A well screen created by taking two existing well screens and placing the one with a larger diameter over the other, then infilling the area between them with granular filtering material, such as ceramic beads, sand, or other media. The end caps keep the two screens in place. Because of its rigidity, pre-pack is difficult to install in horizontal environmental wells and is prone to breaking.


Ramming: An open-ended casing is driven through the ground with a percussion hammer attached. To ensure the casing is left open, the spoil within it is removed.

Rear Locate – The point along the drilling route behind the front locate point and drill bit when a walkover locating system is being used. The point in the sonde's magnetic field that is both vertically directed and negative.

Reamer – Used to enlarge borehole diameter after drilling a pilot bore.

Reception/Exit Shaft/Pit: An excavation that trenchless technology equipment is driven into and recovered from once the product pipe, cable, or conduit has been installed.

Remote Display – When using walkover equipment, a locating technician usually holds a direct reading receiver as well as a remote unit for receiving radio signals that contain locator information. With a remote display, the driller can see the same real time data information as the locator from the drilling machine.

Resin Impregnation (Wet-out): A method for installing cured-in-place pipes in which a plastic-coated cloth fabric tube is saturated uniformly by a liquid thermosetting resin while air is vacuumed out of the coated tube.

Riser – A well casing (non-perforated or non-slotted) extending between the ground surface and a well screen.

Rod Wiper – Synthetic or rubber grommet that goes overtop of drill rods when they are pulled back, stripping excess mud from rods before stowing them.


Screen – A well screen is a perforated or slotted pipe inserted in a borehole that allows hydraulic interaction with the surrounding formation's soil natural gas or ground water. It is attached to a riser drill pipe or a well casing, allowing ground surface access for pump installation, fluid conveyance, sampling, and other purposes.

Shield: A steel cylinder at the face of a utility casing or tunnel, which may or may not be steerable and employ a mechanical excavator while providing hazard protection from the region covered.

Sliplining: (1) A general word that refers to both continuous and discrete pipe lining new technologies. (2) Insertion of a new pipe into an existing pipe by tugging or pushing it in and grouting the annular gap. A continuous pipe or a string of separate pipes can be employed. Segmental Sliplining is a term used to describe the latter.

Slurry: During microtunneling, a fluid, usually water, that is utilized in a closed loop system to remove spoil and maintain groundwater pressure balance.

Sonde – Another word for transmitter – an electronic locating system’s downhole component.

Strike Alert – A drilling safety device that triggers an alarm whenever a drill string comes into contact with an underground electrical utility.

Swab (Bull Plug): A steel plug used to remove cuttings from a horizontal bore by being pulled through it.


Transmitter: Another word for a sonde. The electronic locating system component positioned immediately behind the drill string's drill bit (downhole). Gravity-sensing accelerometers in the device allow the pitch and roll to be determined. This information is sent to the ground surface, where the locating technician receives it via radio signal or direct continuous wire (within the drill rod). When used in conjunction with a walk-over locator, the transmitter also produces a magnetic field that can be utilized to determine an over bit position. Ferrous metals (either on the ground surface or in the ground) can bend the magnetic field's orientation, affecting the precision of the over bit location detection.

Thermoset: Materials like epoxies that will or have already undergone a chemical reaction caused by heat, a chemical catalyst, ultraviolet radiation, or other factors, resulting in their infusible condition.

Thread Compound – To keep the drill rod threads from seizing, an anti-seizing compound, usually a high-pressure copper-oil based lubricant, is utilized. Threads on a casing or threaded well are also treated with it. For remediation or sampling wells, environmental impact friendly, inert chemicals can also be used.

Trenchless Technology: Utility line replacement, renovation, installation, repair, location, rehabilitation, inspection, and leak detection techniques that require minimal ground surface excavation.

Tunnel Boring Machine (TBM): (1) A circular, full-face, mechanized shield machine with a rotating cutting head that is steerable and usually of Man-Entry diameter. It is used to lead a string of jacked pipes during drill pipe installation. It may be manipulated either from inside the shield or from afar. (2) (Mole, Tunneling Head) In a tunnel, a mechanical excavator used to excavate the tunnel's front face.

Tunneling: A method for excavating a beneath-ground hole without disturbing the ground surface and with a large enough diameter to give individuals access and to install a ground support system at the excavation site.


Upsizing: Any procedure involving the replacement of a smaller diameter pipe with a larger diameter pipe to expand the cross sectional area of an existing pipeline.

Utility Tunneling: The procedure of constructing a temporary support liner as a tunnel is excavated. Steel or concrete liner plates, steel ribs with wood lagging, or an all-wood box culvert are all common options for the liner. Personnel must carry out excavation and/or spoil removal inside the tunnel.


Wing Cutters: When turned in a forward direction, these appendages on cutting heads open to expand the cutting diameter of the head, then close when reversed. They're employed to make room for the casing pipe by cutting clearance.

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