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ANNULAR BLOWOUT PREVENTER - An annular blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. It is usually installed above the ram preventers that forms a seal in the annular space between the pipe and well bore or, if no pipe is present, over the well bore itself. 


ANNULAR VELOCITY - The speed at which drilling fluid or cement moves in the annulus. 


ANNULUS - The annulus is the area between two concentric objects, such as, between the wellbore and casing or between casing and tubing, where fluid can flow.


ANTI-BUCKLING GUIDE – A device located between the top of the stripper assembly and the bottom of the injector gripper blocks that will support the coiled tubing as it pushed into the well control stack preventing or reducing the tendency for the tubing to buckle in this area. 


AXIAL FORCE - The force acting along the axis of an object. In wellbore tubulars, axial loading is typically expressed as tension or compression and may result from applied conditions such as set-down-weight, or be induced by operating conditions or variations such as changes in temperature that cause expansion or contraction of components. 


AZIMUTH - The compass direction of a directional survey or of the wellbore as planned or measured by a directional survey. The azimuth is usually specified in degree with respect to North Pole. For example, a well that goes east has an azimuth of 90 degrees. 


AT BALANCE – The pressure at the bottom of the hole exerted by the column of drilling fluid is the same as the formation pressure. 




BACKFLOW – Fluid flow in a direction opposite to the intended direction of flow. 


BEAM PUMPING UNIT / SUCKER ROD PUMP - An artificial-lift pumping system using a surface power source to drive a downhole pump assembly. A beam and crank assembly creates reciprocating motion in a sucker-rod string that connects to the downhole pump assembly. 


BENT SUB - A short cylindrical device installed in the drilling assembly above the downhole motor, the bent sub allows a bend to be put in the assembly to allow the drilling assembly to build or drop inclination . 


BIT – The tool at the bottom of the drilling assembly that is used to cut the formation. The size of the bit determines the size of the Open hole diameter. The most common bits used for CTD are the polycrystalline diamond compact (PDC) bits. 


BLEED OFF - To equalize or relieve pressure from a vessel or system.


BLEED-OFF LINE - A section of manifold containing the valves and piping necessary to bleed off pressure from a vessel or system. 


BLIND RAMS - A thick, heavy steel component of a conventional ram blowout preventer. In a normal pipe ram, the two blocks of steel that meet in the center of the wellbore to seal the well have a hole (one-half of the hole on each piece) through which the pipe fits. The blind ram has no space for pipe and is instead blanked off in order to be able to close over a well that does not contain a drill string. It may be loosely thought of as the sliding gate on a gate valve. 


BLOW OUT PREVENTOR – Large valve that goes on the top of a well. This valve is closed when there is an unexpected and uncontrolled flow of formation fluids form a well. 


BLOWOUT - Uncontrolled flow of formation fluids or gas from a well. 


BOREHOLE – Deliberately drilled hole directed to intersect hydrocarbon reserves. 


BOTTOM HOLE TEMPERATURE – The temperature at Total Depth inside the wellbore. 


BOTTOMHOLE ASSEMBLY – The assemblage of tools and equipment installed on the end of the coiled tubing string used to perform a prescribed service in the well.


BOTTOMHOLE ASSEMBLY CONNECTOR – A mechanical device used to attach the bottomhole assembly tools to the coiled tubing. 


BOTTOMHOLE CHOKE - A hydraulic component whose diameter is deliberately restricted to control flow rate.


BOTTOMHOLE PRESSURE - The pressure at the bottomhole. For non-flowing condition, it is caused by the hydrostatic pressure of the wellbore fluid and by any backpressure held at the surface, if any. When wellbore fluid is being circulated, bottomhole pressure is the hydrostatic pressure plus the friction pressure drop in the annulus. 


BRIDGE PLUG – A tool employed as temporary or permanent barrier in the casing string or open hole; either permanent or retrievable. A downhole tool composed primarily of slips, a plug mandrel, and a rubber sealing element that is run and set in casing or open hole to isolate a lower zone while testing an upper section. 


BUCKLING – The critical compressive load which induces bending failure. It happens when the bending stress is greater than the compressive stress of the material. 


BUILD RATE - The “build rate” is the positive change in inclination over a normalized length (e.g., 3°/100 ft.) 




CASING - Large-diameter pipe lowered into an open hole and cemented in place. Casing is run to protect the wellbore from fluids, pressures, wellbore stability problems, etc and contain well pressures if the well is shut in. 


CASING BURST PRESSURE – The amount of pressure that when applied to a string of casing, causes the wall of the casing to fail. This pressure is critically important when a gas kick is being circulated out because gas on the way to the surface expands and exerts more pressure than it exerted at the bottom of the well.


CASING COLLAPSE PRESSURE - The pressure at which the casing will catastrophically deform as a result of differential pressure acting from outside to inside of the vessel or tube. The collapse-pressure rating of perfectly round tubing is relatively high. However, when the tubing is  not perfectly round, the differential pressure at which the tube will collapse may be  much lower. 


CASING SHOE – A short, heavy, hollow, cylindrical steel section with a rounded bottom that is placed on the end of the casing string to serve as a reinforcing shoe and to aid in cutting off minor projections from the borehole wall as the casing is being lowered.


CEMENT PLUG – A volume of cement that is pumped into the well bore and hardens that has different applications including kicking off for sidetracking. 


CHOKE - A hydraulic component whose diameter is deliberately restricted to control flow rate. 


CHOKE LINE - A line, or pipe, that runs from the blowout preventer stack to the choke manifold through which fluid from the hole is flowed when the well is shut in with the blowout preventer. 


CHRISTMAS TREE – The control valves, pressure gauges, and choke assembled at the top of a well to control the flow of oil and gas after the well has been drilled and completed. Also known as “xmas tree”. 


CIRCULATING PRESSURE – The pressure generated by the mud pumps and exerted on the drilling fluid system. 


COILED TUBING - A continuous string of flexible steel tubing, often hundreds or thousands of feet long, that is wound onto a drum. 


COMPLETE A WELL – To finish work on a well and bring it to productive status. 


CORKSCREW – The buckling of tubing in a large-diameter pipe or casing. 


CUTTINGS - The fragments of rock dislodged by the bit and brought to the surface in the drilling mud. Cuttings samples can be analyzed by geologists to obtain information about the formations drilled Cuttings are conveyed to the surface by drilling mud. 




DEAD WELL – A well that has ceased to produce oil or gas, either temporarily or permanently. A well that has kicked and been killed. 


DEPLOYMENT BARS – Pressure isolated bars installed in the bottomhole assembly used for the safe insertion and removal of tool strings in two or more sections with surface pressure present, if the tool string length exceeds the available height of the well control stack above the tree. 


DEVIATION – Inclination of wellbore from vertical.  This angle may be measured by a deviation survey. 


DIAMETRICAL GROWTH – The enlargement of the coiled tubing diameter due to the effects of applied internal pressure when the coiled tubing is subjected to bend-cycling events. 


DIFFERENTIAL PRESSURE – The difference in pressure between the hydrostatic head of drilling-fluid column and the formation pressure at any given depth. It can be positive, zero or negative with respect to the hydrostatic pressure. 


DIFFERENTIAL STICKING – this occurs when the well bore fluid pressure is greater than the formation pressure. The imbalance in pressure and force can cause the drill string to become stuck against the wellbore wall especially with a slow moving or stationary string. 


DIRECTIONAL DRILLING - Intentional deviation of wellbore from the vertical. Controlled directional drilling allows access to subsurface areas laterally remote from the point where bit enters the earth. 


DIRECTIONAL SURVEY – A logging method that records hole drift, or deviation from the vertical and direction of the drift. A single-shot directional survey instrument makes a single photograph of a compass reading of the drift direction and the number of degrees the hole is off vertical. A multishot survey instrument obtains numerous reading in the hole as the device is pulled out of the well. Very accurate surveys can be taken with gyro based tools. 


DOGHOUSE – Small enclosure on rig floor used as an office for the driller or storehouse for small objects. 


DOGLEG - The severity of a sudden change of 3D wellbore trajectory. It is normally expressed in terms of degree per 100 feet (deg/30m) of wellbore length. 


DRILL BIT – The cutting or boring element used for drilling. 


DRILLING BREAK – A sudden increase in the rate of penetration by the drill bit. It sometimes indicates that the bit has penetrated a high-pressure zone and thus warns of the possibility of a influx of fluid or gas pinto the well from a high porosity or high permeability zone. 


DRILLSTRING – It is formed by the pipe and the drilling BHA. 




ELASTOMER – A seal; a rubber sealing member such as an O-ring, vee ring, face seal etc. 


E-LINE – term associated with electric logging and cables.   An E-line cable serves the dual purpose of physically suspensing the tool in the hole while providing a conduit for electrical power and telemetry.An e-line cable can be run inside coiled tubing to transmit data and instructions from the surface to downhole tools and from the logging tools back to the surface equipment. 




FATIGUE – Weakness in a material caused by the repetition of variations in stress. 


FISH - An object that is unintentionally left in the wellbore during drilling or workover operations and that is normally recovered before work can proceed. The term can be applied to a simple hand tool inadvertently dropped from the surface, and equally to a multi million dollar drilling assembly lost while drilling . 


FISHING - The procedure of recovering lost or stuck equipment in the wellbore. 


FLAPPER VALVE – A hinged closure mechanism operating in a pivot manner, used to shut off tubing flow. 


FLOW TUBE – Internal device commonly found in subsurface safety valves used to protect the tool’s closure mechanism from the wellbore mediums. 


FLUID DENSITY/FLUID WEIGHT – The unit weight of fluid; e.g. Pounds per gallon (lb/gal).


FORMATION - A bed or deposit composed throughout of substantially the same kinds of rock; a lithologic unit. 


FORMATION PRESSURE - The pressure of the subsurface formation fluids, commonly expressed as the density of fluid required in the wellbore to balance that pore pressure. 


FRACTURE PRESSURE - The pressure at which a formation will break down, or fracture. 




GAMMA RAY LOG – a down hole electric log that shows the amount of naturally occurring gamma radiation a rock gives of. 


GAS LIFT – Process of raising or lifting fluid from a well by injecting gas down well through tubing or through tubing-casing annulus. Injected gas aerates fluid to make it exert less pressure than the formation; consequently, higher formation pressure forces fluid out of wellbore. 


GAS WELL – A well that primarily produces natural gas. 


GAS-CUT MUD - A drilling mud that contains entrained formation gas, giving the mud a characteristically fluffy texture. Gas cut mud may unintentionally lower the fluid density. 


GAS-OIL RATIO – A measure of the volume of gas produced with the oil; expressed in cubic feet per barrel or in cubic meters per metric ton, or in cubic meters per cut meter. 


GOOSENECK – A curved section of pipe that goes from the CT reel to the wellbore. It is also called guide arch. 


GRAPPLE – A tool used to fish a stuck tool by grabbing it. The part of a catching tool that engages the fish. 


GUIDE ARCH – See Gooseneck.


GUY LINE – A wireline attached to a mast or derrick to stabilize it. The lines that provide the main support for the structure are load guys; the lines attached to ground anchors for lateral support are wind guys. 




HELICAL BUCKLING - A more extreme form of buckling that occurs when compressive forces pass through sinusoidal buckling and exceed the helical buckling limit. Helical buckling causes contact between the pipe and the wellbore. 


HOOK LOAD – The weight of pipe suspended in the well as read on the rig’s weight indicator. 


HORIZONTAL DRILLING - Deviation of the borehole from vertical so that the borehole penetrates a productive formation in a manner parallel to the formation.


HYDROSTATIC PRESSURE - The force per unit area exerted by a static column of fluid. In US oilfield units, it is calculated using the equation: P (psi) =Density*TVD*0.052, where Density is the fluid density in pounds per gallon (ppg), TVD is the true vertical depth in feet (ft.), and 0.052 is a conversion factor. 





INJECTOR - The injector head incorporates special profiled chain assemblies to grip the coiled tubing string and a hydraulic drive system that provides the tractive effort for running and retrieving the string from the wellbore. The base of the injector head is secured to the wellhead pressure-control equipment by the stripper assembly mounting system. The gooseneck mounted on top of the injector head feeds the tubing string from the reel around a controlled radius into the injector head. 




KICKOFF POINT (KOP) - The depth in a vertical hole at which a deviated hole is started.


KILL – The process of controlling and stopping an influx of water, gas, oil, or other formation fluid into the wellbore during drilling. 


KILL LINE – A high pressure line that connects the mud pump and the blowout-preventer assembly through which drilling fluid can be pumped into the hole to subdue the well pressure while the preventers are closed. 





LOG – A systematic recording of data, as from the driller’s log, mud log, electrical well log, or radioactivity log. Many different logs are run in wells being produced or drilled to obtain various characteristics of downhole formation. To record data. 


LOGGING WHILE DRILLING (LWD) – Measurement of different parameters downhole while the well is created. It is also called Measurement While Drilling (MWD) although MWD sometimes only refers to well inclination and azimuth taken downhole. 


LOST CIRCULATION - A lack of mud returning to the surface after being pumped down a well. Lost circulation occurs when the drill bit encounters natural fissures, fractures or caverns, and mud flows into the newly available space. Lost circulation may also be caused by applying more mud pressure (that is, drilling overbalanced) on the formation than it is strong enough to withstand, thereby opening up a fracture into which mud flows. 




MANIFOLD - An arrangement of piping or valves designed to control, distribute and often monitor fluid flow. 


MEASURED DEPTH (MD) – Total depth reached by the bit when measured following the wellbore trajectory.


MEASUREMENT WHILE DRILLING (MWD) – See Logging While Drilling (LWD). 


MECHANICAL JAR - A percussion tool operated mechanically to give an upward or downward thrust to a fish by the sudden release of a device inside the tool. 


MILLING – Cutting material or equipment inside the wellbore. It usually refers to window milling, which refers to the cutting of a section of the casing in order to drill the sidetrack. 


MOUSEHOLE – A hole in the rig floor in which the BHA is held ready to be picked up.


MUD MOTOR - A positive displacement drilling motor that uses hydraulic horsepower of the drilling fluid to drive the drill bit. Mud motors are used extensively in directional drilling operations. 




NEWTONIAN FLUID - A fluid that has a constant viscosity at all shear rates at a constant temperature and pressure, and can be described by a one-parameter rheological model. 


NIPPLE – Completion component in the shape of a ring with thick walls that sits within the casing string. It is frequently used to land other completion components on it. 


NIPPLE UP - Assembling process for well-control or pressure-control equipment on the wellhead. 


NITROGEN LIFT - The use of nitrogen gas circulated into the production conduit to displace liquids and reduce the hydrostatic pressure created by the fluid column. 


NO-GO NIPPLE – It is a nipple with a very small diameter that does not leave enough space for a tool to go through.



OFFSHORE – Not land based


ONSHORE – Land based.


OPEN HOLE - the uncased portion of a well that is exposed to rock formation. 


OVALITY – The change in roundness of a tube body due to external forces derived from bend-cycling activities.


OVERBALANCE – The amount by which pressure exerted by the hydrostatic head of fluid in the wellbore exceeds formation pressure. 


OVERBURDEN – The pressure of the earth’s crust on a formation. The strata of rock that lie above the stratum of interest in drilling.


OVERPULL – Pull on pipe beyond its weight in either air or fluid.




PACKER - A device that can be run into a wellbore with a smaller initial outside diameter that then expands externally to seal the wellbore. Packers employ flexible, elastomeric elements that expand. 


PACK OFF – A build-up of cuttings around the BHA or drill string which prevents circulation.


PDC BIT - A drilling tool that uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous shearing motion. These cutters are synthetic diamond disks about 1/8-in. thick and about 1/3 to 1 in. in diameter. PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds. 


PERFORATE – To pierce the casing wall and cement to provide holes through which formation fluids may enter or to provide holes in the casing so that materials may be introduced into the annulus between the casing and the wall of the borehole. Perforating is accomplished by lowering into the well a perforating gun, or perforator that fires electrically detonated shaped charges from the surface. 


PIPE HEAVY - An operating condition during a snubbing operation in which the force resulting from the weight of the pipe or tubing string is greater than the wellhead pressure and the buoyancy forces acting to eject the string from the wellbore. The string will drop into the wellbore if the gripping force is lost in the heavy pipe condition. 


PIPE LIGHT – An operating condition during a snubbing operation in which the wellhead pressure and buoyancy forces are greater than the force resulting from the weight of the pipe or tubing string. In the light-pipe condition the string will be ejected from the wellbore if the gripping force of the slips is lost. 


PIPE RAMS - A type of sealing element in high-pressure split seal blowout preventers that is manufactured with a half-circle hole on the edge (to mate with another horizontally opposed pipe ram) sized to fit around drillpipe. Most pipe rams fit only one size or a small range of drillpipe sizes and do not close properly around drillpipe tool joints or drill collars. Another type is the variable bore ram, which is designed and manufactured to properly seal on a wider range of pipe sizes. 


PORE PRESSURE – Pressure exerted by fluids within formation pore space. 


POUNDS PER GALLON US (PPG) – A unit of density equivalent to 119,83 kg/m3. 


PULL OUT OF HOLE (POOH) – Action of taking all the drilling components out of the wellbore. 

PUP – A short section of tubing or drill pipe used for properly spacing out. 




RATE OF PENETRATION (ROP) – Feet per hour or meters per hr at which the drill deepens wellbore. 


RATHOLE – A hole in the rig floor in which a drilling kelly can be stored or, an additional or extra length of open hole to provide a sump for electric logging tools. 


REAM - To move the drilling assembly up and down a well section in order to ensure that it is the required diameter.


RESERVOIR PRESSURE – See formation pressure. 


RISER – A large-diameter pipe that connects the subsea BOP stack to a floating surface rig to take mud returns to the surface. Without the riser, the mud would simply spill out of the top of the BOP onto the seafloor. The riser might be loosely considered a temporary extension of the wellbore to the surface. 


ROLLER CONE BIT – Bit made up of one two orthree rotating cones mounted on rugged bearings. Also called rock bits. The surface of each cone is made up of row of steel teeth or rows of tungsten carbide inserts. 


ROUND TRIP – The action of pulling out and subsequently running back into the hole a string of drill pipe or tubing. It is also called tripping the pipe. 


RUN IN HOLE (RIH) – The action of inserting all the BHA components inside the wellbore on either jointed pipe or on coiled tubing . 




SHEAR RAMS – The rams in a well control stack that cuts through the drill string or casing with hardened steel shears. 


SHUT-IN BOTTOMHOLE PRESSURE – The pressure at the bottom of a well when the surface valves on the well are completely closed, this is caused by fluids that exist in the formation at the bottom of the well.


SIDETRACK – A wellbore created by ‘deviating or ‘kicking off’ from the original wellbore. It can either be horizontal, deviated or vertical. A sidetrack can also have several additional sidetracks from the new sidetracked hole. 


SINUSOIDAL BUCKLING - Sinusoidal buckling occurs when compressive forces on the string become too high, resulting in a snake-like bending in the string.Note that in this mode, the pipe deforms, but still in a 2D plan. 


SKIN – Area around the wellbore where the permeability has changed, generally due to fluids and solids from the wellbore invading this area and causing near bore formation damage. 


SLIP RAMS – The rams in a well control stack which are equipped with tubing slips that, when engaged, prevents movement of the coiled tubing but do not isolate pressure or control flow. 


SLOUGHING SHALES – Reactive type of formation that increases its size and becomes unstable when it is in contact with water or water based fluids. 


SNUB – To put pipe or tools into a high-pressure well that has not been killed (i.e. to run pipe or tools into the well against pressure). 


STRIPPER – A device with a resilient elastomeric element used to effect a seal in the annulus. This device is used primarily to isolate well pressure from the atmosphere when injecting or extracting the coiled tubing in pressurized wellbores. 


STUCK PIPE – Portion of drill string or tubing that cannot be moved or steered. 




TIME DRILLING – Technique by which a low Weight on Bit and bit rotation is applied and the bit is maintained in the same depth for a specific period of time. This action can create a ledge and is used to kick off or sidetrack in open hole or from a cement plug. 


TOOLSTRING – It is formed by the bit, the motor and the BHA. 


TOTAL DEPTH (TD) – Measured Depth at the end of the well. 


TOTAL VERTICAL DEPTH (TVD) – Depth of the well measured vertically from the surface. 


TRIP IN - Move the pipe into the wellbore.


TRIP OUT - Move the pipe out of the wellbore. Sometimes, it is called pick up. A pipe trip is usually because the bit has dulled or has otherwise ceased to drill efficiently and must be replaced. 




UNDERBALANCED DRILLING – Refers to a situation when the pressure (or force per unit area) exerted on a formation exposed in a wellbore is less than the internal fluid pressure of that formation. If sufficient porosity and permeability exist, formation fluids enter the wellbore. The drilling rate typically increases as an underbalanced condition is approached. 


UNLOADING - To initiate flow from a reservoir by removing fluid from the tubing in a well.




WEIGHT ON BIT (WOB) - Weight on bit is the amount of downward force exerted on the drill bit.


WELLBORE – See borehole.


WELLHEAD – The equipment used to maintain surface control of a well. 


WHIPSTOCK – A long, steel tool that uses an inclined plane to cause the bit to deflect from the original wellbore at a slight angle. Whipstocks are commonly used in directional drilling to to sidetrack the well in open hole or from inside casing. Whipstocks or “casing exit guides” are frequently used for re-entry drilling with coiled tubing. The guide can be run in the hole and properly oriented with wire-line or coiled tubing. The guide then serves as a ramp for the milling assembly to create an opening in the casing for the BHA and coiled tubing to subsequently drill the planned extension. 


WINDOW (WINDOW MILLING) – A hole cut in the casing that is used to exit from and sidetrack the well. The action of cutting the aperture is called window milling. 


WIPER TRIP – Short movement of the drill string up in the wellbore to assist in hole cleaning and to check the hole is free from cuttings and obstructions higher up. 


WIRELINE – See e-line.


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