Glossary of Drilling Operations

Back Reaming: This is the process of rotating the drill string in direction up hole to enlarge the wellbore or to clear up obstructions along the well path.

Barite: This is natural occurring barium sulphate used for increasing density of drilling fluids

Barrel: Unit of volume in the petroleum industry. A barrel is 42 US gallon or 38 Imperial gallons

Barrier: An obstacle, process or circumstance that keeps an activity or process from progressing

Bi-center Bit: A bi-center bit consists of a pilot bit similar to a conventional bit and a reamer section with cutters just on one side above the pilot bit. The hole drilled by the pilot hole is enlarged by the reamer above the pilot bit.

Bit Balling: This occurs when cuttings sticks on the bit preventing contact between the formation to be drilled and the bit cutter face.

Bit Dulling: Wearing out of bit as it continues to drill ahead.

Bit Space Out (Conductor Jetting): This is a jetting arrangement where bit does not stick out of the casing shoe while jetting

Bit Stick Out (Conductor Jetting): This is a jetting set up where by drill bit stick out below the casing shoe

Blast Wall: This is a barrier designed to protect vulnerable buildings or other structures and the people inside them from the effects of a nearby explosion

Blow Out: An uncontrolled flow of drilling fluid, oil, gas or water from the well caused by loss of hydrostatic pressure to a value below the formation pressure

Blow Out Preventer: A large device/valve at the top of a well that may be closed if the drilling crew loses control of the well if an influx in taken

Booster Pump: This is an auxiliary pump installed in a system to increase or maintain the pressure or flow within circulatory system

Bore Protector: This is a tool run across the wellhead to protect it from wear during drilling

Bottom Hole Assembly: It is the lower part of the drill string, extending from the bit to the drill pipe. It consists of the drill collars, stabilizers, measuring while drilling (MWD) tools etc.

Bottom Hole Balling: This is when the hole gets clogged up with cuttings preventing contact between the drill bit and virgin rock to be drilled

Bottom Hole Pressure: This is the pressure at the bottom of the hole. It is the larger of the hydrostatic pressure at the bottom due to mud column and the pore pressure of the formation

Bottoms Up (Circulating Bottoms Up): This is the wellbore annular volume from bottom of the hole to the surface. Circulating bottoms up is the displacement of the entire annular volume to surface by pumping into the wellbore.

Breaking Circulation: To establish circulation of drilling fluids after a period of static conditions such as taking a survey or making a connection, or after a prolonged interruption, such as after a round trip

Bridge: An obstruction in the wellbore usually caused by caving of the wall of the wellbore

Bridging Document: This document aligns the requirements and responses of various parties involved in an operation. It is commonly used to align the emergency response procedures for operators and contractors

Buckling: Bending and subsequent spiraling of a pipe due to applied compression load

Buffer Zone: An off limit area set aside during high risk operation to prevent personnel exposure to risk of injury

Build Rate: Also Build Up Rate is the rate at which angle of inclination is increasing with length of wellbore drilled expressed in degrees per 100 feet

Bump (Cementing): This is an increased in pump/surface pressure during cementing indicating that the top plug has landed on top of the bottom plug.

Caliper Log: This is a tool used for measuring the diameter and the shape of the wellbore. It provides continuous measurement of size and shape of the wellbore along its depth.

Capping Stack: A device that is designed to be placed on the wellhead to control, divert and shut in on a flowing well during a well containment operation.

Casing: A large diameter pipe inserted into a recently drilled hole and cemented in place.

Casing Drilling: This is a drilling operation where wellbore is drilled with bit on casing rather than a conventional method of drilling with a drill string and pulling out to run in with casing.

Casing Running: This is drilling activity where casing joints are assembled and inserted into a drilled borehole.

Casing Shoe: A short assembly typically manufactured from a heavy steel and profiled cement interior that is screwed to the bottom of a casing string. The profile (which can be rounded or eccentric) helps guide the casing string past any ledges or obstructions that would lead to casing being stuck. Casing shoe is also the bottom of the casing and including the cement around it.

Cation Exchange Capacity: Also called Methylene Blue Test is used to determine the amount of reactive clay (clay-like materials) in water base mud. A methylene blue dye (a cation dye) is utilized for this test because it powerfully magnetizes the negative ions in the clay. The test is typically reported in terms of the reactive clay concentration in pounds per barrel, bentonite equivalent.

Caving: Unstable formation that disintegrates in large sizes and fall into the wellbore

Cement Bond: This is the measure of the bond between cement and casing as well as between cement and formation

Cementing: This is an operation by which cement slurry is pumped down through the casing and out at the bottom of the casing to fill the annulus between the casing and the wall of the wellbore

Cement Hydration: This is the reaction that occur between cement and water during setting and gel strength development

Cement Plug: A volume of cement placed at some interval within the wellbore to prevent fluid movement and/or pressure transmission

Cement Squeeze: A remedial cementing operation designed to force/inject cement into leak paths in wellbore tubular or wellbore annulus. Squeeze cementing operations may be performed to repair poor primary cement jobs, isolate perforations or repair damaged casing or liner.

Cement Stinger: Small diameter tubing used to place cement in the wellbore during plug cementing operations

Centralizers: A mechanical device attached to the casing to provide clearance between the wellbore and the casing. Also to minimize contact area between the casing and the wellbore

Channeling (Cement): This is the flow of cement through a path of least resistance in the annulus preventing full cement coverage around the entire annulus.

Circulation: This is the movement of drilling fluid from the suction pit through pump, drill string, bit, annulus and back to the suction pit

Circulation, Loss of or Lost Circulation: This is the escape of drilling fluid into the formation leading to reduction of flow out of the wellbore

Clearance: This is the space between the wellbore and drill string or casing.

Clear-by Design: This is an operational design in which an off limit area is set aside during high risk operation to prevent personnel exposure to risk of injury. See Buffer Zone

Compressive Strength: Resistance of a material to break under compression

Connection Back-Off: Unscrewing of two pipes that are fastened together

Connection Make-Up: This is the process of fastening two joints of pipe together

Contact Area: This is the area of contact between drill string or casing and the wellbore

Contamination: This is the presence of any foreign material that negatively affects properties of drilling mud and cement

Control Drilling: This is a drilling process where fixed rate of penetration is maintained by controlling the weight on bit

Critical Speed/RPM: The speed/RPM at which natural frequency of the drill string is excited.

Cutting (s): These are broken bits of solid materials from a drilled borehole

Cutting Bed: Accumulated cuttings on the low side of the wellbore

Cutting Bed Impeller: This is a tool that reduced torque and drag by removing cuttings that settle out on the low side of the wellbore.

Cuttings Loading: This is the fraction of drilled cuttings that are inside of the borehole

Deflocculant: Chemical added to drilling fluid to cause particles to settle out of the fluid

Deflocculation: This is the breakup of flocs of gel structure by use of a thinner

Density: This is the mass of drilling fluid or cement per unit volume expressed in pounds per gallon (ppg)

Differential Pressure: This is the difference in pressure between the hydrostatic mud column and the formation pressure at a given depth in the wellbore.

Diverter (Well Control): A mechanical device installed on top of the wellhead or riser to enable flow from shallow formations to be diverted away from the work area in case of a shallow gas kick

Diverter Tool (Casing Running): A tool run within the landing string during casing running to diverter fluid flow inside the casing (when auto fill valve is used) to the annulus to prevent flow from the inside of the casing at surface.

Dogleg: A sharp bend created along the wellpath while drilling

Dogleg Severity: Dogleg severity is a measure of change in the inclination, and/or azimuth of a borehole, usually expressed in degrees per 100 feet of course length

Downhole Mud Motor: A progressive cavity positive displacement pump placed in the drill string to provide additional power to the bit while drilling. RPM is generated from flow through the motor

Drag: This is the restrictive force above the normal string weight when picking up the drill string

Draw Works: It is a component of a rotary drilling rig whose main function is to provide a means of raising and lowering the traveling blocks

Drill Collars: It is a thick walled tubular used to deliver weight to the bit during drilling. It is run as part of the bottom hole assembly

Drilling Mud or Drilling Fluid: This is a circulating fluid used in rotary drilling

Drilling Out: This is the operation when cement is drilled out of the casing before drilling the new hole section

Drill Pipe: A series of pipes (joined by screwed collars) that connect a drilling platform to the drilling bit; rotates the bit and supplies drilling mud

Drill Pipe Protectors: These are mechanical devices run around the drill pipe to prevent contact between the drill pipe and casing/formation to prevent casing wear and drill pipe wear.

Drill Solids: See Cutting(s)

Drill Strings: Drill pipe and bottom hole assembly connecting the bit to the top drive.

Drop Rate: This is the rate at which angle of inclination is decreasing with length of wellbore drilled expressed in degrees per 100 feet

Dual Gradient Drilling: A drilling technology where bottom hole pressure is maintained using two fluid gradients: seawater inside the riser and heavier mud from wellhead to the bottom of the hole.

Dynamic Position Vessel: is vessel whose position and heading are automatically maintained by a computer-controlled system by using its own propellers and thrusters. Position reference sensors, wind sensors, motion sensors and gyrocompasses provide information to the computer pertaining to the vessel’s position and the magnitude and direction of environmental forces affecting its position. For example mobile offshore drilling units (MODU)

Electrical Stability (Mud): This is a test for synthetic and oil based mud that measures the emulsion qualities of the sample.

Equivalent Circulating Density: This is the density of the circulating fluid plus the total annular pressure gradient in pounds per gallon

Fibrous LCM: This is a tough stringy and spongy material added to drilling fluids and cement to prevent lost circulation

Filter Cake (Mud): The residue deposited on formation face when drilling fluid is forced against it under pressure.

Filter Cake Thickness: This is a measurement of solids deposited on filter paper in 32^nd of an inch during the standard 30 minutes API filter test.

Flat Rheology (Mud):  This is a mud that has a relative consistent value of 6 RPM reading, yield point and 10 minutes gel strength over a range of temperature from 40 to 150 degrees Fahrenheit. Also changes in 10 seconds, 10 minutes and 30 minutes gel strength are relatively small over the temperature range.

Flocculant (Chemical): a chemical substance that promotes the clumping of particles in a fluid.

Flow by Area: Cross-sectional area available for flow. See Annular Flow Area

Flow Check: This is a process of stopping drilling, tripping and circulation operations to monitor the well in order to see whether the well is static or not.

Flow Regime: This is the flow characteristics of the flowing fluid as it flows over available cross-sectional area.

Flow-Show: This shows whether drilling fluid is flowing down the flow line or not or if flow characteristic changes.

Fluid Loss Additive: A chemical additive added to drilling fluid to control the loss of fluid to the formation.

Flush Connection: Connection that forms a smooth joint flush with the outer diameter of the remainder of the pipe (pipe body).

Foam Cement: Light weight cement created by injecting nitrogen into cement slurry

Formation Pressure: This is the pressure of the pore fluids in the rock

Fracture Closure Pressure: This is simply the fracture pressure of the formation. It is the pressure at the formation face which is necessary to hold an existing fracture open, or in other words, the pressure below which fractures will not open

Fracture Closure Stress: See Fracture Closure Pressure

Fracture Gradient: This is the fracture pressure per unit vertical depth expressed in psi/ft or pounds per gallon

Fracture Pressure: See Fracture Closure Pressure

Gas Cut: Fraction of entrained gas in mud

Gauge Length (Bit): Longitudinal length of the bit before the pin end.

Gel Strength (Mud): A measure of the ability of a drilling mud to develop and retain a gel form, based on its resistance to shear. Gel strength of a drilling mud determines its ability to hold solids in suspension.

Gel Strength, 10 min: This is the maximum reading taken from a direct-reading viscometer after the fluid has been inactive for 10 minutes reported in lb/100 ft²

Gel Strength, 30 min: This is the maximum reading taken from a direct-reading viscometer after the fluid has been inactive for 30 minutes reported in lb/100 ft²

Gel Strength, Initial (10 Seconds): This is the maximum reading taken from a direct-reading viscometer after the fluid has been inactive for 10 seconds reported in lb/100 ft²

Gel Up: This is the increase in gel strength and/or viscosity of a drilling fluid or cement

Gravel Packing: This is a sand control method used to prevent production of formation sands. A steel screen is placed in the wellbore and the annulus packed with sized gravel to act as filter against the formation sand.

Hardbanding:  This is a process of depositing hardfacing alloys onto drill pipe tool joints, drill collars, heavy weight drill pipe, and other down-hole components to extend the service life of the tubulars and to protect casing from excessive wear

Hazard and Operability Studies (HAZOP): This is a structured study of a planned or existing process or operation in other to identify and assess potential risks. It is a qualitative technique based on use of guide-words and it is carried out by a multidisciplinary team.

Hole Opener: A downhole tool used to enlarge the size of an existing borehole. It has cuttters arranged on its outside circumference to cut the formation as it rotates

Hook Load: The weight of the drill string and associated components that are suspended from the hook

Hoop Stress: This is the force exerted circumferentially (perpendicular both to the axis and to the radius of the wellbore) on the wellbore

Horizontal Stress: This is the stress acting horizontal around the wellbore.

Horsepower (HP): This is the rate of doing work equivalent to lifting 33,000 pounds at 1 foot per minute. Also 550 ft.lb/sec

Hydrostatic Pressure: This is the pressure exerted by the column of fluid on the wellbore

Inhibited Mud: This is a drilling mud with additive that retards and even prevent swelling or dispersion of formation clays

Jar: A downhole tool operated mechanically or hydraulically to deliver a heavy upward or downward blow to pipe stuck in the borehole.

Jetting: The drilling of wellbores in unconsolidated formations using the erosive power of a small-diameter stream of water forcefully ejected from bit nozzles

Job Safety Analysis (JSA): A Job Safety Analysis (JSA) is a on the job method that can be used to identify, analyze and record 1) the steps involved in performing a specific operation, 2) the existing or potential safety and health hazards associated with each step, and 3) the recommended action(s)/procedure(s) that will eliminate or reduce these hazards and the risk of a workplace injury or illness

Job Safety Environmental Analysis (JSEA): It is a JSA (see Job Safety Analysis) that includes environmental impacts of risks.

Junk Slot (Bit): An area/ groove at the bottom of the bit for debris and/or drill cuttings to accumulate without covering the bit cutting structure

Key Seat: A groove along the wellbore cut by the pressing action of the drill string on the side of wellbore. This groove is potential stuck point for drill string and casing.

Kick Tolerance: This is the maximum kick volume that can be taken into the wellbore and circulated out without breaking down the formation at the open hole weak point or without exceeding the maximum allowable surface pressure given the difference between the pore pressure and mud weight in use.

Laminar Flow: This is a fluid flow regime where fluid passes through a flow area smoothly without lateral mixing.

Leak-Off Pressure: This is the pressure at which rock in the open hole section begins to break down. This is indicated by a change in slope on a pressure vs. volume or time plot during leak-off test.

Liner: A casing string that is not run to the wellhead. Top of the liner is set inside another casing or liner within the wellbore

Logging While Drilling (LWD): It is a technique of running logging tools within drilling bottom hole assembly to acquire petrophysical data such as resistivity, density etc. in real-time while drilling.

Lost Circulation Materials (LCM): A substance added to drilling mud or cement slurries to prevent the loss of mud or cement to the formation

Low Gravity Solids: A type of drilling-fluid solid with density lower than density of barite or hematite that is used to weight up a drilling fluid. This includes drill solids plus added bentonite clay

Low End Mud Rheology: This is mud properties at low shear rate for example properties at 3-RPM and at 6-RPM viscometer reading

Maintenance Register: This is a record of equipment, maintenance work done and next maintenance due dates

Managed Pressure Drilling: This is a drilling technique where desired annulus pressure profile is maintained by controlling surface back pressure exerted on the annulus.

Management of Change (MOC):This is a structured process whereby proposed technical or organizational changes are reviewed for technical and business readiness in a consistent manner that can be relaxed or tightened depending on business needs and experiences

Measurement While Drilling (MWD): This is the acquisition of vital drilling data such as inclination, azimuth, downhole weight on bit and RPM while drilling. Data acquisition tools are run as part of the bottom hole assembly (BHA)

Methylene Blue Test: See Cation Exchange Capacity

Mud: See Drilling Mud

Mud Gas Separator: A vessel that removes gas from the mud coming out of a well when a kick is being circulated out. It is also known as gas buster or poor boy degasser

Mud Logging: This is the creation of a detailed record (well log) of a borehole by examining the cuttings of rock brought to the surface by the circulating drilling fluid. Profiles of gas content, lithology etc. are created from examining the fluid and cuttings.

Mud Motor: See Downhole Mud Motor

Mud Weight: This refers to density of the drilling fluid

Natural Fracture: Mechanical discontinuity representing zone of mechanical failure in a rock.

Off-Bottom: This is when drill string or casing is picked up and not in contact with the bottom of the wellbore

Offset Well/Offset Data: These are wells (or data from wells) analogous to or drilled in similar geographical area with a subject well.

Oil Based Mud: This is a drilling fluid where oil is the continuous phase and water is the dispersed phase

Packed Assembly: This is bottom hole assembly with stabilizers installed within to stiffen the assembly

Pack-Off: obstruction in the wellbore causing a reduction or loss of ability to circulate or ability to move drill string or casing

Pad Mud: Heavy weight mud pumped into the well prior to tripping pipe or prior to setting cement plug

Particle Size Distribution: This is a statistical function that defines the relative amount, typically by mass, of particles present according to size.

Pendulum Assembly: This is a bottom hole assembly in which drill collars without stabilizers are swung below a regular packed assembly

Permeability: This is the measure of ability of fluid to flow through a porous media

pH: Abbreviation for potential hydrogen ion. This is the degree of acidity or alkalinity of a drilling fluid. The number range from 0 to 14, 7 is neutral, below 7 is acidic and above 7 is alkaline

Pilot Hole: This is a small diameter hole drilled prior to hole opening to a larger wellbore

Plastic Viscosity (PV): This is the measure of internal resistance of fluid to flow attributed to the amount, type and size of the solids present in a drilling fluid.

Poisson’s Ratio: This is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force or the ratio of transverse extension strain to longitudinal contraction in the direction of compressive force.

Pore Pressure: See Formation Pressure

Porosity: This is the amount of void space in a formation rock expressed as a percentage or fraction of the bulk volume

Pressure Gradient: This is formation pressure per unit true vertical depth expressed in psi/ft or pounds per gallon

Pressure While Drilling: See Annular Pressure While Drilling

Progressive Rheology: This is a mud that has increasing value of 6 RPM reading, yield point and 10 minutes gel strength over a range of temperature from 40 to 150 degrees Fahrenheit.

Pulling Pipe Wet: This occurs when drill string or casing is pulled out with the mud inside the drill string or casing

Pumping Out: This is when mud is circulated within the wellbore while pulling out pipe

Rate of Penetration: This is the footage of wellbore drilled per minute

Rat Hole: This is a hole of reduced size in the bottom of a regular wellbore

Reamers: A downhole tool run as part of the BHA to smooth the wall of a well, enlarge the hole to the specified size, help stabilize the bit and straighten the wellbore if kinks or doglegs are encountered

Reaming: This is the process of enlarging a wellbore or smoothing the wellbore to remove kinks or doglegs.

Relief Valve: This is a type of valve used to control or limit the pressure in a system or vessel which can build up from a process upset, instrument or equipment failure or fire.

Resistivity: This is the measure of the resistance to flow of current through the drilling fluid and formation

Restoring Force (Centralizer): This is the force the centralizer exerts on the wall of the wellbore to keep casing away from the wall of the wellbore.

Revolutions per Minute (RPM): This is the number of revolution the drill string makes per minute s it rotates.

Reynolds Number: a number characteristic of the flow of a fluid in a pipe or past an obstruction

Rheology (Mud): This is a term used to indicate property of a drilling fluid

Risk Register: This is a risk management tool that lists all identified risks that may affect a project or operation. It should be as comprehensive as possible to include all identifiable items that have probability of occurrences and generally includes estimated probability of the risk event to occur, severity or possible impact of the risk, probable timing and anticipated frequency.

Roller Reamer: See Reamer. It is design to roll so as to minimize torque while reaming

Rotary Speed: See Revolutions per Minute

Rotary Steerable System (RSS): A tool designed to drill directionally with continuous rotation from the surface thus eliminating the need to slide a steerable downhole mud motor.

Rubble Zone: This is a geological weak or sheared zone prone to wellbore instability

Running Force (Centralizer): This is the maximum force required to move a centralizer through a specified wellbore diameter.

Salinity (Mud): Also water phase salinity, is the concentration of salt in the mud expressed in percentage or parts per million (ppm)

Self Verification: Inspection/verification conducted internally within the team

Set Screw (Centralizer): This is a screw used to attach centralizer to the casing to keep it in position

Shale: Sedimentary rock that formed from consolidated mud or clay

Shale Shaker: This is a vibrating screen that removes coarser cuttings from the circulating fluid before it flows into the return mud pit

Side Force: Force exerted by drill string or casing on the wall of the wellbore

Sidetrack: To divert from a well path to reach a productive zone or to avoid and obstruction along the well path

Slack-Off Weight: This is also referred to as set-down weight; it is the weight of the drill string and BHA available at bottom hole.

Slim-Hole Drilling: This means drilling a wellbore with diameter equal to 4-1/2” or less

Slip Crushing: This is the crushing of drill pipe/casing due to bi-axial loading on the pipe/casing when it is set in slips

Slow Pump Pressure (SPP): This is the surface pressure or pump pressure recorded while circulating at a slow rate usually at 10 to 50 strokes per minute

Slow Pump Rate (SPR): Also know as slow circulation rate (SCR), it is a slow flow rate used in circulating out an influx from the wellbore.

Slurry: This is a mixture of Portland cement and water which is pumped into the well annulus to cement casing in place.

Soak Time (Conductor Jetting): This is the period of time after jetting when jetted conductor casing is left in place without any drilling operation to allow the skin friction to increase and improve soil to casing adhesion.

Sonic Logs: This is a measure of the travel time of sound through rocks in micro seconds per foot. It is a type of acoustic log. It can be used to identify abnormally pressure zone, determine porosity of the rock and prediction of compressive strength of formation.

Specific Heat Capacity: This is the heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).

Squeeze: See Cement Squeeze

Stabilizers: It is a piece of downhole equipment used in the bottom hole assembly (BHA) of a drill string. It mechanically stabilizes the BHA in the wellbore in order to avoid unintentional sidetracking, vibrations, and ensure the good hole quality

Stand-Off Subs: A downhole tool attached to the bottom hole assembly to minimize the contact area between the bottom hole assembly and the wall of the wellbore

Standpipe: This is a pipe extending along a derrick leg to point of attachment to the rotary hose

Starting Force (Stabilizer): This is the maximum force required to insert a centralizer into a specified wellbore diameter

Stop Collars: These are accessories run on casing to keep centralizers and other casing attachments in place on the casing thereby restricting movement.

Stop The Job: This is a policy implemented on well site allowing anyone to call the job to halt if there is any safety concern

Strapping Casing: To measure the length of casing before running and/or while running in and out of the wellbore

Surge: This is the increase in bottom hole pressure as pipe moves downward into the wellbore with or without circulation.

Swab: This is the bottom hole pressure reduction as pipe is pulled too fast out of the wellbore

Syntactic Foam: This is a composite material consisting of a polymer matrix containing microspheres only. The presence of microspheres makes the material very compressible hence its application in annular pressure build up mitigation

Tagging (Cement): This is the process of confirming the presence of cement plug by setting weight on the plug with drill string

Thermal conductivity: This is the measure of the ability of a material to allow flow of heat through it.

Thermal Cracking: This is the cracking of a pipe due to its exposure excessive temperature beyond its temperature resistance

Thinner: This is an organic or inorganic agent added to a drilling fluid to reduce its viscosity

Tool Joint: This is drill pipe coupling consisting of a pin and box

Top Drive: It is a mechanical device on a drilling rig that provides clockwise torque to the drill string to facilitate the process of drilling a borehole. It is different from the more conventional rotary table and kelly method of turning the drill string because it enables drilling to be done with three joint stands instead of single joints of pipe. The top drive is suspended from the hook, so the rotary mechanism is free to travel up and down the derrick.

Torque:  This is a measure of twisting force that causes rotation. Mathematically, it is the product of force and the perpendicular distance of the line of action of the force from the axis of rotation

Torsional Strength: This is the measure of the resistance of a pipe to torque.

Tortuosity: This is a measure of deviation from a straight line.

Total Depth (TD): This is the maximum depth reached by the drill bit

Trip Speed: This is the rate at which pipe is moved in feet per minute

Trip Time: This is the time taken to trip the entire pipe out of the hole or trip the entire pipe in the hole

Turbulent Flow: This is a fluid flow regime where fluid passes through a flow area with lateral mixing.

Twist off: This is when drill string parts by falling catastrophically under torsional stress. This may be due to structural weakness of the pipe for continual use.

Ultrasonic Imaging Tool: This is a tool that uses a single rotating ultrasonic sensor to provide a complete image of the casing and cement to aid in evaluation of casing damage and cement quality

Ultrasonic Logs: A type of circumferential log used to measure pipe to cement bonding, casing thickness for casing wear evaluation and downhole pipe condition.

Underbalance: This is a condition where bottom hole hydrostatic pressure due to mud column is less than the formation or pore pressure.

Under-reamer: See Reamer, it is a reamer for hole enlargement from below the casing shoe.

Velocity: This is the rate of movement expressed in feet per minute

Viscosity: See Plastic Viscosity

Vortex Induced Vibrations: These are motions induced on riser joints interacting with external fluid flow around these joints due to water current and periodical irregularities in water current

Vugs: These are unfilled cavities in rocks

Wash-out: This is an enlarged region of a wellbore which is larger than the original hole size or bit size. This enlargement is caused by excessive bit jet velocity and/or turbulent flow inside the annulus in soft or unconsolidated formations, mechanical abrasion caused by drill string and unstable/reactive shale. Wash-out can also be a hole in a pipe caused by erosional velocity of fluid flowing inside or outside the pipe

Water Based Mud: This is a drilling fluid in which water is the continuous phase and carrying medium for weighting material

Weak Zone: See Rubble Zone

Wear Bushing: See Bore Protector

Weight on Bit: This is the amount of downward force placed on the bit to overcome the formation strength

Weighting Material: This is any material with high specific gravity used to increase the density of drilling fluid. See Barite

Wellbore Breakout: This is the enlargement and elongation of the wellbore due to failure of the rock at the wellbore wall. Rock failure is due to magnitude of the stresses or changes in stresses around the wellbore. Elongation and enlargement are in parallel to the direction of minimum horizontal stress

Wellbore Collapse: See Wellbore Breakout

Well Logging: See Logging

Wiper Trip: This is the retrieval of the drill string (at least to make a pass across the open hole section) and then trip back in hole to depth to clear obstructions such as cuttings bed in the wellbore. This can be “short trip” where only a fraction of the drill string is retrieved or “long” trip where the entire drill string is retrieved

Yield Point (YP): This is the resistance to the initial flow of fluid or the stress required to initiate fluid motion

Yield Strength (Pipe): This is the amount of stress needed to permanently deform a pipe