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MWD Glossary

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An MWD Glossary April, 1996
  • An MWD Glossary

    April, 1996

  • International MWD Society (IMS), April 1996 1



  • International MWD Society (IMS), April 1996 0

  • International MWD Society (IMS), April 1996 1

  • International MWD Society (IMS), April 1996 2

    Foreword Effective communication is based on the standardization of the words or expressions used by the different parties, and more importantly, on the standardization of the meaning of these words and expressions. Confusion and poor decisions have resulted from the ambiguity on some terms. The glossary that follows is an attempt to reduce the diversity of jargons in MWD, an emerging technology that is located at the confluence of drilling, geology, logging, metrology and surveying. Jeff Brami, Ron Deady, Mike Donovan, Gaylor Heemink, Billy Hendricks, Dale Heysse, Mark Hutchinson and Leon Robinson have contributed to the completion of this glossary.

    Philippe P. Theys Chairman IMS Glossary Committee

  • International MWD Society (IMS), April 1996 3


    AAPI Apparent API unit. API American Petroleum Institute. BHA Bottomhole assembly BHT Bottom hole temperature CSG Casing DOI Depth of investigation DOT Drill-off test DLS Dogleg severity DT Interval transit time FEWD Formation evaluation while drilling GL Ground level KB Kelly bushing LCM Lost circulation material LIH Lost-in-hole LOT Leak-off test LWD Logging while drilling MAD Measurement after drilling MD Measured depth MSL Mean sea level MWD Measurement while drilling MTBF Mean time between failures OBM Oil-based mud OCS Outer continental shelf OD Outside diameter PDM Positive displacement mud motor POOH Pull out of hole ROP Rate of penetration TD Total depth TF Tool face TVD True vertical depth UTM Universal Transverse Mercator WBM Water-based mud WOB Weight-on-bit

  • International MWD Society (IMS), April 1996 4

    Unit Abbreviations

    API American Petroleum Institute units C degree (Celsius) cm centimeter(s) cps counts per second dB decibel(s) F degree (Fahrenheit) ft foot (feet) FS Full Scale g gram(s) gal gallon(s) hr hour(s) Hz Hertz ID inside diameter in inch(es) kg kilogram(s) l litre(s) lb pound(s) m meter(s) MHz megaHertz min minute(s) mm millimeter(s) mmho millimho(s) msec millisecond(s) mV millivolt(s) sec microsecond (s) nsec nanosecond(s) Ohm-m Ohm-meter(s) Pa Pascal(s) ppm parts per million psi pounds per square inch p.u. porosity unit(s) s / sec second(s) V volt(s)

  • International MWD Society (IMS), April 1996 5


    ACCURACY (of a measurement) The closeness of the agreement between the result of the measurement and the (conventional) true value (of the measurand).

    ACOUSTIC LOG Typically, a measurement of the length of time required for a sound impulse to travel through rock, usually given in msec/ft. The interval transit time is often used to calculate porosity; it may also be used to estimate formation pressure and as an input to seismic data as a velocity control point in a wellbore.

    ANISOTROPY The property of a rock which allows it to different measurements depending on the axis along which it is measured. In terms of petrophysical measurements, the relationship of the axis of measurement to the bedding plane is often tied to anisotropy.

    API GRAVITY The weight / unit volume of crude oil or other liquid hydrocarbon expressed in degrees API, where a specific gravity of 1.0 is equivalent to 10API. API gravity should always be referenced to temperature. Low API gravity oils (e.g. 10 API @ 80F) are called heavy / thick; high API gravity oils (e.g. 40 API @ 80F) are called light / thin.

    API TEST PITS Shallow wells located at the University of Houston in Houston, Texas. The wells serve as standards for a variety of nuclear logging tools. The test pits are currently used primarily as a reference/standard for wireline logging tools because most MWD tools are too large a diameter to fit directly into the pits. The API test wells are used to calibrate Gamma Ray (GR), neutron and spectral GR tools.

    API UNIT A unit of measurement in GR logs (previously neutron logs also). For GR tools, one API unit is equivalent to 1/200th of the total deflection observed between zones of high and low radiation in the test pit. MWD GR tools measure gamma radiation in API units, Counts Per Second (cps) and AAPI (see APPARENT API UNITS). Because MWD GR sensors are housed in thick steel drill collars, the measurements usually are reduced compared to the same measurement by a wireline GR tool. GR measurements may vary from one service company to another.

    API WELL NUMBER A unique well identification number consisting of (from left to right) a two digit state code (or pseudo code for offshore), a three digit county code (or pseudo for offshore), a five digit unique well code, and if applicable, a two digit sidetrack code as defined in API Bulletin D12A.

    APPARENT API UNIT A unit of measurement of total natural GR tools based on counts/second (cps) and scaled to approximate standard API units (see API UNIT).

  • International MWD Society (IMS), April 1996 6

    ATTENUATION The decrease of a physical quantity. Many high frequency MWD resistivity devices (e.g. 2 MHz) measure the attenuation of the propagated wave and relate this attenuation to resistivity. Conversely, many MWD vendors also measure the shift in phase of the propagated wave and relate this shift to resistivity. Attenuation is often expressed in decibels (dB).

    AZIMUTH Direction, as in a compass direction. The clockwise angle of departure from a reference direction (typically geographic) north, measured in a horizontal plane. In dipmeter and directional surveys, it is the clockwise angle from magnetic north to the tool reference point or electrode. This measurement must be corrected for magnetic declination to compute true azimuth. The azimuth is generally expressed in degrees.

    AZIMUTHAL The characteristic of a logging tool to perform separate measurements in different directions (azimuths) around the axis of the tool. Currently, MWD sensors making azimuthal measurements are limited to density and tend to give measurements in quadrants around the borehole. Some MWD GR sensors are shielded on one side so that measurements are taken from only (primarily) the unshielded side. These are oriented measurements rather than true azimuthal measurements.

    BARITE Barium sulfate, a mineral used to increase the weight of drilling mud. The presence of barite can affect some logging sensors, both wireline and MWD. The measurements most significantly affected by barite are spectral gamma-ray and the photoelectric effect (Pe).

    BENDING STIFFNESS The resistance to axial bending of a drill collar (expressed in Nm/Rad or ft-lb/degree of deflection). It is equal to the bending moment required to produce a unit deflection of a collar when one end is fixed. This value is supplied to drilling engineers for the comparison of the angle building characteristic of an MWD drill collar to that of a standard API drill collar.

    BENTONITE A type of clay, composed of the mineral montmorillonite. The major area of interest in this mineral is often its tendency to swell when wet. It is a common component of drilling muds.

    BIAS The systematic or persistent distortion of a measurement process which causes errors in one direction (See SYSTEMATIC ERROR).

    BIT BOUNCE A dynamic axial drillstring motion characterized by the drill bit (bottom hole assembly) periodically lifting off the bottom of the wellbore, in most cases at the naturally resonant frequency of a drillstring component (e.g. when the BHA axial resonant frequency is three times the BHA rpm, when drilling with a tricone bit).

    BIT RESISTIVITY An MWD tool that makes a point resistivity measurement at the bit using a toroidal coil and the bit itself. The bit resistivity measurement technique depends on the BHA configuration. The quality of the measurement is dependent on the mud conductivity and the formation resistivity. The measurement is typically given in Ohm-m. The reciprocal measurement, conductivity, may be expressed in millimhos/m.

  • International MWD Society (IMS), April 1996 7

    BIT RUN The amount of footage made by a bit. It may also mean one trip in the well with the drill string. Typically, the driller will sequentially number all bit runs.

    BLOCK POSITION The distance of the traveling block above the rotary table.

    BOTTOM HOLE ASSEMBLY (BHA) The portion of the drilling assembly below the drill pipe. The Bottom Hole Assembly (BHA) will typically consist of drill collars, stabilizers and drilling tools (e.g. motor and MWD) and the bit.

    BOTTOM HOLE TEMPERATURE The temperature of the formation at the total depth of the well at the time of the activity being performed on the well (e.g., drilling, producing, injecting, logging). This is not necessarily the true temperature of the formation. The Bottom Hole temperature (BHT) is given in F or C.

    BUILD ANGLE The rate of increase in inclination of a wellbore. This is sometimes expressed as Rate-of-Build (ROB) and expressed in degrees/unit length, often degree/100 ft or similar length. BHAs are designed to either build, hold, or drop angle as the well is drilled. Some BHAs, when combined with down-hole motors, are designed to turn in a desired direction.

    CALIBRATION The adjustment of raw measurements back to a known standard.

    CALIBRATION INTERVAL The maximum time period between calibrations during which a sensor remains within its specified accuracy.

    CALIPER LOG A collection of measurements of hole or casing size. This may be in the form of minimum values or it may be an expression of the maximum size that the tool is able to read. The hole size is a critical measurement in many petrophysical measurements as well as an input in calculating cement volumes. Small (e.g., 1/4 in.) errors in hole diameter can cause large errors in porosity measurements.

    CASING SHOE A short length of heavy steel pipe which has a tapered profile. The casing shoe is screwed onto the first joint of casing lowered into the hole. In many cases, sensor measurements made near the casing shoe are of doubtful accuracy due to poor hole conditions near the casing shoe. Conversely, in many wells, but not all, the best cement job (integrity) is closest to the bottom of the well.

    CAVINGS Formation falling into the wellbore. Cavings tend to be larger and, in the case of shales, more elongated than drilled formation. Cavings may pose problems for petrophysical and drilling reasons. In MWD logging, cavings may fall to the low side of the hole and pave the borehole wall with an unnatural layer of material which could confuse estimation of true formation properties.

  • International MWD Society (IMS), April 1996 8

    CENTRAL MERIDIAN A longitude passing through the center of a projection. It is generally a straight line about which the projection is centered (symmetrically). Usually, the central meridian defines the X-origin of the map.

    CLAY A fine-grained aggregate consisting wholly or predominantly of microscopic and submicroscopic mineral particles, derived from the mineral decomposition of rocks. Clays tend to be plastic when wet and hard when dry. Clays usually exhibit a higher GR-value than most sandstones due to the presence of radioactive materials such as Potassium and Thorium.

    CLOSURE The distance between two points projected onto a horizontal plane.


    CORRECTION A value added to a raw measurement in order to improve the accuracy of the measurement.

    CURIE A standard measure of the rate of nuclear transformations or disintegrations: one Curie corresponds to 3.70 x 1010 disintegrations/second.

    CURVE ID An identifier for a log curve which, when used in combination with context information for the logging job, serves to distinguish it from all other log curves. Often this identifier is a code or mnemonic utilized by the logging company.

    CURVE TYPE General classification of log curves, independent of the specific sensor used. Examples are gamma ray, caliper, formation density, resistivity, acoustic, etc.

    DATA RATE The speed at which measurements and data are transmitted and recorded. In MWD, this parameter is important due to its relation to drilling or tripping speeds. If data are recorded slowly by a fast moving sensor, they may be less representative of the environment and difficult to reproduce. See also TRANSMISSION RATE.

    DATA STORAGE CAPACITY The volume of information which may be saved. In MWD, data are usually transmitted to the surface in real time at a lower rate than the data are stored downhole. The real-time log is often replaced by the higher-density, and better resolution data which are stored downhole and down-loaded after the tool is brought to the surface or downlinked from the surface. See also DOWNLINK.

    DECIBEL A unit used to express the relationship between two measurements of power as an interval on a logarithmic scale; 20 log10 of the amplitude ratio or 10 log10 of the power ratio. An amplitude ratio of 2 (power ratio of 4) is approximately 6 dB. 1 dB = 0.1151 Neper.

  • International MWD Society (IMS), April 1996 9

    DENSITY The mass of some material divided by its volume. In petrophysics, formations and drilling fluid densities are measured, primarily as input to equations to derive the porosity of the rock. Most logging tools actually measure bulk density (rb), and express the density in g/cm3. The equation used for determining porosity (f) from bulk density is:

    f = (rma - rb)/(rma - rmf)

    where rma is the assumed density of the matrix (formation) and rmf is the assumed density of the fluid in the pore spaces.

    DEPARTURE Horizontal displacement of one station from another in the horizontal plane. Departure is often expressed as two components - east/west and north/south.

    DEPTH DATUM The zero-depth reference for logging. A location on or above the surface (land or water) at which an elevation can be determined for depth reference. The elevation of this datum is the reference for all depth measurements made in the wellbore. Usually, the top of the kelly bushing is used as depth datum in drilling wells, but it could be ground level, derrick floor, or some other specific depth reference.

    DEPTH ENCODER A device that is generally affixed to the rig drawworks and that generates electric pulses as the drum rotates. After calibration the output of the encoder is converted to depth.

    DEPTH OF INVASION The radial depth from the borehole wall to which mud filtrate has invaded porous and permeable rock. It is usually measured in inches.

    DEPTH OF INVESTIGATION The radial distance from the measure point on a sensor to a circle, usually within the formation, where the predominant tool-measured response may be considered to be centered. It varies from one type of device to another because of different designs, and techniques of compensation and focusing. It also varies from formation to formation due to changes in formation properties. For a better understanding of the volume of investigation of a logging tool, it is recommended to know the depths of investigation corresponding to 10%, 50% and 90% of the cumulative GEOMETRIC FACTOR. See also RADIUS OF INVESTIGATION.

    DEPTH POLICY A set of procedures defined by a data vendor to obtain a consistent depth.

    DEPTH SHIFT (1) Amount of change to a core or log depth scale to align that scale to another log depth scale used as a depth reference. (2) Amount of change to the depth produced by the MWD depth system to match the driller's depth.

    DETECTION RADIUS The distance at which an MWD sensor in one formation can detect another formation parallel to the borehole, measured from the center of the MWD tool. For resistivity or conductivity tools, this is

  • International MWD Society (IMS), April 1996 10

    defined as the distance where the log response changes by 25% of its value in the current formation. For nuclear measurements, it is defined as the distance where the log response changes by at least two standard deviations from the value in the current formation.

    DIAMETER OF INVESTIGATION Geometrical specification that characterizes a logging measurement. It is equal to twice the radius of investigation or twice the depth of investigation plus the tool diameter. See GEOMETRIC FACTOR.

    DIELECTRIC CONSTANT The ratio of a the observed DIELECTRIC PERMITTIVITY of a formation or material to that of a vacuum.

    DIELECTRIC EFFECT Systematic shift observed on resistivities when the actual formation dielectric permittivity differs from the assumed dielectric permittivity value. This effect is larger when resistivities are measured with a high frequency (MHz range) electric field. See also DIELECTRIC PERMITTIVITY.

    DIELECTRIC PERMITTIVITY A measure of the ability of a material to store electrical energy or to be electrically polarized when submitted to an electric field. Dielectric permittivity is frequency dependent.

    DIP DIRECTION The direction of dip (maximum slope in a plane) perpendicular to the DIP STRIKE, expressed relative to compass directions.

    DIP STRIKE The direction or bearing of a horizontal line drawn on the plane of a structural surface. The strike is perpendicular to the DIP DIRECTION.

    DIRECTIONAL DRILLING Intentional drilling of an off-vertical well at a closely controlled, predetermined angle and direction through the use of special equipment .

    DIRECTIONAL SURVEY A well survey that measures the degree of departure of a borehole from vertical and the direction of departure. Measurements are made of azimuth and inclination of the borehole.

    DOGLEG SEVERITY The rate of change of hole angle and/or direction evaluated between the current survey point and the next shallowest survey point. It is expressed in degrees per course length, and is significantly influenced by the course length over which it is calculated.

    DOLOMITE Rock mostly composed of the mineral CaMg(CO3)2.

    DOWNLINK The capability to retrieve data from, and send instructions to the tool when it is located downhole. Four principles are currently used for downlink communications: mechanical (wireline), electrical (inductive coupling), hydraulic (mud pulse) and electromagnetic propagation.

  • International MWD Society (IMS), April 1996 11

    DRIFT ANGLE The deviation of a section of the borehole from vertical.

    DRILL BIT PENETRATION RATE The average or instantaneous distance drilled per unit of time (see also RATE OF PENETRATION.

    DRILL COLLAR Heavy, thick-walled tube, usually steel, employed between the drill pipe and the bit in the drill string to provide weight on the bit in order to improve its performance.

    DRILLING FLUID TYPE The type of mud present in the drilling media, such as chemical gel mud, crude oil, caustic (high pH), gypsum mud, native mud, etc.

    DRILLING LOG A log of drilling parameters such as penetration rate, rotary speed, weight on the bit, pump pressure, pump strokes, etc.

    DRILL-OFF TEST (DOT) A drilling procedure whereby additional weight is applied to the bit, and the drilling brake locked down at various rates of rotation (rpm), and the drilling rate of penetration observed as the weight drills off. This technique is commonly used to determine optimal drilling control parameters.

    DROP ANGLE The average rate of angular decrease at which a deviated hole departs from the hold angle in a directional well.

    ELECTROMAGNETIC PROPAGATION The passing of electromagnetic energy through a medium. Most MWD resistivity logs are based on electromagnetic propagation and typically operate at high frequencies (typically between hundreds of kHz and a couple of Mhz). They are used for correlation and to determine formation electrical properties or invasion characteristics. MWD tools record the phase shift and attenuation of electromagnetic energy through the formation near the borehole, which are then converted into resistivities and dielectric properties.

    ENVIRONMENTAL CORRECTION Correction due to the borehole environment. See CORRECTION.

    EQUIVALENT BENDING THICKNESS The wall thickness that would be required for a drill collar of uniform cross-section that would produce the same bending stiffness as a non-uniform MWD. This number is used by drilling engineers to estimate the angle-build characteristics of a BHA.

    EQUIVALENT CIRCULATING DENSITY The mud density that would be required to produce the same effective borehole pressure that results from the solids / cuttings loading and circulation of the mud system.

  • International MWD Society (IMS), April 1996 12

    FILTER TYPE The type of filter (e.g., box, binomial, truncated binomial, exponential, deconvolving, Kalman, or adaptive) used for smoothing logging curves. Because filtering affects precision and vertical resolution, filtering methods need to be reported.

    FIRST READING The depth, at the onset of a log, of the first meaningful reading of a particular curve. For MWD first readings are usually the shallowest readings, whereas in wireline logging they are generally the deepest.

    FLOW RATE RANGE Range within which a MWD tool functions. Above the maximum flow rate, erosion (and consequent tool damage) can occur. Below the minimum flow rate, telemetry information may not be transmitted, and/or the tool may not be powered to make any measurements.

    FLUID LOSS Rate of drilling fluid dissipation due to seepage into the formation (filtration), commonly measured in cm3/30 min.

    FLUSHED ZONE The zone, at a relatively short radial distance from the borehole and immediately behind the mud cake. It is considered to have had all mobile formation fluids displaced from it by mud filtrate.

    FORMATION (1) Stratigraphic: A body of rock strata, of intermediate rank, in the hierarchy of lithostratigraphic units, which is unified with respect to adjacent strata by consisting dominantly of a certain lithologic type or combination of types or by possessing other unifying lithologic features. The formation is the fundamental unit of lithostratigraphic classification. (2) Drilling: A general term applied by drillers without stratigraphic connotation to a sedimentary rock that can be described by certain drilling or reservoir characteristics.

    FORMATION EXPOSURE TIME The elapsed time between when the bit first penetrates a given section of rock and when a particular sensor logs the same section. Also called TIME SINCE DRILLED and TIME AFTER BIT.

    FORMATION EVALUATION WHILE DRILLING (FEWD) The process of collecting, while drilling, petrophysical data that can be used to evaluate the characteristics of a formation. Also called LOGGING-WHILE-DRILLING (LWD).

    FORMATION WATER RESISTIVITY The resistivity of the in-situ water contained in a formation, usually referred to as Rw.. It should be referenced to a temperature.

    FRACTURE GRADIENT The mechanical strength of a formation that represents the maximum borehole fluid pressure that can be sustained without fracturing the formation, and losing borehole fluid. This gradient is largely dependent upon lithology, the formation pore pressure, and the weight of overlaying sediments (see also LEAK-OFF TEST).

  • International MWD Society (IMS), April 1996 13

    FUNNEL VISCOSITY Viscosity, equal to the time(in integer seconds) it takes one U.S. quart of mud to flow through a Marsh funnel. The measuring unit is seconds.

    GAMMA-RAY LOG A log of the formation natural radioactivity level. It is typically used as an indicator of formation shaliness. It is also used extensively for well-to-well correlation and to correlate cased-hole logs with open-hole logs.

    GEOLOGRAPH A brand name commonly used to refer to a drilling recorder that records particular drilling events as a function of time. Depth and rate of penetration are two drilling parameters derived from its recording.

    GEOMETRIC FACTOR The contribution of a small geometrical region to the total response of a sensor. Geometric factors are used to provide insight as to the spatial response characteristics of a measuring device. Geometric factors are usually integrated and cumulated over spatial volumes.

    GEOSTEERING A technique in which one or more geologically sensitive parameters, measured downhole and transmitted to the surface, are used to guide the well path and keep it in the desired location. In GEOMETRICAL STEERING, the measurements are limited to azimuth and inclination, and the well is steered toward a pre-determined geometrical target. In GEO(logical)STEERING, formation sensitive measurements are used to steer the wellbore in relation to adjacent geological features.

    GRAVITY TOOL FACE The angle between a tool reference axis and a line perpendicular to the hole axis and lying in the vertical plane. Also commonly referred to as HIGHSIDE TOOL FACE.

    HEAVE The vertical motion of a mobile offshore drilling rig. Logging depth systems need to track the rig heave in order to accurately calculate the drillbit position.


    IMPULSE RESPONSE The theoretical sensor response when logging past an infinitely thin formation of infinite contrast to its shoulder beds. See GEOMETRIC FACTOR.

    INTERVAL TRANSIT TIME The travel time of a compressional (usually) wave over a unit distance. It is proportional to the reciprocal of the wave velocity. Also called SLOWNESS. It is usually expressed in the unit of msec (microseconds) per unit length.

    INVADED ZONE The portion of formation surrounding a wellbore into which drilling fluid (typically mud filtrate) has penetrated, displacing some of the native fluids.

  • International MWD Society (IMS), April 1996 14

    KELLY The heavy square or hexagonal hollow steel member, which is suspended from the swivel, that connects to the drillpipe. It is engaged in the rotary table, via the kelly bushing, to rotate the drillstring. Drilling fluid is pumped through the kelly into the drillstring.

    KELLY BUSHING Device, through which the kelly slides, that fits into the rotary table. It transmits the torque of the rotary table to the kelly and consequently to the drillstring. It is sometimes also called the drive bushing or rotary kelly bushing (RKB).

    KICKOFF DEPTH The depth in the vertical part of a well at which the deviated (inclined) portion of the well is started.

    LEAK-OFF TEST A pressure test (usually performed after setting a casing strinng) that determines the maximum pressure (mud weight) that can be contained by the open hole formations without fracturing and losing circulation.

    LIMESTONE Sedimentary rock consisting mainly of calcium carbonate.

    LOG (1) A detailed record, usually correlated with depth, of certain parameters of the formations penetrated during drilling. Data recorded may include electrical and radioactive surveys, description of cuttings, core analyses, etc. (2) A history of operations where drilling time, intervals cored, drillstem test results, etc. are recorded.

    LOGGING SPEED The speed at which the measuring instrument is moving when the log is recorded. In wireline operations, the cable speed typically controls the speed of a particular logging tool. In MWD operations, the rate of bit penetration controls the speed of the logging operation.

    LOGGING TOOL A tool for performing downhole well log data gathering services for determining properties of the formation, or characteristics of the wellbore and its environment.

    LOGGING-WHILE-DRILLING (LWD) Sets of methods used to record formation characteristics while drilling - commonly called LWD. Also called FORMATION EVALUATION WHILE DRILLING.

    LOST CIRCULATION MATERIAL (LCM) Material added to the mud to aid in preventing the downhole loss of mud - also called LCM. Downhole mud pulse telemetry devices and turbine generators may be affected by the presence of this material in large quantities..

    MAGNETIC DECLINATION The angle between geographic north and magnetic north. It can be either a negative or positive number. It is used to transform data referenced to magnetic north to data referenced to geographic north.

  • International MWD Society (IMS), April 1996 15

    MAGNETIC INCLINATION Vertical angle between the direction of the magnetic field and the horizontal plane. Commonly called magnetic dip angle.

    MAGNETIC INTERFERENCE That condition which occurs when extraneous (not due to the earth) magnetic forces affect a magnetically sensitive instrument. Proximity to magnetized casing, magnetized drillstring components, and certain magnetic minerals are potential sources of interference.

    MAGNETIC PERMEABILITY The property of a substance that determines to what degree it modifies the magnetic flux in a magnetic field - assumed to equal unity in most oilfield geological formations. Magnetic permeability is frequency dependent. See also DIELECTRIC PERMITTIVITY.

    MAGNETIC TOOLFACE The angle between magnetic north and the projection of the tools reference axis onto a horizontal plane. See RELATIVE BEARING.

    MAGNETOMETER A geophysical instrument used to measure the intensity, in both the horizontal and vertical directions, of the earth magnetic field.

    MAXIMUM OPERATING TEMPERATURE The published temperature above which a tool is not designed or expected to operate within its performance tolerances.

    MAXIMUM RECORDED TEMPERATURE The maximum (highest) temperature measured in the borehole during a logging operation. This temperature is a function of several parameters (e.g. formation temerature, depth, mud flow rate, and time since circulation).

    MEAN TIME BETWEEN FAILURE (MTBF) Average elapsed time between failures. It is calculated by dividing the number of MWD operating hours by the number of failures. Industry standard practice (see SPE paper #19862) has established two measures of MTBF, one for circulating hours (real-time transmission), and the second for total hours of operation below rotary (while the tool operating and recording data). MTBF statistics are recorded for individual components, for whole MWD systems, and by geographical area. Operators are also interested in the number of times MWD failures interfere with drilling operations and require tripping for the MWD tool. MTBF is significantly affected by the drilling environment (e.g. SHOCK, VIBRATION, mud solids and flow rate) and by MWD maintenance schedules.

    MEASURED DEPTH The actual distance measured along the axis of the borehole from the zero depth reference point to the depth of interest. Sometimes referred to as the Along Hole Depth (AHD).

    MEASUREMENT AFTER DRILLING (MAD) Measurements taken in a borehole after the initial drilling has taken place. MAD is the common acronym for this operation.

    MEASUREMENT ERROR The difference between the true value and that which is reported from a measurement.

  • International MWD Society (IMS), April 1996 16

    MEASUREMENT WHILE DRILLING (MWD) A technique of making downhole measurements of azimuth, inclination, tool orientation, natural radioactivity, resistivity, porosity, temperature, vibration, weight, torque, etc. These measurements are made while drilling by sensors located in the bottomhole assembly close to the drill bit, and can be recorded downhole and/or telemetered to the surface.

    MEASURING RANGE Range of values for a quantity subjected to a measurement, for which the error of a measuring instrument is intended to lie within specified limits. Sometimes called operating range.

    MEMORY DUMP TIME Elapsed time required to retrieve the data stored (recorded) in an MWD tool, once it is returned to the surface. See TURN-AROUND TIME.

    MODULARITY The ability to interchange components of an MWD tool string at the wellsite, either when one component fails, or in order to conform to desired (directional) drilling characteristics.

    MUD A liquid circulated through the wellbore during drilling and workover operations. One purpose of the mud is to remove rock cuttings produced by drilling. The mud also helps cool the bit, it prevents the borehole walls from caving in, constrains high-pressure formation fluids, and provides a medium for MWD mud-pulse transmission signals. See DRILLING FLUID.

    MUD CAKE The sheath of mud solids which forms on the borehole wall opposite permeable formations when the mud filtrate seeps into the formation.

    MUD FILTRATE The liquid portion of the mud that is able to flow into permeable formations.

    MUD MOTOR CAPACITY The actual stalling pressure of a mud motor for a given flowrate, divided by its stalling pressure at the same flowrate when originally manufactured.

    MUD MOTOR DRILLING EFFICIENCY The actual maximum mechanical horsepower of a mud motor divided by its maximum mechanical horsepower when originally manufactured.

    MUD MOTOR EFFICIENCY The output mechanical horsepower (proportional to torque multiplied by RPM) divided by the input hydraulic horsepower (proportional to flowrate multiplied by pressure drop through the motor).

    MUD TYPE The primary component of the mud, commonly water (fresh or saline) or oil. When the mud is a mixture of these fluids, mud type is usually taken as the continuous phase. The mud type may affect MWD measurements and telemetry.

  • International MWD Society (IMS), April 1996 17

    NATURAL GAMMA-RAY SPECTROSCOPY A well log which indicates the concentrations of naturally-occuring radioactive thorium, potassium, and uranium in the formation, by measuring the specific energy of formation-produced gamma rays. This log is used primarily to analyze clay content. Also called a SPECTRAL GAMMA-RAY log.

    NEUTRON LOG A porosity log whose response is primarily related to the hydrogen content of the formation (hydrogen index). Neutrons emitted from the logging tool are scattered off formation nuclei, and the porosity is determined from either counting the neutrons themselves or the gamma rays produced when the neutrons are absorbed by nuclei. Used with other porosity information, the neutron log is used to ascertain the presence of gas, determine mineralogy, and quantify shaliness.

    OBSERVED ERROR The variation of closely spaced data within a sensor intrinsic resolution from a linear trend.

    OFFSET WELL (1) A well drilled on a nearby location to the original well. (2) A well drilled on one tract of land to prevent the drainage of oil or gas to an adjoining tract where another well is being drilled or is already producing.

    OIL SATURATION The percentage of the porosity volume which is filled with oil. It is one of the main objectives of formation evaluation.

    OPERATIONAL CHECKS Rigsite procedures that are used to determine if all parts of the measurement system are functioning properly prior to being run downhole, and again upon return to surface.

    OPERATOR The individual, partnership, firm or corporation who has control or management of operations on the rig. The operator may be a lessee, designated agent, or holder of the operating rights under an approved operating agreement.

    PERMANENT DATUM The permanent elevation reference entity, independent of any equipment at the location. Generally, mean sea level (MSL) or ground level (GL) is used.

    PERMEABILITY A property of rock denoting its ability to pass fluids, and commonly used by the oil industry to distinguish between rocks which will give up no pore fluids and those which will produce oil, gas, and/or water. See also MAGNETIC PERMEABILITY.


    PHASE SHIFT The phase angle between two different signals, typically measured in degrees (e.g. between electromagnetic propagation receivers).

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    POLARIZATION HORN A localized (in true vertical depth), high resistivity deflection anomaly on some propagation resistivity logs, caused by a combination of high relative dip and a large, sharp resistivity contrast at two adjacent formations.

    PORE PRESSURE The pressure of fluids within a porous formation. MWD services are used in certain areas to estimate the formation pore pressure.

    PORE SPACE The open space, or voids, between the individual grains of a rock mass, available for fluid accumulation.

    POROSITY The percentage of bulk rock volume that consists of interstitial spaces or voids, whether isolated or connected. Porosity can be measured, calculated, or inferred. The common unit for porosity is the p.u. (porosity unit). One p.u. is equivalent to 1% porosity.

    PRECISION The closeness of agreement between the results obtained by applying a measurement procedure several times on identical materials and under prescribed measurement conditions. The smaller the random part of experimental error, the more precise the measurement procedure.

    PRESSURE Force per unit area applied to a body (e.g. hydrostatic, flow and pump pressures). It may be gauge or absolute. The kPa (kiloPascal) unit is used in physics. The more common related oilfield unit is the pound per square inch (psi).

    PRINCIPAL MERIDIAN A meridian line accurately located and used as a basis from which to construct interior lines of monuments, called meridians, for use by surveyors.

    P.U. A unit of porosity. One percent by volume of porosity is one p.u.

    PULSE RATE The number of variations caused by mud pulses per unit of time.

    QUALITATIVE Relates to a parameter that can be characterized by a relation, not a value.

    QUALITY ASSURANCE An integrated system of management activities involving planning, quality control, quality assessment, reporting and quality improvement to ensure that a product or service meets defined standards of quality with a stated level of confidence.

    QUALITY PARAMETERS Quantitative parameters that provide an indication of the quality of a measurement (e.g. static error, observed error, resolution, signal to noise and hole size).

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    QUANTITATIVE Relates to a parameter that can be characterized by a value, not a relation.

    RANDOM ERROR Random error is a non-reproducible error that is generally imputable to the physics of the measurement.

    RATE OF PENETRATION (ROP) The interval drilled per unit of time, either averaged over an interval or instantaneous. Also called ROP.

    REAL-TIME DATA (1) Data transmitted to the surface while a drilling operation is in progress. (2) Data acquired during the time that a new borehole section is first being drilled.

    REAMING The process of circulating and rotating drillpipe down (or up) a wellbore section that has been previously drilled.

    RECORDED WHILE DRILLING (RWD) Data which were recorded in the logging tool while drilling, stored in downhole electronic memory, and later retrieved - sometimes referred to as RWD. Because recorded mode data are not sent by mud pulse transmission, the data are generally of higher resolution, more complete, and of greater precision than real-time data.

    REFERENCE POINT (1) Measure point: a mark or position on the MWD tool to which all measurements are related - a tool zero. (2) A depth datum.

    REGULARIZATION A filtering process that shifts unevenly spaced data onto an evenly spaced grid - commonly performed when converting evenly time sampled data onto an even depth grid.

    RELATIVE BEARING The clockwise angle (looking downhole) from the upper side of the survey tool to the sensor face. See also MAGNETIC TOOLFACE.

    REPEAT SECTION Another set of measurements run over a short section of hole, usually run to enable comparison with the main survey and document instrument stability and repeatability.

    REPEATABILITY (1) Qualitatively: The closeness of agreement between independent results obtained in the normal and correct operation of the same method on identical test material, in a short interval of time, and under the same test conditions (same operator, same apparatus and same laboratory).

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    (2) Qualitatively: The representative parameters of the dispersion of the population which may be associated with the results are qualified by the term "repeatability", for example repeatability standard deviation, repeatability variance. (3) Quantitatively: The value equal to, or below which, the absolute difference between two single test results obtained in the above conditions may be expected to lie with a probability of 95%.

    REPLACEABILITY The ability to retrieve and replace a portion of an MWD device (such as a failed electronics component) from the bottom hole assembly, without tripping the drillpipe, and then continue operating the MWD tool.

    REPRODUCIBILITY (1) Qualitatively: The closeness of agreement between individual results obtained in the normal and correct operation of the same method on identical test material, but under different test conditions (different operators, different apparatus and different laboratories). (2) Qualitatively: The representative parameters of the dispersion of the population which may be associated with the results are qualified by the term "reproducibility", for example reproducibility standard deviation, reproducibility variance. (3) Quantitatively: The value equal to or below which the absolute difference between two single test results on identical material obtained by operators in different laboratories, using the standardized test method, may be expected to lie with a probability of 95%.

    RESISTIVITY The resistance per unit volume offered against the passage of electrical current.

    RESOLUTION (1) Intrinsic Sensor Resolution is the length associated with a sensor that relates to its ability to see thin detail (see also Impulse Response Function). It is quantitatively defined as the full width at half maximum of the response of a sensor to an infinitesimally short event of infinite magnitude, and is approximately equal to the minimum distance between two bed boundaries that the sensor can resolve. (2) Spatial Resolution is defined as the minimum formation thickness that can be resolved from a data set, and is a function of the intrinsic sensor resolution, data sampling interval and data filtering. (3) Digital Resolution is the precision with which data are digitized when either transmitted to the surface, or stored in memory. It is related to the number of digital bits used to represent a quantity.

    RETRIEVABILITY The ability to retrieve a portion of an MWD system from downhole while the MWD tool is in the bottom hole assembly. Retrievability is used on various MWD systems to recover electronics or radioactive sources from stuck bottom hole assemblies. See also REPLACEABILITY.

    SAMPLING ERROR The error introduced by the sampling process caused by making measurements on only a limited portion of a formation.

    SAMPLING INTERVAL The time between successive measurements of a sensor.

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    SAND (1) A quartz lithological description. (2) A sedimentary particle with a diameter between 1/16 and 2 mm, based upon the Wentworth scale of measurement.

    SATURATION The fraction or percentage of the pore volume occupied by a specific fluid (e.g. oil, gas or water).

    SCREW IN / OUT A drilling technique used when weight transfer from the surface is difficult. Excess weight is applied at the surface and then drilled off with the drillpipe held in the slips. The drillbit makes no progress as weight is applied, and conversely makes hole with no apparent movement of the pipe at the surface. Screwing out is the reverse technique when there is drag in the borehole, and the driller pulls the drillstring, and then rotates until the amount of pull has reduced.

    SCRIBE LINE A line marked on various drillstring components that relates to the orientation of the bend in a motor or orientation of the azimuthally sensitive sensors.

    SENSOR FACE The orientation of a sensor in the borehole when the drillpipe is not rotating (zero by convention is defined as the gravitational top side of a borehole). See also MAGNETIC TOOLFACE and RELATIVE BEARING.

    SHALE A fine grained, thinly laminated sedimentary rock formed by the compaction and consolidation of clay, silt and mud. SHOCKS Large and sudden, instantaneous forces applied to the BHA, and characterized by a relatively wide frequency band. Shocks are often associated with either resonant vibrations (accumulating large amounts of energy) or chaotic motion of the BHA. Accelerometer sensors are often used to monitor the severity and frequency of axial, lateral and tangential shock loading on an MWD tool in order to help the driller adjust surface drilling control parameters (e.g. rpm and hookload) to reduce the magnitude and frequency of destructive shocks. See also VIBRATION.

    SIDETRACK The drilling of a new and different hole from an existing wellbore.

    SILT A sedimentary particle with a diameter less than 1/128 mm, based upon the Wentworth scale of measurement.

    SLIDING The process of drilling without rotating the drillstring.

    SPECIFICATIONS A set of values that characterize a measurement or define the design operating limits for a system. Typical measurement specifications are accuracy, precision, depth of investigation and resolution. Typical operating limits include flow rate, mud sand content, dogleg severity and temperature.

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    STABILIZER A bladed device that is used to eliminate vibration, centralize and prevent differential sticking of the bottom hole assembly, and to control the directional tendencies of the drilling process. The diameter of some stabilizers can be controlled by adjusting surface drilling parameters.

    STANDOFF The distance between a logging sensor or drill collar and the borehole wall.

    STANDPIPE A pipe used for drilling fluid circulation that extends part the way up the derrick to a height suitable for attaching to the rotary hose.

    STATIC ERROR The amount of measurement variation observed when a sensor is stationary in the borehole.

    STATUS PARAMETERS Information that is recorded or transmitted periodically to the surface that confirms that a sensor is electronically performing within specification.

    STICK-SLIP An uneven rotational motion of the bottom hole assembly when downhole rotation periodically stops and then starts again. Although a drillstring may be turning fairly smoothly at the surface, downhole frictional forces may cause the bottom hole assembly to stop momentarily until sufficient torque can build in the drillstring to overcome the downhole frictional forces.

    STRIKE The direction of a horizontal line in an inclined plane, perpendicular to dip.

    SUB A small drill collar that is significantly shorter than a typical collar.

    SUBSEA DEPTH Depth measurements that have been adjusted to a zero reference at sea level.

    SYSTEMATIC ERROR A reproducible inaccuracy of measurement introduced by either faulty design, failing equipment, inadequate calibration, inferior procedure or a change in the measurement environment.

    TALLY A record of the drillpipe, drillcollars, tubing or casing installed in a well containing the length of each joint, the number of joints, and the overall length of the string.

    TELEMETRY TYPE MWD signals are transmitted in real time either through the fluid in the borehole and casing (mud pulses), or through the earth formations (electromagnetically). MWD signals are either amplitude or frequency modulated. The type of drilling fluid (compressible or incompressible) and the conductivity of geological formations may dictate the appropriateness of one telemetry type or

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    another. The type of telemetry affects data rate, the depth at which an MWD system can transmit in real time back to the surface, and various operational procedures.

    TICK OR TIME MARKS A set of marks associated with a measurement curve that indicate the depths where data were sampled, and which provide a qualitative indication of the variability in the precision and resolution of the data.

    TIME AFTER BIT The elapsed time between the times the bit first penetrates a section of rock and a sensor logs that same section. Also called FORMATION EXPOSURE TIME and TIME SINCE DRILLED.



    TOOLJOINT A threaded coupling for drill pipe, comprised of a male section (pin) and a female section (box).

    TORSIONAL OSCILLATION A periodic variation in the rotational motion of the bottom hole assembly - slowing down and then speeding up. This motion is often associated with the mass and length of sections of the drillstring. In the extreeme case, the bottom hole assembly may briefly stop rotating or instantaneously reverse its rotation. See also STICK-SLIP.

    TRANSMISSION RATE The rate at which data are transmitted to the surface in real time, usually quantified in bits per second. Effective data rate, however, is a function of transmission rate, data resolution, parity checking and data compression. See also RESOLUTION.

    TRAVELING BLOCK The block containing sheaves and a hook that is raised and lowered in a derrick.

    TRIPPING The process of moving drillpipe down (or up) a previously drilled wellbore section without circulating mud or rotating the drillpipe.

    TRUE VERTICAL DEPTH (TVD) The vertical, straight line, distance from a reference elevation to a subsurface point of interest.

    TRUE VERTICAL DEPTH LOG A well log from deviated borehole that has been rescaled from measured depth onto a vertical plane with true vertical depths.

    TURN-AROUND TIME The time required to prepare an MWD system for lowering back into the borehole after tripping it to the surface. This will include such operations as dumping memory, replacing batteries and performing diagnostic operational checks.

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    TWO-WAY COMMUNICATION A system that allows a downhole tool to transmit and receive information from the operator to either query the MWD tool or change some operating parameters such as data sampling times, transmitted data sequence and quality control checks.

    UTM COORDINATE SYSTEM The UTM (Universal Transverse Mercator) system is a convention for transforming a portion of the curved surface of the earth onto a flat plane surface of grid rectangular (x-y) coordinates. The grid system is designed for the identification of locations between the latitudes of 80 degrees south and 84 degrees north.

    VALIDATION A procedure performed after the calibration of a sensor which confirms that the calibration parameters truly produce measurements within a specified accuracy over a range of measurement.

    VERIFICATION A check performed at the rigsite that establishes whether a sensor electronics are operating within their design specification See also OPERATIONAL CHECK.

    VIBRATION Repeatable (quasi-harmonic) motion of the drillstring, MWD tool or other drillstring components, characterized by relatively narrow frequency bands. Vibration is often caused by resonant phenomena or driven energy sources (e.g. mud motors). See also SHOCKS.

    VISCOSITY The property of a substance offering internal resistance to flow; a measure of the degree of fluidity. Viscosity is defined as the ratio of the shear stress applied to a fluid divided by the shear rate resulting from the shear stress application. If the shear stress is expressed in dynes/cm2 and the shear rate is expressed in reciprocal seconds, the viscosity would be calculated in poise.

    WALK (1) Of the bit: The action of a drill bit that causes the direction of the wellbore to tend to drift away from its projected path. (2) Of the hole: The tendency of a wellbore to deviate in the horizontal plane; generally thought to be caused by the bit rotating preferentially into the side of the hole and/or the anisotropic nature of the formation.

    WASHING The process of moving drillpipe down (or up) a wellbore section that has been previously drilled, while circulating mud, but without rotating the drillpipe pipe.

    WATER SATURATION The percentage of the rock pore volume that is filled with water.

    WHIRL An excentered rotation of the center axis of the drillstring in the borehole, induced most usually by either the compressive bending or the rotational mass imbalance of drill collars. Depending upon the frictional forces acting at the borehole wall, and upon the severity of the bending forces, whirl may manifest itself in the same direction as (forward whirl), or in the opposite direction as the

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    rotation of the drillstring (backward whirl). Whirl can also be instable, transitioning between forward whirl and backward whirl states in a chaotic manner.

    WIPER RUN The process of moving drillpipe down (or up) a wellbore section that has been previously drilled, while circulating mud and rotating the drillpipe pipe - also referred to as a wiper trip or short trip.

    YIELD POINT An additional thixotropic measurement of the mud, which is the resistance to internal fluid flow measured as stress.

    ZERO DEPTH REFERENCE The elevation reference from which depths are measured. Frequently used reference points are derrick floor (DF) or kelly bushing (KB).