Education - Grossary

A
Acoustic log: A generic term for well logs which display any of several aspects of acoustic-wave propagation. In some acoustic logs (sonic log, continuous velocity log), the travel time of the compressional wave between two points is measured. In others (amplitude log), the amplitude of part of the wave train is measured. Other acoustic logs (character log, three-D log, VDL-log, microseismogram log, signature log) display part of the wave train in wiggle or variable density form. Still others (cement-bond log, fracture log) are characterized by the objective of measurements rather than their form. The borehole televiewer is also an acoustic log.

Acoustic wave: 1) Sonic wave. An elastic wave train, sometimes restricted to propagation through a fluid. 2) The wave train generated and detected by a sonic-logging sonde. The wave train is a composite of various modes of energy transfer. The first arrival usually results from compressional (P- or longitudinal) waves traveling in the formation; the inverse of its velocity is measured by the sonic log. A second arrival is sometimes identified as shear (S-) wave travel in the formation; it represents a pseudo-Rayleigh wave which travels at approximately the velocity of S-waves. Compressional waves traveling through the mud usually have relatively high frequency content; they are sometimes called fluid waves. One or more modes of high-amplitude, low frequency tube waves (sometimes called Stonely waves) are usually very distinct arrivals. 3) More generally, an elastic wave or seismic wave.

Advanced Piston Corer (APC): A coring device used to obtain near-complete core recovery when sediments are very soft (usually the uppermost 100-200 m of section).

API Units: (1) A unit of counting rate for the gamma-ray log. The difference between the high and low radioactivity sections in the API calibration pit is defined as 200 API units. (2) A unit of counting rate for the neutron log. The reading in the Indiana limestone portion of the API neutron log calibration pit which has 19% porosity and is saturated with fresh water is defined as 1000 API units.

Auxiliary Measuring Sonde (AMS): A Schlumberger tool that can be added to any digital string, yielding measurements of hole temperature and head tension.

B
Borehole effect: A distortion of a well log because of the size and influence of the borehole or (sometimes) the invaded zone.

Bottom Hole Assembly (BHA): The lowest 70-100 m portion of the drillstring, made of thicker steel with a smaller inner diameter than normal pipe. Different BHAs are used for APC/XCB coring and RCB coring.

Bridge: A hole constriction too small for the logging tool to pass through, caused sometimes by clay swelling and sometimes by caving of fractured formations.

Bridle: (1) The insulation covered lower portion of the cable to which a logging tool is connected. (2) To connect in parallel a group of amplifiers to a common input. (3) An arrangement for towing a seismic streamer.

C
Caliper: A tool used for measuring the diameter of a borehole. The measurements are displayed as a caliper log. Open hole caliper logging tools often have four or more arms.

Casing: Tubes or pipes used in boreholes to keep them from caving in. Usually made in pieces of ten feet lengths that screw together.

Cement-bond log: A well log of the amplitude of the acoustic wave which indicates the degree of bonding of cement to the casing and formations. If the casing is poorly cemented, energy which travels through the casing at the fast speed of acoustic waves in steel is strong and little energy travels in the formation; if the casing is well cemented, the casing signal nearly disappears and the formation signal is strong. The log may consist of: (1) an amplitude log (CBL) which represents the amplitude of a portion of the longitudinal acoustic wave train; or (2) a display of the acoustic wave train such as the character log, 3-D, microseismogram, VDL, or acoustic signature log.

Check Shot Survey: Seismic sources shot into a borehole where a seismic recording tool records travel times for checking results of integrating a continuous velocity or sonic log.

Compensated log: A well log made with a sonde designed to correct unwanted effects. The compensated density log uses the signal from a secondary detector to correct for the effect of mud cake and small irregularities in the borehole wall. The compensated sonic log uses a special arrangement of the transducers to correct for irregularities in the borehole size and sonde tilt.

Continuous velocity log: A sonic log; a log of formation velocity against depth. The quantity recorded and graphed is usually the reciprocal of the velocity, the travel time over a short interval, often expressed in µsec/ft.

Customer tape: Tape containing the data used in the processing of standard logs.

Cycle skipping: In acoustic or sonic logging, the first arrival is sometimes strong enough to trigger the receiver closest to the transmitter but not the farthest receiver, which may then be triggered by a later cycle resulting in an erroneously high transit time. This situation is called cycle skipping. Its onset is characterized by an abrupt deflection corresponding to an added cycle of travel between receivers. Short cycle skipping, where the near receiver is triggered by a cycle too late, also can occur, resulting in an abnormally short travel-time.

D
Density log: A well log which records the formation density. The logging tool consists of a gamma ray source (e.g., Cs¹³⁷) and a detector so shielded that it records backscattered gamma rays from the formation. This secondary radiation depends on the density of electrons, which is roughly proportional to the bulk density. The source and detector are on a skid which is pressed against the borehole wall. Compensated density logging tools include a secondary detector which responds more to the mud cake and small borehole irregularities; the response of the second detector is used to correct the readings of the main detector.

Depth of investigation: The radius about a logging sonde within which material contributes significantly to the readings from the sonde.

Dipmeter: A well log from which the magnitude and azimuth of formation dip can be determined. The resistivity dipmeter includes: (a) three or more microresistivity readings made using sensors distributed in azimuth about the logging sonde; (b) a reading of the azimuth of one of these; (c) a reading of the hole deviation or drift angle; (d) its bearing; and (e) one or two caliper measurements. The microresistivity curves are correlated to determine the differences in depth of bedding markers on different sides of the borehole and dip calculations are based on such correlations.

E
Eccentralize: To push a logging tool from the center of the borehole to the borehole wall. This is often accomplished by a mechanical arm in the logging tool, actuated at the beginning of the upward logging run. Nuclear logging tools, for example, need to be eccentralized to make correct measurements.

Environmental Corrections: Log data are adversely influenced by downhole conditions such as pressure, salinity, drilling mud, filter cake, etc. The effects of these environmental conditions on the data may be eliminated post-cruise though environmental correction software.

Extended Core Barrel (XCB): A thin bit which extends beyond the normal bit, for high core recovery when sediments are too firm for use of the advanced piston corer.

F
Flowmeter: A device that measures the flow of fluid in the borehole or casing at specified depth intervals. Sometimes the flowmeter is lowered through the flow stream in a borehole and sometimes it is set in one spot with a packer.

G
Gamma-ray log: A well log which records the natural radioactivity. 1) In sediments the log mainly reflects shale content because minerals containing radioactive isotopes (the most common of which is potassium) tend to concentrate in clays and shales. Volcanic ash, granite wash, and some salt deposits also give significant gamma-ray readings. The log often functions as a substitute for the SP for correlation purposes in cased holes, in conductive muds in open holes, and for thick carbonate intervals.

H
Hydraulic Bit Release (HBR): Equipment inserted next to the bit while "making up" (putting together) an RCB bottom hole assembly, to permit dropping the bit for logging. A "go-devil" is sent down the pipe to release bit latches and seal the bit opening, then the bit is pumped off by applying hydraulic pressure. The HBR usually does not release immediately but needs a fair amount of work to get off; sometimes it refuses to release, and we either cannot log or have to wash a new hole for logging.

I
Induction log: An electrical conductivity/resistivity well log based on electromagnetic-induction principles. A high-frequency alternating current of constant intensity induces current flow in the formation. This Foucault current flowing in the formation ground loop causes an alternating magnetic field which produces a current in a receiving coil. The receiving-coil current is nearly proportional to the conductivity of the formation. Induction sondes may have several transmitting and receiving coils to produce a highly focused log. An induction log can be recorded where the borehole fluid is conductive or nonconductive, as in oil-base muds or gas. A dual induction log measures different depths of penetration.

Interval transit-time: The travel time of a compressional sonic (seismic) wave over a unit distance, hence proportional to the reciprocal of P-wave velocity. Measured in the sonic log, usually in microseconds per foot.

Invaded zone: The portion about a wellbore into which drilling fluid has penetrated, displacing some of the formation fluids. Invasion takes place in porous, permeable zones because the pressure of the mud is kept greater than that of the formation fluids. A mud cake builds on the formation wall, limiting further flow of mud fluid (filtrate) into the formation. Directly behind the mud cake is a flushed zone from which almost all of the formation water and most of the hydrocarbons have been displaced by filtrate. The invasion process alters the distribution of resistivities and other properties and consequently the value which logs read. The depth of invasion is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate. In oil-bearing intervals, the filtrate may push a bank of formation water ahead of it to produce a relatively low-resistivity annulus which is especially important with deep-investigation induction logs.

L
Logging-While-Drilling (LWD): Process by which downhole geophysical logs are acquired during drilling operations. LWD acquires data from sensors integrated into the drill string immediately above the drill bit. LWD records data minutes after cutting the hole, closely approximating in situ conditions. This forefront industry technology provides high quality logging information in environments where standard wireline systems previously acquired either no data or poor quality data. Specifically, LWD provides excellent quality results in deviated holes or unstable environments that may preclude wireline log runs. In addition to the acquisition of logging data in potentially unstable boreholes where high-quality wireline data cannot be acquired, LWD measurements also offer at least two other operational advantages over standard coring and wireline operations: (1) in situ logs and azimuthal borehole images are acquired over the entire drilled interval, providing data over the critical section; and (2) data is either saved in memory or transmitted during drilling, hence data can be obtained without dismantling the drill string and the chances of borehole wall collapse are reduced.

Measurement-While-Drilling (MWD): Drilling and logging technology very similar to LWD (Logging-While-Drilling). MWD data is telemetered to a surface acquisition system in real-time, while LWD data is stored in downhole memory until the tool is pulled to the surface and the data retrieved. The MWD tools are now routinely used in industry, often together with LWD tools, to monitor drilling parameters in real time. A significant advantage of measuring downhole weight-on-bit is that it allows for changes in the rate of penetration to be quantified in terms of formation strength through a simple transform. When calibrated to shear strength measurements on core, this estimate of downhole formation strength together with LWD and core measurements of porosity, density and lithology provides an improved determination of the pore-pressure and effective stress at depth.

N
Neutron Activation: Radioactive sources in density and porosity tools emit neutrons into the formation as part of the routine density and porosity measurements. If a toolstring with radioactive sources is stationary in the hole for any amount of time, residual neutrons will remain in the borehole for a small period of time in the location adjacent to source. This neutron activation does not usually last more than a few hours, but it is detectable with the gamma tool and can be falsely interpreted as a high gamma count interval.

Neutron log: A porosity well log which measures mainly hydrogen density. Fast neutrons emitted by a source in the tool are slowed to thermal speed by collisions with (mainly) hydrogen atoms. The thermal neutrons are then captured by atomic nuclei of the surrounding material (mainly chlorine atoms) at which time a characteristic gamma ray of capture is given off. Porosity calculated from the neutron log is affected somewhat by the formation matrix and by the presence of gas. Neutron logs are used in crossplots to detect gas and determine lithology. Neutron logs are sometimes scaled in API units, sometimes in porosity units assuming a limestone matrix. The neutron log can be recorded in cased holes.

P
Permeability: A measure of the ease with which a fluid can pass through the pore spaces of a formation. Measured in millidarcy (1/1000 darcy) units. The permeability constant k is expressed by Darcy's law as µq/(dp/dx), where µ is fluid viscosity, q is linear rate of flow, and dp/dx is the hydraulic pressure gradient.

Pigtail: A 4-foot long piece of logging cable, modified with electrical connectors at each end, which converts the rope socket (Schlumberger cable termination) via the torpedo to the Gearhart Owen (G.O.) cablehead used on the specialty tools.

Processed Data: Logging data that has been processed using a specific log analysis system (such as GeoFrame or Logos). Processing includes depth shifting, environmental corrections, quality control and the creation of ASCII files for the online database.

Proprietary Data: The entirety of logging data collected by the Schlumberger acquisition system aboard the JOIDES Resolution.

Proprietary Tape: A tapes containing the original log data recorded by the Schlumberger engineer, as well as the calibration counts necessary for some onshore processing of the original count rates.

Pull Out of Hole (POOH): A term used to describe the upward trip of either a logging tool or the drillstring. Most commonly, it refers to the distance from the bottom of the hole to the drill floor, but distances to specific depths below the seafloor can also be specified (e.g., "POOH to 70 mbsf" means raising pipe from the bottom of the hole to 70 mbsf).

R
RCB: Rotary Core Barrel. Used after the APC/XCB core barrels to drill hard rocks. Logging tools cannot pass through an RCB bit; therefore, the bit must be removed prior to logging.

Resistivity: The property of a material which resists the flow of electrical current. Also called specific resistance. The ratio of electric-field intensity to current density. The reciprocal of resistivity is conductivity.

Resistivity logs: Well logs which depend on electrical resistivity: normal, lateral, laterolog and induction log. Most resistivity logs derive their readings from 10 to 100 ft³ of material about the sonde. Microresistivity logs on the other hand derive their readings from a few cubic inches of material near the borehole wall.

Rigup (RU): To assemble a toolstring or piece of equipment in preparation for deployment.

S
Schlumberger Workshop: A room on the JOIDES Resolution immediately beneath the logging winch, containing Schlumberger supplies and the ozalid machine for paper copies of logs.

Scintillation Counter: An instrument for measuring radioactive radiation, especially from gamma rays. Gamma radiation impinging on a sensitive phosphor causes it to emit light (scintillations) which is measured by a photo-multiplier tube.

Secondary porosity: Porosity resulting from the alteration of the formation such as by fractures, vugs, solution channels, dolomitization, etc.

Seismic Source: The sound source used for the collection of seismic reflection data. In the early days of seismic exploration of watered covered areas, the source was always a form of unconfined explosion. However, an unacceptable level of environmental damage resulted from this method and it was soon clear that there was a pressing need to generate seismic waves that did not have sufficiently high peak pressures to cause damage to marine fauna. In addition, explosive sources created a so-called "bubble effect" – an undesirable artifact in a seismic record caused by oscillations of gas bubbles generating repetitions of first arrivals. Because of the consistency of the water medium, it is possible to generate energy within the frequency band used for seismic exploration by a more controlled release of gas pressure (air gun) or by other means of producing a sudden volume increase within the water column (water gun) while at the same time minimizing the bubble effect. The requirements for a marine seismic source are:

Shoulder-bed effect: Effect of adjacent beds on a log reading. Also called the adjacent bed effect. For example, high resistivity beds adjacent to a low resistivity bed may result in more current flowing in the low-resistivity bed than if the high-resistivity bed were not present, thus changing the apparent resistivity of the low-resistivity bed.

Sonic log: A well log of the travel time for acoustic waves over a unit distance, and hence the reciprocal of the longitudinal wave (P-wave) velocity. Also called acoustic velocity log and continuous velocity log. Usually measured in microseconds per foot. Especially used for porosity determination by the Wyllie relationship. The interval transit time is integrated down the borehole to give the total travel time. For the compensated sonic log, two transmitters are pulsed alternately; averaging the measurements tends to cancel errors due to sonde tilt or changes in hole size.

Spontaneous Potential (SP): Also called self potential. 1) A well log of the difference between the potential of a movable electrode in the borehole and a fixed reference electrode at the surface. The SP results from electrochemical SP and electrokinetic potentials which are present at the interface between permeable beds adjacent to shale. In impermeable shales, the SP is fairly constant at the shale base-line value. In permeable formations the deflection depends on the contrast between the ion content of the formation water and the drilling fluid, the clay content, the bed thickness, invasion, and bed-boundary effects, etc. In thick, permeable, clean nonshale formations, the SP has the fairly constant sand line value, which will change if the salinity of the formation water changes. In sands containing disseminated clay (shale), the SP will not reach the sand line and a pseudostatic SP value will be recorded. The SP is positive with respect to the shale base-line in sands filled with fluids fresher than the borehole fluid. 2) The DC or slowly varying natural ground voltage observed between nearby nonpolarizing electrodes in field surveying. In many mineralized areas this is caused by electrochemical at the electrically conducting sulfide body.

Stand: A 30-meter segment of pipe, made up of 3 10-meter pipe joints. This is the usual increment for adding or removing pipe.

Stoneley wave: 1) A type of seismic wave propagated along an interface. 2) A surface wave in a borehole.

Synthetic Seismogram: An artificial seismic reflection record manufactured from velocity-log data by convolving the reflectivity function with a waveform which includes the effects of filtering by the Earth and recording system. Used to compare with an actual seismogram to aid in identifying events or predicting how stratigraphic variation might affect a seismic record. Often constructed from sonic log data alone although density data may also be incorporated. Generally assumes plane interfaces and plane waves, sometimes a point source. Synthetic seismograms sometimes show primary events only, primaries plus selected multiples, or primaries plus all multiples; they may be constructed by analog, digital, or manual methods.

T
Tadpole Plot: A type of plot of dipmeter or drift results, also sometimes called an arrow plot. The position of a dot gives the dip angle versus depth and a line segment pointing from the dot gives the direction of dip, using the usual map convention of North being up.

Telemetry: Communication with a remote acquisition system. Logging tools usually transmit acquired data in real-time via wireline telemetry.

Temperature log: A well log of temperature, often made with a resistance thermometer (thermistor). Used for locating cement behind the casing (because the setting of cement is exothermic and hence raises temperature), intervals which are producing gas (because the expansion of gas as it enters the borehole lowers the temperature), and fluid flows (particularly behind the casing).

V
Vertical Seismic Profile (VSP): A VSP differs from a conventional reflection profile in that the receiver is clamped successively at different borehole depths within the Earth. The seismometer records both the direct, downgoing waves and upgoing waves reflected from acoustic impedance changes below the clamping depth. Interval velocities may be calculated from the difference in arrival time of the direct wave between receiver depths. Processing techniques can be applied to separate the upgoing and downgoing wavefields, which can then be analyzed for attenuation properties of rock, prediction of acoustic properties below the bottom of the hole, and correlation with borehole lithology, wireline logs, and events on conventional seismic reflection profiles.

W
Washout: A borehole feature where the gauge of the hole increases substantially due to caving or erosion during the drilling processes.

Wiper Trip: The action of pulling pipe from the bottom of the hole to logging depth, then lowering pipe back to bottom, or the opposite. Wiper trips are always pipe round trips to clean the hole for logging, usually at a much slower speed than normal pipe trips to avoid damaging the hole.

Wireline: A cable comprising one or conductors which is lowered into a borehole and provides for real-time communication between a tool and the surface.