Oil and Gas Terms Beginning with “V”

An opening at floor level in a side of a derrick or mast, opposite the drawworks, used as an entry to bring in drill pipe, casing, and other tools from the catwalk.

Viscosity-gelmeter. This jargon is used to describe the direct-indicating viscometer, the instrument commonly used to test flow properties of drilling muds.

A copolymer of vinyl acetate (ethylenic polymer) and anhydrous maleic acid (a di-hydroxy acid). The vinyl acetate polymer component is usually high molecular weight. As such, with polar groups on the structure, it is used as a flocculant or bentonite extender.

A non-thermal heavy oilproduction method. Similar in concept to SAGD, in vapor extraction a solvent vapor is used to reduce viscosity of the heavy oil. The injected solvent vapor expands and dilutes the heavy oil by contact. The diluted heavy oil will drain by gravity to the lower horizontal well, to be produced.

Viscosity-gelmeter. This jargon is used to describe the direct-indicating viscometer, the instrument commonly used to test flow properties of drilling muds.

Abbreviation for very large crude carrier.

A class of boreholeseismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. In the more general context, VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals hundreds of meters apart. Also, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver. The check-shot survey uses only the direct path traveltime. In addition to tying well data to seismic data, the vertical seismic profile also enables converting seismic data to zero-phase data and distinguishing primary reflections from multiples.

A non-thermal heavy oilproduction method. Similar in concept to SAGD, in vapor extraction a solvent vapor is used to reduce viscosity of the heavy oil. The injected solvent vapor expands and dilutes the heavy oil by contact. The diluted heavy oil will drain by gravity to the lower horizontal well, to be produced.

The pressure exerted by a vapor escaping from a liquid. It quantifies the tendency of molecules to enter the gaseous phase. The vapor pressure of water increases as temperature increases and reaches one atmosphere pressure (760 mm Hg or 14.7 psia) at the boiling point (100°C or 212°F). The activity of an aqueous solution is the ratio of vapor pressures: aw = p/po, where p = vapor pressure of a solution and po is vapor pressure of pure water. Since this is a ratio of vapor pressures, activity is not a strong function of temperature.

A system composed of a scrubber, a compressor and a switch. Its main purpose is to recover vapors formed inside completely sealed crude oil or condensate tanks.The switch detects pressure variations inside the tanks and turns the compressor on and off. The vapors are sucked through a scrubber, where the liquid trapped is returned to the liquid pipeline system or to the tanks, and the vapor recovered is pumped into gas lines.

A gasflood process in which a lean gas, for example methane, nitrogen or carbon dioxide, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, light and intermediate molecular-weight hydrocarbons transfer from the oil into the gas phase, thus vaporizing into the gas.Formation of miscibility may require several contacts between gas containing vaporized components and fresh reservoir oil. If the injected gas becomes sufficiently enriched with these components that miscibility results with the oil, then the lean gas and oil have multiple-contact miscibility. A forward multiple-contact test is a laboratory evaluation of a vaporizing drive process. In the field, both forward- and backward-contact processes can occur during a given gasflood.

A condition that results when the flow rate varies appreciably during a test period. This can be contrasted to multirate conditions, which describe the step changes in rate demanded by certain test designs. The variable rates observed during drawdown can be measured by a flowmeter at the sandface level in the well. Elegant mathematical methods have been developed to analyze well tests conducted at variable rates.Although, from a practical standpoint, it is much more desirable to conduct constant-rate tests, in reality the only truly constant rate is zero. Sandface flow-rate measurements provide considerable information from drawdown data that cannot be learned from a buildup.

A presentation of the acousticwaveform at a receiver of a sonic or ultrasonic measurement, in which the amplitude is presented in color or the shades of a gray scale. The variable-density log is commonly used as an adjunct to the cement-bond log, and offers better insights into its interpretation; in most cases microannulus and fast-formation-arrival effects can be identified using this additional display. In openhole, it may be displayed alongside the sonic log transit-time as a qualitative presentation of the acoustic wave train, and is sometimes used for fracture detection by examination of the chevron patterns given by Stoneley wave reflections (and other wave reflections) at fractures crossing the borehole.

A method for rotating axes of a plot such that the eigenvectors remain orthogonal as they are rotated. These rotations are used in principal component analysis so that the axes are rotated to a position in which the sum of the variances of the loadings is the maximum possible.

A two-point statistical function that describes the increasing differences or decreasing correlation, or continuity, between sample values as separation between them increases.The term variogram is sometimes used incorrectly in place of semivariogram. The two differ only in that the semivariogram uses each pair of data elements only once, whereas the variogram uses all possible data pairs. Semivariograms are usually used instead of variograms, but opposite vector directions (for example, north and south) are recognized as representing the same thing and having identical ranges, sills, nugget points and the like.

A rhythmic sequence of sediments deposited in annual cycles in glacial lakes. Light-colored, coarse summer grains are deposited by rapid melting of the glacier. The summer layers grade upward to layers of finer, dark winter grains of clay minerals or organic material that are deposited slowly from suspension in quiet water while streams and lakes are icebound. Varves are useful to the study of geochronology because they can be counted to determine the absolute age of some Pleistocene rocks of glacial origin.

The open location on a mast-type rig (nonderrick) that functions like the vee-door. At least two sides are open on most mast rigs. Hence, the open side adjacent to the slide and catwalk is considered the vee-door. The vee-door is really a hole and has no true door that can be closed or locked, so inexperienced visitors to a rigsite are sometimes asked by the rig crew to find the key to the vee-door as a joke.

The open location on a mast-type rig (nonderrick) that functions like the vee-door. At least two sides are open on most mast rigs. Hence, the open side adjacent to the slide and catwalk is considered the vee-door. The vee-door is really a hole and has no true door that can be closed or locked, so inexperienced visitors to a rigsite are sometimes asked by the rig crew to find the key to the vee-door as a joke.

The rate at which a wave travels through a medium (a scalar) or the rate at which a body is displaced in a given direction (a vector), commonly symbolized by v. Unlike the physicist's definition of velocity as a vector, its usage in geophysics is as a property of a medium-distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. Velocity can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant velocity (homogeneous) medium.

The process of calculating seismic velocity, typically by using common midpoint data, in order to better process seismic data. Successful stacking, time migration and depth migration all require proper velocity inputs. Velocity or stacking velocity can be calculated from normal moveout, or the change in arrival time produced by source-receiveroffset.

A feature in seismic data that results from changes in velocity, both laterally and vertically. Pull-up and push-down are examples of velocity anomalies.

A change made in seismic data to present reflectors realistically. Velocity corrections typically require that assumptions be made about the seismic velocities of the rocks or sediments through which seismic waves pass.

A two-dimensional display, using colors or different gray scales, of the bubble velocity around the borehole against depth. The x-axis of the image shows different segments of the borehole, normally inside a casing, displayed from the top of the hole clockwise around through the bottom and back to the top again. Depth is in the z-axis, while the values of bubble velocity are represented by different colors or changes from black to white.The velocity image is constructed from between four and eight local probe measurements using interpolation within constraints. Images, sometimes called maps, are also made for bubble count and holdup.

Those thicknesses of rock or sediment that have a common velocity, as opposed to the sedimentary layering or bedding of the rock or sediments.

(noun) A spatial representation of seismic wave propagation velocities across a survey area at a specific depth or time horizon, derived from seismic data processing and well tie analysis. Velocity maps are used for depth conversion of seismic time data and identification of lateral lithology or pore fluid variations.

A small-diameter tubing string run inside the production tubing of a well as a remedial treatment to resolve liquid-loading problems. As the reservoir pressure in a gas well depletes, there may be insufficient velocity to transport all liquids from the wellbore. In time these liquids accumulate and impair production. Installing a velocity string reduces the flow area and increases the flow velocity to enable liquids to be carried from the wellbore. Velocity strings are commonly run using coiled tubing as a velocity string conduit. Safe live-well working and rapid mobilization enable coiled tubing velocity strings to provide a cost effective solution to liquid loading in gas wells.

Measurements used to determine average velocity versus depth, such as from an acoustic log or check-shot survey. Acquiring a velocity survey is also known as "shooting a well."

The factor linking the velocity of single-phase liquid flow measured in the center of a pipe with the average velocity across the pipe. For vertical pipes with turbulent flow measured by standard flowmeters, the velocity-correction factor varies within a range of 0.75 to 0.95, but is often taken as 0.83. For laminar flow, it is theoretically 0.5.

A method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used.Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D [16m3/d]. They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log.

A check performed at the wellsite to establish whether a logging measurement is functioning properly. Verification is also known as an operational check. The verification may be done before or after the survey and may be presented with the log. Verification is distinct from calibration.

A description of the contents of a digital record.

In a displacement process, the ratio of the cumulative height of the vertical sections of the pay zone that are contacted by injection fluid to the total vertical pay zone height. Vertical displacement efficiency (EI) strongly depends on parameters such as mobility ratio and total volume of fluid injected. Nonuniform permeability may cause an irregular front that affects the vertical displacement efficiency because the injected fluid flows faster in high-permeability zones than in low-permeability zones.

The vertical distance between two points in a horizontal or deviated wellbore. Any calculations relating to wellbore pressure or downhole pump performance will be based on the vertical lift rather than the distance traveled through the wellbore.

The resistivity of a formation measured by flowing current in a vertical plane. In anisotropic formations, the horizontal and vertical resistivities are different. In a vertical well, wirelineinduction logs and measurements-while-drilling propagation logs measure the horizontal resistivity, whereas laterologs measure the horizontal resistivity with some component of the vertical. In deviated and horizontal wells, all these logs measure some mixture of both vertical and horizontal resistivity.

A distance that characterizes the ability of a logging tool to resolve changes parallel to the tool axis. The word vertical implies a vertical well, but the term is used at other wellbore deviations. The vertical resolution summarizes the vertical response of the measurement in one or more distances. Most quoted vertical resolutions assume a homogeneous formation with stated properties. Vertical resolutions can vary considerably in more complex conditions, and at different values of the properties concerned. They should be considered only a qualitative guide to tool response.There are several different definitions of the vertical resolution distance. First, and most commonly, it is the interval within which a large percentage, typically 90%, of the vertical response occurs. Second, it is the minimum bed thickness needed for the measurement to read within a small percentage, typically 10%, of the true value at the center of the bed. Third, it may refer to the smallest bed thickness for which a significant change can be detected by the measurement.For acoustic and electromagnetic propagation measurements, it is taken, with reasonable accuracy, as the span of the receiverarray. For nuclear and nuclear magnetic resonance measurements, which must be acquired during a significant time interval, the vertical resolution also depends on the logging speed and the precision required.

The response of a logging measurement as a function of distance parallel to the tool axis. The word vertical implies a vertical well, but the term is used at other wellbore deviations.Vertical responses are determined by computer simulation or laboratory measurement. For some measurements, mainly resistivity, the vertical response can be shaped as desired through signalprocessing. In general, the vertical response depends on the formation properties throughout the measurement volume. Most quoted vertical responses have been determined in formations that are radially homogeneous and have small vertical changes. They can then be summarized by a geometrical factor or a pseudogeometrical factor. These factors are appropriate for volumetric measurements such as nuclear and resistivity, but not for others such as acoustic propagation.

What Is a Vertical Seismic Profile? A vertical seismic profile (VSP) is a borehole seismic acquisition technique that deploys downhole geophone arrays inside a wellbore to record seismic energy generated by a surface source, producing subsurface images of significantly higher resolution than conventional surface seismic surveys and enabling operators to correlate geological structure directly to well data. Key Takeaways A VSP uses downhole receivers spaced 15-25 m (49-82 ft) apart inside the wellbore, capturing both downgoing direct arrivals and upgoing reflected wavefields for depth-to-time conversion and well-tie calibration. Zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, and salt proximity VSP represent the primary survey geometries, each suited to specific reservoir imaging and structural delineation objectives. VSP data provides critical look-ahead capability, imaging formations 500-2,000 m (1,640-6,562 ft) below the current drill bit position to identify overpressured zones, fluid contacts, and structural hazards before the bit reaches them. Processing VSP data involves separating the upgoing reflected wavefield from the downgoing direct wavefield, applying velocity analysis, and producing depth-migrated images tied directly to the wellbore log suite including gamma-ray, resistivity, and porosity curves. VSP surveys are conducted in vertical wells, deviated wells, and horizontal wells across deepwater basins, tight gas plays, carbonate reservoirs, and presalt formations worldwide, with acquisition standards governed by API and NORSOK guidelines depending on jurisdiction. How a Vertical Seismic Profile Works In a VSP acquisition, a seismic source, typically an airgun array for offshore environments or a vibroseis truck for onshore operations, generates a controlled acoustic pulse at or near the surface. Downhole receivers, usually hydrophone-geophone combinations locked against the borehole wall at predetermined stations, record the resulting seismic wavefield as it travels through the subsurface. Because the receivers are physically inside the formation being imaged, the travel path from source to receiver is dramatically shorter than in conventional surface seismic surveys, which translates directly into higher-frequency signal content and improved vertical resolution. Geophone spacing in a VSP survey is typically 15-25 m (49-82 ft) compared to hundreds of meters for check-shot surveys, which record only the direct-path first arrival for velocity calibration rather than the full reflected wavefield. The raw VSP dataset contains two distinct wavefields superimposed on each receiver record: the downgoing wavefield, which travels directly from the surface source to the receiver, and the upgoing wavefield, which consists of reflections from interfaces below the receiver returning toward the surface. Processing separates these two components using median filtering, f-k filtering, or other wave-separation algorithms. The upgoing wavefield carries the reflection seismic information that, after depth migration and wavelet extraction, produces the VSP image. The downgoing wavefield provides the velocity function used to convert two-way travel time from surface seismic data into true depth, a process called depth conversion or well-seismic tie. This tie anchors the surface seismic interpretation to the known stratigraphy encountered in the wellbore, resolving ambiguities in velocity models that routinely affect surface-only seismic interpretation. Acquisition tools include single-level clamped geophones lowered on wireline, multi-level arrays that record multiple stations simultaneously to reduce acquisition time, and permanent downhole seismic sensors deployed for time-lapse or 4D VSP monitoring. Memory-mode tools store data downhole and retrieve it on a single wireline trip, while telemetry tools transmit data in real time. Tool specifications such as natural frequency, clamping force against the borehole wall, and geophone orientation sensitivity are governed by API Recommended Practice 14C and related standards for downhole seismic acquisition. In Norway, NORSOK D-010 and associated borehole seismic guidelines prescribe minimum data quality requirements for VSP surveys tied to well integrity operations. Vertical Seismic Profile Across International Jurisdictions In Canada, VSP surveys are a standard component of well evaluation programs in the Montney Formation of northeastern British Columbia and northwestern Alberta, where the tight gas siltstone reservoir exhibits strong lateral heterogeneity. Operators including Shell Canada, ConocoPhillips Canada, and Tourmaline Oil have used walkaway VSP to map fracture corridors and define the limits of stimulated reservoir volume around hydraulic fracturing stages. In the Duvernay shale of central Alberta, 3D VSP programs image carbonate reef buildups in the underlying Leduc Formation that act as structural traps influencing Duvernay completion design. The Alberta Energy Regulator (AER) classifies borehole seismic operations under Directive 056, which governs well logging and testing procedures including downhole seismic acquisition. In the United States Gulf of Mexico, VSP surveys are essential for subsalt imaging in deepwater fields where surface seismic velocity models are degraded by thick, irregular salt canopies. The Bureau of Safety and Environmental Enforcement (BSEE) permits borehole seismic operations under 30 CFR Part 250, and operators such as Shell, BP, and Chevron routinely acquire salt proximity VSPs prior to drilling into challenging subsalt targets. Salt proximity VSP uses a series of receiver stations near the interpreted salt flank to precisely locate the salt body edge in 3D, reducing wellbore placement risk. The Mad Dog, Atlantis, and Thunder Horse developments in the deepwater Gulf of Mexico have all relied on extensive VSP campaigns to calibrate subsalt velocity models and reduce drilling uncertainty. In Norway and the North Sea, the Johan Sverdrup field operated by Equinor offshore Stavanger has been the subject of extensive 4D VSP monitoring to track fluid movement during production. NORSOK D-010 "Well Integrity in Drilling and Well Operations" provides the regulatory framework for borehole seismic acquisition, and the Norwegian Petroleum Directorate (now Sodir, the Norwegian Offshore Directorate) maintains data sharing requirements for VSP datasets acquired on the Norwegian Continental Shelf. Offshore VSP programs in the United Kingdom Continental Shelf are governed by the North Sea Transition Authority (NSTA), and operators including TotalEnergies and Harbour Energy have used VSP imaging on complex fault structures in the Central Graben and East Shetland Platform. The Ormen Lange deepwater gas field offshore Norway used walkaway VSP during appraisal drilling to image the highly irregular seabed and constrain velocity models affected by gas-charged sediments. In Australia, the Carnarvon Basin on the Northwest Shelf hosts some of the world's largest LNG projects, including Chevron's Gorgon and Wheatstone developments and Woodside's North West Shelf project. VSP surveys in the Carnarvon Basin must account for complex velocity layering in Jurassic sandstone reservoirs overlain by thick Cretaceous shales. NOPSEMA (the National Offshore Petroleum Safety and Environmental Management Authority) governs borehole seismic operations in Australian waters under the Offshore Petroleum and Greenhouse Gas Storage Act 2006. In the Middle East, Saudi Aramco operates one of the world's most extensive VSP programs targeting carbonate reservoirs in the Arab Formation and Jurassic carbonates of the Ghawar field, the world's largest conventional oil field. VSP surveys in Saudi Arabia are calibrated to core data and fullbore formation microimager logs to characterize fracture permeability in tight carbonate zones, with geophone spacings as close as 10 m (33 ft) in high-value reservoir intervals. Fast Facts: Vertical Seismic Profile Typical geophone spacing: 15-25 m (49-82 ft) for standard VSP; as close as 5 m (16 ft) for high-resolution reservoir VSP Look-ahead range: 500-2,000 m (1,640-6,562 ft) below the deepest receiver station, depending on frequency content and subsurface geology Frequency content: VSP surveys typically preserve signal up to 100-150 Hz versus 60-80 Hz for conventional surface seismic, yielding vertical resolution of 5-10 m (16-33 ft) Source types: airgun arrays (offshore), vibroseis (onshore), weight drop, mini-airgun in shallow water, seismic-while-drilling (SWD) using bit noise Zero-offset VSP source offset: typically less than 50 m (164 ft) from the wellhead Walkaway VSP source range: typically 3,000-8,000 m (9,843-26,247 ft) from the wellhead along a linear or circular receiver array Acquisition standards: API RP 14C, NORSOK D-010, SPE Technical Reports on borehole seismic interpretation VSP Survey Types and Technical Configurations The zero-offset VSP is the most common configuration, placing the seismic source directly above or within a few tens of meters of the wellhead so that raypaths are approximately vertical through the subsurface. This geometry maximizes the quality of the well-seismic tie because the near-vertical raypaths sample the same formation volume as the adjacent well logs. The zero-offset VSP also provides the cleanest separation of upgoing and downgoing wavefields because the two travel nearly parallel paths, simplifying the wave-separation step in processing. Vertical resolution in a zero-offset VSP at typical oilfield depths of 3,000 m (9,843 ft) commonly reaches 5-8 m (16-26 ft), compared to 15-25 m (49-82 ft) for surface seismic at similar depths. The offset VSP moves the source to a lateral position, typically 300-3,000 m (984-9,843 ft) from the wellhead, so that raypaths illuminate dipping reflectors and structural features that a vertical raypath would miss. Offset VSP is particularly effective for imaging faults, salt flanks, and laterally heterogeneous carbonate reservoirs. The walkaway VSP extends this concept by recording at a series of progressively increasing source-to-well offsets along a surface traverse, effectively producing a 2D seismic image along the survey line that is anchored to the wellbore. In a 3D VSP, multiple offset lines or a circle source geometry samples a 3D volume around the well, and migration of the resulting dataset produces a 3D image with higher resolution than the surrounding surface seismic grid. The 3D VSP at Mad Dog Field in the Gulf of Mexico demonstrated that a single wellbore could image a 10 km2 (3.9 sq mi) area with significantly better definition of the salt overburden than the regional 3D surface seismic. The walk-above VSP is designed for deviated and horizontal drilling operations in which the wellbore departs substantially from vertical. The source moves along the surface above the wellbore trajectory, maintaining a nearly constant source-to-receiver offset as the well deviates. This geometry preserves the simplicity of the zero-offset wave-separation problem while accommodating the complex 3D trajectory of a directional well. Walk-above VSPs are standard practice in horizontal Montney and Duvernay wells in Canada and in horizontal Permian Basin targets in the United States. The salt proximity VSP deploys receivers near a salt body flank and shoots from multiple azimuths to precisely triangulate the 3D position of the salt boundary, critical for placing exploratory and appraisal wellbores safely away from unexpected salt re-entrant structures that could cause wellbore instability and loss of the bottom-hole assembly. Seismic-while-drilling (SWD) VSP uses the noise generated by the rotating drill bit as a seismic source, eliminating the need for a surface shot and enabling continuous look-ahead imaging while drilling proceeds. Correlating the drill bit signature with recordings from surface geophones allows processing teams to extract a look-ahead VSP image updated at each pipe connection. SWD systems reduce operational cost and rig time compared to conventional wireline VSP, though signal-to-noise ratios are generally lower. Schlumberger's seismicVISION and Halliburton's equivalent tool represent commercial SWD implementations used in deepwater Gulf of Mexico and North Sea operations where conventional VSP would require interrupting drilling for a dedicated logging run.

A vessel with its cylindrical axes perpendicular to the ground that is used to separate oil, gas and water from the production stream. The vessel can be a two-phase or three-phase separator.

A method to convey or reserve oil, gas, or mineral rights in a defined portion of land such as the Northwest Quarter of a tract.

A supertanker with a capacity between 100,000 and 500,000 deadweight tons. The term is commonly abbreviated as VLCC.

Bubble-shaped cavities in volcanicrock formed by expansion of gas dissolved in the precursor magma.

A type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock.

An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data.

Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data (Vibroseis is a mark of Conoco). The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear "sweep" of at least seven seconds, beginning with high frequencies and decreasing with time ("downsweeping") or going from low to high frequency ("upsweeping"). The frequency can also be changed in a nonlinear manner, such that certain frequencies are emitted longer than others. The resulting source wavelet is not impulsive. Vibrators are employed in land acquisition in areas where explosive sources cannot be used, and more than one vibrator can be used simultaneously to improve data quality.

A copolymer of vinyl acetate (ethylenic polymer) and anhydrous maleic acid (a di-hydroxy acid). The vinyl acetate polymer component is usually high molecular weight. As such, with polar groups on the structure, it is used as a flocculant or bentonite extender.

A class of polymers constructed with the monomer ethylene, H2C=CH2, with hydrogen replaced by various chemical groups. Among the many vinyl-based polymers and copolymers are acrylates, methacrylates, acrylamides, acrylate-acrylamide (PHPA), vinyl acetate and the various oligomers of ethylene, polyalphaolefins, linear alphaolefins and isomerized olefins. The prefix "vinyl" is more correctly "ethenyl" and sometimes "vinylene." "Polyvinyl" is synonymous with vinyl polymers, but not specific to an exact polymer. For example, polyvinyl acetate, polyvinyl alcohol, polyvinyl sulfide are commonly used polymers based on ethylene monomers.

The original, undisturbed pressure of a reservoir prior to fluid production.

(noun) An abbreviation for viscosity, the measure of a fluid's internal resistance to flow under an applied shear stress. In drilling and production operations, viscosity is a critical property of drilling fluids, completion fluids, and crude oils that affects hole cleaning, pump pressure requirements, flow behaviour, and facility design.

A property of fluids and slurries that indicates their resistance to flow, defined as the ratio of shearstress to shear rate. Viscosity can be expressed mathematically as follows: Poise is the unit for viscosity, equivalent to dyne-sec/cm2. Because one poise represents a high viscosity, 1/100 poise, or one centipoise (cp), is used for mud measurements. One centipoise equals one millipascal-second. Viscosity must have a stated or an understood shear rate in order to be meaningful. Measurement temperature also must be stated or understood.

A measure of a fluid's resistance to flow. Viscous forces in a fluid are proportional to the rate at which the fluid velocity is changing in space; the proportionality constant is the viscosity. For Newtonian liquids (liquids that show no variation of viscosity with shear or extension rate), the ratio of extensional viscosity to shear viscosity is 3. This value is Trouton's ratio. For more complex liquids, for example, polymer solutions, Trouton's ratio can be different from 3 and can vary with shear or extension rate.

(noun) Crude oil with high viscosity and low API gravity that resists flow under normal reservoir or surface conditions, typically requiring thermal stimulation, diluent blending, or specialised artificial lift to achieve commercial production rates. Viscous oils generally have API gravities between 10 and 22.3 degrees.

A type of woody kerogen that is relatively uniform in composition. Since vitrinite changes predictably and consistently upon heating, its reflectance is a useful measurement of source rock maturity. Strictly speaking, the plant material that forms vitrinite did not occur prior to Ordovician time. Also, because vitrinite originated in wood, its occurrence in marine rocks might be limited by the depositional processes that act in a given depositional environment.

A measure of the thermal maturity of organic matter. This analytical method was developed to rank the maturity of coals and is now used in other rocks to determine whether they have generated hydrocarbons or could be effective source rocks. The reflectivity of at least 30 individual grains of vitrinite from a rock sample is measured under a microscope. The measurement is given in units of reflectance, % Ro, with typical values ranging from 0% Ro to 3% Ro, with values for gas-generating source rocks typically exceeding 1.5%. Strictly speaking, the plant material that forms vitrinite did not occur prior to Ordovician time, although geochemists have established a scale of equivalent vitrinite reflectance for rocks older than Ordovician.

A surface feature of the Earth that allows magma, ash and gas to erupt. The vent can be a fissure or a conical structure.

The cross section of a material to photoelectric absorption, in barns/cm3. The volumetric cross section, U, is from the product of the photoelectric factor, PEF or Pe , and the electron density. In practice, U is usually calculated using the bulk density instead of the electron density. U is a volumetric quantity, whereas Pe is not. U is more useful in log interpretation since it can be used in a linear mixing law in terms of the volumes of the formation components.

(noun) The ratio of the actual volume of fluid displaced per pump stroke to the theoretical displacement volume of the pump cylinder, expressed as a percentage. In sucker rod pumping systems, volumetric efficiency is reduced by gas interference, fluid slippage past the plunger, and mechanical inefficiencies.

A cavity, void or large pore in a rock that is commonly lined with mineral precipitates.

Pore space consisting of cavities or vugs. Vugular porosity can occur in rocks prone to dissolution, such as limestone, in which case it is secondary porosity.