Glossary

facies	Appearance, character. Characteristics of a bed which reflect the conditions of its origin. Such characteristics may change relative to the same bed. or other beds which are deposited at the same time, reflecting changes in the depositional environments.

Fahrenheit scale	A temperature scale devised by Gabriel Fahrenheit. in which 32° represents the freezing point and 212° the boiling point of water at standard sea-level pressure. Fahrenheit degrees may be converted to centigrade degrees by using the following formula: °C = 5/9 (°F – 32)

fault	Fracture or break in subsurface strata. Strata on one side of the fault line have been displaced (upward, downward, or laterally) relative to their original positions.

fault block	A mass bounded on at least two opposite sides by faults. It may be elevated or depressed relative to the adjoining regions, or it may be elevated relative to the region on one side and depressed relative to that on the other. See illustration in fault definition.

fault trap	A subsurface hydrocarbon trap created by faulting, which causes an impermeable rock layer to be moved to a location opposite the reservoir bed.

feldspar	A group of abundant rock-forming minerals of the general formula, MAl (Al,Si)₃O₈ where M can be K, Na, Ca, Ba, Rb, Sr, and Fe. Most widespread of any mineral group, feldspar may constitute 60% of the earth's crust, occurring in all types of rock. When the positive ion is K⁺, the mineral is orthoclase; when it is Na⁺, it is albite; when it is Ca⁺², it is anosthite.

felsic	A mnemonic term derived from "fe" for feldspar, "l" for lenads or feldspathoids, and "s" for silica. The term is applied to light-colored rocks containing an abundance of one or all of these constituents. Also applied to the minerals themselves. The chief felsic minerals are quartz, feldspars, feldspathoids, and muscovite. Compare mafic.

FFI	Free fluid index. See nuclear magnetism log.

field	A geographical area in which a number of oil or gas wells produce from a continuous reservoir. A field may refer to surface area only or to underground productive formations as well. In a single field, there may be several separate reservoirs at varying depths.

field print	A preliminary print of the well log presented in the field at the completion of the survey operation.

field tape	A magnetic tape on which well-log data were recorded during the logging operation.

film badge	A small piece of x-ray or similar photographic film in a light-proof paper usually crossed by lead or cadmium strips, carried in a small metal or plastic frame. The badge is used to estimate the amount of radiation to which an individual wearing the badge has been exposed.

filter	A porous medium through which a fluid is passed to separate it from material suspended in it.

filter cake	See mud cake.

filter loss	The amount of tluid that can be delivered through a permeable filter medium after being subjected to a differential pressure for a specified time.

filter press	A device used in separating the suspended colloidal material from the liquid of drilling mud.

filtrate	Usually refers to mud filtrate.

filtration	The process of filtering a fluid.

filtration loss	The escape of the liquid part of a drilling mud into permeable formations.

final print	A print generally supplied as the permanent well-log record. This is a composite log (if the current survey is the last of a series) which has been printed on high quality reproduction paper after final entry of all pertinent log heading information, addition of calibration tails, and necessary drafting.

fines	Fragments or particles of rock or mineral which are too minute to be treated as ordinary coarse material. When found in pore spaces of reservoir rock, fines sometimes can be moved by produced fluids to cause formation damage. See skin effect.

fire flood	In-situ combustion.

first curve	The self potential curve, SP. See further discussion under electrical survey and SP.

first reading	FR (1) The depth of the first useable reading or value recorded on a curve at the onset of the survey. (2) Usually the depth in the hole of the deepest reading for any given curve normally recorded in the bottom-to-top direction. See also pick-up.

Fischer assay	A prescribed laboratory procedure for assaying the production of liquid and gaseous products upon pyrolysis of organic matter contained in rock. The destructive distillation from this process can give the content of water, oil, and ash. The residue rock can contain coke.

fish	(1) A foreign object (such as tool or pipe) lost in the borehole which obstructs routine functions performed in the well. Usually must be removed or by-passed. (2) To attempt to retrieve the object lost in the well bore. (3) A portable surface electrode, attached to the end of a line, making a remote ground electrical connection. The fish is usually placed in a shallow hole dug into the ground and filled with mud. Compare ground stake.

fishing bell	The cable end of the housing enclosing the connectors at the downhole end of the survey cable. The housing has a neck or reduced diameter at the cable end which is designed to enter an overshot and be caught securely during fishing operations.

fishing neck	Cable end of the fishing bell.

fissure	A crack or fracture in a subsurface formation.

flat-topping	Forming a plateau of maximum recorded values. A result of the loss of sensitivity caused by saturation of some part of the measuring or recording system.

float collar	A special coupling device. inserted one or two joints above the bottom of the casing string, that contains a check valve to permit fluid to pass downward but not upward through the casing. The float collar prevents drilling mud from entering the casing while it is being lowered, allowjng the casing to float during its descent and also decreasing the load on the derrick. The float collar also prevents a backflow of cement during the cementing operation.

floating pad	A term used to refer to the pad of any contact logging tool that does not make sufficiently good contact with the formation wall to record quality information. For example. dipmeter pads in oval shaped holes and microlaterolog pads in slant holes where the weight of the tool might pull the pad away from the borehole wall.

float shoe	A short, heavy, cylindrical steel section with a rounded bottom, attached to the bottom of the casing string. It contains a check valve and functions similarly to the float collar but also serves as a guide shoe for the casing.

flood	To drive oil from a reservoir into a well by injecting water under pressure into the reservoir formation. See water flood.

flow check	An interruption in circulation with no or very little movement of the drill pipe. It is performed in order to determine if there is an influx of gas, oil, or water into the hole and drilling mud.

flowing pressure	The pressure registered at the wellhead of a flowing well.

flowing well	A well that produces oil or gas by its own reservoir pressure rather than by use of artificial means (as pumps).

flowline	Large-diameter pipe which conducts mud coming out of the hole from the annulus to the shale shaker and mud tanks.

flowmeter	A downhole tool used to measure the rate-of-flow and sometimes direction-of flow of borehole fluids. Usually these tools utilize impellers. During use the tools are run continuously with or against the direction of fluid flow. Sometimes the flowmeter measurements are made at stationary levels when fluid is diverted past the impeller blades by an inflatable packer. See also continuous flowmeter, fullbore-spinner flowmeter, packer flowmeter, spinner survey, and radioactive-tracer log.

flow tube	The lower end of the hydraulic packing head (control head) of a lubricator. Grease or other viscous material is injected into flow tubes (containing seals, etc.) providing additional seal and lubrication. Flow tubes can be stacked for additional pressure control through step-down pressure reduction.

fluid	Any substance which will undergo continuous deformation when subjected to shear stress. Liquids and gases are fluids.

fluid level	A misnomer. Often used incorrectly to refer to liquid level in the open or cased borehole.

fluid loss	The loss of the liquid part of the drilling mud into a formation, often minimized or prevented by the blending of additives with the mud.

fluid travel log	FTL. A record of borehole fluid flow rate. See radioactive-tracer log. Compare flowmeter.

fluid sampler	A device used to recover accurately depth-controlled fluid samples of wellbore fluids for PVT analysis. The taking of the fluid in the well bore and the transfer of the fluids to a PVT receptacle is accomplished in a way to avoid pressure loss.

fluid wave	A compressional wave in the liquid column. The wave form arrival which has been transmitted to the receiver directly through the liquid column within the well bore.

flushed zone	The zone at a relatively short radial distance from the borehole, immediately behind mud cake, which is considered to be flushed by mud filtrate (i.e., is considered to have all mobile formation fluids displaced from it). See also invaded zone.

focused log	Refers to a well log produced by any well-logging device in which survey-current flow is focused or otherwise controlled. Examples of focused logs are: laterolog, guard log, microlaterolog, induction log, and spherically focused log. There are two main purposes for focusing survey current of resistivity-measuring devices: (1) To increase vertical resolution of the logging tool; i.e., improve its capability to resolve thin beds. That reduces the influence of adjacent beds on the measurements. (2) To reduce the influences of borehole and mud cake in the presence of saline drilling mud.

fold	A flexure of rock strata into arches and troughs, produced by earth movements. See anticline and syncline.

formation	(1) A general term applied in the well-logging industry to the external environment of the drilled well bore without stratigraphic connotation. (2) The basic or fundamental rock-stratigraphic unit in the local classification of rocks, consisting of a body of rock (usually a sedimentary stratum of strata, but also igneous and metamorphic rocks) generally characterized by some degree of internal lithologic homogeneity or distinctive lithologic features (such as chemical composition, structures, textures, or gross aspect of fossils). Formations may be combined in groups or subdivided into members and beds. A formation name should preferably con sist of a geographic name followed by a descriptive lithologic term (usually the dominant rock type) or by the word formation if the lithology is so variable that no single lithologic distinction is appropriate

formation damage	See skin effect.

formation density log	See density log.

formation dip	The angle at which a formation bed inclines away from the horizontal. See dip. See illustration at strike.

formation evaluation	The analysis and interpretation of well-log data, drill-stem tests, etc. in terms of the nature of the formations and their fluid content. The objectives of formation evaluation are (1) to ascertain if commercially producible hydrocarbons (or other forms of energy and minerals) are present, (2) to determine the best means for their recovery, and (3) to derive lithology and other information on formation characteristics for use in further exploration and development.

formation factor	See formation resistivity factor.

formation factor log	A log in which the formation resistivity factor curve derived from a resistivity or porosity estimating device is shown as a function of depth. Usually recorded on a logarithmic grid.

formation fracturing	See hydraulic fracturing. A less common means of formation fracturing employs the use of explosives.

formation interval tester	A formation tester.

formation pressure	The pore pressure existing within reservoir rock or non-reservoir rock at a specified time. The pressure exerted by fluids in a formation, recorded in the hole at the level of the formation with the well shut in. It is also called reservoir pressure or shut-in bottom-hole pressure. See reservoir pressure. Compare geopressure.

formation resistivity factor	Formation factor, F. Equal to the ratio of the resistivity of the l00% water-saturated rock framework to the resistivity of the water solution contained in the rock. The limiting formation factor is an intrinsic characteristic of the rock, obtainable with reliability only when the interpore water solution is highly salt saturated. The apparent formation factor. most often obtained, is a function of porosity, salinity of water filling the pores, pore geometry, clay content, and presence of electrically conductive solid matter. See Archie's formulas.

formation signal	The signal related to the formation. with the logging tool at a given depth in the borehole, as opposed to signals generated from the tool itself (e.g., diode error, sonde error) or coming from borehole fluids.

formation strike	See description and illustration under strike.

formation tester	A wireline tool used for recovering fluid samples from the formation and recording hydrostatic, flowing, and shut-in pressures versus time.

formation testing	The gathering of data on a formation to determine its potential productivity before installing casing in a well. The conventional method is the drill-stem test. Incorporated in the drill-stem-testing tool are a packer. valves, or ports that can be opened and closed from the surface, and a pressure-recording device. The tool is lowered to bottom on a string of drill pipe and the packer set, isolating the formation to be tested from the formations above and supporting the fluid column above the packer. A port on the tool is opened to allow the trapped pressure below the packer to bleed off into the drill pipe, gradually exposing the formation to atmospheric pressure and allowing the well to produce to the surface, where the well fluids can be sampled and inspected. From a record of the pressure readings, a number of facts about the formation can be inferred.

formation volume factor	The ratio of the volume of gas or liquid with its dissolved gas at reservoir conditions of tempcrature and pressure to its volume at standard conditions.

formation water	See interstitial water. Compare connate water.

FoRxo Log	A focused resistivity log recorded from a pad which contains a small button electrode surrounded by a guarding electrode and which is forced against the side of the bore hole. The current from the button electrode is forced to flow out into the first few inches of the formation, which would be the R_xo zone in a permeable formation. FoRxo is a Welex trademark.

fossil fuel	A deposit of organic material containing stored solar energy that can be used as fuel. The most important are coal, natural gas, and petroleum.

fourth curve	A name given to a deep investigation resistivity curve appearing on electrical surveys in the 1930s and 1940s. Usually, this was a lateral curve. See further discussion under electrical survey.

FR	See first reading.

frac job	See hydraulic fracturing.

fracture	A break, parting, or separation in brittle rock.

Fracture Finder log	An acoustic well log used in the location of fractures. Usually the log consists of one or more curves in which the amplitudes of the compression and/or shear wave forms are shown across a formation segment. Fractures may produce attenuation of both compressional and shear waves, if the fractures are properly oriented. Fracture Finder is a Welex trademark.

fracture gradient	Is the pressure per unit depth required to fracture or cause the rock of the formation to separate. See also hydraulic fracturing.

fracture log	See Fracture Finder log, an acoustic method. Other types of fracture logs employ resistivity measurements to show possible fractures.

fracture porosity	Porosity resulting from the presence of openings produced by the breaking or shattering of brittle rocks.

free-air correction	A correction for the elevation of a gravity measurement required because the measurement was made at a different distance from the center of the earth than the datum. The first term of the free air correction is 0.09406 mgal/ft or 0.3086 mgal/m.

free fluid index	FFI. The percent of the bulk volume occupied by fluids which are free to flow, as recorded on the nuclear magnetism log. Gas gives a low FFI.

free interstitial water	Mobile interstitial water. Compare bound water.

free pipe	Pipe or casing in a well bore which is free to vibrate or respond to stress. Casing or tubing which is free of the restraint of a cement sheath or formation materials.

free point	The deepest depth in the well bore that stuck casing or drill pipe is free and can be salvaged.

free-point indicator	A tool designed to measure the amount of stretch in a string of stuck pipe and to indicate the deepest point at which the pipe is free. The free-point indicator is lowered into the well on a conducting cable. Each end of a strain-gauge element is anchored to the pipe wall by friction springs or magnets, and, as increasing strain is put on the pipe, an accurate measurement of its stretch is transmitted to the surface. The stretch measurements indicate the depth at which the pipe is stuck.

frequency	(1) The number of cycles or waves completed in a unit of time. In electronics, one cycle per second is a frequency unit called a hertz. (2) The number of occurrences or events over a specified period of time or length of borehole.

fresh mud	A relative, though inaccurate, term which describes the condition where the make-up water for the drilling mud is fresher than the water in the formations to be drilled.

fresh water	Very low in dissolved salts. Sometimes used comparatively with respect to normal sea water (which has 35,000 parts of dissolved salts per million). Sometimes used comparing mud filtrate with formation water.

froth flow	In producing wells, a fluid-flow condition in well bores in which larger bubbles and slugs of gas have united to move up the center of the column. The gas carries some oil droplets, although most of the oil flows up along the pipe walls.

fullbore-spinner flowmeter	A flowmeter with retractable impeller blades which can be used below the bottom of tubing where the impeller blades open to almost full inside diameter of the casing. The measurement made is related to the velocity of the fluid relative to the tool, which in turn is related to the volumetric flow rates. In polyphasic flow, however, much higher threshold flow rates are needed for useful measurements. A continuous log can be recorded with or against the flow of fluid.

full waveform recording	A representation of the acoustic wave train in the amplitude-time mode. A trace in the X-Y plane illustrating the wave amplitude vs. time. See acoustic log. Compare Micro-Seismogram or intensity modulated-time. See wave train and illustration at wave train display.

full wave train	See wave train.

funicular saturation	See description and illustration under saturation.

funnel viscosity	Viscosity as measured by the Marsh funnel, based on the number of seconds it takes for 1,000 cm³ of drilling fluid to flow through the funnel.