Glossary

machine computation	A computed log analysis.

macroscopic anisotropy	See anisotropy.

macroscopic capture cross section	See capture cross section.

magma	Molten rock material within the earth from which an igneous rock results by cooling.

magmatic water	Water which exists in or is derived from molten igneous rock or magma.

Magnaflux	A trade name for the equipment and processes used for detecting cracks and other surface discontinuities in iron or steel. A magnetic field is set up in the part to be inspected. and a powder or paste of magnetic particles is applied. The particles arrange themselves around discontinuities in the metal, revealing defects.

magnetic declination	The acute angle between the direction of true north and magnetic north determined at or for a specific geographic location.

magnetostriction	The change in dimension of a body when subjected to a magnetic field. A magnetostrictive transducer, for example, is one which converts electromagnetic energy to mechanical energy and vice versa.

make a trip	To hoist the drill stem out of the wellbore to perform one of a number of operations such as changing bits, attaching core barrel, etc. and then to return the drill stem to the wellbore.

mandrel	A support member for sensors or actuator assemblies of a downhole tool.

manometer	A continuous, borehole-pressure measuring device used in some production-logging tools.

manual shift	Usually an intentional electrical or mechanical shift given to a specific curve by the logging engineer during a survey in order to maximize the utility of a curve or otherwise keep the curve within a given track.

marine drilling riser	Marine riser, drilling riser. A string of specially designed steel pipe which extends from a drill ship or platform to the subsea wellhead. Marine risers are used to provide a return fluid-flow conductor between the well bore and the drill vessel and to guide the drill string to the wellhead on the ocean floor. The riser is made up of several sections including flexible joints and a telescoping joint to absorb the vertical motion of the ship or platform caused by wave action.

mark	(1) A magnetic mark or metal shim on a survey cable. Usually located every 100 feet for depth-control purposes. (2) To measure cable. The act of placing magnetic marks at 100 foot intervals over the useful length of the survey cable. (3) One of a number of marks on a log used to compute logging speed. See minute-mark. (4) tic-mark. See tic mark.

mark detector	A device utilizing a moving-coil pickup which produces an electric output when it is moved in the magnetic field of the magnetic marks on survey cable, or vice versa. Often portable.

marker bed	A distinctive, easily identified rock stratum, especially one used as a guide for drilling.

marl	A semisolid or unconsolidated clay, silt, or sand.

Marsh funnel	A calibrated funnel used in field tests to determine the viscosity of drilling mud.

mass absorption	The absorption of nuclear particles or photons by the mass of material through which the energy must pass. In nuclear logging, this refers to the loss of radiation caused by the collective mass of materials in the wellbore environment through which the radiation energy must pass before being detected. With respect to the natural radioactivity level of some rock formations, it is sometimes referred to as self-absorption.

mast	See explanation under derrick.

master bushing	The rotary bushing in the rotary table of a drilling rig, which mates with the kelly bushing. The master bushing transmits the torque of the rotary table to the kelly bushing during the drilling process. The kelly bushing, in turn, rotates the kelly and drill string.

matrix	(1) The solid framework of rock which surrounds pore volume. (2) In a rock in which certain grains are distinctly larger than the others, the grains of smaller size comprise the matrix. (3) The natural material in which any metal, fossil, pebble, or crystal is imbedded. (4) In mathematics, a rectangular array of numbers which obey certain rules.

maximum pressure rating	Pertaining to well logging, this is the maximum pressure for satisfactory operation of a specific downhole instrument.

maximum-reading thermometer	A mercury-filled thermometer with a constriction in the capillary tube which registers the maximum temperature attained. When in use, it is contained in a protective case and attached to the downhole instrument or bridle.

maximum temperature rating	Pertaining to well logging, this is the maximum temperature for satisfactory operation of a specific downhole tool.

md	Millidarcy. 1/1000 darcy.

measured depth	MD. Depth measured along the drilled hole. Reported in drilling records, measured by well-logging cables, and shown on well logs. This depth has not been corrected for hole deviation. Compare true vertical depth.

measurements-while-drilling	MWD. A wireless system for making downhole measurements of azimuth, inclination, and tool facing from sensors mounted inside of a nonmagnetic drill collar located near the drill bit. Formation properties can be measured, such as: natural radioactivity, resistivity, and temperature. These measurements are sent to the surface by means of pressure pulses induced in the drilling mud by a mud-pulse transmitter, or stored downhole and brought to the surface when the bit and MWD collar are brought out of the hole.

measure point	A depth reference point on a logging tool at which measurements are taken. Usually the lowermost sensor or lowermost measure point. Static measure point. Compare dynamic measure point.

mechanical shift	See manual shift.

mechanical zero	The reading of a galvanometer at rest, or the reading of any other recording system when no signal is input. With no potential applied, this is a zero reference for galvanometer deflections.

M electrode	The potential-measuring electrode nearest the A electrode in the electrode configuration of a resistivity measuring device. Compare N electrode. See also normal and lateral devices.

memorizer	An electronic storage device which delays the measured signal. Different logging measurements made simultaneously by one downhole instrument quite often do not have the same measure point. For example, one system may be located above another and, therefore, when logging "up," the upper will record entry into a formation while the other is still below the formation. The upper logging system measurements must be delayed or memorized to play back at the measure point of the lower device so that all measurements will be recorded on film simultaneously and at the same depth.

merged data	Refers to well-log data derived from different well logging measurements which have been adjusted for differences in depths of the measurements.

metamorphic rock	An igneous or sedimentary rock which has partially or completely recrystallized in response to elevated temperature, pressure, and chemical environnment. The change the metamorphic rocks have undergone generally occurs in the solid state and at depth in the earth.

meteoric water	Water which is found in or is derived from the atmosphere.

metric system	A decimal system of weights and measures based on the meter as the unit of length. the gram as the unit of weight, the cubic meter as the unit of volume, the liter as the unit of capacity. and the square meter as the unit of area.

metric ton	A measurement equal to 1000 kg or 2204.6 lb avoirdupois. In many oil-producing countries, production is reported in metric tons. One metric ton is equivalent to about 7.4 barrels (42 U.S. gal = 1 bbl) of crude oil with specific gravity of 0.84, or 36 °API gravity.

mho	A unit of electrical conductance. Equal to the reciprocal of ohm.

mho per meter	1000 mmho per nmeter. A unit of electrical conductivity. The conductivity of a cubic meter of material which offers a resistance of one ohm between opposite faces. Reciprocal of ohm-meter.

mica	A silicate mineral characterized by sheet cleavage. Biotite is ferromagnesian black mica. and muscovite is potassic white mica. Sometimes mica is used as a lost-circulation material in drilling.

microannulus	A type of impairment in cement bond quality. Can occur opposite poorly consolidated formations after cemented casing has been subjected to internal pressure. The application of internal pressure results in an expansion of the casing and the cement sheath into soft formations. Removal of the pressure and the resultant contraction of the pipe leaves a microannulus between the casing and the cement sheath. Can be produced. for example, by pressure testing, perforating, fracturing, or cement squeeze operations.

microinverse	A very short lateral electrode arrangement used in obtaining microresistivity measurements for one of the curves of a microlog.

microlaterolog	A microresistivity log made from a tool of the laterolog type with a bucking electrode and two monitor electrodes arranged concentrically on a sidewall pad which is pressed against the formation. The survey current flow is concentrated into a gradually flaring tube shape. Because the spacing is small, the measurement is responsive to the resistivity of a small volume of formation in front of the pad. Measurements of the resistivity of the flushed zone are made. A caliper curve is recorded simultaneously. Compare proximity log and microspherically focused log.

microlog	A type of microresistivity log recorded from a tool that uses three button electrodes spaced 1 in. apart in a line, located on a pad that is pressed against the formation wall. The lower electrode is the A current electrode. The potential of the upper electrode with respect to a remote reference electrode gives a 2 in. micronormal, and the difference between the two upper electrodes gives a 1.5 in. microinverse (lateral type) measurement. Because mud cake usually has appreciably lower resistivity than the formation, the microinverse will read less than the micronormal when mud cake is present. This difference (called positive separation) usually indicates a permeable formation. A caliper curve is usually recorded at the same time.

micronormal	A very short normal-electrode arrangement used in obtaining microresistivity measurements for one of the curves of a microlog.

microresistivity log	A log of the resistivity of the flushed zone around a borehole, measured with electrodes on a pad pressed against the formation wall. See microlog, microlaterolog, proximity log, and microspherically focused log.

Micro-Seismogram	MSG. An acoustic log showing the wave train in the intensity modulated-time mode. Micro-Seismogram is a Welex trademark.

microspherically focused log	A microresistivity log produced by a tool of the spherically-focused type which has electrodes mounted on a sidewall pad which is pressed against the drilled formation. Because of the kind of focusing method used, the tool gives improved flushed zone measurements over those made from microlaterolog- and proximity log-type tools. Mud cakes with thicknesses up to 3/4 in. have little effect on the measurements of the microspherically focused tool; and, resistivity measurements are made from the region just behind the mud cake where the flushing is most effective. A caliper curve usually is recorded simultaneously.

migration	The movement of oil from the area in which it formed to a reservoir rock where it can accumulate.

millidarcy	md. Equal to 1/1000 of a darcy.

milligal	mgal. A unit of acceleration used in gravity measurements. Equivalent to 10^-3 gal or 10 microns per second which is approximately one millionth of normal acceleration of gravity at the earth's surface.

millivolt box	A source for calibrated emf which usually is applied to offset unwanted signals during logging or calibration procedures. See manual shift and electropolarization potential.

mineral	A naturally occurring material having a definite chemical composition and, usually, a characteristic crystal form. May be disseminated in some other mineral or rock. Most mineralogists include the requirements that a mineral have inorganic origin and internal crystalline structure. In accord with the latter requirement, an amorphous compound such as opal would be considered to be mineraloid.

mineral composition of rocks	The crust of the earth contains only eight elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium) with concentrations greater than l% by weight. Minerals are compounds of these elements. each mineral having a specified crystalline structure. The most abundant minerals represent only five types of chemical compounds: silicates, carbonates, sulfates, halides, and oxides. Of these minerals, the silicates are more abundant than all the rest combined, comprising 95% of the rest.

Minerals Management Service (MMS) and Royalty Management Operations (RMO)	The Minerals Management Service, an agency of the Department of the Interior, was created January 19, 1982. It is responsible for the Nation's Federal Offshore leasing program as well as collection and management of all Federal and Indian mineral revenues. The Minerals Management Service is headquartered in Washington D.C. Offshore leasing activities are conducted largely through the MMS's four Outer Continental Shelf Regional Offices located in Anchorage, Alaska; Los Angeles, California; Metairie, Louisiana; and Vienna, Virginia. All onshore leasing is the responsibility of the Bureau of Land Management. Royalty and Accounting activities are conducted in Lakewood, a suburb of Denver, Colorado. Satellite offices, near principal energy development areas, augment the program. Responsibility for overseeing mineral revenue activities on Indian lands is shared by the Bureau of Indian Affairs and the MMS. Indian mineral revenues, like Federal mineral revenues, are administered under the Royal Management Program but are given priority attention.

minute-mark	Tattle tale. A mark or grid line interruption which is placed on the film every 60 seconds during the survey. Usually found near the outside edge of track 1. Used in documenting logging speed.

miscible drive	A method of enhanced recovery in which various hydrocarbon solvents or gases (as propane, LPG, natural gas, carbon dioxide, or a mixture thereof) are injected into the reservoir to reduce interfacial forces between oil and water in the pore channels and thus displace oil from the reservoir rock.

mist flow	A producing flow condition in a well bore in which gas and oil flow at very high velocities. The oil film on the pipe wall becomes very thin and most of the oil is transported in the form of very small droplets nearly homogeneously dispersed in the gas. Therefore, the two phases move at essentiallv the same velocity.

MKS	Meter, kilogram, solar second system of fundamental standards.

mmho	Millimho. A unit of electric conductance equal to 1/1000 of a mho, the reciprical of ohm.

MMS	See Minerals Management Service.

MN spacing	The distance between the two potential-measuring electrodes in the electrode configuration of an electrical resistivity-measuring device. See normal and lateral devices.

mobility	Of a fluid, a measure of the ease with which a fluid can be moved in a specific medium i.e., rock). Equal to the ratio between the effective permeability to the fluid and the viscosity of the fluid.

modified Schmidt diagram	A plot of dipmeter information on polar chart paper where 0° dip is represented on the circumference and 90° at the center. Dips close to structural form a group near the circumference. The center of this group represents the structural dip. Compare Schmidt diagram.

mol	Abbreviation of mole. See mole.

mold	See moldic porosity.

moldic porosity	Porosity formed by the preferential dissolution of a former constituent of sedimentary rock. Once the constituent (such as a shell. other detrital material, or oölith) has dissolved, a void or empty mold remains which bears the shape of the former material.

mole	The quantity of a substance whose unit weight is numerically equal to the molecular weight of the substance. For example, one mole of water (H₂O) is 18 weight units (pounds, grams, tons, etc.) because the molecular weight of water is 18 (two atoms of hydrogen. each of which is 1 unit; one atom of oxygen, the weight of which is l6 units).

molecule	The smallest part of a substance that can exist on its own. It usually consists of a group of atoms that are either different (e.g., water, H₂O, consists of two atoms of hydrogen and one of oxygen) or that are alike (e.g., hydrogen, H₂ in which each molecule is made up of two atoms of hydrogen).

molecular weight	The sum of the atomic weights in a molecule. For example, the molecular weight of water, H₂O, is l8 because the atomic weight of each of the hydrogen atoms is 1 and the atomic weight of oxygen is 16.

mole percent	The ratio of the number of moles of one substance to the total number of moles in a mixture of substances, all multiplied by l00 (to put the number on a percentage basis).

monitor curve	A curve recorded on a well log which is a measure of some aspect of tool performance or stability. An indicator to the quality of measurements being made by the instrument.

monocable	An armored single conductor cable for surveying or perforating.

monoelectrode	A single electrode for measuring formation resistance in electrically-conductive, liquid-filled boreholes. See single-point resistance log.

montmorillonite	It is a hydrous aluminum silicate capable of reacting with such substances as magnesium and calcium. A clay mineral often used as an additive to a drilling mud. See bentonite.

MOP	Movable oil plot.

movable oil plot	MOP. A computed log, based on several logging operations. prepared for the purpose of determining the presence and quantity of movable hydrocarbon (usually oil) at difterent parts of a formation. For example: (1) The first curve of the log will be said to represent the pore volume of the rock framework. It must be determined by a technique in which derived porosity is least influenced by the type of fluid occupying pore volume (sometimes a single porosity curve; i.e., sonic; sometimes computed from several porosity curves). (2) The second curve will represent the pore volume occupied by the water in the flushed zone (i.e., the product of water saturation in the flushed zone and porosity from the first curve). Focused microresistivity tools are usually used to determine flushed-zone water saturation (e.g., microlaterolog, FoRxo, etc.). (3) The third curve represents the pore volume occupied by water in the uninvaded zone (i.e., the product of water saturation beyond the depth of invasion and porosity from the first curve). Deep resistivity measuring tools are usually used, such as deep induction tools or deep laterolog tools. The first curve represents void volume or pore volume. The difference between the first and second curves is related to the volume occupied by immovable hydrocarbon. The difference between the second and third curves represents the bulk volume of movable hydrocarbon. And the third curve represents the bulk volume of water present (including adsorbed water, if part of the original porosity measurement).

mud	Drilling mud. A drilling mixture consisting of one or two liquid phases and dissolved and undissolved solids with properties tailored to a particular problem. It cools and lubricates the drilling bit, conditions the formation wall. removes cut tings from the borehole, brings traces of formation fluids to the surface, and, when it possesses certain characteristics, it provides a wellbore medium suitable for some electrical, acoustic, and nuclear well-logging measurements. See also oil emulsion, invert oil emulsion, oil-base mud, and water-base mud.

mud ball	An accumulation of mud solids that sometimes builds up on the drilling bit during circulation prior to logging. It can present a problem to logging if the ball should become dislodged in the borehole or scrape off on the face of porous and permeable reservoir rock.

mud cake	Filter cake. The residue deposited on the borehole wall as the mud loses filtrate into porous, permeable formations. The mud cake generally has very low permeability and hence tends to retard further loss of fluid to the formation. See invaded zone.

mud cup	(1) A cup-like device containing electrodes which is used to measure mud resistivity. A common item on logging trucks. (2) A receptacle for drilling-mud samples.

mud density	The density of the drilling mud usually measured in pounds per U.S. gallon or pounds per cubic foot.

mud filtrate	The effluent of the continuous (external) phase liquid of drilling mud which penetrates porous and permeable rock, leaving a mud cake on the drilled face of the rock.

mud hose	See kelly hose.

mud log	(1) See mud resistivity log. (2) A record of information derived from examination of drilling fluid and cuttings from the formation.

mud logging	Hydrocarbon well logging. The analysis of samples of the circulated drilling fluid (or mud) and formation cuttings to detect signs of fluids which have entered the mud from the formations. Hydrocarbons may be detected by fluorescence, by chromatographic analysis, gas, and other ways. Also, the mud may be monitored for salinity and viscosity. Plots of such data often include a sample log and a drill time curve.

mud pit	Excavation or tank near the rig into which drilling mud is circulated. Mud pumps withdraw the mud from one end of the pit as the circulated mud (bearing rock chips from the borehole) flows in at the other end. As the mud moves to the suction line, the cuttings drop out leaving the mud "clean" and ready for another trip to the bottom of the borehole.

mud resistivity log	A log made with a microlog- or microlaterolog-type sonde with the arms collapsed so that the measuring pad loses contact with the formation wall. Used to record the mud resistivity at downhole conditions.

mud pump	A large, reciprocating pump used to circulate the mud on a drilling rig. A typical mud pump is a double- or triple-acting, two- or three-cylinder piston pump whose pistons travel in replaceable liners and are driven by a crankshaft actuated by an engine. A mud pump also is called a slush pump.

mud viscosity	Viscosity of drilling mud. See viscosity.

mud weight	A misnomer, see mud density.

multiphase flow	A flow regime in which gas, oil, and water are all flowing.

multiple completion	An arrangement for producing a well in which one wellbore penetrates two or more petroleum-bearing reservoirs that lie one over the other. The tubing strings are suspended side by side in the production casing string. each a different length and each packed off to prevent the commingling of different reservoir fluids. Each reservoir is then produced through its own tubing string.

multishot survey	See directional survey.

MWD	See measurements-while-drilling.