Well logging, also known as borehole logging is the practice of making a detailed record (log) of the geologic formations penetrated by a borehole. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole (geophysical logs). Well logging is done when drilling boreholes for oil and gas, groundwater, minerals, and for environmental and geotechnical studies. Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations Geotechnical engineering is the branch of Civil engineering concerned with the engineering behavior of earth materials

Contents 1 Electric or geophysical well logs 1.1 Types of Electric Logs 1.2 History 1.3 Logging While Drilling 1.4 Logging measurement types 2 Geological logs 3 Information use 4 References 5 See also 6 External links 6.1 Commercial links

Electric or geophysical well logs

The oil and gas industry records rock and fluid properties to find hydrocarbon zones in the geological formations intersected by a borehole. Petroleum ( L petroleum, from Greek πετρέλαιον, lit Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. The logging procedure consists of lowering a 'logging tool' on the end of a wireline into an oil well (or hole) to measure the rock and fluid properties of the formation. West Texas PumpjackJPG|thumb|right|300px|This Pumpjack located south of Midland TX is a common sight in West Texas. An interpretation of these measurements is then made to locate and quantify potential depth zones containing oil and gas (hydrocarbons). Determining the true Porosity of a gas filled formation has always been a problem Logging tools developed over the years measure the electrical, acoustic, radioactive, electromagnetic, and other properties of the rocks and their contained fluids. Logging is usually performed as the logging tools are pulled out of the hole. This data is recorded to a printed record called a 'Well Log' and is normally transmitted digitally to office locations. Well logging is performed at various intervals during the drilling of the well and when the total depth is drilled, which could range in depths from 300 m to 8000 m (1000 ft to 25,000 ft) or more. A drill (from Dutch Drillen) is The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit

Types of Electric Logs

There are many types of electric logs, and they can be categorized either by their function or by the technology that they use. "Open hole logs" are run before the oil or gas well is lined with pipe or cased. "Cased hole logs" are run after the well is lined with casing or production pipe ^[1].

Electric logs can also be divided into two general types based on what physical properties they measure. Resistivity logs measure some aspect of the specific resistance of the geologic formation. Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of Electric current. There are about 17 types of resistivity logs, but they all have the same purpose which is to measure the electric conductivity fluid in the rock.

Porosity logs measure the fraction or percentage of pore volume in a volume of rock. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Most porosity logs use either acoustic or nuclear technology. Acoustic logs measure characteristics of sound waves propagated through the well-bore environment. Nuclear logs utilize nuclear reactions that take place in the downhole logging instrument or in the formation. Nuclear logs include density logs and neutron logs, as well as gamma ray logs which are used for correlation. ^[2].

History

Conrad and Marcel Schlumberger are considered the inventors of electric well logging. Conrad Schlumberger (2 October 1878 in Guebwiller – 9 May 1936 in Stockholm) and Marcel Schlumberger (21 June 1884 in Guebwiller – 9 May Conrad developed the Schlumberger array which was a technique for prospecting for metal ore deposits, and the brothers adopted that surface technique to subsurface applications. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining On September 5, 1927, a crew working for the Schlumberger brothers, lowered an electric sonde or tool down a well in Pechelbronn, Alsace France creating the first well log. Conrad Schlumberger (2 October 1878 in Guebwiller – 9 May 1936 in Stockholm) and Marcel Schlumberger (21 June 1884 in Guebwiller – 9 May Sonde may also be an abbreviation for Radiosonde for the town in Togo see Sonde Togo Sonde is a music design and Alsace (Alsace alzas Alsatian and Elsass pre-1996 German: Elsaß; Alsatia is one of the 26 Regions of France, located on the eastern This article is about the country For a topic outline on this subject see List of basic France topics. In modern terms, the first log was a resistivity log that could be described as 3. 5 meter upside-down lateral log ^[3].

In 1931, Henri G. Doll and G. Dechatre, working for Schlumberger, discovered that the galvanometer wiggled even when no current was being passed through the logging cables. Schlumberger Limited is the world's largest Oilfield services corporation operating in approximately 80 countries with about 84000 people of 140 nationalities A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. This led to the discovery of the spontaneous potential (SP) which was as important as the ability to measure resistivity. In the field of Formation evaluation, spontaneous potential (SP is a naturally occurring electric potential difference in the Earth measured by an electrode in a Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of Electric current. The SP effect was produced naturally by the borehole mud at the boundaries of permeable beds. In computer gaming, a MUD ( Multi-User Dungeon, Domain or Dimension) is a multi-player computer game that combines elements of By simultaneously recording SP and resistivity, loggers could distinguish between permeable oil-bearing beds and impermeable nonproducing beds ^[4]. Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of Electric current.

In 1940, Schlumberger invented the spontaneous potential dipmeter, which greatly improved the vertical resolution of the open hole logs. Schlumberger Limited is the world's largest Oilfield services corporation operating in approximately 80 countries with about 84000 people of 140 nationalities In the field of Formation evaluation, spontaneous potential (SP is a naturally occurring electric potential difference in the Earth measured by an electrode in a "Dipmeter" could also refer to an influential early commercial Expert system called Dipmeter Advisor. This tool allowed the calculation of the dip and direction of the dip of a layer. The basic dipmeter was later enhanced by the resistivity dipmeter (1947) and the continuous resistivity dipmeter (1952). "Dipmeter" could also refer to an influential early commercial Expert system called Dipmeter Advisor. Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of Electric current.

Oil-based mud (OBM) was first used in Rangely Field, Colorado in 1948. The State of Colorado ( or chiefly by nonresidents) is a state located in the Rocky Mountain region of the United States of America. Normal electric logs require a conductive or water-based mud, but OBMs are nonconductive. The solution to this problem was the induction log, developed in the late 1940s.

The introduction of the transistor and integrated circuits in the 1960s made electric logs vastly more reliable. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside Computerization allowed much faster log processing, and dramatically expanded log data-gathering capacity. The 1970s brought more logs and computers. These included combo type logs where resistivity logs and porosity logs were recorded in one pass in the borehole.

The two types of porosity logs (acoustic logs and nuclear logs) date originally from the 1940s. Sonic logs grew out of technology developed during World War II. Nuclear logging has largely replaced acoustic logging, but acoustic or sonic logs are still run on some combination logging tools.

Nuclear logging was initially developed to measure the natural gamma radiation emitted by underground formations. However, the industry quickly moved to logs that actively bombard rocks with nuclear particles. A subatomic particle is an elementary or composite Particle smaller than an Atom. The gamma ray log was introduced by Well Surveys Inc. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions in 1939, and the WSI neutron log came in 1941. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. These logs were important because they could be used in cased wells (wells with production casing). WSI quickly became part of Lane-Wells. During World War II, the US Government gave a near wartime monopoly on open-hole logging to Schlumberger, and a monopoly on cased-hole logging to Lane-Wells. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Schlumberger Limited is the world's largest Oilfield services corporation operating in approximately 80 countries with about 84000 people of 140 nationalities Nuclear logs continued to evolve after the war.

The nuclear magnetic resonance log was developed in 1958 by Borg Warner. Initially the NMR log was a scientific success but an engineering failure. However, the development of a continuous NMR logging tool by Numar (now a subsidiary of Halliburton is a promising new technology. Halliburton Energy Services ( is a US -based Multinational corporation with operations in more than 70 countries

Many modern oil and gas wells are drilled directionally. At first, loggers had to run their tools somehow attached to the drill pipe if the well was not vertical. Modern techniques now permit continuous information at the surface. This is known as logging while drilling (LWD) or measurement-while-drilling (MWD). Logging while drilling (LWD is a technique of measuring Geologic formation properties in real-time while drilling an Oil well. MWD logs use mud pulse technology to transmit data from the tools on the bottom of the drillstring to the processors at the surface.

Logging While Drilling

In the 1980s, a new technique, Logging While Drilling (LWD), was introduced which provided similar information about the well. Logging while drilling (LWD is a technique of measuring Geologic formation properties in real-time while drilling an Oil well. Instead of sensors being lowered into the well at the end of wireline cable, the sensors are integrated into the drill string and the measurements are made while the well is being drilled. A drill string on an Oil rig is a column or string of drill pipe that transmits Drilling fluid (via the Mud pumps and rotational power (via the While wireline well logging occurs after the drill string is removed from the well, LWD measures geological parameters while the well is being drilled. However, because there are no wires to the surface, data are recorded downhole and retrieved when the drill string is removed from the hole. A small subset of the measured data can also be transmitted to the surface in real time via pressure pulses in the well's mud fluid column. This mud telemetry method provides a bandwidth of much less than 100 bits per second, although, as drilling through rock is a fairly slow process, data compression techniques mean that this is an ample bandwidth for real-time delivery of information.

Logging measurement types

Logging measurements are quite sophisticated. The prime target is the measurement of various geophysical properties of the subsurface rock formations. Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic Of particular interest are porosity, permeability, and fluid content. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated Porosity is the proportion of fluid-filled space found within the rock. It is this space that contains the oil and gas. Permeability is the ability of fluids to flow through the rock. The higher the porosity, the higher the possible oil and gas content of a rock reservoir. Determining the true Porosity of a gas filled formation has always been a problem The higher the permeability, the easier for the oil and gas to flow toward the wellbore. Logging tools provide measurements that allow for the mathematical interpretation of these quantities.

Beyond just the porosity and permeability, various logging measurements allow the interpretation of what kinds of fluids are in the pores — oil, gas, brine. In petrophysics, Archie's law relates the in-situ Electrical conductivity of sedimentary rock to its Porosity and Brine saturation In addition, the logging measurements are used to determine mechanical properties of the formations. These mechanical properties determine what kind of enhanced recovery methods may be used (tertiary recovery) and what damage to the formation (such as erosion) is to be expected during oil and gas production. Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind

The types of instruments used in well logging are quite broad. The first logging measurements consisted of basic electrical logs (resistivity) and spontaneous potential (SP) logs, introduced by the Schlumberger brothers in the 1920's. The spontaneous potential log, commonly called the self-potential log or SP log, is a measurement taken by Oil industry well loggers to characterise Conrad Schlumberger (2 October 1878 in Guebwiller – 9 May 1936 in Stockholm) and Marcel Schlumberger (21 June 1884 in Guebwiller – 9 May Tools later became available to estimate porosity via sonic velocity and nuclear measurements. Tools are now more specialized and better able to resolve fine details in the formation. Radiofrequency transmission and coupling techniques are used to determine fluid conductivity (brine is more conductive than oil or gas). Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz Sonic transmission characteristics (pressure waves) determine mechanical integrity. Nuclear magnetic resonance (NMR) can determine the properties of the hydrogen atoms in the pores (surface tension, etc. ). Nuclear scattering (radiation scattering), spectrometry and absorption measurements can determine density and elemental analysis or composition. Radiation scattering is the deflection of Radiation ( electromagnetic or products of Nuclear decay) from its original path as a result of interaction Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Elemental analysis is a process where a sample of some material (e High resolution electrical or acoustical imaging logs are used to visualize the formation, compute formation dip, and analyze thinly-bedded and fractured reservoirs.

In addition to sensor-based measurements above, robotic equipment can sample formation fluids which may then be brought to the surface for laboratory examination. Also, controlled flow measurements can be used to determine in situ viscosity, water and gas cut (percentage), and other fluid and production parameters.

Geological logs

Geological logs, use data collected at the surface, rather than during an electric survey. The geological logs include drilling time logs, sample logs, and mud logs. A Mud log is a graphical representation of the ROP ( R ate O f P enetration Lithology, Hydrocarbons and other drilling parameters Though mud logs have become the industry standard. A Mud log is a graphical representation of the ROP ( R ate O f P enetration Lithology, Hydrocarbons and other drilling parameters The petroleum industry includes the global processes of exploration, extraction, refining, transporting (often by Oil tankers and pipelines

Drilling time logs record the time required to drill a given thickness of rock formation. A change in the drilling rate or penetration rate usually means a change in the type of rock penetrated by the bit. The drilling time is expressed as minutes per foot, while the rate of penetration is usually expressed as feet per hour. Therefore, drilling time is the inverse of penetration rate.

Sample logs are made by examining cuttings, which are bits of rock circulated to the surface by the drilling mud in rotary drilling. The cuttings have traveled up the wellbore suspended in the drilling fluid or mud which was pumped into the wellbore via the drill string/pipe and they return to the surface via the annulus, then to the shale shakers via the flow line. A drill string on an Oil rig is a column or string of drill pipe that transmits Drilling fluid (via the Mud pumps and rotational power (via the The annulus (yellow area in diagram of an oil well refers to any void between any piping tubing or casing and the piping tubing or casing immediately surrounding Shale shakers are devices that remove Drill cuttings from the Drilling fluid while circulating and drilling Cuttings are then separated from the drilling fluid as they move across the shale shakers and are sampled at regular depth intervals. These rock samples are analyzed and described by the wellsite geologist or mudlogger. A mudlogger, or mud logger in the modern oil field positions hydrocarbons with respect to depth

Mud logs are prepared by a mud logging company contracted by the operating company. A Mud log is a graphical representation of the ROP ( R ate O f P enetration Lithology, Hydrocarbons and other drilling parameters Mud logging, also known as hydrocarbon well logging is a type of borehole logging performed by the Mud logger, that provides well owners with information about the Lithology One parameter a typical mud log displays is the formation gas (gas units or ppm). "The gas recorder usually is scaled in terms of arbitrary gas units, which are defined differently by the various gas-detector manufactures. In practice, significance is placed only on relative changes in the gas concentrations detected^[5]. " The current industry standard mud log normally includes real-time drilling parameters such as rate of penetration (ROP), lithology, gas hydrocarbons, flow line temperature (temperature of the drilling fluid) and chlorides but may also include mud weight, estimated pore pressure and corrected d-exponent (corrected drilling exponent) for a pressure pack log. The petroleum industry includes the global processes of exploration, extraction, refining, transporting (often by Oil tankers and pipelines A Mud log is a graphical representation of the ROP ( R ate O f P enetration Lithology, Hydrocarbons and other drilling parameters The Rate of penetration, abbreviated as ROP as used in the drilling industry, is the speed at which a drill bit breaks the rock under it to deepen In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Drilling a very deep hole is a very costly business The hole size is kept very small as the drilled depth increases because it is to cased and cemented after wards The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus Mud weight as described in the oilfield is the density (weight of the Drilling fluid and is normally measured in pounds per gallon (lbm/gal or (ppg Estimated pore pressure, as used in the Oil industry and Mud logging, is an approximation of the amount of force that is being exerted into the Borehole The Corrected d-exponent, also known as cd-exponent or more correctly dc-exponent (dc-exponent as used in Mud logging in the Oil industry, is an extrapolation Other information that is normally notated on a mud log include lithology descriptions, directional data (deviation surveys), weight on bit, rotary speed, pump pressure, pump rate, viscosity, drill bit info, casing shoe depths, formation tops, mud pump info, to name just a few. A Mud log is a graphical representation of the ROP ( R ate O f P enetration Lithology, Hydrocarbons and other drilling parameters In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which A Deviation survey, more generally known in the Oil industry as simply a ' Survey ' is the measurement of a Borehole 's departure from the Vertical Weight on bit (WOB as expressed in the Oil industry, is the amount of downward force exerted on the drill bit and is normally measured in thousands of pounds A Rotary table is a mechanical device on a drilling rig that provides clockwise (as viewed from above rotational force to the Drill string to facilitate the process of drilling Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress.

Information use

The well and mud logs are usually transferred in 'real time' to the operating company, which uses these logs to make operational decisions about the well, correlative formation depths analysis to surrounding wells and to make interpretations about the quantity and quality of hydrocarbons present. Specialists involved in well log interpretation are called log analysts.

References

See also

• Geosteering
• Drilling mud
• Drilling rig
• List of oilfield service companies
• Formation evaluation
• MWD/LWD
• Wireline

External links

• Society of Petrophysicists & Well Log Analysts
• Basic Description of LWD-MWD Well Logging

Commercial links

• TecWel
• Petris Recall
• Geoservices

In the process of drilling a borehole, geosteering is the act of adjusting the borehole position ( Inclination and Azimuth angles on the fly In Geotechnical engineering, drilling mud, also known as spud mud (when beginning the drilling process is a Drilling fluid used to drill Boreholes For a detailed diagram of a Petroleum drilling rig, See Drilling rig (petroleum A drilling rig is a machine which creates holes (usually called The list of oilfield service companies includes companies that provide services to the Petroleum exploration and production industry but that do not typically produce petroleum In Petroleum exploration and development formation evaluation is used to determine the ability of a borehole to produce Petroleum. Measurement while drilling is a tool that transmits information in real time from the tool located near the drill bit to the surface Logging while drilling (LWD is a technique of measuring Geologic formation properties in real-time while drilling an Oil well.

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