crain's petrophysical handbook - what is a well log

7/21/2019 Crain's Petrophysical Handbook - What is a Well Log

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1/9/2015 Crain's Petrophysical Handbook - Whar Is A Well Log?

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WHAT IS A LOG?

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Logging Basics Log Types Log Scales Log LayoutsSee Also Integration Analysis Basics Courses Site Map

LOGGING BASICSWell logging is the process of recording various physical, chemical,electrical, or other properties of the rock/fluid mixtures penetrated bydrilling a borehole into the earth's cruste. A log is a record of a voyage,similar to a ship's log or a travelog. In this case, the ship is ameasuring instrument of some kind, and the trip is taken into and outof the wellbore.

In its most usual form, an oil well log is a record displayed on a graph

with the measured physical property of the rock on one axis and depth(distance from a near-surface reference) on the other axis. More thanone property may be displayed on the same graph.

Well logs are recorded in nearly all oil and gas wells and in many mineral and geothermal exploration anddevelopment wells. Although useful in evaluating water wells, few are run for this purpose.

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The traditional well log is a record of various measurements of the physical properties of rocks recorded versus depth (left side of illustration). Each wiggly line is a log curve, representing a particular roc property.

More recently, imaging logs have appeared in which colour and position versus depth are used to display datain more intuitive formats (right half of illustration).

None of the logs actually measure the physical propertiesthat are of most interest to us, such as how much oil or gasis in the ground, or how much is being produced. Suchimportant knowledge can only be derived, from themeasured properties listed in the box on the left (andothers), using a number of assumptions which, if true, willgive reasonable estimates of hydrocarbon or mineral

resources.

Thus, analysis of log and related laboratory data is required. The art and science of log analysis is mainlydirected at reducing a large volume of data to more manageable results, and reducing the possible error in theassumptions and in the results based on them. When log analysis is combined with other physical





measurements on the rocks, such as core analysis or petrographic data, the work is called petrophysics or petrophysical analysis. The results of the analysis are calledpetrophysical properties or mappable reservoir properties.The petrophysical analysis is said to be “calibrated” whenthe porosity, fluid saturation, and permeability resultscompare favourably with core analysis data. Further confirmation of petrophysical properties is obtained byproduction tests of the reservoir intervals.

The use of well logs for evaluating mineral deposits other than oil and gas, such as coal, potash, uranium, and hardrock sequences has been practiced since the early 1930’sand is widespread today. Although the vast majority of logsare run to evaluate oil and gas wells, an increased number are being run for other purposes, including evaluation of geothermal energy and ground water. A large portion of thisHandbook is aimed at oil and gas, but the other topics arenot ignored. Most Chapters apply to both hydrocarbon andmineral exploration.

When logs are used for purposes other than evaluation of oiand gas, they are often called geophysical logs instead of

well logs. The science is called borehole geophysics insteadof petrophysics. This difference is merely a matter of semantics and training. The theory doesn't change - justthe nomenclature, and sometimes the emphasis.

To perform a logging operation, the measuring instrument, often called a probe or sonde, is lowered into theborehole on the end of an insulated electrical cable. The cable provides power to the downhole equipment.Additional wires in the cable carry the recorded measurement back to the surface. The cable itself is used asthe depth measuring device, so that properties measured by the tools can be related to particular depths in theborehole.

<== The wireline logging operation showing logging truck (right), logging cable strung into the rig, then lowered into the borehole (left), with loggingtools at the end of the cable (bottom). Logs are usually recorded while

being pulled upward by the winch in the logging truck. Most logs can alsobe run as an integral part of the drill string (logging while drilling or LWD)or attached to coiled tubing. These methods are useful in deviated,

horizontal, or other hostile well environments.





Cutaway view of a modern logging truck showing winch with logging cabl(left) and computerized operator's station (center)

A logging tool is made up of a sonde and a cartridge. The sonde is theportion of the tool which gives off energy, receives energy, or both. Thecartridge contains the electrical circuitry or computer components neededto control the downhole equipment, and to transmit data to and from thesurface.

Combination logging tools consist of more than one sonde and cartridge,

so that more than one log can be recorded on a single trip into thewellbore.

Surface equipment is mounted in a logging truck, van, or skid unit fromwhich all logging operations are controlled. The logging unit containshoisting equipment for lowering and raising the tools in the hole, andelectronic or computer equipment for controlling and recording thedownhole measurements.





Recording the well log involves a number of steps, beginning with sensing and pre-processing themeasurement in the logging tool itself, transmission of this information to the surface over several miles of

wireline, further processing in the logging truck computer, data storage on disc or magnetic tape, and finally display of the data on film or paper.

Measurements are recorded in two forms, analog and digital. The analog data may be recorded onphotographic film, electronic plotter, or chart recorder. The same data are captured in digital form on magnetitape or disc for later use in computer aided analysis. Many instrument control and calibration functions arenow handled by the same computer used to record the digital data, with some human control. The result is a

log, as seen below.

All logging tools and surface equipment must be properly calibrated. Service companies have calibrationprocedures for most tools, some of which are based on standards established by the American PetroleumInstitute (API). Each tool must be calibrated at the surface before placing it in the hole to make measurementsand must pass certain calibrations after the measurements are complete to verify that measurement accuracyhas not drifted. Some tools also have downhole calibration checks.

After reaching total depth, or some other location of interest in the borehole, measurements are made whilepulling the tool upward over several hundred feet of the borehole. This is called the repeat run, and is used todetermine the repeatability of the measurements when compared to the main logging pass. After the repeat runis complete, the tool is lowered to the bottom of the hole, and the main logging pass is commenced. Duringthe early portion of these measurements, the responses are compared to those of the repeat run to determine





that no instrument drift has occurred. Results of all field calibrations and repeats are attached to the bottom ofthe well log record.

In addition to the actual measurements, the well log itself contains information about the logging processwhich supports use and interpretation of the data. The well name, location, date, surface measurements on themud system, drill bit size, casing information, and logging equipment data are found on the log heading, Anypertinent information or comments regarding the logging job may be recorded in the remarks section.

The logging equipment is carried to the wellsite on a truck (for land based operations near roads), or transported by helicopter on skids (for remote land operations) or are permanently mounted on offshore rigs.

Some typical logging units are shown below.

Logging Trucks and Skid Units

Computerized surface equipment is now the rule rather than the exception. Such units, on a truck and withlogging tools on board, can cost over $1,000,000.





Inside a Modern Logging Unit

TYPES OF LOGSLogs run in a hole which has just been drilled, and before it is cased, are called open-hole logs. Logs run aftethe well is cased are called cased-hole logs. Open hole logs are mainly used to determine the petrophysicalproperties of the rocks. Some cased hole logs are used for the same purpose. Others are used to assess the

integrity of the well completion; others are used to assess fluid flow into the well.

Other types of logs require no cable, such as a mud log which may record up to 5 or 10 properties of thedrilling fluid, or a drilling log which records the rate of penetration and other functions of the drilling process.

The geology log, often called the stratigraphic log, strat log, or sample description log, is a record of the rocksamples retrieved from the drilling mud, and is one of the primary sources of rock and fluid descriptions for the well. It consists of a verbal description of the rock type as well as qualitative or interpretive dataconcerning evidence of the fluid content of the rock. These are all useful logs and are used in any analysis of well, if they are available.

Most logs can now be recorded while drilling is going on or while tripping the drill pipe. This is calledmeasurements while drilling (MWD) or logging while drilling (LWD). Open-hole logs require that the drill string





be removed from the well bore before the logging tools can be lowered into the hole. MWD does not have thisneed, so measurements are available continuously as drilling proceeds.

A composite log is made up of measurements and interpretations from many sources of data. It is usuallymade up in the office in a standard format (for the company or agency who owns the well). Since itcompresses a great deal of data onto one log, it is often one of the most used items in the well file.

Most open and cased-hole logs are recorded continuously as the tools are pulled out of the hole. A few logs,however, may only be recorded when the tool is stationary in the hole, such as the gravity meter survey. Suchlogs are called station-by-station logs as opposed to continuous logs. Some early station-by-station logs, by

virtue of significant improvements in measuring and recording techniques, have become continuous logs. Thfirst electrical log run in 1927 was station-by-station, but soon after, electrical logs were run as continuouslogs.

Most open hole logs are run in a conductive mud system. Muds with relatively high resistivity are called freshmuds, and those with low resistivity are called salt muds. Salt muds may be salted on purpose to reduceerosion in shales or solution of salt beds while drilling through them.

Oil-base muds are non-conductive and cause a few problems, but not many are serious. You cannot run SP,microlog, microlaterolog, or laterlog because they need conductive mud. Dipmeter and Formation MicroScanners need scratcher pads but otherwise operate normally. Sonic, density, neutron, gamma ray, NMR,caliper, induction logs all work normally.

Logs are used for a variety of purposes depending on the nature of the data gathered. Correlation from well towell is the oldest and probably the most common use of logs. It allows the subsurface geologist to mapformation depths and thicknesses and then to identify conditions that could trap hydrocarbons.

Correlation is usually based on the shapes of the recordedcurves versus depth. Correlation in complex geologic areasmay be difficult or impossible, and in any event requirescorroboration from actual rock samples for the initialcorrelations in an area. After the curve shape patterns arerecognized, they can often be used in subsequent wellswithout relying too heavily on rock sample data.

Identification of the lithology of the rock sequence is anotheimportant use of logs. A log shows many variations from topto bottom. Each wiggle has significance, but it can berelated to the rocks being logged only by comparing the logwith actual rock samples or a core from the well. After acquiring experience in an area it is possible for a loganalyst to make an educated guess as to lithology bylooking at the log. Modern analytical methods permit moreaccurate lithology identification, but this requires charts or mathematical solutions in addition to the curve shapes.

One of the important uses of logs today is the determinationof rock porosity. This measurement is significant because ittells how much storage space a rock has for fluids. No logactually measures porosity directly, but many analytical

methods are available to help estimate this important property.





Another of the routine uses of logs is the determination of the water, oil, or gas saturation in the rock pores. When theporosity, oil or gas saturation, the thickness and extent of the reservoir are known, then it is possible to tell how muchhydrocarbon is in place in the reservoir. Again, no logactually measures the fluid saturation directly, so analysis of indirect measurements is required. The logs most often runfor the above purposes are resistivity, sonic travel time,density, neutron, gamma ray and spontaneous potentiallogs.

One of the older, but very useful, surveys is the caliper log.In open hole logging, it is used to determine hole volumeand aids the engineer in designing a cementing program. Italso indicates mud cake build-up and hole wash-out. Both of these indications are of interest to the log analyst when heconsiders the other logs. In cased hole work the caliper isoften used to find casing damage and separated casing.

A more recent development in logging is fracture-finding. Itis important because fractures will often produce largequantities of fluid even though the rock the fractures are inwould not otherwise produce commercially. Many open-hole

logs have some artifacts caused by fractures, but theformation micro-scanner and borehole televiewer are themost useful.

When it is time to perforate the casing to allow fluid to flowinto the well, there may be some doubt about how well theperforator depths match the log depths. To overcome thisuncertainty, a casing collar gamma ray log is often run. Thislog is correlated to the open-hole log. Even though theactual depths may not agree, the zone of interest on theopen hole logs is related to the collar log depth. Then theperforating gun is positioned in relation to the collars in thecasing and perforating accuracy is assured.

The measurement of fluid flow in and near the wellbore isoften of vital importance. Such measurements can indicatechannels behind casing, casing leaks, packer leaks, tubingleaks, water influx problems, cross flow from one reservoir to another, and other production problems.

Another common use for this type of measurement is thedetermination of water input profiles in water injection wellsA thief zone may take most of the water and leave the rest ofthe reservoir unflooded. Surveys of this type point out the

type of remedial action that is necessary to establish a more desirable water input distribution.

Generally it is not advisable to complete a well in a zone that has poor bond between cement and casingwithout first squeezing in more cement to seal the casing to the rock formation. A cement bond or cementevaluation log is used to identify this problem.

The temperature log is commonly used to indicate the top of cement behind a newly cemented string of casing. The setting cement liberates heat and warms up the well bore, which is thus recognizable on atemperature log of the well.

Another use for the temperature log is the location of points of fluid entry in a well bore or of fluid flow behindcasing. As the fluid enters the well it expands and cools creating abnormally low temperature in the well at thepoint of entry. Acoustic noise logs also find flow entry and flow behind pipe by the noise caused by theflowing fluid.





The most significant change in the use of logs, in recent years, is production monitoring. The thermal decaytime log (often called a pulsed neutron log) allows for the interpretation of porosity and fluid saturation behindcasing. The fluid saturation will change over time as a reservoir is depleted by production, and the changesmay be monitored by logging at regular intervals, say once a year. If the production pattern is not aspredicted, remedial action may be possible. The log is also used to provide porosity and fluid saturation datain wells which are not, or could not, be logged in the more conventional open-hole manner.

A large suite of logging instruments is available to evaluate fluid type, fluid flow, and mechanical conditionsin producing or injecting wells, in addition to those already mentioned. These are generically calledproduction logs and are usually run in cased holes, but some are also effective in open hole or "bare-foot"

completions. Production log analysis is not described in this book as excellent treatments are availableelsewhere.

The same logs that are used to evaluate porosity and water saturation in oil and gas wells are also used toevaluate other resources such as ground water, coal, potash, salt, uranium, oil shale, gypsum, sulfur,geothermal energy, tar sands, and hard rock minerals. Logs are also used to explore the earth's surface ingeneral, such as in the Deep Sea Drilling Program which has helped to document the theory of plate tectonicssea floor spreading, and continental drift.

LOG SCALES and LAYOUT

Logs can be run on a number of vertical (depth) scales and quite a variety of horizontal (curve value) scales.

Common Logging Scales

English Metric

Often Called Terminology TerminologyDetail scale or

large scale 5" = 100 ft 1:240 1:200 is also very commonCorrelation scale or 2" = 100 ft 1:600 1:500 is also very comm onsmall scale 1" = 100 ft 1:1200 1:1000 is also very commonSuper detail scale 10" = 100 ft 1:120 1:100 is also very common* 25" = 100 ft 1:48 1:50 is also very commonDipmeter scale 60" = 100 ft 1:20





Correlation and Detail Scale Logs

The spacing between depth grid lines is 10 feet or 5 meters for correlation scales and 2 feet or 1 meter for

other scales. Heavier grid lines appear every 10 feet (5 meters) and every 50 feet (25 meters).





(Meters)Metric and English (USA) Log Grids – Detail Scale

Logs are presented in the field in a three track presentation. The pair of tracks 2 and 3 is often called track 4,which is used to record curves with a large range in values.

Four (or more) track presentations (with all tracks to the right hand side of the depth numbers) is created in the

course of computer processed log analysis. These can be generated in the computer on the logging truck or ithe office. Some logs recorded prior to 1946 have only two tracks, and logs run for special purposes (eg.potash) have three tracks all to the right of the depth numbers.





Various Log Grid Styles

The space where the depth numbers are printed is called the depth track and is often used for annotation of tops, DST and core data by the analyst. Both right and left hand margins can also be used for annotation.

Note that logarithmic scales can also vary in presentation. The standard scale is 4 decades wide starting at 0.2

and going to 2000. This can be shifted one decade to give a 2 to 20000 scale or a partial decade to give 0.1 to1000 or 0.3 to 3000.

Linear scales can be ANY RANGE. So check every time to find out what scale is current on the section of logyou are analyzing.

Back-up scales are shown on the log heading underneath the primary scale. Back-up scales take over when thprimary curve goes off-scale. Back-ups can be the same sensitivity (ie. 140 to 240 is the back-up scale to 40 to140) or a multiple of the primary scale (ie. 0 to 500 is the back-up to a 0 to 50 scale). Usually, but not always,the multiple is 10 to 1 and the first one tenth of the back-up scale is blanked out to prevent a confusion of curve. Logs exist with back-ups with multiples of 5, 10, 100 and 1000 (all could be on the same log) whichgives a considerable range of answers if the wrong scale is selected by the analyst.





Primary, Backup, and Amplified Log Curves

The use of logarithmic scales has reduced the need for back-up scales on resistivity logs, but a back-up mayalso be seen to augment a logarithmic scale. It will add another four decades to the scale range. Back-ups arestill common on other linear scales such as the sonic, density, neutron and gamma-ray logs.

Amplified scales are often presented on resistivity or sonic logs. For resistivity logs the short normal curvecan be shown amplified on a 0 to 2 or 0 to 4 scale, while the primary scale stays at 0 to 20 or 0 to 50 scale. Thesonic log may show an amplified scale of 40 to 80 or 40 to 100 scale while the primary scale is 40 to 140.Amplified scales are not used on logarithmic presentations, or on metric sonic logs (scales of 100 to 300 or 100 to 500 usually give sufficient detail with back-ups).

There are many variations in the presentation of well logs. There are a few standard conventions, but localneed often creates its own conventions.





Usually a log will be composed of several separate piecesspliced together (splice can be physical, as separate piecesof film taped together, or virtual as created by a computer playback):

Each piece of log is recorded on film and the pieces arespliced together in the order described above. A heading,with basic well data, is spliced to the top of the correlationscale film and a scale insert is spliced (usually) betweeneach separate piece of film. These inserts show the scale of

each of the recorded curves below the insert (and oftenabove the insert as well). On computerized logging units,the inserts may be an integral part of the film.




The Parts of a Log

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