c. 1978 - 2008 E. R. (Ross) Crain, P.Eng.
Rocky Mountain House, Alberta Canada T4T 2A2
403-845-2527 email us
Please be fair to the author - pay your Shareware Fee HERE
and receive a copy of Crain’s Petrophysical Handbook on CD-ROM at no extra cost.
Updated 10 June 2005
CHAPTER THREE: AN OVERVIEW OF WELL LOGGING PRACTICE Table
Of Contents Click here to go to NEXT CHAPTER Publication History: This Chapter formed Chapter Three of the Log Analysis Handbook, Pennwell 1986. Extensive updates have been added to this eText edition. Sections 3.06 through 3.08, 3.11, and 3.12 were added, and the balance was edited and revised February 2001. CHAPTER THREE: AN OVERVIEW OF WELL LOGGING PRACTICE 3.00
Introduction To This Chapter
3.01
What Is Well Logging? 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 the surface) on the other axis. More than one property may be displayed on the same graph.
Thus, analysis of log data is required. The art and science of log analysis is mainly directed at reducing a large volume of data to more manageable results, and reducing the possible error in the assumptions 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 called petrophysical properties or mappable reservoir properties. The petrophysical analysis is said to be “calibrated” when the porosity, fluid saturation, and permeability results compare favourably with core analysis data. Further confirmation of petrophysical properties is obtained by production 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 hard rock sequences has been practiced since the early 1930’s and is widespread today. Although the vast majority of logs are run to evaluate oil and gas wells, an increased number are being run yearly for other purposes, including evaluation of geothermal energy and ground water. A large portion of this handbook is aimed at oil and gas, but the other topics are not ignored. Most Chapters apply to both hydrocarbon and mineral exploration. When logs are used for purposes other than evaluation of oil and gas, they are often called geophysical logs instead of well logs. The science is called borehole geophysics instead of petrophysics. This difference is merely a matter of semantics and training. The theory doesn't change - just the nomenclature, and sometimes the emphasis.
3.02
Creating the Well Log
A logging tool is made up of a sonde and a cartridge. The sonde is the portion of the tool which gives off energy, receives energy, or both. The cartridge contains the electrical circuitry or computer components needed to control the downhole equipment, and to transmit data to and from the surface. 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 the wellbore. Surface equipment is mounted in a logging truck, van, or skid unit from which all logging operations are controlled. The logging unit contains hoisting equipment for lowering and raising the tools in the hole, and electronic or computer equipment for controlling and recording the downhole measurements. Measurements are recorded in two forms, analog and digital. The analog data may be recorded on photographic film, electronic plotter, or chart recorder. The same data are captured in digital form on magnetic tape or disc for later use in computer aided analysis. Many instrument control and calibration functions are now 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 calibration procedures for most tools, some of which are based on standards established by the American Petroleum Institute (API). Each tool must be calibrated at the surface before placing it in the hole to make measurements, and must pass certain calibrations after the measurements are complete to verify that measurement accuracy has 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 while pulling the tool upward over several hundred feet of the borehole. This is called the repeat run, and is used to determine the repeatability of the measurements when compared to the main logging pass. After the repeat run is complete, the tool is lowered to the bottom of the hole, and the main logging pass is commenced. During the 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 of the well log record. In addition to the actual measurements, the well log itself contains information about the logging process which supports use and interpretation of the data. The well name, location, date, surface measurements on the mud system, drill bit size, casing information, and logging equipment data are found on the log heading, Any pertinent 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.
Computerized surface equipment is now the rule rather than the exception. Such units, on a truck and with logging tools on board, can cost over $1,000,000.
3.03 Types of Logs Other types of logs require no cable, such as a mud log which may record up to 5 or 10 properties of the drilling 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 rock samples 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 data concerning evidence of the fluid content of the rock. These are all useful logs and are used in any analysis of a well, if they are available. Most logs can now be recorded while drilling is going on or while tripping the drill pipe. This is called measurements 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 this need, 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 usually made up in the office in a standard format (for the company or agency who owns the well). Since it compresses a great deal of data onto one log, it is often one of the most used items in the well file. More details on specific open-hole logging tools are found later in this Chapter. 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. Such logs 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. The first electrical log run in 1927
was station-by-station, but soon after, electrical logs were run
as continuous logs.
3.04 Present Uses of Logs Correlation is usually based on the shapes of the recorded curves versus depth. Correlation in complex geologic areas may be difficult or impossible, and in any event requires corroboration from actual rock samples for the initial correlations in an area. After the curve shape patterns are recognized, they can often be used in subsequent wells without relying too heavily on rock sample data. Identification of the lithology of the rock sequence is another important use of logs. A log shows many variations from top to bottom. Each wiggle has significance, but it can be related to the rocks being logged only by comparing the log with actual rock samples or a core from the well. After acquiring experience in an area it is possible for a log analyst to make an educated guess as to lithology by looking at the log. Modern analytical methods permit more accurate 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 determination of rock porosity. This measurement is significant because it tells how much storage space a rock has for fluids. No log actually 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 the porosity, oil or gas saturation, the thickness and extent of the reservoir are known, then it is possible to tell how much hydrocarbon is in place in the reservoir. Again, no log actually measures the fluid saturation directly, so analysis of indirect measurements is required. The logs most often run for the above purposes are resistivity, sonic travel time, density, neutron, gamma ray and spontaneous potential logs. One of the older, but very useful, surveys is the caliper log. In open hole logging, it is used to determine hole volume and aids the engineer in designing a cementing program. It also indicates mud cake build-up and hole wash-out. Both of these indications are of interest to the log analyst when he considers the other logs. In cased hole work the caliper is often used to find casing damage and separated casing. A more recent development in logging is fracture-finding. It is important because fractures will often produce large quantities of fluid even though the rock the fractures are in would not otherwise produce commercially. Many open-hole logs have some artifacts caused by fractures, but the formation micro-scanner and borehole televiewer are the most useful. When it is time to perforate the casing to allow fluid to flow into the well, there may be some doubt about how well the perforator depths match the log depths. To overcome this uncertainty, a casing collar gamma ray log is often run. This log is correlated to the open-hole log. Even though the actual depths may not agree, the zone of interest on the open hole logs is related to the collar log depth. Then the perforating gun is positioned in relation to the collars in the casing and perforating accuracy is assured. The measurement of fluid flow in and near the wellbore is often of vital importance. Such measurements can indicate channels behind casing, casing leaks, packer leaks, tubing leaks, water influx problems, cross flow from one reservoir to another, and other production problems. Another common use for this type of measurement is the determination of water input profiles in water injection wells. A thief zone may take most of the water and leave the rest of the 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 casing without first squeezing in more cement to seal the casing to the rock formation. A cement bond or cement evaluation 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 a temperature 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 behind casing. As the fluid enters the well it expands and cools creating abnormally low temperature in the well at the point of entry. Acoustic noise logs also find flow entry and flow behind pipe by the noise caused by the flowing fluid. The most significant change in the use of logs, in recent years, is production monitoring. The thermal decay time log (often called a pulsed neutron log) allows for the interpretation of porosity and fluid saturation behind casing. The fluid saturation will change over time as a reservoir is depleted by production, and the changes may be monitored by logging at regular intervals, say once a year. If the production pattern is not as predicted, remedial action may be possible. The log is also used to provide porosity and fluid saturation data in 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 conditions in producing or injecting wells, in addition to those already mentioned. These are generically called production 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 available elsewhere. The same logs that are used to evaluate porosity and water saturation in oil and gas wells are also used to evaluate 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 in general, such as in the Deep Sea Drilling Program which has helped to document the theory of plate tectonics, sea floor spreading, and continental drift. The wealth of logging tool types, analysis methods and applications of log data is so great that no-one (and no book) can cover all topics adequately. This dilemma provides a unique opportunity for ambitious and talented geologists, engineers, and scientists to overcome the data deluge and become practical analysts in spite of the often confusing and conflicting information.
3.05
Logging Tools Available To The Analyst The tool names and abbreviations shown are those used by Schlumberger. Other service companies use other names and abbreviations. The curve name abbreviations used in this handbook are shown following each curve name along with the usual units of measurements. Many other slim hole tools used in the mineral industry are not listed here. Consult your local service companies for up-to-date information. Curves marked with an asterisk are optional and may not be present on a particular log. The fact that many of the tools listed are obsolete should not discourage you from knowing what they measured, because you will find all these logs in existing well files and you will have to know what the curves are and how to use them.
3.06
Resistivity Logs Resistivity logs can be classified into three distinct categories.
2. Because the current flow in the borehole influences the resulting resistivity value, especially in very conductive muds, the laterolog was developed. It uses two matched sets of electrodes, similar to electrical survey electrodes, one of which is arranged upside down compared to the other. This provides some focusing for the current flow so that it cannot as easily flow up and down the borehole. As a result, some service companies refer to these logs as focused or guard logs. These logs also require a conductive mud, preferably very conductive. Some tools measure conductivity; others measure resistivity. All present resistivity on the log. 3. Induction logs impress a current into the formation by electromagnetic radiation from one or more coils. The current rings the borehole, and in turn creates its own electromagnetic field. This field induces a current in one or more receiving coils, the value of which is proportional to the conductivity of the formation. Fortunately, the received signal is out of phase from the transmitted signal, so the two can be segregated by the receiver circuits. This log does not require conductive mud in the borehole, but is often used in fresh mud. Although conductivity is measured, the usual presentation of the log converts conductivity to resistivity so that the logs are visually compatible with ES and laterologs.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
None
of the logs actually measure the physical properties that are
of most interest to us, such as how much oil or gas is in the
ground, or how much is being produced. Such important knowledge
can only be derived, from the measured properties listed above
(and others), using a number of assumptions which, if true, will
give reasonable estimates of hydrocarbon reserves.
of
newer tools, so use some editorial judgment - you might try for
a demonstration run in your well before you recommend the service
to management.
1.
Electrical logs use electrodes to impress a current through the
mud system into the formation. Other electrodes are used to measure
the voltage set up by the current over a specified distance, namely
the electrode spacing. The current, voltage and geometry of the
tool define the resistivity of the formation. Depending on the
relative arrangement of the current and voltage electrodes, the
log may be termed a lateral or a normal curve. The borehole must
contain a conductive fluid so that the current can get to the
formation.
