Well Logging
Well Logging Definition
Well logging, field technique used in mineral exploration to analyze the geologic formations penetrated by a drill hole. If the hole has been drilled by using coring techniques, the core provides a visual record of the formations and rock types encountered. The description (log) of the core provides the basic data used in geologic analysis, interpretation, and resource calculations.
Types of Well Logging
1. Acoustic logging
Acoustic logging includes those techniques that use a transducer to transmit an acoustic wave through the fluid in the well and surrounding elastic materials. Several different types of acoustic logs are used, based on the frequencies used, the way the signal is recorded, and the purpose of the log. All these logs require fluid in the well to couple the signal to the surrounding rocks. Four types will be described here: acoustic velocity, acoustic waveform, cement bond, and acoustic televiewer.
2. Electrical Methods
Spontaneous Potential Log
Spontaneous potential (SP) is one of the oldest logging techniques. It employs very simple equipment to produce a log whose interpretation may be quite complex, particularly in freshwater aquifers. This complexity has led to misuse and misinterpretation of spontaneous potential (SP) logs for groundwater applications. The spontaneous potential log (incorrectly called self-potential) is a record of potentials or voltages that develop at the contacts between shale or clay beds and a sand aquifer, where they are penetrated by a drill hole. The natural flow of current and the SP curve or log that would be produced under the salinity conditions.
Single-Point Resistance Log
The single point resistance log has been one of the most widely used in non petroleum logging in the past; it is still useful, in spite of the increased application of more sophisticated techniques. Single point logs cannot be used for quantitative interpretation, but they are excellent for lithologic information. The equipment to make single point logs usually is available on most small water well loggers, but it is almost never available on the larger units used for oil well logging. The resistance of any medium depends not only on its composition, but also on the cross sectional area and length of the path through that medium. Single point resistance systems measure the resistance, in W, between an electrode in the well and an electrode at the surface or between two electrodes in the well. Because no provision exists for determining the length or cross sectional area of the travel path of the current, the measurement is not an intrinsic acteristic of the material between the electrodes. Therefore, single point resistance logs cannot be related quantitatively to porosity, or to the salinity of water in those pore spaces, even though these two parameters do control the flow of electric current.
Normal Resistivity Log
Among the various multi-electrode resistivity-logging techniques, normal resistivity is probably the most widely used in groundwater hydrology, even though the long normal log has become rather obsolete in the oil industry. Normal-resistivity logs can be interpreted quantitatively when they are properly calibrated in terms of Ώm. Log measurements are converted to apparent resistivity, which may need to be corrected for mud resistivity, bed thickness, borehole diameter, mudcake, and invasion, to arrive at true resistivity. ts for making these corrections are available in old logging manual
Lateral Resistivity Log
Lateral logs are made with four electrodes like the normal logs but with a different configuration of the electrodes. Lateral logs are designed to measure resistivity beyond the invaded zone, which is achieved by using a long electrode spacing. They have several limitations that have restricted their use in environmental and engineering applications. Best results are obtained when bed thickness is greater than twice AO, or more than 12 m for the standard spacing. Although correction ts are available, the logs are difficult to interpret. Anomalies are asymmetrical about a bed, and the amount of distortion is related to bed thickness and the effect of adjacent beds. For these reasons, the lateral log is not recommended for most engineering and environmental applications.
Focused Resistivity Log
Focused resistivity systems were designed to measure the resistivity of thin beds or high-resistivity rocks in wells containing highly conductive fluids. A number of different types of focused resistivity systems are used commercially such as "guard" or "laterolog." Focused or guard logs can provide high resolution and great penetration under conditions where other resistivity systems may fail. Focused-resistivity devices use guard electrodes above and below the current electrode to force the current to flow out into the rocks surrounding the well
Microresistivity Log
A large number of microresistivity devices exist, but all employ short electrode spacing so that they have a shallow depth of investigation. They can be divided into two general groups: focused and non-focused. Both groups employ pads or some kind of contact electrodes to reduce the effect of the borehole fluid. Non-focused sondes are designed mainly to determine the presence or absence of mud cake, but they also can provide very high-resolution lithologic detail. Names used for these logs include microlog, minilog, contact log, and micro-survey log. Focused microresistivity devices also use small electrodes mounted on a rubber-covered pad forced to contact the wall of the hole hydraulically or with heavy spring pressure.
Dipmeter Log
The dipmeter includes a variety of wall-contact microresistivity devices that are widely used in oil exploration to provide data on the strike and dip of bedding planes.
The modern dipmeter provides a large amount of information from a complex tool, so it is an expensive log to run. Furthermore, because of the amount and complexity of
the data, the maximum benefit is derived from computer analysis and plotting of
the results. Interpretation is based on the correlation of resistivity anomalies detected by the individual arms, and the calculation of the true depth at which those anomalies occur. The log from a four-arm tool has four resistivity curves and two caliper traces,
which are recorded between opposite arms, so that the ellipticity of the hole can be determined.
Induction Logging
Induction logging devices originally were designed to make resistivity measurements in oil-based drilling mud, where no conductive medium occurred between the tool and the formation. A simple version of an induction probe contains two coils: one for transmitting an AC current, typically 20 to 40 kHz, into the surrounding rocks, and a second for receiving the returning signal. The transmitted AC generates a time-varying primary magnetic field, which induces a flow of eddy currents in conductive rocks penetrated by the drill hole. These eddy currents set up secondary magnetic fields, which induce a voltage in the receiving coil. That signal is amplified and converted to DC before being transmitted up the cable. Magnitude of the received current is proportional to the electrical conductivity of the rocks. Induction logs measure conductivity, which is the reciprocal of resistivity. Additional coils usually are included to focus the current in a manner similar to that used in guard systems. Induction devices provide resistivity measurements regardless of whether the fluid in the well is air, mud, or water, and excellent results are obtained through plastic casing.
3. Flow Logging
The measurement of flow within and between wells is one of the most useful
well-logging methods available to interpret the movement of groundwater and contaminants. Flow measurement with logging probes includes mechanical methods, such as impellers, chemical and radioactive tracer methods, and thermal methods (Crowder, Paillet, and Hess, 1994). Their primary application is to measure vertical flow within a single well, but lateral flow through a single well or flow between wells also may be recorded by borehole geophysical methods.
4. Hole-to-Hole Logging
Crosshole Seismic/Sonic Logging Survey
Crosshole Sonic Logging (CSL) uses compressional seismic waves as the energy source. Seismic waves passing through concrete are influenced by the density and elastic modulus of the concrete. Fractured or "weak" concrete zones lower the velocity of the seismic waves and can, therefore, be detected. In addition, the amplitude of a seismic pulse is affected by these defects although this is not extensively used at the present time. The frequency content of the seismic energy pulse determines the resolution and penetration of the signal. High frequencies have high-amplitude attenuation but can image small targets. Conversely, lower frequencies have less attenuation but image larger targets. The seismic source produces an impulse whose frequency content is usually 30 to 40 kHz.
Crosshole Seismic/Sonic Tomography Survey
If more drilled holes are available in the shaft, then CSL can be conducted using these holes, producing a better definition of the location of the defects. Recording readings from a number of offsets also helps to define the location of an anomaly. Data recording with two holes and a constant source-receiver offset is called CSL. If a number of offsets between the source and receiver are used, then tomographic calculations can be done and the method is called CSL Tomography (CSLT). The probes (source (S) and receiver (R)) are lowered to the bottom of a tube pair. Before the logging begins, one of the probes (e.g., the receiver) is lifted or lowered a specific offset distance (or angle) above or below the source level (respectively). Both probes are then pulled simultaneously up to maintain that offset distance. When the receiver is above the source, the records are defined as "positive" offset data. Conversely, when the receiver probe is lower than the source level, the records are defined as "negative" offset data. The offset, either positive or negative, is maintained during that logging run.
5. Hydrophysical Logging
Fluid replacement and fluid-column conductivity logging, or "Hydrophysical" logging (Pedler, et al., 1990; Pedler, Head, and Williams, 1992; Tsang, Hufschmied, and Hale, 1990) involves fluid-column conductivity logging over time after the fluid column has been diluted or replaced with environmentally safe deionized water. Hydrophysical logging results are independent of borehole diameter, and the method does not require a flow concentrating diverter or packer. The logging probe involves relatively simple and readily available technology and has a small diameter allowing it to be run through an access pipe below a pump. Hydrophysical logging is used to determine flow magnitude and direction during pumping and under ambient conditions, and to identify hydraulically conductive intervals to within one well bore diameter
6. Nuclear Logging
Nuclear logging includes all techniques that either detect the presence of unstable isotopes, or that create such isotopes in the vicinity of a borehole. Nuclear logs are unique because the penetrating capability of the particles and photons permits their detection through casing and annular materials, and they can be used regardless of the type of fluid in the borehole. Nuclear-logging techniques described in this manual include gamma, gamma spectrometry, gamma-gamma, and several different kinds of neutron logs. Radioactivity is measured by converting the particles or photons to electronic pulses, which then can be counted and sorted as a function of their energy. The detection of radiation is based on ionization that is directly or indirectly produced in the medium through which it passes.
Gamma logging
Gamma logs, also called gamma ray logs or natural-gamma logs, are the most widely used nuclear logs for most applications. The most common use is for identification of lithology and stratigraphic correlation, and for this reason, gamma detectors are often included in multi-parameter logging tools. Gamma logs provide a record of total gamma radiation detected in a borehole and are useful over a very wide range of borehole conditions.
Gamma-Gamma Logging
Gamma-gamma logs, also called density logs, are records of the radiation from a gamma source in the probe after it is attenuated and backscattered in the borehole and surrounding rocks. The logs can be calibrated in terms of bulk density under the proper conditions and converted to porosity if grain and fluid density are known
Neutron Logging
Neutron logs are made with a source of neutrons in the probe and detectors that provide a record of the interactions that occur in the vicinity of the borehole. Most of these neutron interactions are related to the amount of hydrogen present, which, in groundwater environments, is largely a function of the water content of the rocks penetrated by the drill hole. Neutron probes contain a source that emits high-energy neutrons. The most common neutron source used in porosity logging tools is americium-beryllium, in sizes that range from approximately 1 to 25 Curies. Moisture tools may use a source as small as 100 millicuries
Well logging, field technique used in mineral exploration to analyze the geologic formations penetrated by a drill hole. If the hole has been drilled by using coring techniques, the core provides a visual record of the formations and rock types encountered. The description (log) of the core provides the basic data used in geologic analysis, interpretation, and resource calculations.
Types of Well Logging
1. Acoustic logging
Acoustic logging includes those techniques that use a transducer to transmit an acoustic wave through the fluid in the well and surrounding elastic materials. Several different types of acoustic logs are used, based on the frequencies used, the way the signal is recorded, and the purpose of the log. All these logs require fluid in the well to couple the signal to the surrounding rocks. Four types will be described here: acoustic velocity, acoustic waveform, cement bond, and acoustic televiewer.
2. Electrical Methods
Spontaneous Potential Log
Spontaneous potential (SP) is one of the oldest logging techniques. It employs very simple equipment to produce a log whose interpretation may be quite complex, particularly in freshwater aquifers. This complexity has led to misuse and misinterpretation of spontaneous potential (SP) logs for groundwater applications. The spontaneous potential log (incorrectly called self-potential) is a record of potentials or voltages that develop at the contacts between shale or clay beds and a sand aquifer, where they are penetrated by a drill hole. The natural flow of current and the SP curve or log that would be produced under the salinity conditions.
Single-Point Resistance Log
The single point resistance log has been one of the most widely used in non petroleum logging in the past; it is still useful, in spite of the increased application of more sophisticated techniques. Single point logs cannot be used for quantitative interpretation, but they are excellent for lithologic information. The equipment to make single point logs usually is available on most small water well loggers, but it is almost never available on the larger units used for oil well logging. The resistance of any medium depends not only on its composition, but also on the cross sectional area and length of the path through that medium. Single point resistance systems measure the resistance, in W, between an electrode in the well and an electrode at the surface or between two electrodes in the well. Because no provision exists for determining the length or cross sectional area of the travel path of the current, the measurement is not an intrinsic acteristic of the material between the electrodes. Therefore, single point resistance logs cannot be related quantitatively to porosity, or to the salinity of water in those pore spaces, even though these two parameters do control the flow of electric current.
Normal Resistivity Log
Among the various multi-electrode resistivity-logging techniques, normal resistivity is probably the most widely used in groundwater hydrology, even though the long normal log has become rather obsolete in the oil industry. Normal-resistivity logs can be interpreted quantitatively when they are properly calibrated in terms of Ώm. Log measurements are converted to apparent resistivity, which may need to be corrected for mud resistivity, bed thickness, borehole diameter, mudcake, and invasion, to arrive at true resistivity. ts for making these corrections are available in old logging manual
Lateral Resistivity Log
Lateral logs are made with four electrodes like the normal logs but with a different configuration of the electrodes. Lateral logs are designed to measure resistivity beyond the invaded zone, which is achieved by using a long electrode spacing. They have several limitations that have restricted their use in environmental and engineering applications. Best results are obtained when bed thickness is greater than twice AO, or more than 12 m for the standard spacing. Although correction ts are available, the logs are difficult to interpret. Anomalies are asymmetrical about a bed, and the amount of distortion is related to bed thickness and the effect of adjacent beds. For these reasons, the lateral log is not recommended for most engineering and environmental applications.
Focused Resistivity Log
Focused resistivity systems were designed to measure the resistivity of thin beds or high-resistivity rocks in wells containing highly conductive fluids. A number of different types of focused resistivity systems are used commercially such as "guard" or "laterolog." Focused or guard logs can provide high resolution and great penetration under conditions where other resistivity systems may fail. Focused-resistivity devices use guard electrodes above and below the current electrode to force the current to flow out into the rocks surrounding the well
Microresistivity Log
A large number of microresistivity devices exist, but all employ short electrode spacing so that they have a shallow depth of investigation. They can be divided into two general groups: focused and non-focused. Both groups employ pads or some kind of contact electrodes to reduce the effect of the borehole fluid. Non-focused sondes are designed mainly to determine the presence or absence of mud cake, but they also can provide very high-resolution lithologic detail. Names used for these logs include microlog, minilog, contact log, and micro-survey log. Focused microresistivity devices also use small electrodes mounted on a rubber-covered pad forced to contact the wall of the hole hydraulically or with heavy spring pressure.
Dipmeter Log
The dipmeter includes a variety of wall-contact microresistivity devices that are widely used in oil exploration to provide data on the strike and dip of bedding planes.
The modern dipmeter provides a large amount of information from a complex tool, so it is an expensive log to run. Furthermore, because of the amount and complexity of
the data, the maximum benefit is derived from computer analysis and plotting of
the results. Interpretation is based on the correlation of resistivity anomalies detected by the individual arms, and the calculation of the true depth at which those anomalies occur. The log from a four-arm tool has four resistivity curves and two caliper traces,
which are recorded between opposite arms, so that the ellipticity of the hole can be determined.
Induction Logging
Induction logging devices originally were designed to make resistivity measurements in oil-based drilling mud, where no conductive medium occurred between the tool and the formation. A simple version of an induction probe contains two coils: one for transmitting an AC current, typically 20 to 40 kHz, into the surrounding rocks, and a second for receiving the returning signal. The transmitted AC generates a time-varying primary magnetic field, which induces a flow of eddy currents in conductive rocks penetrated by the drill hole. These eddy currents set up secondary magnetic fields, which induce a voltage in the receiving coil. That signal is amplified and converted to DC before being transmitted up the cable. Magnitude of the received current is proportional to the electrical conductivity of the rocks. Induction logs measure conductivity, which is the reciprocal of resistivity. Additional coils usually are included to focus the current in a manner similar to that used in guard systems. Induction devices provide resistivity measurements regardless of whether the fluid in the well is air, mud, or water, and excellent results are obtained through plastic casing.
3. Flow Logging
The measurement of flow within and between wells is one of the most useful
well-logging methods available to interpret the movement of groundwater and contaminants. Flow measurement with logging probes includes mechanical methods, such as impellers, chemical and radioactive tracer methods, and thermal methods (Crowder, Paillet, and Hess, 1994). Their primary application is to measure vertical flow within a single well, but lateral flow through a single well or flow between wells also may be recorded by borehole geophysical methods.
4. Hole-to-Hole Logging
Crosshole Seismic/Sonic Logging Survey
Crosshole Sonic Logging (CSL) uses compressional seismic waves as the energy source. Seismic waves passing through concrete are influenced by the density and elastic modulus of the concrete. Fractured or "weak" concrete zones lower the velocity of the seismic waves and can, therefore, be detected. In addition, the amplitude of a seismic pulse is affected by these defects although this is not extensively used at the present time. The frequency content of the seismic energy pulse determines the resolution and penetration of the signal. High frequencies have high-amplitude attenuation but can image small targets. Conversely, lower frequencies have less attenuation but image larger targets. The seismic source produces an impulse whose frequency content is usually 30 to 40 kHz.
Crosshole Seismic/Sonic Tomography Survey
If more drilled holes are available in the shaft, then CSL can be conducted using these holes, producing a better definition of the location of the defects. Recording readings from a number of offsets also helps to define the location of an anomaly. Data recording with two holes and a constant source-receiver offset is called CSL. If a number of offsets between the source and receiver are used, then tomographic calculations can be done and the method is called CSL Tomography (CSLT). The probes (source (S) and receiver (R)) are lowered to the bottom of a tube pair. Before the logging begins, one of the probes (e.g., the receiver) is lifted or lowered a specific offset distance (or angle) above or below the source level (respectively). Both probes are then pulled simultaneously up to maintain that offset distance. When the receiver is above the source, the records are defined as "positive" offset data. Conversely, when the receiver probe is lower than the source level, the records are defined as "negative" offset data. The offset, either positive or negative, is maintained during that logging run.
5. Hydrophysical Logging
Fluid replacement and fluid-column conductivity logging, or "Hydrophysical" logging (Pedler, et al., 1990; Pedler, Head, and Williams, 1992; Tsang, Hufschmied, and Hale, 1990) involves fluid-column conductivity logging over time after the fluid column has been diluted or replaced with environmentally safe deionized water. Hydrophysical logging results are independent of borehole diameter, and the method does not require a flow concentrating diverter or packer. The logging probe involves relatively simple and readily available technology and has a small diameter allowing it to be run through an access pipe below a pump. Hydrophysical logging is used to determine flow magnitude and direction during pumping and under ambient conditions, and to identify hydraulically conductive intervals to within one well bore diameter
6. Nuclear Logging
Nuclear logging includes all techniques that either detect the presence of unstable isotopes, or that create such isotopes in the vicinity of a borehole. Nuclear logs are unique because the penetrating capability of the particles and photons permits their detection through casing and annular materials, and they can be used regardless of the type of fluid in the borehole. Nuclear-logging techniques described in this manual include gamma, gamma spectrometry, gamma-gamma, and several different kinds of neutron logs. Radioactivity is measured by converting the particles or photons to electronic pulses, which then can be counted and sorted as a function of their energy. The detection of radiation is based on ionization that is directly or indirectly produced in the medium through which it passes.
Gamma logging
Gamma logs, also called gamma ray logs or natural-gamma logs, are the most widely used nuclear logs for most applications. The most common use is for identification of lithology and stratigraphic correlation, and for this reason, gamma detectors are often included in multi-parameter logging tools. Gamma logs provide a record of total gamma radiation detected in a borehole and are useful over a very wide range of borehole conditions.
Gamma-Gamma Logging
Gamma-gamma logs, also called density logs, are records of the radiation from a gamma source in the probe after it is attenuated and backscattered in the borehole and surrounding rocks. The logs can be calibrated in terms of bulk density under the proper conditions and converted to porosity if grain and fluid density are known
Neutron Logging
Neutron logs are made with a source of neutrons in the probe and detectors that provide a record of the interactions that occur in the vicinity of the borehole. Most of these neutron interactions are related to the amount of hydrogen present, which, in groundwater environments, is largely a function of the water content of the rocks penetrated by the drill hole. Neutron probes contain a source that emits high-energy neutrons. The most common neutron source used in porosity logging tools is americium-beryllium, in sizes that range from approximately 1 to 25 Curies. Moisture tools may use a source as small as 100 millicuries
Written by
Rujipas Piriyapanich
Edited by
Nipaporn Poonsawat
Rujipas Piriyapanich
Edited by
Nipaporn Poonsawat
