Caliper Logging

Basic Concept

Caliper logs provide a continuous record of borehole diameter and are used widely for groundwater applications. Changes in borehole diameter may be related to both drilling technique and lithology. Caliper logs are essential to guide the interpretation of other logs, because most of them are affected by changes in well diameter. They also are useful in providing information on well construction, lithology, and secondary porosity, such as fractures and solution openings. Many different types of caliper probes are described in detail by Hilchie (1968). The most common type of probe used for logging water wells has three arms approximately the diameter of a pencil, spaced 120° apart. Arms of different lengths can be attached to this type of tool to optimize sensitivity over the hole-diameter range expected. Mechanical caliper probes have been used that will measure to a maximum hole diameter of 1 m. The typical water-well caliper employs arms that are connected to move a linear potentiometer, so changes in resistance transmitted to the surface as voltage changes are proportional to average hole diameter. Single-arm calipers commonly are used to provide a record of hole diameter while running another type of log. The single arm also may be used to decentralize a probe, such as a side-collimated gamma-gamma tool, but logs made with this type of probe are usually not high resolution. High-resolution caliper-logging devices usually employ three or four independent arms, and they are compass oriented in some tools. The difference in resolution between logs made with a four-arm device and the more common types is shown in figure 386. The high-resolution logs on the left were made with four independent arms. The three-arm averaging tool is typical of that used in engineering and environmental applications, and the single-arm log on the right was recorded during the running of a compensated gamma-gamma log. The apparent erratic response on the four-arm caliper logs in part of the well is repeatable and is caused by solution openings in the carbonate rock. Digital sample interval should be close spaced, such as 0.03 m if high-resolution logs are desired. Acoustic calipers may use the time-of-travel data from an acoustic televiewer to provide compass-oriented, high-resolution traces.

Figure 386. Caliper logs from probes having four independent arms, three averaging arms,
and a single arm. Madison limestone test well No 1, Wyoming.

Data Acquisition

Calibration. Calibration of calipers is carried out most accurately in cylinders of different diameters. Large cylinders occupy a considerable amount of room in a logging truck, so it is common practice to use a metal plate for onsite standardization of three-arm averaging or single-arm probes. The plate is drilled and marked every inch or two and machined to fit over the body of the probe and accept one caliper arm in the holes. Because values obtained with a calibration plate are not as accurate as those obtained with a cylinder, usually log scale is checked using casing of known diameter in the well.

Data Interpretation

A valid caliper log is essential to guide the interpretation of the many different types of logs that are affected by changes in hole diameter, even those that are labeled borehole compensated. Differences in hole diameter are related to drilling technique and lithology and structure of the rocks penetrated. The shallower part of a hole is usually larger diameter than the deeper part, because it has been exposed to more drilling activities. Couplings, welds, and screens may be located on a high-resolution caliper log.

Applications. Caliper logs have been used to correlate major producing aquifers in the Snake River Plain in Idaho (Jones, 1961). Vesicular and scoriaceous tops of basalt flows, cinder beds, and caving sediments were identified with three-arm caliper logs. In the Snake River, basalt caliper logs also were used to locate the optimum depth for cementing monitoring wells and to estimate the volume of cement that might be required to achieve fill of the annulus to a pre-selected depth (Keys, 1963). Similarly, a caliper log can be used to calculate the volume of gravel pack needed and to determine the size of casing that can be set to a selected depth. Caliper logs are particularly useful for selecting the depths for inflating packers. Packers can be set only over a narrow specified range of hole diameters and may be damaged if they are set in rough or irregular parts of a well. Packers set under these conditions may explode; if they are set on a fracture, they may implode or be bypassed by flow. Caliper logs are useful for determining what other logs can be run and what range of diameters will be accepted by centralizers or decentralizers. Hole diameter information is essential for the calculation of volumetric rate from many types of flowmeter logs. Because of the usefulness of a caliper log to the interpretation of other logs, it needs to be run before casing is installed in a hole that is in danger of caving. When hole conditions are questionable, the first log run is usually the single-point resistance log, because it will provide some lithologic information; if it is lost, the tool is relatively inexpensive to replace. If no serious caving problems are detected during the single-point log, a caliper log needs to be run before casing is installed so it can be used to aid the analysis of nuclear logs made through the casing. Very rough intervals of a drill hole, with changes in hole diameter of several inches, cannot be corrected based on caliper logs, and they need to be eliminated from quantitative analysis.

Lithology and Secondary Porosity. Caliper logs can provide information on lithology and secondary porosity. Hard rocks such as limestone will show on the log as a smaller diameter than adjacent shales. Shales may produce an irregular caliper trace, caused by thin bedding. Secondary porosity, such as fractures and solution openings, may be obvious on a caliper log, although the acter will not be uniquely defined, as it would be on an acoustic-televiewer log. Open fractures are detected readily by three-arm averaging calipers, but the true character of the fractures may not be correctly interpreted from a caliper log. If an open fracture is dipping at a sufficient angle so that the three arms enter the opening at different depths, the separate anomalies produced will indicate three fractures rather than one.

Fluid

Fluid logging includes those techniques that measure characteristics of the fluid column in the well; no direct signal is derived from the surrounding rocks and their contained fluids. The fluid logs that are described here are temperature, electrical conductivity, and flow. Fluid logs are unique in that the recorded characteristics of the fluid column may change rapidly with time and may be altered by the logging process.

Temperature Logging

Basic Concept

Temperature probes used in groundwater and environmental studies employ a glass-bead thermistor, solid-state IC device, or platinum sensor mounted in a tube that is open at both ends to protect it from damage and to channel water flow past the sensor. The sensor may be enclosed in a protective cover, but it must be made of materials with a high thermal conductivity and small mass to permit fast response time. Thermistor probes used by the U.S. Geological Survey have an accuracy, repeatability, and sensitivity on the order of 0.02°C. They also are very stable over long periods of time, but they have the disadvantage of a nonlinear temperature response. For high-temperature logging in geothermal wells, platinum sensors may be used that have an accurate, stable, and linear response. Two general types of temperature logs are in common use: the standard log is a record of temperature versus depth, and the differential-temperature log is a record of the rate of change in temperature versus depth. The differential-temperature log, which can afford greater sensitivity in locating changes in gradient, can be considered to be the first derivative of the temperature. It can be obtained with a probe with two sensors located from 0.3 to 1.0 m apart or by computer calculation from a temperature log. A differential log has no scale, and log deflections indicate changes from a reference gradient.

Calibration. Calibration of temperature probes needs to be carried out in a constant temperature bath, using highly accurate mercury thermometers. The bath and probe need to reach equilibrium before a calibration value is established. Onsite standardization cannot be carried out with great accuracy because no portable substitute exists for a constant-temperature bath. The only temperature that can be achieved and maintained for sufficient time to permit a valid calibration is 0°C in an ice bath.

Data interpretation

Temperature logs can provide very useful information on the movement of water through a well, including the location of depth intervals that produce or accept water; thus, they provide information related to permeability. Temperature logs can be used to trace the movement of injected water or waste and to locate cement behind casing. Although the temperature sensor only responds to water or air in the immediate vicinity, recorded temperatures may indicate the temperatures of adjacent rocks and their contained fluids if no flow exists in the well.

Applications. Temperature logs can aid in the solution of a number of groundwater problems if they are properly run under suitable conditions, and if interpretation is not oversimplified. If there is no flow in, or adjacent to a well, the temperature gradually will increase with depth, as a function of the geothermal gradient. Typical geothermal gradients range between 0.47 and 0.6°C per 30 m of depth; they are related to the thermal conductivity or resistivity of the rocks adjacent to the borehole and the heat flow from below. The geothermal gradient may be steeper in rocks with low intrinsic permeability than in rocks with high intrinsic permeability.

Thermal Gradient. The sensor in a temperature probe only responds to the fluid in its immediate vicinity. Therefore, in a flowing interval, measured temperature may be different from the temperature in adjacent rocks. Under these conditions, a thermal gradient will exist from the well outward. Only in a well where no flow has occurred for sufficient time to permit thermal equilibrium to be established does a temperature log reflect the geothermal gradient in the rocks. If vertical flow occurs in a well at a high rate, the temperature log through that interval will show little change. Vertical flow, up or down, is very common in wells that are completed through several aquifers or fractures that have different hydraulic head, although the flow rate is seldom high enough to produce an isothermal log. Movement of a logging probe disturbs the thermal profile in the fluid column. Unless rapid flow is occurring, each temperature log will be different. High logging speed and large-diameter probes will cause the greatest disturbance. The most accurate temperature log is made before any other log, and it is recorded while moving slowly down the hole. Convection is a major problem in the interpretation of temperature logs, particularly in large-diameter wells and in areas of high thermal gradient. Convective cells in large-diameter wells can cause major temperature anomalies unrelated to groundwater movement (Krige, 1939).

Example. Identification of fractures producing groundwater from Triassic sedimentary rocks is illustrated in figure 387. The temperature log on the left shows several changes in gradient that are clearly defined by the computer-derived differential‑temperature log. The caliper log suggests that water production may come from fractures; this interpretation is substantiated by the acoustic-televiewer logs on the right.

Movement of Injected Water

Temperature logs can be used to trace the movement of injected water (Keys and Brown, 1978). A sampling of several hundred temperature logs run during a 7--day recharge test in the high plains of Texas is shown in figure 388. Water from a playa lake was injected into an irrigation well, and logging was used to determine the movement of the recharge water and the extent of plugging of the Ogallala aquifer. Several monitoring holes were drilled and completed with 2-in steel pipe, capped on the bottom, and filled with water. The logs in figure 388 were of a monitoring hole located 12 m from the injection well. Most of the time, the water in the playa lake was warmer than the groundwater, and the lake temperature fluctuated several degrees each day. The passing of a cold front caused a marked decrease in temperature of the lake water. The first warm water was detected in the monitoring hole less than 4 hours after recharge started. The temperature logs indicated that the interval of highest permeability was located at a depth of approximately 50 m. Water did not arrive at a depth of 55 m until the third day. Diurnal temperature fluctuations and buildup of a recharge cone can be observed in figure 388. Data plotted in figure 389 were calculated from temperature logs of the same borehole; however, logs from other holes gave similar results. The solid line in the upper half of figure 389 shows the diurnal temperature fluctuations of the recharge water obtained from a continuous recorder on the recharge line. The other three lines represent fluctuations at three depths in the monitoring well, as obtained from temperature logs. The points shown by symbols represent the calculated center of the thermal waves. Travel times of the centers of the waves did not decrease during the life of the test, except possibly at the end. Test results show that the aquifer was not plugged by reging water with a high content of suspended solids and entrained air, and the well yield was greatly increased.

Figure 387. Optical Televiewer (OTV), acoustic televiewer (ATV), caliper, fluid temperature, and
fluid conductivity logs from an open bedrock borehole in North Carolina

Temperature logs can be used to trace the movement of water that has been injected from a tank that has been allowed to heat in the sun. In a similar fashion, temperature logs can be used to locate plumes of wastewater that result from injection if sufficiently different from the groundwater. Temperature logs also can be used to determine the location of cement grout outside of casing. The casing is filled with water, and the log usually is run within 24 hours of grout injection; however, anomalous temperatures may persist for several days.

Figure 388. Selected temperature logs of a monitoring hole 12 m from a recharge well, high plains, Texas.

Figure 389. Diurnal temperature cycles and travel times, based on temperature
logs of a monitoring hole 12 m from a recharge well.

Conductivity Logging

Basic Concept

Logs of fluid electrical conductivity, which is the reciprocal of fluid resistivity, provide data related to the concentration of dissolved solids in the fluid column. Although the fluid column may not reflect the quality of adjacent interstitial fluids, the information can be useful when combined with other logs. Fluid-conductivity or resistivity logs are records of the capacity of the borehole fluid that enters the probe to transmit electrical current. The probe should not be affected by changes in the conductivity of adjacent fluids or solid materials because it is constructed with the electrodes inside a housing. Ring electrodes are installed on the inside of a steel tube that is open at both ends, so water will flow through as the probe moves down the well. The electrodes are usually gold or silver to reduce changes in contact resistance caused by chemical reactions, and they are insulated from the steel housing. Conductivity is recorded in μmho/cm or μS/cm, which is equal to 10,000 divided by the resistivity in Ωm. Specific conductance is measured at the standard temperature of 25°C. Calibration usually is done empirically in solutions of known sodium chloride concentration, because most ts are based on this salt, and conversion factors are available to correct for the presence of other ions. The salinity of the calibration solution may be calculated by adding a known amount of salt to distilled water and converting to conductivity, or by measuring with an accurate laboratory conductivity meter. Temperature of the calibration solution is recorded while the measurement is being made, and it needs to be uniform and stable. Onsite standardization may be carried out using several fluids of known concentration in plastic bottles sufficiently large to allow submersion of all electrodes in the probe. A laboratory conductivity cell or a less accurate mud resistivity kit also can be used. Disturbance of the fluid column in the borehole can make fluid-conductivity logs difficult to interpret. Disturbance of an equilibrium-salinity profile can be caused by the movement of logging probes or by convective flow cells. Because of the possibility of disturbance by logging, the most accurate fluid-conductivity log is made on the first trip down the well. This recommendation also pertains to temperature logs, so an ideal probe is capable of making simultaneous fluid-conductivity and temperature logs.

Data Interpretation

The interpretation of fluid-conductivity logs is complicated by the flow regime in a well. Unless the flow system is understood, analysis of the conductivity profile is subject to considerable error. Information on the construction of the well, flowmeter logs, and temperature logs are useful in the interpretation of conductivity logs. When both fluid conductivity and temperature are known, the sodium chloride concentration can be determined. Water samples must be analyzed to determine the concentrations of the various ions so corrections can be made.

Applications. Regional patterns of groundwater flow and recharge areas may be recognized from fluid-conductivity logs of the wells in an area. Fluid-conductivity data can be used to map and monitor areas of saltwater encroachment. Similarly, the logs can be used to monitor plumes of contaminated groundwater from waste-disposal operations. Commonly, chemical waste or leachate from solid-waste‑disposal operations produces groundwater with a higher than normal conductivity. Conductivity logs provide the basis for selecting depths from which to collect water samples for chemical analyses. Fluid-conductivity logging equipment can be used to trace the movement of groundwater by injecting saline water or deionized water as a tracer. Small amounts of saline water may be injected at selected depths, and conductivity logs may be used to measure vertical flow in a single well, or larger amounts may be detected in nearby wells. Another important use for fluid-conductivity logs is to aid in the interpretation of electric logs. Spontaneous potential, single-point resistance, and many types of multi-electrode-resistivity logs are affected by the salinity of the fluid in the well.