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Appendix D

Electrical Logging System

D-1. Logging Unit. The DR-74 Electrical Logging System has been designed for use in logging shallow, vertical wells. It is a hand-operated unit. You manually lower and raise the well probe in and out of the well. The operator takes readings point by point up the well bore and records these readings. The data is plotted on a logging form to get the graphical log needed for interpretation. When using the gear, you can get a spontaneous potential (SP) curve and three normal and two lateral resistivity curves.

a. Well Probe. The well probe consists of one brass current electrode and three lead-oxide potential electrodes. Each electrode is internally insulated with epoxy resin, and each is connected to the surface by a separate conductor. The potential electrodes are spaced at 0.25, 2.5, and 10 feet from the current electrode. The current electrode is drilled so that an insulated sinker rod can be attached to the probe with a leather thong if more weight is needed to carry the probe to the bottom of the well. With the resistivity instrument at the surface, the operator can read the SP in millivolts and the resistivity in ohm-feet. The reel holds about 500 feet of cable. (An additional 500 feet of cable can be attached to work at depths of 1,000 feet.) A test set is provided for checking the operation of the instrument.

b. Resistivity Instrument. This instrument uses direct current and is of the null reading type. The instrument reads directly in ohm-feet. The use and operation of the various controls on the instrument panel follow:

• Galvanometer. This is a zero-centered micrometer. All operations of the instrument involve returning the meter needle to the zero position.
• SP shutoff switch. This is a spring- return switch located in the upper left comer of the panel. The switch automatically shuts off the 1-1/2-volt C-battery (used in the SP circuit) when the instrument lid is closed.
• Probe selector switch. This switch is located directly below the SP shutoff switch on the units modified for mud logging. When using the switch, the operator can select either the normal probe with the 0.25-, 2.5-, and 10-foot electrode spacings or the mud probe for monitoring conditions in the mud pit or borehole.
• Self-potential potentiometer. This instrument balances out the SP that exists between any pair of potential electrodes. You must obtain a balance before making any resistivity reading. Balance is indicated when the galvanometers needle goes to zero. If you want an SP curve, record the potentiometer reading. Each division is one millivolt; a full scale is 1,000 millivolts.
• SP polarity-reversing switch. This switch is located directly below the SP potentiometer and shows the polarity of the particular cable electrode being used. When using the switch, the operator can change the polarity of the injected voltage as required by borehole conditions.
• Current switch. This is the ON-OFF switch located directly below the function switch; it is a momentary spring-return toggle switch. The spring-return is an automatic shutoff safety feature.
• Function switch. This is the three-position selector switch located directly beneath the galvanometers. The three positions are CUR (current), CAL (calibrate), and LOG (logging). Use the CUR position when checking the system to determine if you are using the correct amount of current. Use the CAL position when calibrating the instrument. Calibration is required at the start of the logging operation and after every 50 feet of logging). Use the LOG position during the logging operation.
• Calibration adjustment. The CAL ADJUST is located in the lower right-hand comer of the panel and is used to zero the galvanometers when calibrating the instrument.
• Ohmmeter. This instrument is located directly above the CAL ADJUST and indicates earth resistivity directly in ohm-feet. A reading at 0.25-foot NORMAL equals 1 ohm-foot for each division and 1,000 ohm-feet at full scale. A reading at 2.5-foot NORMAL equals 10 ohm-feet for each division and 10,000 ohm-feet at full scale. A reading at 10-foot NORMAL equals 40 ohm-feet for each division and 40,000 ohm-feet at full scale.
• Electrode selector switch. This five-position switch is located directly above the ohmmeter. On the right side, it makes circuit connections for either the 0.25- or 2.5-foot NORMAL arrangement. When the switch points straight up, the connections are for the 10-foot NORMAL arrangement. On the left side, the connections are for the 0.25- and 2.5-foot LATERAL arrangement, as marked. In the LATERAL arrangements, the 10-foot electrode is the reference for both positions.
• Cable jack. This is the four-pronged receptacle located in the upper right comer of the panel. The jumper cable from the logger cable case plugs into the jack thereby connecting the instrument to the well probe.
• Potentiometer (POT). This is a black plug receptacle used to connect the surface-potential ground wire (lead-oxide flag) to the instrument.
• CUR. This is a red plug receptacle used to connect the surface-current ground wire (steel stake) to the instrument.
• Test set. With the test set, you can check the instrument operation and condition of the batteries without using the logging cable. The test set is normally kept in the base of the instrument. To use the set, do the following: plug the set into the cable jack, the red plug into CUR, and the black plug into POT. Follow the usual logging procedure. You will have little or no SP. The electrode selector switch may be in any of the NORMAL logging positions.

c. Power Supply. Three power sources of 1 1/2, 9, and 45 volts are requited. You can use a C-size battery for the 1 1/2-volt source and a 9-volt battery for the 9- and 45-volt sources.

d. Logging Cable. This cable used to lower the probe into the well. The cable has four conductors that are made of copper-coated steel wire and are covered with a tough, durable 60 percent natural-rubber jacket. The cable is very resistant to abrasion, is flexible at low temperatures, and has a high breaking strength of 320 pounds. The cable is marked every five feet with numbered markers.

e. Cable Reel and Reel Case. The reel case is made of drawn aluminum. The reel is made of aluminum and PVC. The reel ends ride in bronze bearings attached to the reel case. Whenever possible, nonferrous metals have been used to construct the logging gear. Before removing or installing the reel, unreel the cable to lighten the reel. To remove the reel from the case, unscrew the six screws from each bearing block and carefully lift the reel and bearings from the case.

f. Extender Cable-Reel Assembly. Use this cable when logging past 500 feet. The assembly has an S stamped on the flange to which you mount the jumper connector. This cable must be used first. When you reach the 500-foot marker, attach the second waterproof connector to the cable reel. This connector will be located at the hub of the cable reel assembly. The second cable is then connected to the instrument by means of the jumper.

g. Surface Lines. Two small lengths of stranded-conductor, insulated wire are included with the unit. The red wire has a plug on one end for connecting to CUR on the instrument panel and a clamp at the other end for fastening to a steel stake. (The steel stake is not furnished.) The black wire has a plug on one end for connecting to POT on the instrument panel and a piece of oxidized lead on the other end. If these lines become worn or broken, replace them with any 18- to 20-gauge stranded-conductor, insulated wire. These lines and reels are too large for the instrument cases, so they are carried separately.

a. With the Normal Arrangement.

• Logging instrument.
• Logging cable.
• Surface-potential wire (black) with lead-oxide flag.
• Surface-current wire (red).
• Steel rod that is about 1/4 to 1/2 inch in diameter and 2 to 3 feet in length.

• Step 1. Plant the lead-oxide flag and the steel rod on opposite sides of the well about 75 feet from the well. Bury the lead-oxide flag, which has been moistened with water and tamped, in a hole about 1-foot deep. Plug the black wire into POT and the red wire into CUR on the instrument panel.
• Step 2. Connect the probe assembly to the logging cable only if the waterproof connectors and their pins and sockets are clean. If the connectors do not mate easily after the pins are aligned, apply silicone (spray or grease) to the surfaces. The connectors are mated properly when you hear a pop.
• Step 3. Secure the locking rings.
• Step 4. Lower the probe to the bottom of the well and plug the jumper cable into the cable receptacle on the instrument panel. On units equipped with a mud probe, place probe selector switch NORMAL.

b. With the Lateral Arrangement. In areas that have highly resistive surface materials (deserts or dunes) or large and varying ground potentials (highly industrialized areas using large direct-current generators), you may not be able to make a normal log. Use a lateral log to overcome these difficulties.

The setup is similar to the NORMAL arrangement except you will not need the lead-oxide surface electrode (POT). You can complete the current circuit (CUR) by attaching the red wire to the well casing or any other good ground. The electrode-selector switch must be in one of the two LATERAL positions. To complete the reading, use the procedure as for the NORMAL arrangement. However, the meter does not provide a direct reading. For the 0.25-foot lateral, the factor is 1.025 (essentially direct); for the 2.5-foot lateral, the factor is 13.33.

a. Calibrating. Unit calibration is a rear panel adjustment and is adjusted for the mud logger probe furnished with the unit. However, the CAL ADJUST potentiometer will effect some adjustment to accommodate changes due to temperature and battery condition. If the rear panel adjustments are not tampered with and the galvanometers needle remains within ± 2 minor divisions of zero when calibrating the instrument in the CAL position of the function switch, the calibration of the instrument will be ± 10 percent of the obtained reading.

Unit checkout of the instrument in the mud mode of operation with the test set may be done as in the normal mode with the results obtained. Electrode switch must be in one of the following NORMAL positions:

• CUR position - at 10 divisions.
• CAL position - within ± 2 divisions of zero.
• LOG position - reading on ohmmeter of about one-tenth the value marked on test set or as indicated on resistivity chart furnished with instrument.

Note: Self-potential potentiometer must be at zero.

b. Setting up the Equipment. Since the electrode array is contained within the mud probe, the surface lines are not used. A special jumper makes the proper electrical connections between the unit and the cable reel. Use the following procedures for calibrating:

• Step 1. Place probe selector switch to MUD.
• Step 2. Connect the mud probe to the 500-foot cable. Press the connectors until you hear a pop, which indicates that the waterproof sealing surfaces are mated. Secure the threaded locking rings.
• Step 3. Pull desired length of cable from the reel and attach the mud logger jumper to the instrument and cable reel. Place unit in any of the NORMAL logging positions.
• Step 4. Pack the probe with mud or lower it into the borehole.
• Step 5. Place the function switch at CAL, energize the current switch, and make sure that the galvanometers needle is centered. If needle swings full left, place function switch at CUR and check for probe current. The needle should deflect about ten divisions to the right of zero, indicating that about 1 milliampere (ma) of current is flowing. If you note a zero deflection, the probe electrodes have not made contact with material to be measured.
• Step 6. If you get proper indication at CAL or CUR, place the function switch at LOG and adjust the SP or the 0 on the galvanometers.
• Step 7. Energize the current switch while adjusting the ohmmeter potentiometer until you notice minimal or zero deflection of the galvanometers. If the current switch keeps energizing, the galvanometers needle may start to swing to the left as an induced voltage is set up in the probe. You can verify this by turning the galvanometers needle back to the right an equal distance as the left swing that shows up as an SP on releasing the current switch.

D-4. Troubleshooting Procedures. The logging unit has a test set that you can use to check the instrument's operation independent of the cable assembly. If the instrument reads the correct value for the test set, then the instrument is functioning properly and the trouble is somewhere other than in the electronics of the instrument. See above for operating instructions for the test set. The electrode selector switch must be in any NORMAL logging position.

a. Checking Batteries. With the test set plugged into the instrument, turn the function switch to CAL and try to calibrate the instrument. If you cannot calibrate the instrument, turn the function switch to CUR and check the current by throwing the current switch. If the current flowing is less than 8 ma, replace all six 9-volt batteries. Refer to battery voltage checking procedure to check the batteries with a voltmeter. The following lists some common problems with the corrective procedures:

b. Using a Voltmeter. Conduct the following checks with the test set:

• Place the voltmeter function switch at +direct current (DC) volts and the range switch at full scale reading closest to, but no lower than 10 volts.
• Connect the red (+) lead to the positive (male) and the black (-) lead to the negative (female) battery terminals. Record this open circuit voltage.
• Place function switch at CUR or CAL mode, energize the current switch, and record the battery voltage under load. The voltage should remain at or slightly below the open-circuit voltage.
• Replace the battery if the voltage should continue to change value when the current switch is energized.

• Place the function switch at +DC volts and the range switch at full scale reading closest to, but not lower than 50 volts.
• Connect the red (+) lead to the exposed positive terminal (male) and the black (-) lead to the exposed negative terminal (female) of the battery string. Record this open-circuit voltage.
• Place the function switch at CUR or CAL mode, energize the current switch, and record the battery voltage under load. The voltage should remain at or slightly below the open-circuit voltage.
• Replace all the batteries if the level should continue to change value when the current switch is energized.

c. Calibrating the DR-74 Mud Logger (with a Known Salt Solution).

• Place instrument in mud mode.
• Connect probe to unit.
• Lock SP potentiometer at zero.
• Place function switch at LOG.
• Put probe in known concentration of sodium chloride and distilled water.
• Take temperature of water sample.
• Clean probe in distilled water, when necessary.
• Refer to resistivity, concentration, and temperature chart to determine the resistivity of the sample known concentration and temperature. If in ohmmeters, multiply reading by 3.28 to convert to ohm-feet. Set ohmmeter potentiometer to this reading and lock.
• Place function switch at LOG.
• Center CUR ADJUST potentiometer on front panel to center of travel.
• Energize CUR switch, momentarily, and the CAL ADJUST potentiometer marked log, which is on the circuit board on the back of the galvanometers, to zero the needle to that position when CUR switch is not energized.
• Place function switch to CAL. Rotate the CAL ADJUST potentiometer on same bored to zero the needle in the same manner as above (when energizing the current switch).
• Recheck the log position. If necessary, readjust both potentiometers until you get proper readings.

D-5. Maintenance. Keep all plugs, panel connections, and cable and reel case clean and dry. Moisture on the panel plugs or cable plug can cause current leakage and cause the gears to operate improperly. When pulling the cable out of the well, wipe it clean. Do not let water accumulate in the reel case.

a. Of Resistivity Measurement. Required maintenance is cleaning and changing batteries. The test set will indicate when to replace the 9-volt batteries. Replace the 1 1/2-volt cell every two months. The battery box is at the bottom of the case. Lift the instrument panel to access the batteries.

When operating the instrument, be careful not to slam the care ohmmeter and self-potential potentiometer dials against their zero stops. When the dials are turned completely counterclockwise, they should read zero. If they do not, loosen the two setscrews, set them 90 degrees apart, and reset the knob to zero. (To set the screws, use the allen wrench that is taped on the potentiometer.)

b. Of Cable and Reel Case. When handling the cable, be careful not to damage the insulation. Always clean the cable before storing it. Store the reel case in a dry place. Keep the reel case lid left open until the cable is thoroughly dry.

a. Preparation of the Log. When preparing the log, plot the 0.25-foot reading, NORMAL arrangement at the depth you read from the marked cable. Plot the 2.5-foot readings about one foot above that point. You plot the readings in this manner because the cable markings have been measured from the current electrode. For the LATERAL arrangement, plot the values as for the NORMAL arrangement. If you use the 10-foot NORMAL, plot the readings about 5 feet above the marked cable reading.

b. Significance of 0.25-Foot Spacing. The reading you get from the 0.25-foot spacing is greatly influenced by the fluid in the well bore. The reading is only a fraction of the formation resistivity. However, the short spacing lets you to see changes in resistivity with greater detail. With this electrode spacing, you can detect formations with a thickness of about 6 inches or more. Because you can see more detail, use the 0.25-foot curve to pick formation boundaries.

c. Significance of 2.5-Foot Spacing. The 2.5-foot electrode spacing gives you the closest true formation resistivity for wells with diameters up to 16 inches and for formations thicker than 5 feet. For larger diameter wells or thinner formations, the measured resistivity will depart somewhat from the true. For qualitative interpretation, this departure is not significant. Because the 2.5-foot curve gives you the formation resistivity, use it to identify the type of material penetrated.

d. Significance of the Lateral Log. You get a lateral log by combining either the 0.25- or 2.5-foot electrode with the 10-foot electrode. Because the distance of the 10-foot electrode is larger than the other electrodes, you can interpret the log as for the normal log after using the appropriate correction factors. For the 0.25-foot lateral log, the meter factor is 1.025. For the 2.5-foot lateral log, the meter factor is 13.33.

e. Interpretation of Resistivity Values. When interpreting the resistivity values, the following generalities may apply: clays and shales will have low resistivity; sands, grovels, sandstones, and limestones will have high resistivity; igneous and metamorphic rocks (granites and gneisses) will have extremely high resistivity. The exact range of numerical values will depend on the following:

• Type of earth material making up the formation.
• Degree of cementation of the formation.
• Water quality of the formation water.
• Porosity of the formation.
• Diameter of the well born.
• Resistivity of the fluid in the well bore.

Granular materials will have high resistivity compared to fines such as silt and clay; crystalline materials (such as limestone or granite) will have high resistivity compared to the granular materials. The quality of the formation water will affect the measured resistivity. In general, the resistivity of a formation will vary in an inverse proportion to the total dissolved solids. For example, if all conditions remain the same but the total solid content increases, the formation resistivity will decrease. Hence a clean sand filled with salty water may actually have extremely low resistivity.

Porosity of the formation also has an effect on the resistivity. In the logging of chemical precipitates, such as limestone, changes in porosity may enable you to detect the water-producing zones. Increased porosity will lower the formation resistivity and hence in such material a low resistive zone (where no shale is present) is indicative of increased porosity. This is then indicative of possible water production.

In the midwestem United States, clean sand and grovel generally exhibit resistivity values of 350 to 1,000 ohm-feet. The lower values apply to formations having water quality in the range of 300 to 400 ppm total solids and the upper values apply for formation waters having 100 to 150 ppm total solids.

f. Selection of Formation Contacts. When selecting the formation boundaries, use the 0.25-foot curve, when possible. The inflection point of the resistivity curve (the point midway between changes in curvature of the resistivity curve) is used to mark the contact between different formations.

g. Correlation of Electrical Logs. You can use the electrical logs to correlate formation thicknesses and depths from one well to another. For example, two wells within a few feet of each other invariably will give identical electrical logs. When the wells are farther apart, you should still be able to recognize the correlation. Studying the changes (thickening or thinning of beds) could be useful for further exploration. In bedrock formation, correlation is possible with distances of thousands of feet. However, such distances are the exception.

h. Effect of Metal on the Resistivity Log. Because metal is a good conductor, its presence in the measurement zone will cause a major decrease in the resistivity and make the log unusable for determining formation type. However, using metal could help locate steel in the well.

In making the log, the bottom of the well casing will be detected when the probe enters it. The effect on the curves will be that both fall off to extremely low values, 5 to 20 ohm-feet, and then remain fairly constant. Where the casing is seated into very low-resistive shale, it may be rather difficult to determine the exact position of the casing by this method.

i. SP Curve. This curve shows a great deal of character and can be related to relative changes in formation permeability. When logging in freshwater horizons, the SP curve will usually be featureless and provide little or no useful information.