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Methods: Downhole Logging

Have you ever taken on a project at a site with existing wells that have lithologic boring logs that have been prepared previously by several different geologic consultants, each with their own interpretation of lithologic boundaries? And have you found that the existing wells are cased and have a diameter too small to allow running standard logs such as SP and resistivity to resolve the discrepancies? Spectrum can help. Instead of drilling a new well, downhole electromagnetic (EM) induction and natural gamma logs can be run after PVC casing is installed and with or without water in the hole – at a cost much lower than that of drilling and installing a new well. What’s more, the EM induction and natural gamma signature of a lithologic layer or rock type is repeatable and characteristic for a specific locality, so that when correlated with geologic data for that locality, stratigraphic sections can be constructed directly from the logs.
    These methods are commonly used for a variety of environmental and geotechnical site investigations where the following information is needed:
    • Unbiased, high-resolution electrical resistivity logs (EM induction) that allow stratigraphic correlation from well to well
    • Delineation of thin layers of silt or sand that were missed during initial logging of the hole(s)
    • Delineation of potassium-bearing sand or clay layers (natural gamma)
    • Monitoring of contaminant plumes where aquitards and thin aquifers may be controlling contaminant transport
  • WHAT IT MEASURES

    EM Induction Log


    Downhole induction logging is a method that uses the inductive properties of a continuous primary electromagnetic field to measure the conductivity (reciprocal of resistivity) of the material through which the field passes. Downhole induction logging was originally developed in the petroleum industry as an alternative to traditional resistivity logging methods (such as 16-inch and 64-inch normal) in order to have a way to measure formation resistivity when the borehole was filled with resistive petroleum fluids and the coupling between the resistivity probe and the formation could not be made. However, it is now commonly used for many environmental and engineering types of applications.


    Because the material of which subsurface lithologic layers are composed controls the ability of these layers to carry an electrical current (or hold a secondary field), the conductivity log is highly sensitive to small changes, such as grain size, chemistry and degree of sorting in sediments and rocks. In the case of grain size (where an increase in conductivity correlates with a decrease in grain size) conductivity logs can be used both to discriminate between silts, sands and clays (for instance) as well as to identify the thickness of these layers to a high degree of accuracy. In the case of chemistry (where an increase in conductivity correlates with an increase in dissolved salts or other ionic constituents) conductivity logs can be used to discriminate between fresh-water-saturated versus saline or high-TDS-water-saturated sediments or rocks. EM Induction logs are particularly useful where geologic layers of contrasting conductivity are juxtaposed, such as sand vs. clay, sandstone vs. shale and saltwater vs. freshwater. The EM induction tool may be used in both open and PVC-cased wells, and can provide conductivity measurements in the vadose zone.


    Natural Gamma Log


    Natural gamma (or gamma) logging uses the scintillation properties of certain crystals to detect the presence of gamma radiation from unstable isotopes in the formations adjacent to the well or borehole. In aquifers that are not contaminated by artificial radioisotopes the most significant naturally-derived radioisotopes that emit gamma radiation are potassium-40 (K40) and daughter products of the uranium and thorium series. Like other nuclear logging methods, natural gamma logging was originally developed in the petroleum industry for lithologic and stratigraphic correlation; however, and particularly because it is not an active source log, it is now the most widely used nuclear method for groundwater applications. Because of its sensitivity to clay minerals, one of the most common uses for this log is to delineate clays and shales; however, it is also very effective for the identification of potassium-bearing, uranium-bearing and thorium-bearing sediments and rocks under certain conditions. The natural gamma log is particularly useful where geologic layers of contrasting natural gamma radiation are juxtaposed, such as sand vs. clay, arkose vs. limestone, and granite vs. schist; and for mineral exploration. The natural gamma tool has no casing restrictions and is capable of measuring natural gamma radiation in the vadose zone.


    HOW IT WORKS

    Spectrum uses the Mount Sopris MGXII logging system for EM induction and natural gamma logging. This system consists of a cable wound on an automated winch. An EMP-2493 EM induction sonde is used for downhole induction logging, and a natural gamma sonde designed for groundwater applications is used for natural gamma logging. The cable can be connected to the desired logging tool and lowered down the borehole using the automated winch system, which in turn is connected to a laptop computer for recording. A description of how each logging tool works follows.


    EM Induction Tool


    The EMP-2493 induction sonde is a modified version of the Geonics EM-39 downhole induction tool (EM-39). The EM-39 downhole tool consists of three coils mounted on a probe: one transmitter coil, one focusing coil, and one receiver coil. The transmitter and receiver coil are separated by 50 centimeters, and a focusing coil is centrally located on the probe to negate the response from conductive drill fluids. During data logging, an alternating current is applied to the transmitter coil, which sends a primary electromagnetic (EM) field into the surrounding formation. By Faraday’s Law of Induction, this primary field induces eddy currents in conductive material that is encountered, and these eddy currents generate a secondary magnetic field. This secondary magnetic field is measured at the receiver and compared to the primary field. At low induction numbers this secondary magnetic field is linearly proportional to the electrical conductivity of surrounding materials, and is converted to read conductivity in millimhos per meter. The EMP-2493 is a tool proven to be very effective for lithologic interpretation and delineation of groundwater contamination and is designed to detect materials between 30 cm and 100 cm from the borehole axis. The vertical resolution of the tool is approximately one foot.


    During a typical EM induction logging run, the EMP-2493 probe is connected to the MGXII, the MGXII is then connected to a laptop computer, and the calibration of the probe is verified. Once this is done, continuous readings of conductivity are recorded at a rate of 5 feet per minute as the probe is lowered down the borehole. Once the probe reaches the bottom of the hole, the probe direction is set to the “up” position and another conductivity log is collected at a rate of 5 feet per minute until the probe reaches the surface. The end result is two conductivity logs of the same well that should be identical. Once collected the data are downloaded, edited where necessary, and plotted using a graphing software package.


    Natural Gamma Tool


    The natural gamma tool contains sodium iodide scintillation crystals for gamma ray detection at borehole temperatures below 150 degrees F. During operation, the probe traverses the borehole on an insulated metal-jacketed conductor cable with a device for recording the digital pulses from the scintillation crystal. When the probe detects a gamma ray, the sodium iodide crystals produce a flash of light (or scintillation) where the number of scintillations determines the amount of gamma radiation. The scintillations are then amplified in a photomultiplier tube, converted to digital pulses, and counted by a digital ratemeter for a specific time interval, which transmits a proportional signal to the digital recording system in counts per second. For groundwater applications the natural gamma tool has been proven effective for the determination of formation shale or clay content, and for detection and delineation of potassium-bearing rocks such as potassium feldspars. The vertical resolution of the tool depends somewhat on the speed of logging; however it is generally regarded as two feet.


    During a typical natural gamma logging run, the natural gamma probe is connected to the MGXII, the MGXII is then connected to a laptop computer, and continuous readings of natural gamma radiation are then recorded at a rate of 10 or 15 feet per minute as the probe is lowered down the hole. Once the probe reaches the bottom of the hole, the probe direction is set to the “up” position and another log is collected at a rate of 10 or 15 feet per minute until the probe reaches the surface. The end result is two natural gamma logs of the same well that should be identical. Once collected the data are downloaded, edited where necessary, and plotted using a graphing software package.