Submit concise one-page overviews of two radiometric tools (Geiger counter and gamma-ray spectrometer). Use figures to illustrate your answers.
Geiger counter was developed by Hans Geiger in collaboration with Walther Muller to detect ionizing radiation. Basically, the tool is designed with a metal tube fitted with a glass or mica window on one extreme while a wire with a strong positive charge runs at the center of the metal tube. The tube is usually filled and sealed with an inert gas like argon at low pressure. The device works in such a manner that when it comes into contact with ionizing particles, the electrons are knocked from atoms in the filler glass. The free electrons are then attracted to the positively charged wire at the center of the tube and they acquire energy through attraction. The electrons that are accumulated create a cascade effect resulting in a pulse of current that is detectable. Since the casing of Geiger-Muller acts as a cathode, and the central wire acting as an anode, the Geiger counter is able to detect the pulse created by ionizing particles. Although the device is unable to measure the type, vector as or energy of the particles, they adequately detect alpha, beta, gamma and x-radiations.
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Gamma-ray Spectrometer is an advancement of the scintillation counter and is able to identify the source elements. The source elements can be identified using this device because the spectra are from potassium K 40 , Thallium, Th and Uranium, U with peaks representing various stages in decay series. The device expresses spectra in terms of energy levels. The energy levels are monitored in a kind of window where the top limits are already predetermined and the lower levels are observed and analyzed to provide a way of discriminating between various sources. Depending on the materials of manufacture, there are several types of gamma-ray spectrometers. Examples include sodium iodide and cesium iodide
Describe the basic principles of a) natural gamma ray logging, b) gamma-ray density logging and c) neutron logging. Use at least one illustration per method!
Natural Gamma-ray logging determines radioactivity that originates within decimeters of the borehole. The gamma ray tool obtains measurements of radiations that are found naturally in rock formations. Estimates of clay content of sedimentary rocks and shale horizons can be detected through the natural gamma log radiation. The small quantities of radioactive elements such as K 40 , U 238 and Th 232 contained in shale which are detectable gamma radiation
Gamma-ray density logging utilizes artificial gamma rays from various components. Density is calculated by determining the proportion of gamma radiation returned to the detector by Compton scattering. Compton scattering occurs when gamma-ray photons collide with electrons ending up in reduced energy. The elements present determines the formation density
Ρf and electron density index Ρe. i.e ρf = ρew/2ΣN
For example, in an instance where rock formation density is estimated as ρj matrix density as ρm and pore fluid density as ρw , the formation is
ρf = φρw + (1 - φ)ρm therefore φ = (ρm - ρf)/(ρm - ρw)
Neutron logging
Gamma rays, which provide information on porosity, are emitted by stimulating nuclei due to bombardment of nonradioactive elements with neutrons. Gamma radiation is produced when neutrons and atomic nuclei collide on the rocks on the wall. Since the nucleus is more massive than neutrons, the elasticity of the rebounds ensures little loss of kinetic energy and therefore slows down the neutron to the point that they can be absorbed by larger nucleus.
Why do operators (oil and gas especially) generally run a suite of resistivity logs in a borehole (as opposed to running a single log)? How can resistivity data be used to provide information on the nature of subsurface pore fluids? Explain your answers. Make sure you incorporate relevant equations and illustrations!!
The suite of resistivity log is run in the borehole by the operators because it is able to record several different parameters simultaneously in a single pass of the borehole. The tool records ten different parameters unlike the single log. Resistivity data is significant in defining the nature of subsurface pore fluids since reduction in resistivity towards the top of a carbonate unit gives an implication of leveled upward carbonate sediments. The information can also be obtained based on the fact that conductivity of pore fluids and the amount and connectivity of pore space controls resistivity.
k = (cφ3/Sirr)2
where φ is porosity and c is a constant dependent on the pore fluids lithology and grain size of the formation.
Describe the principles of the SP logging technique. Use figures to illustrate your answer.
The principles of self-potential (SP) logging hold that measurements of potential difference are established in boreholes that are containing drilling fluids which must be conductive between electrode on the surface and that on the sonde. Ions which are in fluids that have varied concentrations and moving at different speeds stimulate the effects of self-potential. As a result of the contact between pore fluid and mud filtrated of varied salinity in the shale near the borehole, the SP effect is communicated across the boundary in linking sandstone and shale. The membrane polarization effect in the clay minerals of the shale blocks the movements of the ion that can balance the difference created by self-potential. SP is very significant during the determination of boundaries in shale horizons and very porous beds, the relationship with the boreholes and establishing the volume of the shale in porous beds.
SP Log
What is a borehole acoustic televiewer? How does it work? Use figures to illustrate your answer. Answer should be at least one page in length.
A borehole acoustic televiewer is a borehole tool for fracture analysis. It is an acoustic scanner weighing approximately 150Kgs and is approximately 3.6m long which at great temperatures and pressure transmits ultrasound pulses that produce images of the borehole walls. The main component of the device is piezoelectric transducer acting both as a transmitter and receiver. The ordinary operation of the device is estimated at the logging speed of 18 m per hour. The acoustic televiewer works in such a way that the piezoelectric transducer is swiveled by a motor so that it scans the walls of the borehole through ultrasonic pulses. The device then generates images of the wall of the borehole which are then sent back and converted into monochromatic images. The rotating sensor of the device and the recording signals are able to capture the images of various interfaces of the walls of the borehole and provide details in the images that can be clearly interpreted. The device produces strong reflections and portrays smooth borehole walls as light areas on the images that are transmitted from the sensor. On the contrary, the device captures on the images as dark patches sections of borehole walls with fractures, vugs or generally irregular and rough walls. The result remitted to the user differentiates on the images the nature of walls of the borehole. The amplitude and the travel time of the signals as recorded by the sensor reflect between the mud and formation. The maximum pressure at which the device operates is 1200 PSI while the maximum temperatures recorded can be 275 degrees Celsius for a period of ten hours.
