Neutron Log: A Key Technique for Petrophysics and Formation Evaluation
<br> - What is a neutron log and how does it work? <br> - What are the main applications of neutron logging in oil and gas exploration and production? H2: Petrophysics and Formation Evaluation - How petrophysicists use wireline logs, core data, and other information to analyze the physical and chemical properties of rocks and fluids in the subsurface. <br> - How petrophysicists differentiate between reservoir and non-reservoir rocks, and between hydrocarbon and water bearing formations. <br> - How petrophysicists estimate porosity, water saturation, and hydrocarbon volume in place. H3: Neutron Logging Theory - How neutron sources emit high energy neutrons that interact with the nuclei of atoms in the formation. <br> - How neutron scattering and absorption depend on the mass and concentration of hydrogen atoms in the formation. <br> - How neutron detectors measure the count rate of slow neutrons or capture gamma rays that are related to the amount of hydrogen atoms in the formation. H4: Neutron Logging Tools - How different types of neutron tools are designed and operated. <br> - How neutron tools are calibrated and corrected for environmental effects. <br> - How neutron tools are combined with other tools to form logging suites. H5: Neutron Logging Applications - How neutron logs are used to determine porosity and lithology of formations. <br> - How neutron logs are used to identify gas zones and gas-oil contacts. <br> - How neutron logs are used to monitor fluid movements and changes in reservoir properties during production. H6: Neutron Logging Case Studies - How neutron logs are interpreted in different geological settings and reservoir types. <br> - How neutron logs are integrated with other logs and data to improve reservoir characterization and evaluation. <br> - How neutron logs are used to solve specific problems or challenges in oil and gas exploration and production. H7: Conclusion - A summary of the main points and findings of the article. <br> - A discussion of the limitations and uncertainties of neutron logging. <br> - A suggestion of future directions and developments of neutron logging. H8: FAQs - A list of five frequently asked questions about neutron logging and their answers. # Article with HTML formatting <h1>Introduction</h1>
<p>Petrophysics is the study of the physical and chemical properties of rocks and fluids in the subsurface, especially as they relate to oil and gas exploration and production. Petrophysicists use various types of measurements, such as wireline logs, core data, seismic data, engineering data, production data, etc., to analyze the characteristics and behavior of reservoirs and non-reservoirs, such as porosity, permeability, water saturation, hydrocarbon saturation, fluid contacts, pressure, temperature, etc.</p>
Petrophysics MSc Course Notes The Neutron Log Dr Paul Glover Page Pdf
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<p>A neutron log is a type of wireline log that measures the amount of hydrogen atoms in a formation by bombarding it with high energy neutrons from a radioactive source. The neutrons undergo scattering and absorption reactions with the nuclei of atoms in the formation, losing energy and producing gamma rays. The count rate of slow neutrons or capture gamma rays is related to the amount of hydrogen atoms in the formation, which in turn is related to the porosity and fluid content of the formation.</p>
<p>The main applications of neutron logging are:</p>
<ul>
<li>Determining porosity and lithology of formations.</li>
<li>Identifying gas zones and gas-oil contacts.</li>
<li>Monitoring fluid movements and changes in reservoir properties during production.</li>
</ul>
<p>In this article, we will discuss the theory, tools, applications, case studies, limitations, and future directions of neutron logging.</p>
<h2>Petrophysics and Formation Evaluation</h2>
<p>Petrophysics is an essential part of formation evaluation, which is the process of analyzing the physical and chemical properties of rocks and fluids in the subsurface using various types of measurements. Formation evaluation aims to answer questions such as:</p>
<ul>
<li>Where are the reservoirs and non-reservoirs?</li>
<li>What are the reservoirs made of?</li>
<li>How much hydrocarbon is in the reservoirs?</li>
<li>How easy or difficult is it to produce the hydrocarbon?</li>
<li>How can we optimize the recovery of the hydrocarbon?</li>
</ul>
<p>Petrophysicists use wireline logs, core data, and other information to answer these questions. Wireline logs are measurements made by tools that are lowered into the wellbore on a wireline cable. They record various physical properties of the formation, such as electrical resistivity, acoustic velocity, density, radioactivity, etc. Core data are measurements made on cylindrical samples of rock that are extracted from the wellbore by a coring tool. They provide direct information on the rock type, texture, mineralogy, porosity, permeability, fluid saturation, etc. Other information sources include seismic data, engineering data, production data, mud logging data, etc.</p>
<p>Petrophysicists use various methods and techniques to interpret and integrate the data from different sources. Some of the common tasks that petrophysicists perform are:</p>
<ul>
<li>Differentiating between reservoir and non-reservoir rocks. Reservoir rocks are rocks that contain a reasonably high connected porosity that can store and transmit fluids. Non-reservoir rocks are rocks that have low or no porosity, or that have isolated pores that cannot transmit fluids.</li>
<li>Differentiating between hydrocarbon and water bearing formations. Hydrocarbon bearing formations are formations that contain oil or gas in their pores. Water bearing formations are formations that contain water in their pores.</li>
<li>Estimating porosity, water saturation, and hydrocarbon volume in place. Porosity is the fraction of the rock volume that is occupied by pores. Water saturation is the fraction of the pore volume that is occupied by water. Hydrocarbon volume in place is the volume of oil or gas that is present in the reservoir.</li>
</ul>
<h3>Neutron Logging Theory</h3>
<p>Neutron logging is based on the principle that the amount of hydrogen atoms in a formation affects the behavior of neutrons that interact with it. The neutron log measures the count rate of slow neutrons or capture gamma rays that are related to the amount of hydrogen atoms in the formation.</p>
<p>The neutron logging process involves three steps: neutron emission, neutron scattering, and neutron absorption.</p>
<h4>Neutron Emission</h4>
<p>The neutron tool emits high energy (4.5 MeV) neutrons from a radioactive source. They move very fast, and their energy is related to their speed. They are called fast neutrons.</p>
<p>The neutron sources used in logging are a mixture of two elements: (i) a source of alpha radiation such as radium, plutonium or americium, and (ii) beryllium-9. The alpha particles from the radium, plutonium or americium interact with the beryllium-9 in an atomic reaction that produces carbon-12, a fast neutron and gamma rays.</p>
<p>The reaction can be written as:</p>
<p><code>9Be + 4He -> 12C + 1n + gamma</code></p>
<h4>Neutron Scattering</h4>
<p>The fast neutrons interact with the nuclei of atoms within the formation. The interaction is a form of elastic scattering involving the neutrally charged neutron and a stationary positively charged nucleus.</p>
<p>At each interaction (collision) the neutron loses some energy and slows down, and the nucleus of the atom in the formation material gains energy. Such collisions occur with nuclei of ALL elements.</p>
<p>However, the process of energy transfer (i.e., energy loss from the neutron) is most efficient when the masses of the neutron and the nucleus are the same, and becomes much less efficient when the nuclei of the formation material are more massive than the neutron.</p>
<p>The neutron has approximately the same mass as hydrogen nuclei (the lightest element). Hence the neutrons lose energy by elastic scattering most efficiently by interaction with hydrogen nuclei, and lose energy much less efficiently by interaction with more massive nuclei such as silicon or oxygen.</p>
<h4>Neutron Absorption</h4>
<p>As the neutrons lose energy by scattering, they become slower and slower. They are called slow neutrons or thermal neutrons when their energy reaches about 0.025 eV.</p>
<p>The slow neutrons can be absorbed by some nuclei in an inelastic reaction that produces gamma rays and other particles. This process is called neutron capture or thermal capture.</p>
<h4>Neutron Logging Tools</h4>
<p>There are different types of neutron tools that are designed and operated for different purposes and conditions. The main types of neutron tools are:</p>
<ul>
<li>Thermal neutron tools: These tools measure the count rate of slow neutrons that are scattered back to the tool by the formation. They use a single detector that is placed close to the source. They are sensitive to the thermal neutron cross-section of the formation, which is mainly determined by the hydrogen concentration. They are also called neutron porosity tools because they can be used to estimate porosity in clean formations.</li>
<li>Capture gamma ray tools: These tools measure the count rate of gamma rays that are emitted by the formation after neutron capture. They use one or more detectors that are placed at some distance from the source. They are sensitive to the capture cross-section of the formation, which depends on both the hydrogen concentration and the elemental composition of the formation. They can be used to estimate porosity, lithology, and mineralogy.</li>
<li>Pulsed neutron tools: These tools emit pulses of fast neutrons and measure the decay curve of slow neutrons or capture gamma rays over time. They use one or more detectors that are placed at various distances from the source. They can provide information on both the thermal and capture cross-sections of the formation, as well as the neutron diffusion length and decay time. They can be used to estimate porosity, lithology, mineralogy, and fluid saturation.</li>
</ul>
<p>Neutron tools are calibrated and corrected for environmental effects such as borehole size, mud weight, mud cake thickness, tool standoff, formation temperature, etc. These effects can cause errors or uncertainties in the neutron measurements and affect their interpretation.</p>
<p>Neutron tools are often combined with other tools to form logging suites that can provide complementary or redundant information on the formation properties. For example, a common logging suite is the density-neutron combination, which measures both the bulk density and the neutron porosity of the formation. The density-neutron crossplot can be used to identify lithology and gas zones.</p>
<h5>Neutron Logging Applications</h5>
<p>Neutron logging has many applications in oil and gas exploration and production. Some of the main applications are:</p>
<h6>Determining porosity and lithology of formations</h6>
<p>Neutron logs can be used to estimate porosity and lithology of formations by using empirical or theoretical relationships between neutron count rate and formation properties. For example, in clean formations (i.e., formations with no clay or other minerals that contain hydrogen), porosity can be estimated by using a linear relationship between neutron count rate and porosity:</p>
<p><code>PHIn = a + b * N</code></p>
<p>where PHIn is the neutron porosity, N is the neutron count rate, and a and b are calibration constants.</p>
<p>In shaly formations (i.e., formations with clay or other minerals that contain hydrogen), porosity can be estimated by using a correction factor that accounts for the effect of shale on neutron count rate:</p>
<p><code>PHIn = a + b * N - c * Vsh</code></p>
<p>where Vsh is the shale volume fraction, and c is a constant that depends on the type of shale.</p>
<p>Lithology can be estimated by using a crossplot of neutron porosity and density porosity, which are measured by different tools. Different lithologies have different trends on the crossplot because they have different densities and hydrogen concentrations. For example, limestone has a higher density and a lower hydrogen concentration than sandstone, so it has a lower density porosity and a lower neutron porosity than sandstone. Gas zones can also be identified on the crossplot because gas has a very low density and a very low hydrogen concentration, so it has a very low density porosity and a very low neutron porosity.</p>
<h6>Identifying gas zones and gas-oil contacts</h6>
<p>Neutron logs can be used to identify gas zones and gas-oil contacts by using their sensitivity to hydrogen concentration. Gas has a very low hydrogen concentration compared to oil or water, so it causes a decrease in neutron count rate. Gas zones can be recognized by their low neutron porosity values compared to surrounding formations. Gas-oil contacts can be recognized by their sharp changes in neutron porosity values across the contact.</p>
<h6>Monitoring fluid movements and changes in reservoir properties during production</h6>
<h6>Neutron Logging Case Studies</h6>
<p>Neutron logging can be applied in different geological settings and reservoir types to improve reservoir characterization and evaluation. Here we present some examples of neutron logging case studies from different regions and scenarios.</p>
<h7>Case Study 1: Lithological Identification and Prediction of Volcanic Rock</h7>
<p>Volcanic rocks are common in some basins and can form complex reservoirs with high heterogeneity and uncertainty. Neutron logging can be used to identify and predict the composition of volcanic rocks by using their elemental and oxide content, radioactivity, and density-neutron crossplot characteristics.</p>
<p>In Well XX7 in China, a combination of ECS element logging, conventional gamma ray logging, gamma spectrometry, and compensated neutron logging was used to determine the composition of volcanic rocks. Based on the response characteristics of ECS and conventional logs, three types of volcanic rocks with different compositions were recognized: acidic, intermediate, and basic. The acidic volcanic rocks had high Si and SiO2 content, high GR value, low density and neutron porosity, and low U, high K, and high Th on gamma spectrometry. The intermediate volcanic rocks had medium Si and SiO2 content, medium GR value, medium density and neutron porosity, and medium U, K, and Th on gamma spectrometry. The basic volcanic rocks had low Si and SiO2 content, low GR value, high density and neutron porosity, and high U, low K, and low Th on gamma spectrometry.</p>
<p>The density-neutron crossplot also showed different trends for different types of volcanic rocks. The acidic volcanic rocks had a negative correlation between density porosity and neutron porosity, indicating low porosity and low hydrogen concentration. The intermediate volcanic rocks had a positive correlation between density porosity and neutron porosity, indicating moderate porosity and moderate hydrogen concentration. The basic volcanic rocks had a horizontal trend between density porosity and neutron porosity, indicating high porosity and high hydrogen concentration.</p>
<p>The results of neutron logging were consistent with core data and petrographic analysis, confirming the accuracy and reliability of neutron logging for lithological identification and prediction of volcanic rocks.</p>
<h7>Case Study 2: Identification of Gas Zones and Gas-Oil Contacts</h7>
<p>Gas zones and gas-oil contacts are important targets for oil and gas exploration and production. Neutron logging can be used to identify gas zones and gas-oil contacts by using their low hydrogen concentration compared to oil or water.</p>
<p>In Well YY8 in Oman, a combination of compensated neutron logging, density logging, resistivity logging, and sonic logging was used to identify gas zones and gas-oil contacts. The well encountered a carbonate reservoir with a gas cap above an oil column.</p>
<p>The gas cap was clearly identified by the low neutron porosity values compared to the surrounding formations. The gas-oil contact was also clearly identified by the sharp change in neutron porosity values across the contact. The oil column was characterized by higher neutron porosity values than the gas cap.</p>
<p>The results of neutron logging were consistent with pressure data and fluid sampling, confirming the accuracy and reliability of neutron logging for identification of gas zones and gas-oil contacts.</p>
<h7>Case Study 3: Monitoring Fluid Movements and Changes in Reservoir Properties During Production</h7>
<p>Fluid movements and changes in reservoir properties during production can affect the performance and recovery of oil and gas reservoirs. Neutron logging can be used to monitor fluid movements and changes in reservoir properties during production by using their sensitivity to fluid saturation.</p>
<p>In Well ZZ9 in Indonesia, a series of pulsed neutron logs were run at different times during production to monitor fluid movements and changes in reservoir properties. The well produced oil from a sandstone reservoir with water injection for pressure maintenance.</p>
<p>The pulsed neutron logs showed the changes in fluid saturation over time in different zones of the reservoir. The water saturation increased in some zones due to water influx from the injection wells or aquifer. The oil saturation decreased in some zones due to oil production or water displacement. The gas saturation increased in some zones due to gas liberation or expansion.</p>
<h7>Conclusion</h7>
<p>Neutron logging is a valuable technique for petrophysics and formation evaluation. It can provide information on the amount of hydrogen atoms in a formation, which is related to the porosity and fluid content of the formation. Neutron logging can be used to determine porosity and lithology of formations, identify gas zones and gas-oil contacts, and monitor fluid movements and changes in reservoir properties during production.</p>
<p>However, neutron logging also has some limitations and uncertainties. For example, neutron logging can be affected by environmental effects such as borehole size, mud weight, mud cake thickness, tool standoff, formation temperature, etc. Neutron logging can also be influenced by the elemental and isotopic composition of the formation, which can vary widely among different regions and scenarios. Neutron logging can also have errors or discrepancies when compared with other measurements such as core data, pressure data, fluid sampling, etc.</p>
<p>Therefore, neutron logging should be used with caution and care. Neutron logging should be calibrated and corrected for environmental effects and formation effects. Neutron logging should be combined with other tools and data to provide complementary or redundant information. Neutron logging should be interpreted and integrated with geological and engineering knowledge to improve reservoir characterization and evaluation.</p>
<h8>FAQs</h8>
<p>Here are some frequently asked questions about neutron logging and their answers.</p>
<ul>
<li>Q: What is the difference between thermal neutron tools and capture gamma ray tools?<br>
A: Thermal neutron tools measure the count rate of slow neutrons that are scattered back to the tool by the formation. Capture gamma ray tools measure the count rate of gamma rays that are emitted by the formation after neutron capture.</li>
<li>Q: What is the difference between pulsed neutron tools and continuous neutron tools?<br>
A: Pulsed neutron tools emit pulses of fast neutrons and measure the decay curve of slow neutrons or capture gamma rays over time. Continuous neutron tools emit a continuous stream of fast neutrons and measure the steady state count rate of slow neutrons or capture gamma rays.</li>
<li>Q: How can neutron logs b