1. Explain the principle of Gamma Ray Log. Describing clearly the different sources: Gamma ray is a high-energy electromagnetic waves which are emitted by atomic nuclei as a form of radiation (Gamma-Radiation) Gamma ray logging is measurement of natural radioactivity in the formation versus depth This radiation is in fact emitted from three main types of source elements: 40K (Potassium), 232Th (Thorium) or 238U (Uranium) and their decay products as sources. Use of Gamma Ray log: Depth determination Depth correlation within the well and between wells and logging runs Lithology identification: shales, evaporites, uranium Qualitative evaluation of shaliness or clay content Qualitative evaluation of radioactive mineral deposits Cased hole perforating depth control Positioning for open-hole sampling tools 2. What is the main difference between Gamma Ray and Spectral Gamma Ray: For standard GR logs the value measured is calculated only from thorium and potassium. Due to the weight of uranium concentration in the calculation, concentrations of uranium can cause clean sand reservoirs to appear shaley Misinterpretation. Therefore, Spectral Gamma Ray measures not only the radioactivity of the formation, but also provides individual reading of every element; thorium, uranium and potassium. The use of spectral gamma ray (NGS) log: Detail analysis of the sources of the natural gamma radiation gives us added information concerning the composition and lithology of the formation. The total gamma ray spectra measured is resolved into the three most common components of naturally occurring radiation in sands and shales—potassium, thorium, and Uranium (K, Th, and U, respectively). These data are used to distinguish important features of the clay type or sand around the wellbore e.g. identification of Feldspar or Glimmer, different clay formations, etc. 1. 3. Read the attached gamma ray log and identify the clean and clay zones:
The clean zone defines as where the value of GR is low (clean sand reservoir). The clay zone defines as where the value of GR is high, see Figure of General GR Response. Typical ranges are 50 - 120 API Units in shales (clay), and 20 - 40 units in clean formations.
Factors affecting tool and Gamma Ray response: (1). Radiation intensity of the formation (2). Time constant (sec) (3). Logging speed: The faster the logging speed, the less time the tool can sufficiently react and properly count the radiation intensity. This will affect the bed resolution. (5). Borehole environment: borehole-fluid and present of any Bed tubing, casing, should cementmore type,than etc. the diameter of the sphere of investigation. (6). thickness: 4. Why SP log is used? State clearly the principle, readings, source, receiver.
Objective: Primarily used as lithology indictor and as correlation tool. Other uses are as permeability and porosity indicator (determine permeable beds), measurement of formation water resistivity Rw, hence formation water salinity and formation clay content. Principle: The SP log records the naturally occurring electrical potential [in mV] produced by interaction of formation connate water, conductive drilling mud and shale. It reflects a difference of electrical potential between a moveable electrode in the borehole with a fixed electrode reference at the surface. Readings: A positive SP normally indicates shales, negative SP indicates sandstones, etc. Source: Liquid junction or diffusion potential Membrane potential Filtration potential Interpretation of SP-logs Fixing of a shale baseline as the highest Sp-values in regions of membrane active shale and fixing the sand base line as the lowest Sp-values in diffusion potential. Calculating the maximum potential difference with respect to the shale baseline. By calculating the potential difference, start from shale baseline (+ value) to the sand baseline (- value). 5. Explain the detailed procedure of obtaining resistivity from SP Log.
After we have been able to determine the potential difference of SP-value (shale and sand baseline) from SP log, the water formation resistivity Rw can be obtained using the following procedure below: Determining the formation water resistivity (Rw): 1. Obtain the resistivity ratio of equivalent formation water resistivity Rweq and equivalent mud filtrate resistivity Rmfeq to the SP value for a particular zone at the given reservoir temperature (BHT) using a calculation or by graphic (Rmfeq / Rweq). 2. Determine whether Rmf > 0.1 @75°F by using resistivity nomograph for NaCl solutions. 3. If Rmf > 0.1 @75°F, then calculate the equivalent mud filtrate resistivity Rmfeq using equation Rmfeq = 0.85 Rmf@ BHT. 4. If Rmf < 0.1@75°F, then determine Rmfeq @BHT using chart SP-2 (Rw vs. Rweq and Formation temperature chart) with the calculated Rmf @75°F 4. Determine the equivalent formation water resistivity Rweq from the resistivity ratio given (number 1) graphically or using calculation. 5. Now, determine the formation water resistivity Rw using chart Sp-2 @BHT and estimated Rweq from step number 4. 6. What are main porosity-lithology logs. Explain in detail any one of the log. Elaborate the equations and describe the parameters. Define the difference of changing one parameter with respect to others. Popular methods of obtaining the formation porosity are sonic, neutron and density logs. Sonic Log – Is acoustic log that measure the speed of sound waves in subsurface formations. It determines the integrated travel time using Slowness Time Coherence (DT) of P, S and Stoneley waves in μsec./ft. Beside determining the porosity esp. in consolidated formation, it also indicates lithology, correlation with other wells, detect fractures and over-pressure, etc.
To derive the porosity from sonic log, a Wyllie-equation is used: Neutron Log – A continuous measurement of induced radiation produced in the formation with the neutron sources contained in the logging tool. These sources emit fast neutrons that are eventually slowed by collisions with hydrogen atoms until they are captured. The capture results in the emission of a secondary gamma ray (neutron - gamma log). Other tools detect epithermal/ thermal neutrons (neutron – neutron logs). Objective: Used to detect the gas in the formation by evaluating the hydrogen density or hydrogen index. Moreover, it also evaluates the porosity in the formation. Density Log – Density logging is a well logging tool that can provide a continuous record of a formation's bulk density (electron density) along the length of a borehole. Source emits gamma rays which are considered energy protons, interacting with electrons (Compton’s scattering) causing loss of energy. From the loss of energy, the density of the formation can be obtained. The high density formation absorbs many gamma rays, while low density formation absorbs fewer. Thus, the high-count rates at the detectors indicate low-density formations, whereas the low count rates indicate highdensity formations. To derive the porosity from the density curve, a calculation or graph can be used using the formula:
7. What are main electrical measurements carried out in a well. Describe the procedure and different source receiver configurations.
Conventional electric logs: Consists of two current electrodes A or B and the voltage measuring electrodes M and N Normal device Lateral device:
Procedure: With constant current I the measured potential at electrode M is direct proportional to the resistivity Ra. This type of tool measures the potential at electrode M with respect to an infinite electrode N (pole-pole-configuration), we call this type of tool a normal device. The tool measures the potential gradient, which for constant current is directly proportional to the resistivity, we call this tool a gradient tool or lateral device. The tool is often designated with the distances of the electrodes, Source receiver:
Laterolog-3: The laterolog-3 contains only three current electrodes, a centered electrode A0 and two long guard-electrodes. The vertical resolution of this tool is approximatly given by the length of A0. The sensitivity of the tool decreases with increasing resistivity of the formation (current → 0), the resolution is best for high conductive formations; therefore the log often is called conductivity-laterolog. Dual Laterolog: The Dual Laterolog (DLL) is a focusing laterolog tool containing 9 electrodes, 5 current electrodes and 4 potential electrodes. Microlaterolog: The microlaterolog contains two current electrodes and two potential electrodes located on a pad, pressed against the borehole wall. The electrodes are shaped as concentric circles, with current electrode A0 in the center and bucking electrode A1 as the outer circle. The depth of investigation of the microlaterolog is 5 . . . 10 cm. Inductionlog IL: The Inductionlog tool contains two coils, one transmitter-coil and one receiver-coil. 8. Describe the cased and open hole logs with example. Comment on the sources used.
Cased hole: Gamma ray log, Resistivity Induction Log, CBL Open hole: Acoustic log GR logs measure the natural gamma ray emission from subsurface formations. Because GR can path through steel casing, measurement can be made in both open and cased holes. Acoustic tools measure the speed of the sound waves in subsurface formations only employed in open holes. 9. What is borehole environment and how to get information from the true formation avoiding flushed zone and annulus.
Borehole environment refers to borehole-fluid and present of any tubing, casing, cement type, etc. The formation around the borehole is divided into 3 parts 1. Flushed zone 2. Transition zone 3. Uninvaded zone
The exchange of the fluid in the pores has large influence on the electrical behavior of the formation. The influence on other measurement mainly depends on mechanical destruction of the formation in the vicinity of the well bore. Only resistivity measurements exist for the measurement in the uninvaded zone and can distinguish between the difference zones in the borehole. 10. How gas pore filling effects to the sonic log, neutron log, density log and resistivity log?
Gas has a very marked effect on both density and neutron logs. Gas is light compared to oil causing density logging (gamma ray emitting sensors) based measurements to produce anomalous signals. Similarly, measurements that rely on detecting hydrogen (neutron emitting sensors) can miss detecting or correctly interpreting the presence of gas because of the lower hydrogen concentration in gas, compared to oil. , in the case of a reservoir where there is gas instead of water or oil in the pore space, the two porosity logs separate, to form what is referred to as gas crossover. Under these conditions, the true formation porosity lies between the measured neutron and density values. Log interpreters often find it difficult to accurately estimate the true formation porosity from these two curves. GAS --- Bulk density too low, density porosity too high GAS --- Neutron --- More hydrogen – lower neutron count -- porosity too low
11. Describe the term porosity and density and name the unity in which they are applied: Porosity: Porosity is the percentage of void space in a rock. It is defined as the ratio of the volume of the voids or pore space divided by the total volume. It is written as either a decimal fraction between 0 and 1 or as a percentage. For most rocks, porosity varies from less than 1% to 40%.
The porosity of a rock depends on many factors, including the rock type and how the grains of a rock are arranged. For example, crystalline rock such as granite has a very low porosity (<1%) since the only pore spaces are the tiny, long, thin cracks between the individual mineral grains. Sandstones, typically, have much higher porosities (10–35%) because the individual sand or mineral grains don’t fit together closely, allowing larger pore spaces. Density: The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by volume:
where ρ is the density, m is the mass, and V is the volume.
The relationship between density and porosity is an indirect proportionality. The higher the density of a rock, the lower the porosity of that rock. In a more explicit way, the weight of over-lying rocks increase the density of the rock beneath, which in turn decreases the amount of pore spaces (porosity) of the rock through processes such as compaction, cementation, etc. 12. Which methods can be used to measure porosity? Which equations do you know to calculate porosity from other petrophysical parameters? a) Sonic acoustic log (Wyllie equation):
b) Density log
c) Neutron log (CNL): No formula d) indirectly, Electrical resistivity log : but we have to determine the water saturation first 13. How can the permeability be measured? Which types of logs can be used? Permeability can be measured using empirical correlation with porosity (e.g. acoustic/sonic log measurement, SP log, induction resistivity log, caliper log, microlog) and using NMR or MDT log. The separation between resistivity curve in the depth of investigation is an indicator of filtrate invasion, that can only happen in permeable zones Filter cake can be built only in the permeable zone. The caliper log and electrical microlog can determine the present of filter cake. Nuclear magnetic resonance (NMR) logging is a type of Nuclear logging (LWD) that uses the NMR response of a formation to directly determine its porosity and permeability, providing a continuous record along the length of the borehole. NMR techniques are typically used to predict permeability for fluid typing and to obtain formation porosity, which is independent of mineralogy. The former application uses a surface-relaxation mechanism to relate measured relaxation spectra with surface-to-volume ratios of pores, and the latter is used to estimate permeability. 14. What is the difference between density and litho density log? What else values are measured in litho density log? Density log: Density logging is a well logging tool that can provide a continuous record of a formation's bulk density (electron density) along the length of a borehole. Source emits gamma rays which are considered energy protons, interacting with electrons (Compton’s scattering) causing loss of energy. From the loss of energy, the density of the formation can be obtained. The high density formation absorbs many gamma rays, while low density formation absorbs fewer. Thus, the high-count rates at
the detectors indicate low-density formations, whereas the low count rates indicate high-density formations.
Litho density log: The modern formation density determination is more accurate than the basic formation density tool because the harder gamma rays are less prone to decrease by borehole effects. Two effects: Compton scattering and Photo-electrical effect give specifically reaction in the formation. The Compton scattering effect is the reaction between gamma radiation and electron in the rock. The gamma radiation will be deflected and slowed/attenuated after the reaction together with electron. The photo-electric effect will absorb the gamma radiation completely. This effect can be used to provide a parameter which is dependent upon the atomic number of the formation, and therefore immensely useful in lithological recognition. Pe = the photo-electric absorption index (barns/electron) se = the photo-electric cross-section (barns) Z = the atomic number (number of electrons) K = a coefficient dependent upon the energy at which the photo-electric The lower Pe, the lowerabsorption the meanisatomic number of the formation. observed (no units). Moreover, litho density log is used to determine the mineralogical Composition of the formations and Detection of fractures fractures can be recognized by their high values for Pe, building peaks on the Pe-curve. 15. Describe physical origin of SP in a well. Which qualitative and quantitative amount can you get from the value? Physical origin: Liquid junction or diffusion potential: the difference in the conductivity between the mud filtrate and the formation water. Membrane potential: Clay minerals are negatively charged at their surface, so that the Na+ ions from solution are attracted and allowed to pass through the shale, but the Cl− ions are repelled. Filtration potential: results from the forced flow of an electrolyte with resistivity ρel and viscosity through a porous medium Quantitative usage of SP log: - Formation water resistivity determination by knowing the mud filtrate resistivity and formation temperature (BHT) - Shale volume by knowing the shale line Qualitative usage of SP log:
- Indicate the permeability by knowing the shale line - Shalliness by knowing the clay and grain size
16. What method can be used to determine the water saturation behind the casing? Explain the principle? SP log and resistivity induction log (Cased Hole Formation Resistivity Tool). The CHFR gives deep-reading resistivity measurements from behind steel casing. It introduces current to the casing so that extremely small amounts escape into the formation. The resulting voltage drop is measured by tool electrodes and used to determine formation resistivity or truth resistivity Rt. Formation resistivity by 100% water saturated Ro is determined by graph or calculation using equation: Ro = FR x Rw
The calculation of Sw behind the casing uses following Archie equation or graph:
17. Why do we use spectral gamma ray? Does it helps you to calculate the clay content and how do you do this? Yes, see question 2 18. Read the attached logs and determine the lithology, porosity, resistivity and water/hydrocarbon saturation Excercise!
