Q921 log lec4 v1
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This document provides an overview of well logging concepts and techniques. It discusses key well log formats and presentations, including standard tracks used for depth, resistivity, porosity, and other measurements. Common logs like SP, gamma ray, resistivity, neutron, and density are examined. Signatures indicating lithology, fluid content, and invasion are described. Proper interpretation requires understanding scales, crossovers caused by lithology changes, and matrix effects. An example of logs in a horizontal well is also provided.
![The signature of shale on
a neutron and density combination log
Shale produces
the opposite effect
[rather than gas]
the neutron porosity
may far exceed
the density porosity
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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Q921 log lec4 v1
- 1. Well Logging Course (1st Ed.)
- 2. 1. Rudimentary definitions 2. hydrocarbons presence determination 3. hydrocarbons quantity and recoverability determination 4. The Borehole Environment
- 3. 1. Reading A Log 2. Examples of Curve Behavior And Log Display 3. Electrical Properties Of Rocks And Brines
- 5. Standard log presentation formats Reading a log with ease requires familiarity with some of the standard log formats. The formats for traditional logs and most field logs are shown in Figure. It contains three tracks. A narrow column containing the depth is found between track 1 and tracks 2 and 3. The latter are contiguous Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 5
- 6. Different scale types In the normal linear presentation, the grid lines in all three tracks having linear scales each with ten divisions In the logarithmic scale We have logarithmic presentation for tracks 2 and 3 Four decades are drawn to accommodate the electrical measurements, which can have large dynamic ranges scale begins and ends on a multiple of two rather than unity In a hybrid scale We have a logarithmic grid on track 2 and a linear in track 3 Electrical measurements that may spill over from track 2 into track 3 will still be logarithmic even though the indicated scale is linear Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 6
- 7. SP and GR log headings used for clean formation determination Figure shows the typical log-heading presentation for several of the basic logs. The upper two presentations show two variations for SP, which is always in track 1. the SP decreases to the left The bottom presentation shows the caliper, a one-axis measurement of the borehole diameter, the gamma ray, which are also generally presented in track 1. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 7
- 8. clean sections determination The rule given for finding clean sections was that the SP becomes less negative for increasing shale, so that deflections of the SP trace to the right will correspond to increasing shale content The GR curve, as it is scaled in increasing activity (in American Petroleum Institute (API) units) to the right, will also produce curve deflections to the right for increasing shale content. Thus the two shale indicators can be expected to follow one another as the shale content varies. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 8
- 9. The induction log heading and schematic of the formation Although modern tools have a larger selection of curves with different depths of investigation, the displays are similar A traditional resistivity log heading along with a schematic indication of the zones of investigation is shown in the figure three zones corresponding approximately to the simultaneous electrical measurements of different depths of investigation Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 9
- 10. dual induction-SFL The particular tool associated with this format (previous slide) is referred to as the dual induction-SFL and will normally show three resistivity traces (units of ohm-m) The trace coded ILD (induction log deep) the deepest resistivity measurement and correspond to Rt when invasion is not severe The curve marked ILM (induction log medium) is an auxiliary measurement of intermediate depth of penetration and is highly influenced by the depth of invasion The third curve, in this case marked SFLU (spherically focused log), is a measurement of shallow depth of investigation and reads closest to the resistivity of the invaded zone Rxo. By combining the three resistivity measurements, it is possible, in many cases, to compensate for the effect of invasion on the ILD reading Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 10
- 11. Log headings for three porosity devices The top two correspond to two possible formats for simultaneous density and neutron logs The porosity is expressed as a decimal (v/v) or in porosity units (p.u.), each of which corresponds to 1% porosity The bottom is the sonic log format It is with the apparent transit time Δt increasing to the left. In all three presentations, the format is such that increasing porosity produces curve deflections to the left Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 11
- 12. matrix setting in neutron and density logs For the neutron and density logs, another point to be aware of is the matrix setting This setting corresponds to a rock type assumed in a convenient pre-interpretation that establishes the porosity from the neutron and density device measurements the matrix setting SS, means that the rock type is taken to be sandstone If the formations being logged are indeed sandstone, • then the porosity values recorded on the logs will correspond closely to the actual porosity of the formation if the actual formation matrix is different, say limestone, • then the porosity values will need to be shifted or corrected in order to obtain the true porosity in this particular matrix Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 12
- 14. An SP log over a clean section bounded by shales shale sections The intervals of high SP above 8,500 ft and below 8,580 ft The value of the typical flat response is called the shale base line Sections of log with greater SP deflection (with a more negative value than the shale base line) are taken as clean, or at least cleaner, zones One clean section is the zone between 8,510 and 8,550 ft Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 14
- 15. A GR and caliper log over the same section as previous slide Note the similarity between the GR trace and the SP trace GR (solid) In the clean sections, the gammy ray reading is on the order of 15 to 30 API units, while the shale sections may read as high as 75 API units the caliper (broken) It follows much of the same trend as GR because the shale sections can “wash out,” • increasing the borehole size compared to the cleaner sand sections that retain their structural integrity Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 15
- 16. An induction log over a water zone with a HC zone above it The shallow, deep, and medium depth resistivity curves are indicated. The zone below 5,300 ft is possibly water, Assuming the resistivity of the formation water is much less (i.e., the water is much more saline) than the resistivity of the mud Mud resistivity effect: the shallow resistivity curve, which for the most part stays around 2 ohm-m Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 16
- 17. An induction log over a water zone with a HC zone above it (Cont.) At 5,275 ft, a possible hydrocarbon zone ILD is much greater than in the supposed water zone However, this increase in resistivity may not be the result of hydrocarbon presence. A decrease in porosity could produce the same effect for a formation saturated only with water The real clue here is that even though the Rxo reading has also increased (means the porosity has decreased), there is less of a separation between the Rxo and Rt curves than in the water zone. This means that the value of Rt is higher than should be expected from the porosity change alone. By this plausible chain of reasoning, we are led to expect that this zone may contain hydrocarbons. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 17
- 18. Sample neutron and density logs the density-porosity estimate (φd , or DPHI, on the log heading), in solid, the dotted neutron porosity, the compensation curve Δρ (or DRHO) (The auxiliary curve Δρ) indicates little borehole irregularity is the correction which was applied to the density measurement in order to correct for mudcake and borehole irregularities It can generally be ignored if it hovers about zero, as is the case at certain depths. Note, once again, the built-in assumption that the matrix is sandstone. Density and neutron derived porosity equality: the presence of liquid-filled sandstone is confirmed. (for the 20 ft section below 700 ft) Density and neutron derived porosity separation: caused by an error in the assumed matrix or by the presence of clay or gas Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 18
- 19. A neutron and density log exhibiting gas in the formation presence of gas from a comparison of the neutron and density logs. With gas in the pores the formation density is less than with oil or water, so that the apparent density porosity is higher. At the same time the hydrogen content of gas is less than oil or water so the neutron porosity is lower. Thus, in the simplest of cases, gas is indicated in any zone in which the neutron porosity is less than the density porosity. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 19
- 20. The signature of shale on a neutron and density combination log Shale produces the opposite effect [rather than gas] the neutron porosity may far exceed the density porosity Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 20
- 21. Neutron and density crossover caused by changes in lithology All of these generalities are true only if the principal matrix corresponds to the matrix setting on the log. The effect of having the wrong matrix setting on the log (or having the matrix change as a function of depth) is shown in Fig figure. Several sections show negative density porosity. These are probably due to anhydrite streaks, • which, because of their much higher density, are misinterpreted as a negative porosity. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 21
- 22. An example of an LWD log in a horizontal well In track 1 is the familiar GR along with three curves indicating the time delay between drilling and the three types of measurements made; depth track the tool rotation rate is there Track 2 contains two types of resistivity measurements, each with multiple depths of investigation that overlay in this example. The third track contains the LWD versions of the neutron measurement (TNPH), the density measurement (ROBB), and the density correction (DRHB). Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 22
- 23. A basic set of logs for performing a wellsite interpretation clean and possibly permeable zones identification an inspection of the SP and GR four clean, permeable zones labeled A through D resistivity readings are contained in the second track. What is the fluid in each zone? the lowest resistivity values =water Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 23
- 26. electrical property measurements An important component of the well logging suite is the measurement of electrical properties of the formation. These measurements deal with • the resistivity of the formation or • the measurement of spontaneously generated voltages. o These voltages are the result of an interaction between the borehole fluid and the formation with its contained fluids. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 26
- 27. spontaneous potential Historically, the first logging measurements were electrical in nature. The first log was a recording of the resistivity of formations as a function of depth and was drawn painstakingly by hand. Unexpectedly, in the course of attempting to make other formation resistivity measurements, “noise” was repeatedly noted and was finally attributed to a spontaneous potential. • It seemed most notable in front of permeable formations. Both of these measurements are still performed on a routine basis today. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 27
- 28. Resistivity Resistivity is a general property of materials, as opposed to resistance, which is associated with the geometric form of the material the dimensions of resistivity are ohms-m2/m, or ohm-m The units of its reciprocal, conductivity, are Siemens per meter. In well-logging, milli Siemens per meter (mS/m) a material of resistivity 1 ohm-m with dimensions of 1 m on each side will have a total resistance, face-to-face, of 1 ohm. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 28
- 29. Resistivity measurement Thus a system to measure resistivity would consist of a sample of the material to be measured contained in a simple fixed geometry. If the resistance of the sample is measured, the resistivity can be obtained from the relation: which becomes, using Ohm’s law: This constant k, referred to as the system constant, converts the measurement of a voltage drop V, for a given current I , into the resistivity of the material. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 29
- 30. A schematic diagram of a mud cup for determination of its resistivity A current, I , is passed through the sample of drilling fluid and the corresponding voltage, V, is measured. the system constant can be calculated to be 0.012 m. The resistivity, ρ, in ohm-m, is then obtained from the measured resistance R by: a sample of salt water with a resistivity of 2 ohm-m in the chamber would yield a total resistance of 166 ohms Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 30
- 32. Resistivity values There are two general types of conduction: electrolytic and the mechanism is dependent upon the presence of dissolved salts in a liquid • such as water electronic Examples of electronic conduction are provided by metals, which are not covered here Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 32
- 33. Resistivity in different materials Notice the range of resistivity variation for salt water, which depends on the concentration of NaCl. Typical rock materials are in essence insulators. The fact that reservoir rocks have any detectable conductivity is usually the result of the presence of electrolytic conductors in the pore space. The conductivity of clay minerals is also greatly increased by the presence of an electrolyte. In some cases, the resistivity of a rock may result from the presence of metal, graphite, or metal sulfides. Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 33
- 34. sedimentary rocks resistivity the resistivity of formations of interest may range from 0.5 to 103 ohm-m, nearly four orders of magnitude. The conductivity of sedimentary rocks is primarily of electrolytic origin. It is the result of the presence of water or a combination of water and hydrocarbons in the pore space as a continuous phase will depend on the resistivity of the water in the pores and the quantity of water present. To a lesser extent, it will depend on lithology of the rock matrix, its clay content, and its texture (grain size and the distribution of pores, clay, and conductive minerals). will depend strongly on temperature Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 34
- 35. Determination of the resistivity of an NaCl solution f(NaCl concentration, T) the resistivity of saltwater (NaCl) solutions is a function of the electrolyte concentration and temperature G/G is grains per gallon Fall 13 H. AlamiNia Well Logging Course: Getting familiar with logs 35
- 36. 1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007. Chapter 2 and 3