Well logging part by Mr lauben Twinomujuni

this is a pdf that takes about well logging

1. PGP3105: WELL LOGGING By Mr. Lauben Twinomujuni , BSc. MSc. Geology DGPS – Makerere University

2. DGPS – Makerere University Well logging basics Basic relationships of well log interpretation Spontaneous potential log Resistivity logs Gamma ray log Porosity logs Content 2

3. DGPS – Makerere University Defn: A well log is a Well log is a continuous record of measurement made in bore hole in response to variation in some physical properties of rocks through which the bore hole is drilled. The Basics 3

4. DGPS – Makerere University 1912 Conrad Schlumberger gave the idea of using electrical measurements to map subsurface rock bodies. In 1919 Conrad Schlumberger and his brother Marcel began work on well logs. The first electrical resistivity well log was taken in France, in 1927. In 1929 the electrical resistivity logs were introduced on commercial scale in Venezuela, USA and Russia. History 4

5. DGPS – Makerere University The photographic – film recorder was developed in 1936. The dip meter log was developed in 1930 The Gamma Ray and Neutron Logs were begun in 1941 Nowadays , there are many sophisticated tools that are run and the interpretation has greatly improved. History… 5

6. DGPS – Makerere University Logging units 6

7. DGPS – Makerere University Logging units… Logging service companies utilize a variety of logging units, depending on the location (onshore or offshore) and requirements of the logging run. Each unit will contain the following components: Logging cable Winch to raise and lower the cable in the well Generator 7

8. DGPS – Makerere University Set of surface control panels Set of downhole tools Digital recording system Logging units… 8

9. DGPS – Makerere University 9

10. DGPS – Makerere University Depth to lithological boundaries Lithology identification Minerals typing Inter-borehole correlation Structure mapping Dip determination Applications 10

11. DGPS – Makerere University In-situ stress orientation Fracture frequency Porosity Fluid salinity Applications… 11

12. DGPS – Makerere University Depth Of Investigation Vs. Resolution 12

13. DGPS – Makerere University Log interpretation objectives The objective of log interpretation depends very much on the user. Quantitative analysis of well logs provides the analyst with values for a variety of primary parameters, such as: Porosity Water saturation, fluid type (oil/gas/water) Lithology P ermeability 13

14. DGPS – Makerere University From the above parameters , many corollary parameters can be derived by integration (and other means) to arrive at values for: Hydrocarbons-in-place. Reserves (the recoverable fraction of hydrocarbons in-place ). Mapping reservoir parameters. Log interpretation objectives… 14

15. DGPS – Makerere University But not all users of wireline logs have quantitative analysis as their objective. Some may be concerned with the geological and geophysical aspects . These users are interested in interpretation for: Well-to-well correlation Facies analysis Regional structural and sedimentary history   Log interpretation objectives… 15

16. DGPS – Makerere University In qualitative log analysis, the objective is to define: The type of reservoir (lithology) Its storage capacity (porosity) Its hydrocarbon type and content (saturation) Its producibility (permeability) Log interpretation objectives… 16

17. DGPS – Makerere University Logging While Drilling (LWD) Open hole Wireline logging Cased hole wireline logging Basic Logging Types 17

18. DGPS – Makerere University Traditionally , petrophysicists were concerned only with wireline logging, that is, the data acquired by running tools on a cable from a winch after the hole had been drilled. However, advances in drilling/logging technology have allowed the acquisition of log data via tools placed in the actual drilling assembly . Logging While Drilling (LWD) 18

19. DGPS – Makerere University These tools may transmit data to the surface on a real-time basis or store the data in a downhole memory from which it may be downloaded when the assembly is brought back to the surface. Logging While Drilling (LWD)… 19

20. DGPS – Makerere University Real-time information is required for operational reasons, such as steering a well. Acquiring data prior to the hole washing out or invasion occurring. Safeguarding information if there is a risk of losing the hole. The trajectory is such as to make wireline acquisition difficult (e.g., in horizontal wells). LWD - Justification 20

21. DGPS – Makerere University Drilling mode Memory size Tool failure Battery life LWD - Limitations 21

22. DGPS – Makerere University Gamma ray Density Neutron porosity Sonic Resistivity LWD - Tools 22

23. DGPS – Makerere University Gamma ray Natural gamma ray spectroscopy Spontaneous potential (SP) Caliper Density Neutron porosity Full-waveform sonic Wire line Openhole Logging 23

24. DGPS – Makerere University Resistivity Microresistivity Borehole Imaging Formation pressure/sampling Sidewall sampling Sidewall coring NMR Wire line Open hole Logging… 24

25. DGPS – Makerere University Thermal decay tool Gamma ray spectroscopy tool (GST ) Production logging Cement bond log (basically acoustic log): Casing collar locator (CCL ) Wireline Cased Hole Logging 25

26. DGPS – Makerere University Typically, an oil company will set up logging contracts with one or more contractors for the provision of logging services. Usually some kind of tendering process is used to ensure competitive bidding among various companies able to provide such services. The service company which wins the tender will sign a contract with the oil company. This contract will clearly stipulate the terms and conditions (terms of reference). Logging Contracts 26

27. DGPS – Makerere University At the FDP stage in the development of a field, the outline of the logging strategy should be developed. Based on the type of well being proposed, decisions have to be made about whether to go principally for a LWD type of approach or conventional wireline and about the types of tool to be run . Preparing a logging program 27

28. DGPS – Makerere University In general, early in the life of a field, particularly during the exploration phase, data have a high value, since they will be used to quantify the reserves and influence the whole development strategy . Moreover , lack of good-quality data can prove to be extremely expensive, particularly for offshore developments, if facilities are designed that are either too big or too small for the field. Preparing a logging program … 28

29. DGPS – Makerere University Later in the life of a field , particularly in tail-end production, the value of log data becomes much less . The FDP should lay down the broad strategy for data acquisition. Discussions should take place between the petrophysicist and the geologist about the need for the different logging runs and coring, and the various analyses that will be performed once the logs and cores are acquired. Preparing a logging program … 29

30. DGPS – Makerere University For a particular well, the detailed logging requirements will first be specified within the well proposal, which will be agreed upon with other partners and any government supervisory bodies. The proposal will specify the general types of tools for the individual hole sections. There will be many items that are conditional on hydrocarbons being encountered, based on shows encountered while drilling. Preparing a logging program … 30

31. DGPS – Makerere University During the drilling of a well there will typically be a mud-logging unit on the rig. This unit has two main responsibilities: To monitor the drilling of the parameters and gas/liquids/solids returns from the well to assist the drilling department in the safety and optimization of the drilling process. Wellsite mud logging 31

32. DGPS – Makerere University To provide information to the petroleum engineering department that can be used for evaluation purposes. Wellsite mud logging… 32

33. DGPS – Makerere University Typically the mud-logging unit will produce a daily “mud log,” which is transmitted to the oil company office on a daily basis. Items that will be included are: Gas readings as measured by a gas detector/chromatograph . A check for absence of poisonous gases (H 2 S, SO 2 ) A report of cuttings received over the shale shakers, with full lithological descriptions and relative percentages. Wellsite mud logging… 33

34. DGPS – Makerere University ROP Hydrocarbon indications in samples. The mud log may be of great use to the petrophysicist and geologist in operational decision making and evaluation. Areas in which the mud log may be particularly important include : Identification of the lithology and formation type being drilled. Identification of porous/permeable zones. Wellsite mud logging… 34

35. DGPS – Makerere University Picking of coring, casing, or final drilling depths. Confirmation of hydrocarbons being encountered and whether they are oil or gas. Wellsite mud logging… 35

36. DGPS – Makerere University Cuttings Description The mud-logging unit will generally take a sample of the cuttings received over the shale shakers at regular time intervals, calculated to correspond to regular changes in formation depth (e.g., every 5m). Wellsite mud logging… 36

37. DGPS – Makerere University Some of these samples are placed into sealed polythene bags as “wet samples” and retained . Other samples are washed, dried, and retained as “dry samples.” Washed samples are examined under a microscope in the mud-logging unit and a description made that may be communicated to the office . Wellsite mud logging… 37

38. DGPS – Makerere University In order for the information received from the rig to be useful, it is essential that rigid standards for reporting are followed that are agreed upon between the rig and the office. Standards will typically vary among companies. Items that should be included are: Grain properties Texture (muddy/composite ) Wellsite mud logging… 38

39. DGPS – Makerere University Type ( pelletoid / micropelletoid ) Color Roundness , or sphericity Sorting Hardness Size Additional trace minerals (e.g., pyrite, calcite, dolomite, siderite) Wellsite mud logging… 39

40. DGPS – Makerere University Carbonate particle types Skeletal particles (fossils, foraminifera) Nonskeletal particles ( lithoclasts , aggregates, rounded particles) Coated particles Porosity and permeability hydrocarbons Wellsite mud logging… 40

41. DGPS – Makerere University Hydrocarbon detection in the mud logging unit: Natural fluorescence Solvent cut Acetone test Visible staining Odor HC gas detection through chromatography or gas filament Wellsite mud logging… 41

42. DGPS – Makerere University Rock properties Borehole environment and resistivity profiles Basic information needed for log interpretation Basic relationships of well log interpretation 42

43. DGPS – Makerere University Rock properties/characteristics that affect logging measurements: Porosity Lithology Mineralogy Permeability Water saturation Rock properties 43

44. DGPS – Makerere University Borehole Environment and Resistivity In log interpretation , the HCs, the rock, and fresh water in the formation are assumed to act as insulators. Salt water has a low resistivity. Resistivity is measured by electric logs, commonly known (in the west) as later-logs and induction logs . From Archie, R o = FxR w R o= resistivity of water-filled formation 44

45. DGPS – Makerere University F = resistivity factor R w = Resistivity of saline water Still from Archie, F=a/ φ m ……………………………………………………… i m is cementation exponent. a is the tortuosity factor . Borehole Environment and Resistivity… 45

46. DGPS – Makerere University The F value varies with grain size , grain-size distribution , and the complexity of the paths between pores (tortuosity ). a is commonly set to 1.0 but can be allowed to vary by some petrophysicists . Water saturation (S w ) is determined from water-filled resistivity (R o ) and the actual (true) formation resistivity (R t ) by : Borehole Environment and Resistivity… 46

47. DGPS – Makerere University S w =( R o / R t ) 1/n …………………………………………….ii n is saturation exponent and varies from 1.8-2.5 but commonly assumed to be 2 . Still from Archie, R o = FxR w …………………………………………..ii By combining the equations above, S w =( axR w / R t x φ m ) 1/n Borehole Environment and Resistivity… 47

48. DGPS – Makerere University Borehole Environment and Resistivity… 48

49. DGPS – Makerere University The rock plus its fluids are affected by drilling mud in the vicinity of the borehole. This affects the logging measurements. Borehole environment 49

50. DGPS – Makerere University Important symbols used Hole Diameter: (d h ) Determined by outside diameter of the drill bit. But the borehole size may be : In gauge: borehole diameter is equal to drilling bit size. Borehole reduction: mud cake buildup, precursor to permeable formation Borehole enlargement: swelling and sloughing of shales , collapse of poorly cemented porous rock, dissolution of salts, evaporates. Borehole Environment… 50

51. DGPS – Makerere University Commonly borehole sizes normally vary from 7 7 / 8 in,. To 12 in., and modern logging tools are designed to operate within these size ranges. The size of the borehole is measured by caliper log . Drilling mud Resistivity (R m ) Mud is used as a circulation fluid. Help to: remove cuttings, Borehole Environment… 51

52. DGPS – Makerere University lubricate and cool the bit, and maintain an excess of borehole pressure over formation pressure (to prevent blowouts). The excess mud pressure forces some of the drilling fluid to invade porous and permeable formations. As invasion occurs, many of the solid particles (i.e., clay minerals from the drilling mud) are trapped on the side of the borehole and form mud cake (with resistivity, R mc ). Borehole Environment… 52

53. DGPS – Makerere University Fluid that filters into the formation (mud filtrate) has resistivity R mf . The resistivity values for drilling mud, mud cake, and filtrate are recorded on a log’s head and are used for interpretation . Invaded zone The zone in which much of the original fluid is replaced by mud filtrate is called the invaded zone . It is divided into flushed and transition/annulus zone. Borehole Environment… 53

54. DGPS – Makerere University The depth of mud filtrate invasion into the invaded zone is referred to as diameter of invasion (d i and d j ). The diameter of invasion is measured in inches or expressed as a ratio: d j /d h (d h is borehole diameter). The amount of invasion depends on the permeability of the mud cake not on the porosity of the rock. The diameter of inversion depends on the porosity of the rock. Borehole Environment… 54

55. DGPS – Makerere University Generally, D j /d h =2 ; for high-porosity rocks D j /d h =5; for intermediate-porosity rocks; and D j /d h =10; for low-porosity rocks. Borehole Environment… 55

56. DGPS – Makerere University Diagramatic, theoretical, cross-sectional views of subsurface conditions moving away from the borehole into a formation. They illustrate the horizontal distributions of the invaded and uninvaded zones and their corresponding relative resistivities . Invasion and Resistivity Profiles 56

57. DGPS – Makerere University Invasion and Resistivity Profiles… 57

58. DGPS – Makerere University Transition profile – the more realistic model. Three resistivities are measured in a transition model; R xo (flushed zone resistivity) Ri (resistivity of the transition zone) and Rt (uninvaded zone resistivity) Invasion and Resistivity Profiles… 58

59. DGPS – Makerere University Exercise 1 Sketch: Resistivity profiles for Water-bearing zones for: Oil based muds Salt water based muds Hydrocarbon -bearing zones Salt water based muds Oil based muds Invasion and Resistivity Profiles… 59

60. DGPS – Makerere University Lithology: in quantitative log analysis, there are several reasons why it is important to know lithology (i.e., sandstone, limestone, or dolomite ). Porosity logs require a lithology or a matrix constant before the porosity of the zone can be calculated. The formation factor used to calculate water saturation also changes with lithology. Basic Information Needed in log Interpretation 60

61. DGPS – Makerere University Formation temperature Formation temperature ( T f ) is important in log analysis, because the resistivities of the drilling mud ( R m ), the mud filtrate ( R mf ), and the formation water resistivity ( R w ) vary with temperature. Temperature of the formation is determined by knowing: Formation depth Bottom hole temperature (BHT) Basic Information Needed in log Interpretation… 61

62. DGPS – Makerere University Total depth of the well (TD) Surface temperature A reasonable value of the formation temperature can be determined by using these data and by assuming a linear geothermal gradient (fig. bellow) Basic Information Needed in log Interpretation… 62

63. DGPS – Makerere University 63

64. DGPS – Makerere University The formation temperature is also calculated by using the equation of the line: Y= mx+c Where: X= depth y= temperature M=slope (geothermal gradient) C= a constant (mean annual surface temperature) Basic Information Needed in log Interpretation… 64

65. DGPS – Makerere University After the formation temperature is determined either by chart or by calculation, the resistivities of the different fluids (R m , R mf , or R w ) can be corrected to formation temperature. Fig. below is a chart that is used for correcting fluid resistivities to the formation temperature. The chart is closely approximated by the Arp’s formula. Basic Information Needed in log Interpretation… 65

66. DGPS – Makerere University 66

67. DGPS – Makerere University R TF = R temp (Temp+6.77)/ (T f +6.77) R TF = R temp (Temp+21.0)/(T f +21.0) for depth in meters. R TF = resistivity at formation temperature R temp = resistivity at temperature other than formation temperature. Temp = temperature at which resistivity was measured ( usually Fahrenheit for depth in feet, Celsius for depth in meters) Borehole Environment and Resistivity… 67

68. DGPS – Makerere University Given: Surface temperature = 80 F Bottom hole temperature (BHT) = 200 F Total Depth (TD) = 10,000ft Formation depth = 7000ft Using both chart and equation, find out formation temperature. Exercise 2 68

69. DGPS – Makerere University Given: Resistivity of drilling mud (R m ) = 1.2 ohm-m at 75 F Formation temperature = 160 F Find R m at formation temperature using equation and chart . Exercise … 69

70. DGPS – Makerere University First run in 1931. One of the earliest measurements used in the petroleum industry, and it has continued to play a significant role in well log interpretation. Primarily, the SP log is used for determining gross lithology (i.e., reservoir vs. non-reservoir ) Spontaneous potential log 70

71. DGPS – Makerere University The SP log is a record of direct current (DC) voltage or potential that develops between a movable electrode in the wellbore and a fixed electrode located at the surface. It is measured in millivolts (mV). Electric voltages arising primarily from electrochemical factors within the borehole and the adjacent rock create the SP response. Spontaneous potential log… 71

72. DGPS – Makerere University These electrochemical factors are brought about by the differences in salinities between mud filtrate and formation water within the permeable beds. Because a conductive fluid is needed in the borehole for the generation of these voltages, the SP cannot be used in nonconductive (e.g., oil-base) drilling muds or in air-filled holes. Spontaneous potential log… 72

73. DGPS – Makerere University Principle SP arises due to salinity contrast between formation water and mud filtrate against permeable beds. No current is sent into the formation. The SP log is recorded by measuring the potential difference in milli -volts between an electrode in the borehole and a grounded electrode at the surface. Spontaneous potential log… 73

74. DGPS – Makerere University The change in voltage through the well bore is caused by a build up of charge on the well bore walls. Shales and clays will generate one charge and permeable formations such as sandstone will generate an opposite one. This build up of charge in turn is caused by differences in salt content and formation water. Spontaneous potential log… 74

75. DGPS – Makerere University Applications Detect permeable beds Detect boundaries of permeable beds Determine formation-water resistivity Determine the volume of shale in permeable beds . Spontaneous potential log… 75

76. DGPS – Makerere University Static Spontaneous potential, SSP The concept of the static spontaneous potential (SSP) is important because SSP represents the maximum SP that a thick, shale-free, porous, and permeable formation can have for a given ratio between R mf and R w . SSP is determined by formula or chart and is a necessary element for determining accurate values of R w and volume of shale. Spontaneous potential log… 76

77. DGPS – Makerere University Formation Water Resistivity ( R w ) Determination Procedure: After determining the formation temperature, correct resistivities of the mud filtrate and drilling mud (obtained from log heading) to formation temperature. To minimize the effect of bed thickness, the SP is corrected to static SP (SSP). Spontaneous potential log… 77

78. DGPS – Makerere University The data needed are: Bed thickness, Resistivity from the shallow-reading resistivity tool ( R i ), The resistivity of the drilling mud at formation temperature Once the value of SSP is determined, it is used on the chart to obtain a value for the R mf / R we ratio. Spontaneous potential log… 78

79. DGPS – Makerere University Equivalent resistivity ( R we ) is obtained by dividing R mf by the R mf / R we value from the chart. The value of R we is then corrected to R w using the chart for average deviation from sodium chloride solutions, and for the influence of formation temperature. Note: R w can also be got from equations: Normally the SP curve has less deflection in HC-bearing zones (HC-suppression). Spontaneous potential log… 79

80. DGPS – Makerere University EXERCISE 3 Determine the formation water resistivity, R w from the given data. R mf = 0.51 @ 135 F (BHT) R m = 0.91 @ 135 F (BHT) Surface Temperature = 60 F Total Depth (TD) = 8007 ft Bottom hole Temperature (BHT) = 135 o F Spontaneous potential log… 80

81. DGPS – Makerere University Shale Volume calculation The volume of shale in a sand can be used in the evaluation of shaly sand reservoirs and a mapping parameter for both sandstone and carbonate facies analysis. The SP log can be used to calculate the volume shale in a permeable zone by: V shale = 1-(PSP/SSP) Spontaneous potential log… 81

82. DGPS – Makerere University Where: V shale is volume of shale PSP is pseudostatic SP (maximum SP of shale formation). Spontaneous potential log… 82

83. DGPS – Makerere University Basics about the Resistivity: Resistivity measures the electric properties of the formation, The ability to conduct electric current depends upon : The Volume of water, The Temperature of the formation , The Salinity of the formation Clays and conducting minerals Geologic Strike Resistivity Logs 83

84. DGPS – Makerere University Application Determine HC-bearing vs. water-bearing zones- most important use by far. Indicate permeable zones Determine porosity Resistivity logs… 84

85. DGPS – Makerere University Because rock matrix or grains are nonconductive and HCs present are nonconductive too, the ability of a rock to transmit electric current is entirely dependent upon the water present in the pores. As HC saturation increases (water saturation decreases), the resistivity increases. Resistivity Logs… 85

86. DGPS – Makerere University As salinity of the water decreases, resistivity increases. From φ (from porosity logs), a, m, n and R t , a geologist is able to determine formation’s water saturation S w from the Archie equation: S w = ( a.R w /R t . φ m ) 1/n R t is got from a deep reading resistivity log. Resistivity logging tools produce a current in the adjacent formation and measure the response of the formation to that current. Resistivity Logs… 86

87. DGPS – Makerere University Current can be produced and measured from two methods: Electrode tools (also called galvanic devices or, for presently available versions, laterologs ) have electrodes on the surface of the tool to emit current and measure the resistivity of the formation . Induction tools use coils to induce a current and measure the formation’s conductivity. Resistivity Logs… 87

88. DGPS – Makerere University Resistivity Logs… 88

89. DGPS – Makerere University Lateral logs These are designed to measure formation resistivity in boreholes filled with saltwater muds ( R mf ~ R w ). Laterolog consists of a single laterolog measurement and sometimes a microlaterolog measurement. It has a linear scale. Resistivity Logs… 89

90. DGPS – Makerere University Dual Laterolog consists of deep-reading measurement (R LLD ) and a shallow reading measurement (R LLS ). These are displayed on a four-cycle logarithmic scale ranging from 0.2-2000 ohm-m. The third resistivity measurement is the micro spherically focused resistivity (R MSFL ) (for very shallow depth). Resistivity Logs… 90

91. DGPS – Makerere University When these three resistivity measurements (shallow deep and very shallow) are combined, the deep laterolog can be corrected for invasion effects to produce R t . A tornado chart is used to graphically correct R LLD to R t and to determine the diameter of invasion d i and the ratio R t / R xo , from which R xo , can be determined. Resistivity Logs… 91

92. DGPS – Makerere University Induction logs These measure formation conductivity. Relation to formation resistivity: C =1000/R C is conductivity in millisiemens . From the dual induction log, we determine true formation resistivity by a technique, called R t minimum (R t min ). R t min = R i ( R w / R mf ). R t can also be got from a dual induction log tornado chart. Resistivity Logs… 92

93. DGPS – Makerere University Exercise 4 a ) Determination of d i , Corrected R t and corrected R xo from dual lateralog-R xo tonardo chart. Given: LLD = R LLD = 16 ohm-m LLS = R LLS = 10 ohm-m M FSL = R MSFL = 4.5 ohm-m Resistivity Logs… 93

94. DGPS – Makerere University b) Determination of d i , Corrrected R t and corrected R t dual induction- sfl tonardo chart. Given: LLD = R LLD = 70 ohm-m LLS = R LLS = 105 ohm-m M FSL = R MSFL = 320 ohm-m Resistivity Logs… 94

95. DGPS – Makerere University Resistivity-derived porosity HCs and grains are non-conductive. Conduction of electric current depends on water present in pores. Thus, resistivity measurements can be used to determine porosity. Normally resistivities close to the bore hole ( flushed zone and transition zone ) are used to calculate porosity (assuming water based muds). Resistivity Logs… 95

96. DGPS – Makerere University Porosity in a water-bearing formation can be related to shallow resistivity ( R xo ) by: S xo = √( F.R mf / R xo ). F is formation factor. In a water bearing zone, S xo = 1 (100%), thus: 1=√( F.R mf / R xo ) Remember: F = a/ φ m Squaring both sides, and solving for φ : Φ = [( a.R mf )/ R xo ] 1/m Resistivity Logs… 96

97. DGPS – Makerere University In HC bearing zones , residual oil affects the S xo (water saturation in flushed zone) hence resistivity-derived porosity. Thus S xo ( S xo <1.0) must be known or estimated. Thus φ = (a/S 2 xo . R mf / R xo ) 1/m Resistivity Logs… 97

98. DGPS – Makerere University Gamma ray logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones . Shale-free sandstones and carbonates have low concentrations of radioactive minerals and give low gamma ray readings. Note: clean sands (i.e., with low shale content) might produce a high gamma ray response if the sandstone contains potassium feldspars, micas, glauconite or uranium-rich waters . Gamma Ray Log… 98

99. DGPS – Makerere University In zones where a geologist knows that there is presence of k-feldspars, micas, or glauconite , a spectral gamma ray log is run instead of a gamma ray log. If a zone has high potassium content coupled with a high gamma ray log response, the zone might not be shale. It might be a feldspathic , glauconitic, or micaceous sandstone ( check the mud log ). Like the SP log , the gamma ray log is not only used for correlation but also for determination of shale (clay) volumes. Gamma Ray Log… 99

100. DGPS – Makerere University Unlike the SP log , the gamma ray log can be used in cased hole and in open holes containing nonconductive drilling fluids (i.e., oil-based muds or air). The gamma ray log is usually displayed in the left track (track 1) of a standard log display, commonly with the caliper curve . Tracks 2 and 3 are usually resistivity and porosity curves. Gamma Ray Log… 100

101. DGPS – Makerere University Gamma Ray Log… 101 Shale volume calculation Because shale is usually more radioactive than sand or carbonate, gamma ray logs can be used to calculate volume of shale in porous reservoirs. It is expressed as decimal or percentage and is called V shale . The value can be used in analysis of shaly sands.

102. DGPS – Makerere University Shale volume calculation… The first step is calculation of gamma ray index . I GR = ( GR log -GR min )/( GR max -GR min ). I GR = gamma ray index GR log = gamma ray readings of formation GR min = minimum gamma ray (clean sand or carbonate) GR max = maximum gamma ray (shale) Gamma Ray Log… 102

103. DGPS – Makerere University Shale volume calculation… For a linear response, V shale = I GR . Larionov (1965) for Tertiary rocks: V shale = 0.083(2 3.7IGR - 1) Steiber (1970): V shale = I GR /3-2I GR Clavier (1971): V shale =1.7- [3.38-(I GR -0.7) 2 ] 1/2 Larionov (1969) for older rocks: V shale = 0.33(2 2IGR -1) Gamma Ray Log… 103

104. DGPS – Makerere University Applications Determing shale (clay) volume (V shale ) in sandstone reservoirs that contain uranium minerals, potassium feldspars, micas, and/or glauconite . Differentiating radioactive reservoirs from shales. Source-rock evaluation: Source rocks generally have low water content, and often exhibit abnormally high concentrations of uranium. Gamma Ray Log… 104

105. DGPS – Makerere University Evaluation of potash deposits Geologic correlations Clay typing Fracture detection ( e.g. use of radioactive tracers to determine fracture height ). Rock typing in crystalline basement rocks. Gamma Ray Log… 105

106. DGPS – Makerere University Exercise 5 Calculate shale volume for the formations at depths; 13,534 ft , 13,570 ft , 13,701 ft. Consider linear, Larionov (for older rocks than tertiary) and Steiber . Use charts attached. Gamma Ray Log… 106

107. END OF PART ONE