Density Log Interpretation

The density determined by the density tool is called bulk volume. The rock structure of minerals such as sandstone or limestone is called the matrix. The density of this structure is called the matrix density. This is the density that the tool would read if the formation had zero porosity. The bulk volume is equal to the sum of the matrix

volume and the fluid volume contained in the pore space.

Density logs have a choice of matrix presentation. Classic presentations are

sandstone or limestone. If the matrix is known to be dolomite then a dolomite matrix should be chosen. If the matrix is in doubt use a limestone matrix presentation. 114.Neutron Log

The original neutron tool bombarded the formation with neutrons from a chemical source in the logging tool. This tool could measure the response of the formation as a function of the number of hydrogen atoms present. Because most of the hydrogen present is in the water and oil, and because one or both of these fluids are present in the pores of the rocks, the porosity can be determined by counting the hydrogen atoms.

The compensated neutron device uses two detectors to compensate for hole rugosity or roughness. In addition, it measures the ratio of the detector responses and converts this ratio to a linear porosity reading instead of the non-linear response of the single-detector neutron tool.

115.Gamma Ray Log (GR)

The gamma rays that are measured with this tool are naturally occurring rays rather than induced gamma rays from a source, as in the density tool. These "natural" gamma rays emanate from radioactive potassium thorium, and uranium, the three elements that account for most of the radiation in sedimentary formations. Potassium and thorium are closely associated with shale (illite, kaolinite, montmorillonite), while uranium may be

found in sands, shales, and some carbonates.

The gamma ray curve is almost unaffected by porosity and is an excellent indicator of shale. However, formations containing radioactive contaminants such as volcanic ash, granite wash or have formation waters with a high level of dissolved potassium salts will indicate a higher level of gamma ray similar to a shale or clay.

Gamma ray logs are also used as the substitute for the SP log in cased holes or in open holes where the SP log is unsatisfactory.

The gamma ray log can be recorded through casing or drillpipe so is very useful for correlation.

The gamma ray log is often used with the casing collar log (CCL) to correlate the location of casing collars, packers, liner hangers, or other downhole equipment with the exact depth on the open hole logs.

116.Natural Gamma Ray Spectroscopy Log (NGS)

The natural gamma ray curve has its source essentially in three radioactive isotopes: uranium, thorium and potassium. Thorium and potassium are usually found in shales and clays. Uranium compounds may be found in practically any formation. The natural gamma ray measurement has long been used as a shaliness indicator, but it can be misleading because uranium may be associated with both shale and reservoir rock. A better shaliness determination may be made from the thorium and potassium

measurements. The clays (montmorillonite, illite, kaolinite) can also be identified with these measurements because different clays have different ratios of thorium to potassium.

Table 5.1 Characteristics of different callipers Arms Phasing Max Dia Remarks

Sonic 3 120 406 mm 3 Arms Coupled

1 Reading Microlaterolog

Proximity

2 180 508 mm 2 Arms Coupled

1 Reading

MicroSFL 4 90 558 mm 4 Arms Coupled

2 x 2 2 Pair Reading

Density 2 180 Short Arm

Long Arm 2 Arms Coupled 1 Reading Dipmeter 4 90 D Type E Type 4 Arms Coupled 2 x 2 2 Independent Readings Borehole Geometry 4 90 Standard

Special

4 Arms Coupled Idependent Readings

Dual Axis 2 180 406 mm 2 Arms Coupled

1 Reading 117.Bottomhole Surveys

The hole diameter is usually recorded in conjunction with the following surveys: 62. Sonic log

63. Microresistivity log 64. Density log

65. Dipmeter log

The readings given by different callipers, in the same hole, may be different depending on the calliper design combined with the hole cross-section.

See Table 7.1 for the characteristics of the different callipers.

118.Calliper readings

The mudcake is a good reason to have different callipers reading different values: 67. If the arm of the calliper is the blade type, it will cut into the cake and this arm will

"ignore" the thickness of the mudcake.

68. If the arm is of the pad type, it will skid over the cake and the mudcake thickness will be taken into account.

Assuming there is no mudcake, the reading of different callipers in a perfectly round hole will be identical. But round holes are not always the case. In clearly ovalized holes, 2 arm, 3 arm and 4 arm callipers will read different hole diameter values, mostly because of the way these arms are coupled together. If the logging tool is fairly free to rotate inside the hole:

69. Two arm callipers will ride using the larger diameter of the hole.

70. Four arm callipers will ride with one pair of coupled arms using the larger diameter of the hole.

The following example shows different callipers in an ovalized hole:

71. The sonic calliper (3 arms linked together) shows an "average" hole diameter. 72. The density calliper (2 arms) is applied on the wall with strength. Its back-up arm

will cut into the mudcake. It will orient itself to read the largest diameter. 73. The two independent callipers on the dipmeter clearly show the oval section of

the borehole. Arm pressure is also quite strong and the mudcake will not be recorded.

74. The proximity calliper (2 arms) will probably orient itself to read the larger

diameter, and its pads will skid on the mudcake. This is the case in the upper part and lower part of this section.

In deviated wells callipers may partially collapse under their own weight and give readings that are too low.

119.Cement Volume Log

This wellsite computed product can be easily computed after running a Dual axis calliper device.

75. Required Inputs:

Four-arm calliper

76. Dipmeter

77. Borehole Geometry Tool 78. Density with Dual axis calliper

79. Outside diameter of the proposed casing. Desired length of casing to be cemented.

80. Outputs:

Cement volume in cubic metres.

Log indicating borehole area, bit size area and area of proposed casing (square metres).

Integrated hole volume and cement volume (in depth track). 120.Dual Dipmeter

azimuth (the direction in which the bed is dipping) of the formations, a great deal can be inferred about the processes that moved the beds to their present positions.

The dip of a bedding plane is represented by two components. The stratigraphic dip is the angle at which the sediment was originally deposited, and the structural dip is the result of subsequent tilting or deformation.

The Dual Dipmeter tool features two side-by-side configured electrodes on each pad. Highly accurate information on tool deviation and azimuth is obtained from a triaxial accelerometer and three magnetometers. Borehole geometry and hole volume are determined from two calliper measurements 90 degrees apart.

121.Fullbore Formation Microlmager (FMI)

In conductive muds, the FMI tool provides electrical images almost insensitive to

borehole conditions and offers quantitative information, particularly for fracture analysis. The FMI provides complete fracture network evaluation.

Processed borehole images and dip data are provided in real time with the MAXIS 500 unit.

122.Wireline Formation Tester

An empty chamber is lowered until it is opposite the zone to be tested. The chamber has an opening so fluids can flow into it. A valve controls the opening, and a pressure

transducer with a surface recorder measures the formation pressures. The opening in the chamber is in the centre of a rubber pad that is pressed against the formation. This pad seals out the mud in the wellbore and allows only formation fluids to enter the chamber. Once the seal is made, the tool is opened and fluids flow in. The flowing pressure is measured during this time. When the chamber is full, the pressure builds up until it approaches reservoir pressure. The tool is closed, the seal to the formation is broken, and the tool is pulled back to the surface. The contents of the sample chamber are measured and analysed, and the pressure build-up curve is used to calculate permeability and reservoir pressure.

The permeability that is measured is the permeability very close to the wellbore, which may not be representative of conditions farther into the formation. However, the wireline tester can be of great value when it is needed or when permeability is fairly high and a full sample chamber is likely.

123.Repeat Formation Tester

Permeability can be estimated from the recovery data and pressure recording of the RFT tools. The RFT is a wireline formation tester that can be set any number of times during a single trip in the well. At each setting depth, a "pre-test" is made in which small

samples of fluid are withdrawn from the formation. During this pre-test the fluid pressure in the formation next to the wellbore is monitored until equilibrium formation pressure is reached. These pressures are recorded at the surface on both analog and high-

resolution digital scales. The pre-test fluid samples are not saved. However, after a successful pre-test in a zone of interest, a larger fluid sample can be taken and retained. In one trip, "virgin formation" fluid can be recovered from one test depth using a

segregated sample technique, or two samples can be recovered from different depths. RFT recoveries can be analyzed to establish reservoir fluid characteristics such as oil gravity, gas-oil ratio, and water cut.

When set for a test, the rubber packer is hydraulically forced against the formation to provide a seal from the wellbore fluids. Formation fluids enter the tool through the probe, which is forced into the formation.

When the tool is set, the pre-tests are automatically and sequentially activated. The lowflow rate pre-test withdraws 10cm3 of fluid from the formation by movement of a piston in Chamber 1. The second pre-test follows immediately and withdraws 10cm3 at a higher flow rate into Chamber 2. In permeable zones the two chambers are filled in approximately 25 seconds. In some cases, the high flow rate pre-test may be

deactivated in low-permeability reservoirs to reduce the amount of fluid withdrawn and speed up the tests. During the retract cycle, the pre-test chambers are pumped closed. 124.Modular Formation Dynamics Tester

The MDT tool performs a mini-DST on wireline. It can measure static pressure faster than any other tool of its type and it can collect more, and better, formation fluid samples in a single trip than any other sampling tool. Sampling pressure and pre-test flow rate and volume can be controlled from the MAXIS 500 service unit. The MDT tool collects formation fluid through a probe that is placed hydraulically against the borehole wall. Thus, the formation can flow into the sample chamber until hydrocarbon enters

(determined by resistivity) and then the sample is taken. Controls from the MAXIS 500 service unit direct this fluid into any selected sample chamber. By equipping the tool with additional sample chamber modules, a large number of fluid samples can be collected in a single trip into the hole. This permits testing a number of different zones on a trip, or collection of a number of samples from a single zone.

The MDT Tool can be many different tools, depending on what a particular test requires. Individual modules, each with a specific purpose, can be assembled in a variety of ways, right at the wellsite.

125.Casing Wear Evaluation Tools:

81. The multi-fingered callipers use 15 - 30 fingers to measure the I.D. of the casing for wear. The resolution is +/- 1/16 inch.

82. The magnetic tools detect variations in the magnetic flux fields and thereby

determine wall thickness reduction. They are used for corrosion detection and leak detection (holes).

83. The electromagnetic tools can detect the actual depth of the casing defect, whether it is internal or external (pitting or grooves). This is accomplished by detecting flux leakage or eddy currents.

84. The current flow tools can detect areas of metal loss and can be used in dry holes or with dielectric fluids.

85. The acoustic tools use the strength of the returning signal to provide a qualitative measure of wear.

86. The imager tools use high-frequency sound and provide excellent resolution of the wall surface, but no thickness.

126.Sidewall Sampling 87. General

Sidewall samples (SWS) are mainly used for lithological and geochemical analysis. Especially for the latter, unshattered samples are of particular importance as mud infiltration may carry microfauna and flora of all ages into the sample.

The sidewall sample programme is prepared in the Main Office. In shales, samples are usually selected at 10-25m intervals (depending upon shale interval length); in sands, selection depends upon requirements. Upon receipt of the programme, the

Well Site Drilling Engineer should check the individual sample depths against the sands and mark them on the GR log. The Logging Engineer must then choose charge size (powder type), release ring and fastener length. Decision will be based on

lithology, hole size and previous experience with similar formation types.

88. For SWS guidelines refer to the SIPM Production Handbook, Volume I, Chapter 3.

89. Where guns are to be preloaded prior to transportation to the rig, a proposed SWS programme should be provided to the logging company by the Company. 90. Samples should be taken from bottom to top.

91. Carrier wire and head ring should be checked before running in.

92. If the number of samples to be taken is less than a full gun (generally 30 shots), the number may be made up by double shots over zones of particular interest. 93. If the number of samples to be taken exceeds one full gun but is not sufficient for

two full guns, the second run can be used to re-test the intervals of poor recovery from the first run.

94. When shooting permeable formations sampling on the run may be used to minimise the potential for differential sticking.

95. SWS should be closely supervised by the wellsite geologist or engineer to ensure samples are correctly numbered when cut loose from the gun, and that the samples are correctly boxed.

96. Whenever possible the samples should not be touched as this contaminates the sidewell core.

97. Samples should be intact and longer than 1/2” to be acceptable. If a double-shot

was performed on one zone and the two recoveries are less than 1/2” each, they

can be combined and accepted as one sample. If altering gun parameters does not provide better than poor recoveries, a lower acceptance criteria may be considered.

98. Reporting

99. During sampling and sample recovery a record should be kept giving shot number, depth recovery, accepted (yes/no), lost, misfire, empty or broken samples and a short sample description. One copy of this list is to be boxed with the samples, one copy to the Company and another retained at well-site for reference.

100.Sample jars should state well number, shot number and sample depth. Depth of sample must also be scratched on the lid with a sharp instrument.

101.Samples are to be transported to a Geological Laboratory specified by the Company Representative as soon as possible following recovery and packaging. 102.Lost SWS Bullets

103.If bullets are not recovered and a wiper trip is required, run in with a rockbit and junksub. Rotate and circulate past depths where the sampling was attempted. 104.On the bottom drill +/- 1 meter, if there is no significant torque, continue as per

programme.

105.If torque indicates junk on bottom make a basket run and fish for the junk. 106.If bullets are not recovered, further sampling runs may be made without wiper

trips but the guns must not be lowered below the point where bullets were lost.

In document Drilling Manual (Page 176-181)