Induction Log

The induction log is most useful in wells with a fresh water drilling fluid and in

formations with medium resistivity. However, the induction tool is also useful in wells that have been drilled with air, gas or oil-based drilling fluids. The induction logs have generally come to replace the conventional resistivity log. They were originally made to measure resisitivity in wells with oil-base drilling fluids and air or gas-filled holes. Induction logs are also focused to minimise the effects of borehole fluids and borehole size, invaded zone fluids and adjacent formation resistivities. The dual induction log has been developed for those areas that had lower porosities and deeper invasion. Induction logs have much the same application as the conventional resistivity logs. A standard induction log will consist of the SP curve, in the left-hand track, a deep induction curve measuring the Rid, the medium induction curve, measuring the Rim, and the spherically focused curve measuring the Rsfl. The Rsfl curve indicates the resistiviy of the flushed zone. The Rim curve indicates the resistivity of the flushed and invaded zones. The Rid curve indicates the resistivity of the uncontaminated zone and can be corrected or adjusted to give the true resistivity or Rt.

The conductivity log is also useful in detecting abnormal pressures. As with the "d” exponent, the conductivity values of normally pressured sections develop a trend which will be usually a straight or slightly curved line. This trend line is caused by the conductivity values decreasing with depth. Shale compaction results in lower shale porosity, which in turn results in less interstitial water to conduct the induced current. The conductivity trend therefore, decreases with depth. Abnormally pressured shales though, will retain more water and will therefore act as a better conductor. The conductivity readings opposite abnormally pressured formations will increase. 52. Spontaneous Potential (SP)

SP is generally included on the resistivity logs but it is not a resistivity measurement. The SP curve is a continuous recording (versus depth) of the difference in electric potential between a moveable electrode in the borehole and a fixed (zero) potential of a surface electrode. The SP curve is recorded in the left- hand track but cannot be recorded in an oil based drilling fluid or an air-filled wellbore because of the lack of electrical continuity between the SP electrode and the formation.

Uses of Sp:

53. Detect permeable beds (a qualitative indication only). 54. Determine Rw, formation water resistivity.

55. Give an indication of the bed shaliness.

56. Locate the permeable bed boundaries and permit correlation of these beds. The magnitude of SP deflections is always measured from the shale line. Opposite a permeable formation, the SP curve shows deflection from the shale base line. In thick,

clean beds the SP deflection tends to reach an essentially constant deflection defining a clean line.

Shut-down of rig generator distorts the SP curve. The Logging Engineer should be informed of any changes in rig power generation.

111.Microresistivity Logs

Microresistivity tools are designed to read Rxo, the resistivity of the flushed zone. These tools are all very shallow reading. The electrodes are mounted on flexible pads pressed against the borehole wall, thereby eliminating most of the effects of the mud on the measurement.

The following are Microresistivity Tools: 57. Micrologs

The first microresistivity tool was called the microlog. On this tool, a pad carrying electrodes is filled with an insulating oil. The pad is pressed against the wall of the hole by the backup pad. The current flows along a path.

The microlog is best used to indicate permeability and formation thickness in low to medium resistivity formations.

58. Microlaterologs

The microlaterolog(MLL) is similar in operation to its big brother, the laterolog (LL). The tool carries small concentric electrodes on a flexible pad that is pressed against the borehole wall. The outer guard electrodes force the current into the formation and prevent short-circuiting by the mudcake. For this reason, the microlaterolog is used in high-resistivity formations.

59. Microspherically Focused Logs

The MSFL uses the same principle of operation as the spherically focused log but on a smaller scale. It is also a pad device and is often combined with other

measurements.

The MSFL is helpful for very thin beds not adequately displayed by normal resistivity logs.

112.Sonic Log Interpretation

A sonic tool has transmitters and receivers. The transmitters are pulsed alternately spherically outward in all directions. The mud column and the tool have slower travel times (sonic velocities) than the formations. The first sound energy to arrive at the two receivers is the compressional wave (P-wave), which travels through the formation near the borehole. The difference in the times at which the signal reaches the two receivers is divided by the spacing of the receivers. This time, recorded in microseconds per foot, is called sonic interval transit time(t) for the difference in arrival times between the two receivers. Sonic tools have multiple transmitters and receivers to compensate for sonde tilt, washed-out hole, and alteration of the rock properties near the wellbore due to drilling processes. The logs produced by these tools are called borehole compensated (BHC) sonic logs.

If the interval transit time and the type of formation is known, and the porosity is uniformly distributed (intergranular as opposed to vugular or fracture porosity),the porosity can be determined. The lithology must be assumed to make an estimate. Shale has a strong effect on the sonic log. In shaly formations (Vsh>5%) the sonic porosity must be corrected for the presence of shale. Gas also has a strong effect on the apparent sonic porosity: it raises the apparent porosity. If gas-bearing formations are anticipated, at least one other porosity device, preferably a compensated neutron log, is

run. Finally, the sonic wave does not "see" the vugular and fractures porosity (secondary) as well as it "sees" the intergranular (primary) porosity. This lowers the apparent porosity in vugular and/or fractured formations.

The Sonic Log is important to geophysicists to tie in seismic data.

113.Density Log

60. Measuring Density

Density is the weight of a unit volume of a substance. 1cc of water weighs 1g, so water has a density of 1.0g/cc and limestone's density is 2.71g/cc. Formation or bulk density cannot be measured directly from the borehole. However, electron density can be measured by using Compton scattering reactions, and electron density is nearly the same measurement as bulk density. The density-measuring tool bombards electrons in the formation adjacent to the wellbore with gamma rays from a caesium source and Compton scattering takes place. The gamma rays are counted by two detectors mounted on a skid pressed against the borehole wall. The two detectors allow compensation for the effects of hole roughness and mudcake while the bulk density measurement is made.

The newest generation of density tools measures the photoelectric absorption cross- section of the formations. The photoelectric effect is simply another way the formation reacts to the bombarding gamma rays. This reaction occurs at a much lower energy level than Compton scattering. By measuring the energy level of the formation reactions, it is possible to separate photoelectric reactions from other reactions. The photoelectric response is then used to help identify lithology.