The document provides information about well logging techniques. It discusses how the borehole and surrounding rock can be invaded by drilling mud, affecting measurements. It describes the invaded zone and different resistivity measurements that can be taken. It then discusses various well logging tools - gamma ray, spontaneous potential, resistivity, density, neutron, and sonic logs - and how they are used to evaluate properties like lithology, porosity, fluid content, and hydrocarbon saturation.

1. 1
Well logging
What is a log?
 Well log is a continuous record of measurement made in bore-hole
that respond to variation in some physical properties of rocks
through which the bore hole is drilled.

2. 2
 Write about Bore-hole Geometry/Environment OR Invasion effect..

3. 3
 Where a hole is drilled into a formation, the borehole and the rock surrounding it are
contaminated by the drilling mud, which affects logging measurements.
1. Invaded Zone:
 The zone in which much of the original fluid is replaced by mud filtrate is called the invaded
zone.
 It consists of a flushed zone (of resistivity Rxo) and a transition or annulus zone (of resistivity
Ri). The flushed zone occurs close to the borehole. Where the mud filtrate has almost
completely flushed out a formation’s hydrocarbons and/or water (Rw).
 The transition or annulus zone, where a formation’s fluids and mud filtrate are mixed,
occurs between the flushed zone and the uninvaded zone (of resistivity Rt).
 The uninvaded zone is defined as the area beyond the invaded zone where a formation’s
fluids are uncontaminated by mud filtrate.
2. Flushed zone Resistivity (Rxo):
 The flushed zone extends only a few inches from the wellbore and is part of the invaded
zone.
 If invasion is deep or moderate, most often the flushed zone is completely cleared of its
formation water by mud filtrate (of resistivity Rmf).
 When oil is present in the flushed zone, the degree of flushing by mud filtrate can be
determined from the difference between water saturations in the flushed (Sxo) zone and
the uninvaded (Sw) zone
3. Uninvaded zone Resistivity (Rt):
 The uninvaded zone is located beyond the invaded zone.
 Pores in the uninvaded zone are uncontaminated by mud filtrate; instead, they are
saturated with formation water (Rw), oil, and/or gas.
 By using water saturation data, a geologist can determine a reservoir’s hydrocarbon
saturation. Sh = 1 – Sw (Sh = hydrocarbon saturation - water saturation of the uninvaded zone ).

4. 4
Logging tools
 Gamma ray log:
 All rocks contain some radioactive material, although marine shales have a higher gamma
ray emission level than sandstone, limestone and dolomite. This difference makes the
gamma ray log especially useful for distinguishing shales from non-shales, therefore the
gamma ray is essentially a lithology log.
 Gamma ray log : Measures Natural radioactivity and main radioactive elements are
Potassium, Thorium and Uranium.
 K and TH mostly in shales while U from other sources. Clean formations have low
radioactivity level.
 Importance of GR log:
 Correlation between wells,
 Determination of bed boundaries,
 Evaluation of shale content within a formation,
GR log - Gr clean
Vsh =
GR shale - Gr clean

5. 5
2. Spontaneous potential log (SP):
o Record the difference in voltage between a
moving electrode in the borehole and a
reference electrode at the surface usually
located at the mud pit.
o In order to record a potential the hole must
contain conductivemud, as it cannot be recorded
in air or oil-base mud.
o Logging rate is approximately 1500m per hour
and recordings are continuous.
 Factors affect the Sp:
 In sand A, Rw is less than Rmf.
 In sand B, the SP deflection is less than in sand A.
 In sand C, the SP is reversed, (Rw > Rmf).
 We may guess that, at about 7000 ft, Rmf and Rw are equal.

6. 6
Shale Volume Calculation (Vsh) :
V shale = (SPclean – SPlog)/ (SPclean-SPshale)
Vshale: shale volume
SPclean: maximum Sp deflection from clean wet zone
SPlog: Sp in the zone of interest (read from the log)
Spshale: SP value at the shale baseline (often considered to be zero).

7. 7
3. Resistivity log :
 Resistivity: The Degree to which the subsurface resist flow of current to pass through.
 Resistivity is the inverse of conductivity.
 Measures resistance of flow of electric current.
 Is function of porosity & pore fluid in rock.
 Resistivity is measured as, R in ohm per m,
 The ability to conduct electric current depends upon:
 The Volume of water,
 The Temperature of the formation,
 The Salinity of the formation.
 The Usage:
 Determine Hydrocarbon versus Water-
bearing zones,
 Indicate Permeable zones.
 There are two general types of resistivity tools:
 Electrode: forces a current through the rock and measures resistivity.
 Induction: Uses electro-magnetic field to induce electrical currents in the rock; it
measures conductivity which is converted to resistivity.
 Various electrode logs and depth of measurement:

8. 8
 Water saturation calculation (Sw):
 Sw, water saturation can be estimated from a resistivity measurement using the Archie
Equation.

9. 9
 Resistivity Analysis:

10. 10
Porosity log types
A. Density log:
 The formation density log is a porosity log that measures
electron density of a formation.
 A radioactive source, applied to the borehole wall in a shielded
portion of the tool, emits medium-energy gamma rays into the
formations. These gamma rays may be thought of as high-
velocity particles that collide with the electrons in the formation.
At each collision a gamma ray loses some, but not all, of its
energy to the electron, and then continues with diminished
energy.
 Dense formations absorb many gamma rays, while low-density
formations absorb fewer. Thus, high-count rates at the detectors
indicate low-density formations, whereas low count rates at the
detectors indicate high-density formations.
 Therefore, scattered gamma rays reaching the detector is an
indication of formation Density.

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B. Neutron logs:
 The Neutron Log can be summarized as the continuous measurement of the induced
radiation produced by the bombardment of that formation with a neutron source
contained in the logging tool whose sources emit fast neutrons that are eventually slowed
by collisions with hydrogen atoms until they
are captured.
 The Neutron Log is primarily used to
evaluate formation porosity.
 It is used to detect gas in certain situations.
 Measures lithology when used with Density
Log.
 Porosity from Neutron log:
 Density-Neutron log:
 Can be an Indicator for Gas (Gas zone boundary).
 We can see in (Density – Neutron) log a Cross over + SEPARATION and the case may
reversal in case of Oil zone.
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12. 12
C. Sonic (Velocity) log:
 Acoustic energy emitted by a transmitter, travels through the formation/fluids, detected
by multiple detectors.
 Log displays the interval transit time (Dt) in msec/ft.
 Measures speed of sound in the formation in
microseconds/ft.
 Each rock type (lithology) has a characteristic DT.
 Fluids have a much slower DT, gas even slower.
 Used to determine porosity and lithology.
 Porosity from Sonic log: