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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.

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

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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 ).

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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

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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 conductive mud, 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.

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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).

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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:

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Water saturation calculation (Sw):

Sw, water saturation can be estimated from a resistivity measurement using the Archie

Equation.

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Resistivity Analysis:

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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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NhcxoNmfxoN

V1V

S1S

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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: