Asst. Lecturer: Amir I. AbdelazizHelwan University

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Porosity

• Porosity: is the pore volume per unit

volume of formation; it is the fraction of the

total volume of a sample that is occupied by

pores or voids.

• The symbol for porosity is f. A dense,

uniform substance, such as a piece of glass,

has zero porosity; a sponge, on the other

hand, has a very high porosity.

• Porosities of subsurface formations can vary

widely.

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Porosity

• Dense carbonates (limestones and dolomites)

and evaporites (salt, anhydrite, gypsum,

sylvite, etc.) may show practically zero

porosity; well-consolidated sandstones may

have 10 to 15% porosity; unconsolidated

sands may have 30%, or more, porosity.

• Shales or clays may contain over 40% water-

filled porosity, but the individual pores are

usually so small the rock is impervious to the

flow of fluids.

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Porosity log types

3 Main Log Types

• Bulk density

• Sonic (acoustic)

• Compensated neutron

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These logs do not measures porosity directly. To accurately calculate

porosity, the analyst must know:

• Formation lithology

• Fluid in pores of sampled reservoir volume

Porosity log types

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Neutron tool

• Neutron source. • High energy neutrons are slowed down by hydrogen

atoms in water (or oil) and detected by tool• Porosity is function of rock type.

Density tool

• Gamma ray source.• Electrons reflect gamma rays back to detector in tool.• Electrons in formation proportional to density.• Porosity is function of rock type and density.

Sonic tool

• Measures speed of sound in formation.• Porosity slows sound.• Porosity is function of rock type and measured speed

of sound.

Porosity from logging

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Gamma

ray

Resisitivity Porosity

Increasing

radioactivity

Increasing

resistivityIncreasing

porosity

Shale

Shale

Density logs

Measures formation’s bulk density

Used as a porosity measure Differentiates lithologies with Neutron

Log.

Used with Sonic Logs to generate synthetic seismic traces to

match to seismic lines.

Uses radioactive source to generate gamma rays.

Gamma ray collides with electrons in formation, losing energy.

Detector measures intensity of back-scattered gamma rays, which

is related to electron density of the formation.

Electron density is a measure of bulk density. 8

Density log

Bulk density, b, is dependent upon:

Lithology

Porosity

Density and saturation of fluids in pores.

Saturation is fraction of pore volume occupied by a

particular fluid (intensive)

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Density log

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The formation density log is a porosity log that measures electron density of a formation.

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.

Scale and units:

The most frequently used scales are a range of 2.0 to 3.0

gm/cc or 1.95 to 2.95 gm/cc across two tracks.

Formation (b)

Long spacing detector

Short spacing detector

Mud cake(mc )

Source

Density log

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Formation (b)

Long spacing detector

Short spacing detector

Mud cake(mc )

Source

• 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.

• The scattered gamma rays reaching the detector, at a

fixed distance from the source, are counted as an

indication of formation density.

• Electron density is related to the true bulk density b,

which, in turn, depends on the density of the rock

matrix material, the formation porosity, and the

density of the fluids filling the pores.

Density log

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GRC0 150

SPCMV-160 40

ACAL6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

Bulk Density

Log

RHOC

1.95 2.95

Density log

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• For a clean formation of known matrix density, ma, having a porosity f, that

contains a fluid of average density f the formation bulk density b, will be:

bmabflb ff )1(

bmabfl

bmabdensity

f

DENSITY POROSITY

Clean formation

Density log

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DENSITY POROSITY

EXAMPLE

?

/31.2

/10.1

/71.2

density

ccgb

ccgfl

ccgma

f

Density log

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SOLUTION

bmabfl

bmabdensity

f

25.0248.061.1

4.0

71.210.1

71.231.2

density

density

f

f

Density logs

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For liquid-filled sandstones,

limestones, and dolomites the tool

reading, a, is practically identical

to actual bulk density, b. For a few

substances, such as sylvite, rock salt,

gypsum, anhydrite; coal, and gas-

bearing formations, the corrections

shown in the picture are needed to

obtain bulk density values from the

density log readings.

Neutron logs

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Measures porosity of formation.

detect quantity of hydrogen

present.

Measures lithology when used

with Density Log.

Neutron logs

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The Neutron Log is primarily used to

evaluate formation porosity.

It is used to detect gas in certain

situations.

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.

Neutron logs

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Porosity from Neutron log

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Theoretical equation

Nmashshsh

NhcxoNmfxoN

V1V

S1S

fff

fffff

fN = Recorded parameter

f Sxo fNmf = Mud filtrate portion

f (1 - Sxo) fNhc = Hydrocarbon portion

Vsh fNsh = Shale portion

(1 - f - Vsh) fNhc = Matrix portion where f = True

porosity of rock

fN = Porosity from neutron log measurement,

fraction

fNma = Porosity of matrix fraction

fNhc = Porosity of formation saturated with

hydrocarbon fluid, fraction

fNmf = Porosity saturated with mud filtrate, fraction

Vsh = Volume of shale, fraction

Sxo = Mud filtrate saturation in zone invaded

by mud filtrate, fraction

Neutron logs

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• Gas zones can often be

identified by comparing the

neutron log with another

porosity log or a core

analysis.

• A combination of the neutron

log with one or more other

porosity logs yields even

more accurate porosity

values and lithology

identification even an

evaluation of shale content.

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Porosity from Neutron log

Porosity log crossplots are one of

the basic tools used in formation

evaluation, since each of the

common porosity logs such as the

acoustic, density and neutron

responds uniquely to different

lithologies.

Crossplotting multiple porosity

responses also defines actual

porosity more accurately.

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Density-Neutron log

• Can be an Indicator for Gas ( Gas zone boundary).

• We can see in ( Density – Neutron) log an OVERLAP + SEPARATION

and the case may reversal in case of Oil zone.

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• Determine the different lithologic units in the following composite log.

Detect the OWC and The GOC.

Sonic (Velocity) log

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• Measures speed of sound in the formation in microseconds/ft.

• Each rock type (lithology) has a characteristic DT 40-70 us/ft.

• Fluids have a much slower DT of 180-230 us/ft, gas even slower.

• Links seismic depth in time to log depth in ft or m.

• Used to determine porosity and lithology.

Sonic (DT)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 (actually an inverse velocity)

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f f

f

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Subsurface temperature

• Temperature Increase with depth.

• This temperature-depth relationship is commonly a

linear function of the following form.

TD = Ts + α D

• TD : Temperature of the reservoir at any depth ;

Ts: Average surface temperature ; α : Temperature

gradient (degree/ft) and D : Depth, ft.

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Subsurface temperature

• Find the temperature at depth of 5000 ft when

the average surface temperature is 75 F and the

temperature gradient is 1.5 F/100 ft.

• Calculate the Geothermal gradient of a sandstone

layer at depth 2200 ft where, the surface

temperature=80 0F and the formation

temperature=122 0F.