4 Density Log (Cont.)

Report
BASIC WELL LOGGING ANALYSIS –
DENSITY LOG
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Hsieh, Bieng-Zih
Fall 2009
DENSITY LOG

The formation density log is a porosity log that measures
electron density of a formation.

It can assist the geologist to (Schlumberger, 1972):
detect gas-bearing zones,
 determine hydrocarbon density,
 evaluate shaly sand reservoirs and complex lithologies.

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DENSITY LOG (CONT.)

The density logging device is a contact tool which consists
of a medium-energy gamma ray source that emits
gamma rays into a formation.

The gamma ray source is either Cobalt-60 or Cesium137.
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DENSITY LOG (CONT.)

Gamma rays collide with electrons in the formation;the
collisions result in a loss of energy from gamma ray
particle.

Tittman and Wahl (1965) called the interaction between
incoming gamma ray particles and electrons in the
formation, Compton Scattering.
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DENSITY LOG (CONT.)

Scattered gamma rays which reach the detector, located
a fixed distance from the gamma ray source, are counted
as an indicator of formation density.

The number of Compton Scattering collisions is a direct
function of the number of electrons in a formation
(electron density).

Consequently, electron density can be related to bulk
density (ρb) of a formation in gm/cc.
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DENSITY LOG
•
•
•
密度井測是由探測針放射
出中等強度的伽瑪線,至
地層內撞擊地層中之電子
後,由接收器偵測伽瑪線
的剩餘強度。
在電子密度愈高之地層
(地層密度高之地層),
伽瑪線撞擊地層中的電子
之機會及次數就愈多,接
收器所偵測之伽碼線的強
度愈少。
利用密度電測可以量測地
層孔隙率。
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FDC LOG SURVEY
Track #1 contains a gamma ray log and a caliper (Fig.
28).
 The bulk density curve is recorded in tracks #2 and #3
(Fig. 28), along with a correction curve (Δρ).

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FDC LOG SURVEY (CONT.)
Because the modern density log is a compensated log
(dual detectors), the correction curve (Δρ) records how
much correction has been applied to the bulk density
curve(ρb), due to borehole irregularities.
 Whenever the correction curve (Δρ) exceeds 0.20 gm/cc,
the value of the bulk density obtained from the bulk density
curve (ρ ) should be considered invalid.

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FDC LOG SURVEY (CONT.)

Formation bulk density (ρb) is a function of matrix density,
porosity, and density of the fluid in the pores (salt mud,
fresh mud, or hydrocarbons).

To determine density porosity, either by chart (Fig. 29) or
by calculation, the matrix density (Table 7) and type of
fluid in the borehole must be known.
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TABLE. MATRIX DENSITIES OF COMMON LITHOLOGIES







Sandstone
Limestone
Dolomite
Anhydrite
Salt
ρb (gm/cc)
2.648
2.710
2.876
2.977
2.032
Constants presented here are used in the Density Porosity
Formula (after Schlumberger, 1972).
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(1) DERIVE DENSITY POROSITY BY CHART
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2.56 gm/cc
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Derive density porosity by chart
Given:
ρma=2.87 gm/cc (Dolomite)
ρf=1.1 gm/cc (salt mud)
From FDC log:
ρb=2.56 gm/cc @ 9310 ft
Note:
The formation's matrix
density must be known
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EXERCISE – FIND DENSITY POROSITY BY FORMULA
(1)
 Depth = 9310 ft
 Lithology [=] Dolomite (ρma=2.87 gm/cc)
 Mud [=] salt mud (ρf=1.1 gm/cc)
 Density porosity = ?

(2)
 Depth = 9320 ft
 Lithology [=] Limestone (ρma=2.71 gm/cc)
 Mud [=] fresh mud (ρf=1.0 gm/cc)
 Sonic porosity = ?

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(2) DERIVE DENSITY POROSITY BY FORMULA
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DERIVE DENSITY POROSITY BY FORMULA

The formula for calculating density porosity is:
den
 ma  b

 ma   f
Where:
 Фden = density derived porosity
 ρma = matrix density (see Table)
 ρb = formation bulk density (= density log reading)
 ρf
= fluid density

(1.1 salt mud, 1.0 fresh mud, and 0.7 gas)

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GAS EFFECT
Where invasion of a formation is shallow, low density of
the formation’s hydrocarbons will increase density
porosity.
 Oil does not significantly affect density porosity, but gas
does (gas effect).
 Hilchie (1978) suggests using a gas density of 0.7 gm/cc
for fluid density (ρf) in the density porosity formula if gas
density is unknown.

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EXERCISE – FIND DENSITY POROSITY BY FORMULA
(1)
 Depth = 9310 ft
 Lithology [=] Dolomite (ρma=2.87 gm/cc)
 Mud [=] salt mud (ρf=1.1 gm/cc)
 Density porosity = ?

den
(2)
 Depth = 9320 ft
 Lithology [=] Limestone (ρma=2.71 gm/cc)
 Mud [=] fresh mud (ρf=1.0 gm/cc)
 Sonic porosity = ?

 ma  b

 ma   f
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DENSITY POROSITY FOR SHALY SANDS
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SONIC POROSITY FOR SHALY SANDS

After the volume of shale (Vsh) is determined, it can be
used to correct the porosity log for shale effect. The
formula for correcting the density log for volume of shale
is (Dresser Atlas, 1979):
Dsh

 ma  b
 ma   sh
(
)  Vsh (
)
 ma   f
 ma   f
Where ρsh = bulk density of adjacent shale
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HOMEWORK #4 --
DENSITY LOG
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HOMEWORK #4 – DENSITY LOG
FDC
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HOMEWORK #4 – DENSITY LOG
Depth
FDC
ФDen
Vsh
ФD-sh
7600
7610
7620
….
….
….
….
Den
 ma  b

 ma   f
Dsh  (
 ma  b
   sh
)  Vsh ( ma
)
 ma   f
 ma   f
Information:
….
….
ρma = 2.65 gm/cc (Sandstone)
….
ρf = 1.0 gm/cc (Fresh mud)
ρf = 1.1 gm/cc (Salt mud)
7840
7850
ρsh = ? μsec/ft (Shale)
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