Basic well Logging Analysis – Density Log

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BASIC WELL LOGGING ANALYSIS –

DENSITY LOG

Hsieh, Bieng-Zih

Fall 2009

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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 Cesium-137.

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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.

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

ρb (gm/cc) Sandstone 2.648 Limestone 2.710 Dolomite 2.876 Anhydrite 2.977 Salt 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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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

Derive density porosity by chart

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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 ：

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)

fma

bmaden

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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 = ?

(2) Depth = 9320 ft Lithology [=] Limestone (ρma=2.71 gm/cc) Mud [=] fresh mud (ρf=1.0 gm/cc) Sonic porosity = ?

fma

bmaden

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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):

Where ρsh = bulk density of adjacent shale

)()(fma

shmash

fma

bmashD V

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HOMEWORK #4 -- DENSITY LOG

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HOMEWORK #4 – DENSITY LOG

FDC

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HOMEWORK #4 – DENSITY LOGDepth

FDC ФDen Vsh ФD-sh

760076107620….….….….….….….78407850

Information:

ρma = 2.65 gm/cc (Sandstone)

ρf = 1.0 gm/cc (Fresh mud)ρf = 1.1 gm/cc (Salt mud)

ρsh = ？ μsec/ft (Shale)

)()(fma

shmash

fma

bmashD V

fma

bmaDen