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B ASIC WELL L OGGING A NALYSIS D ENSITY L OG Hsieh, Bieng-Zih Fall 2009 1

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D ENSITY L OG 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. 2

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D ENSITY L OG (C ONT.) 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. 3

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D ENSITY L OG (C ONT.) 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. 4

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D ENSITY L OG (C ONT.) 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 ( b ) of a formation in gm/cc. 5

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D ENSITY L OG

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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 ( ). 8

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FDC LOG SURVEY (C ONT.) Because the modern density log is a compensated log (dual detectors), the correction curve ( ) records how much correction has been applied to the bulk density curve( b ), due to borehole irregularities. Whenever the correction curve ( ) exceeds 0.20 gm/cc, the value of the bulk density obtained from the bulk density curve ( ) should be considered invalid. 9

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FDC LOG SURVEY (C ONT.) Formation bulk density ( b ) 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. 10

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T ABLE. M ATRIX D ENSITIES OF C OMMON L ITHOLOGIES b (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). 11

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(1) DERIVE DENSITY POROSITY BY CHART 12

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13 2.56 gm/cc

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14 Given: ma=2.87 gm/cc (Dolomite) f=1.1 gm/cc (salt mud) From FDC log: b=2.56 gm/cc @ 9310 ft Note: The formation's matrix density must be known Derive density porosity by chart

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E XERCISE FIND DENSITY POROSITY BY FORMULA (1) Depth = 9310 ft Lithology [=] Dolomite ( ma =2.87 gm/cc) Mud [=] salt mud ( f =1.1 gm/cc) Density porosity = ? (2) Depth = 9320 ft Lithology [=] Limestone ( ma =2.71 gm/cc) Mud [=] fresh mud ( f =1.0 gm/cc) Sonic porosity = ? 15

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(2) DERIVE DENSITY POROSITY BY FORMULA 16

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DERIVE DENSITY POROSITY BY FORMULA The formula for calculating density porosity is Where den = density derived porosity ma = matrix density (see Table) b = formation bulk density (= density log reading) f = fluid density (1.1 salt mud, 1.0 fresh mud, and 0.7 gas) 17

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G AS EFFECT Where invasion of a formation is shallow, low density of the formations 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 ( f ) in the density porosity formula if gas density is unknown. 18

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E XERCISE FIND DENSITY POROSITY BY FORMULA (1) Depth = 9310 ft Lithology [=] Dolomite ( ma =2.87 gm/cc) Mud [=] salt mud ( f =1.1 gm/cc) Density porosity = ? (2) Depth = 9320 ft Lithology [=] Limestone ( ma =2.71 gm/cc) Mud [=] fresh mud ( f =1.0 gm/cc) Sonic porosity = ? 19

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D ENSITY POROSITY FOR SHALY SANDS 20

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S ONIC 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 sh = bulk density of adjacent shale 21

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H OMEWORK #4 -- DENSITY LOG 22

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H OMEWORK #4 DENSITY LOG 23 FDC

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H OMEWORK #4 DENSITY LOG DepthFDC Den Vsh D-sh 7600 7610 7620 . 7840 7850 24 Information: ma = 2.65 gm/cc (Sandstone) f = 1.0 gm/cc (Fresh mud) f = 1.1 gm/cc (Salt mud) sh = sec/ft (Shale)