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BASIC WELL LOGGING ANALYSIS – DENSITY LOG 1 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. 2 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. 3 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. 4 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. 5 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。 7 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). 8 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. 9 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. 10 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). 11 (1) DERIVE DENSITY POROSITY BY CHART 12 2.56 gm/cc 13 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 14 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 = ? 15 (2) DERIVE DENSITY POROSITY BY FORMULA 16 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) 17 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. 18 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 19 DENSITY POROSITY FOR SHALY SANDS 20 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): Dsh ma b ma sh ( ) Vsh ( ) ma f ma f Where ρsh = bulk density of adjacent shale 21 HOMEWORK #4 -- DENSITY LOG 22 HOMEWORK #4 – DENSITY LOG FDC 23 HOMEWORK #4 – DENSITY LOG Depth FDC ФDen Vsh ФD-sh 7600 7610 7620 …. …. …. …. Den ma b ma f Dsh ( 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) 24