Porosity Logs (Density Log) PDF

College of Petroleum Engineering & Geosciences Petroleum Engineering Dept. PETE 313 Well Logging Porosity Logs (Density Log) Dr. Talal Al Shafloot Credits to Dr. Khaled Ibrahim for the slides  Porosity Measurement...

College of Petroleum Engineering & Geosciences Petroleum Engineering Dept. PETE 313 Well Logging Porosity Logs (Density Log) Dr. Talal Al Shafloot Credits to Dr. Khaled Ibrahim for the slides  Porosity Measurements ̶ Log Porosity Sonic Log Provides an estimate of connected porosity 2  Density Log (1) “ Porosity from porosity logs is never MEASURED, it is CALCULATED from what we think might represent the porosity in the rock”  Density Log Introduction to Density Porosity  The density log doesn't actually measure porosity or the formation density directly “Measure what is measurable, and make measurable what is not so” “Galileo Galilei” 4 Density  Density Log Density Porosity  The density of a mixture of components is a linear function of the densities of its individual constituents 5  Density Log Example 1. ϕD from ρb for various formations and pore fluids 6  Density Log Matrix: Mixed Formation Lithology  𝝆𝒎𝒂 Best obtained from core analysis by statistical analysis of core grain density data. Density Clay Density Matrix (g/cm3) mineral (g/cm3) Limestone 2.71 Smectite 2.0-2.6 Dolomite 2.87 Illites 2.6-2.9 Sandstone 2.65 Kaolinites 2.61-2.68 Anhydrite 2.96 Chlorites 2.6-3.3 7  Density Log Matrix 2: Multimineral lithology  For multimineral lithology a known value for each "ingredient“ is required etc. 8  Density Log Example Assume log reading of each porosity log; Density = 2.4 gr/cc; Neutron = 0.2, Sonic = 75 us/ft. If the formation matrix contains a mixture of limestone sandstone and dolomite, what is the multimineral percentage of sandstone, limestone, and dolomite and the true porosity in this reservoir. Solution Limestone ~ 20%, Volume of Sandstone ~ 37%, Dolomite ~ 24% Porosity of 19%. 9  Density Log Fluid  𝝆𝒇 (apparent) Primarily influenced by the mud filtrate density 𝝆𝒇 = 0.76 g/cm3 From plot of core porosity against density log per fluid type. 𝝆𝒇 = 𝑺𝒙𝒐 𝝆𝒎𝒇 + 𝟏 − 𝑺𝒙𝒐 𝝆𝒉𝒄 10  Density Log Density Log Video 11  Density Log Measurement Principle  Gamma rays emitted from radioactive source  Gamma rays collide with electrons in formation, losing energy  Detectors measure intensity of backscattered gamma rays  High energy GR (0.5-2) ==> Electron Density ==> Bulk Density ==> Porosity  Low energy GR ( Photoelectric absorption ==> Lithology Lithologic Density Tool The Lithology Density Pad (LDP) 12 12  Density Log Gamma ray interactions Three types of gamma ray interactions in earth formations  The photoelectric effect A specific gamma ray interaction will depend on  Compton scattering the atomic number of the material & the energy of the gamma ray.  Pair production ………. 13 13  Density Log Gamma Ray Scattering Compton Effect Nucleus Medium to High Energy GR’s - scattered by electrons in formation - each interaction loses energy Eo=ho E’=h’ - more electrons => more scattering e- Photoelectric Effect Eo=ho X ray Low Energy GR’s - absorbed by atoms - more electrons => more absorption - Indicates the atomic number - lithology e- 14  Density Log Notes  Chemical source  Cesium -137 material  33 year half-life  662 keV GR energy Source Type of interaction, depend on GR energy & atomic number of the absorber. The upper limit of Z for common minerals encountered in logging. 15  Density Log Measurement Principle GR Detector Electronics Spectrum Count Rate Energy After interaction, GR reaches crystal at different energy levels (both energy and counts of the GR are of interest) Detector pulse height proportional to GR energy The pulses are then sorted and counted by electronics Lithology Window The count rate vs energy distribution is called Spectrum Region of Photoelectric Effect which will vary with formation. ( & Z Information) Density Window Count/Sec Region of Compton Scattering (  Information Only) Source Energy 662 keV 16 Energy keV  Density Log What the Detector “Sees” Lithology Region (Photoelectric effect) Energy Spectrum Density Region (Compton scattering) 0.66 MeV Source 17  Density Log Electron Density  Density tool measures electron density. 18  Density Log Example: Bulk Density from Electron Density  For water which has formula of H2O, 𝝆 = 𝟏 𝒈𝒎/𝒄𝒄 what is the electron density of water  For Limestone which has formula of CaCO3, 𝝆 = 𝟐. 𝟕𝟏 𝒈𝒎/𝒄𝒄 what is the electron density of Limestone. 19 19  Density Log Example: Bulk Density from Electron Density 3.00 Sandstone RohB = 1.070 Rohe - 0.1885 Electron Density RohB Phi 2.50 RohB Poro 2.000 1.9527 0.442865 1.956 0.421 2.00 2.100 2.05976 0.380257 2.064 0.355 Bulk Density 2.200 2.16682 0.317649 2.171 0.290 1.50 2.300 2.27388 0.255041 2.278 0.225 1.00 2.400 2.38094 0.192433 2.386 0.160 0.50 2.500 2.488 0.129825 2.493 0.095 2.600 2.59506 0.067216 2.601 0.030 0.00 0.00 1.00 2.00 3.00 Electron Density 20  Density Log Example: Bulk Density from Electron Density 0.50 0.50 0.45 0.45 Sandstone 0.40 0.40 Corrected Density Porosity 0.35 Corrected Density Porosity 0.35 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 Sandstone 0.10 Anhydrite 0.10 0.05 Dolomite 0.05 0.00 0.00 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Density Porosity Using Limestone Matrix density Density Porosity Using Limestone Matrix density 21  Density Log Various Minerals and fluids  All the materials do not have the same (2Z/A) as water filled limestone. 22  Density Log Apparent Log Density to True Bulk Density  Given ROHB=2.40 g/cm3 in a sandstone formation containing dry gas, find the corrected bulk density.  The correction from y-axis is 0.022  Add the correction value to ROHB  Corrected ROHB = 2.422 gm/cm3 Dens-2 Schlumberger Chart book page 57 23  Density Log Borehole Correction 24 24  Density Log Bulk Density Interpretation  The density correction (D) curve is “measurement quality”  Poor pad contact gives D > 0.05 D  Often correlates with caliper -0.25 0 +0.25 CAL If correction > 0.20 g/cc, bulk curve is invalid 25  Density Log Factors Affecting Density Log Response  Shales and clays  May cause porosity reading to be too high or too low  Vsh and sh can be obtained from log readings in shale zones  Hydrocarbons ― In oil zones, hc = o which can be measured from fluid samples ― In gas zones, hc = g which can be measured or calculated using gas properties ― Gas will cause anomalously low density and, thus, high density porosity 26 26  Neutron & Density Logging Presentation and scales  Neutron porosity is presented in linear scale  The lithology must be assumed (limestone or sandstone)  Count rate from near and far detectors and the ratio (Near/Far) is available. 𝒎𝒂𝒙 𝑩𝒖𝒍𝒌 𝑫  𝜙N is overlaid on 𝜙D or bulk density. 𝒎𝒂𝒙 𝒇𝒍𝒖𝒊𝒅  Two primary overlays are compatible limestone and sandstone  Compatible scales facilitate lithology identification and gas detection 1.65 Bulk Density 2.65 øD =60 % øD =0 % 60 Neutron Porosity (Sandstone) 0 1.95 Bulk Density 2.95 øD =45 % øD =-15% 45 Neutron Porosity (Limestone) -15 27 The matrix density of limestone is 2.71 g/cc  Neutron & Density Logging Presentation and scales 28  Neutron & Density Logging TYPICAL LOG The particular scheme assumes that the lithology is expected to be predominantly sandstone. The neutron porosity, indicated to be in “sand” units, is scaled from 0.45 to−0.15 v/v, increasing to the left. The density scale is from 1.9 to 2.9 g/cm3, increasing to the right. The offset simply shifts the zero point on the neutron track to the density of the matrix; in this case of quartz sandstone, 2.65 g/cm3. 45 NPHI-Sand -15 1.9 Bulk Density 2.9 29  Density Log (2) “ Porosity from porosity logs is never MEASURED, it is CALCULATED from what we think might represent the porosity in the rock”  Density Log  In addition to ρb, we measure the photoelectric absorption index (Pe) of the formation.  Pe enables lithological interpretation without prior knowledge of porosity 𝟑.𝟔  The probability of this reaction taking place depends upon the energy of the incident gamma rays and the type of atom. Photoelectric Effect Low Energy GR’s Eo=ho X ray - absorbed by atoms - more electrons => more absorption - Indicates the atomic number - lithology e- 31  Density Log The Photoelectric Factor (PE) Log  Was introduced in the early-1980's as a supplementary measurement to the bulk density measurement and records the absorption of low- energy gamma rays by the formation in units of barns per electron. 𝟑.𝟔  The logged value is a direct function of the aggregate atomic number (Z) of the elements in the formation, and so is a sensitive indicator of mineralogy  PE for any mineral can be calculated, given its formula, by summing the separate elemental contributions multiplied by their weight.  PE is an excellent indicator of mineralogy because it is only mildly affected by pore volume magnitude or fluid/gas content. 32  Density Log Photoelectric Effect  The photoelectric absorption index of an atom 𝟑.𝟔 increases as its atomic number, Z, increases 𝒆  Photoelectric absorption index of an atom Material Pe Sand 1.81 Shale 3-4 Limestone 5.08 Dolomite 3.14 Salt 4.65 Anhydrite 5.05 33  Density Log Photoelectric Effect  The Compton scattering occurs over a wide energy range  Photoelectric adsorption only affects lower energy gamma ray Lithology Window Region of Photoelectric Effect ( & Z Information) Density Window Count/Sec Region of Compton Scattering (  Information Only) 662 keV Energy keV 34  Density Log The Compton scattering Vs. Photoelectric adsorption 35  Density Log Density Log  Photoelectric factor values for some minerals common in sedimentary rocks  Much less sensitive to pore volume changes than either the neutron porosity or density logs Photoelectric Absorption index 36 36  Density Log Photoelectric Effect  Measurement of Pe will give indication of Z of the formation mineral.  The lithologies can be discriminated independent of porosity. Average atomic number (Z) value for SS, LS, and DL for porosity of 0 to 40% 37  Density Log Capture cross-section  The logging curve is Pe  The product ePe = U, capture cross-section (Barns/cm3) U  (1   )U ma   U fl  This looks like the density equation  We don’t solve for f because Ufl