Density log


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Density log

  1. 1. 1
  2. 2. PresentationonDensity logMuhammad Zubair Idrees2
  3. 3. 1.0 INTRODUCTION 1.1 Well logging 32.0 THE DENSITY LOG 2.1 Definition 4 2.2 Introduction 5 2.3 Principle 8 2.4 Log presentation scale and units 11 2.5 Tool 14 2.6 Log characteristics 17 2.7 Geological Applications 18 2.8 References 423
  4. 4.  Definition:The continuous recording of ageophysical parameter along a boreholeproduces a geophysical well log . Main objective of well-logging is formationevaluation. Well-logging is done in most oilwells, mining exploration wells, and in manywater wells.4
  5. 5. DefinitionDensity logging is a well logging tooldetermining rock bulk density along awellbore.5
  6. 6.  Geologically, bulk density is a function of thedensity of the minerals forming a rock (i.e.matrix) and the enclosed volume of freefluids (porosity). Density is one of the most important piecesof data in formation evaluation.6
  7. 7.  In the majority of the wells drilled, density isthe primary indicator of porosity. In combination with other measurements, itmay also be used to indicate lithology andformation fluid type.7
  8. 8.  A radioactive source applied to the boreholewall emits gamma rays into the formation sothese gamma rays may be considered as highvelocity particles which collide with theelectrons in the formation. At each collision the gamma ray loses someof its energy to the electron, and thencontinues with diminished energy.8
  9. 9. 9
  10. 10.  This type of interaction is known as Comptonscattering. The scattered gamma raysreaching the detector, at the fixed stationfrom the source, are counted as an indicationof formation density. The denser the formation, the more electronsare presented, and more energy is lost due tocollisions10
  11. 11. 11
  12. 12.  The density log is generally plotted on alinear scale of bulk density. The log is run across track 2 and 3. Most often its scale is between 1.95 and2.95 g/cm3. The main log is accompanied by a curve thatshows the borehole and mud-cakecorrections that have been applied.12
  13. 13.  A record of cable tension may also beincluded, as the density tool tends to stick inpoor holes. A correction curve, is sometimes displayed intrack 3 and less frequently in track 2. The gamma ray and caliper curves usuallyappear in track 1.13
  14. 14. 14Figure showing density log
  15. 15.  The standard density tool has a collimatedgamma ray source (usually radio cesiumwhich emits gamma rays, radio cobalt is alsoused). It has two detectors (near and far) which allowcompensation for bore hole effects whentheir readings are combined and compared incalculated ratios.15
  16. 16.  The near detector response is essentially dueto borehole influence which, when removedfrom the far detector response enhance theformation effects. The most recent density tools use moreefficient scintillation detectors which separatehigh and low energy gamma levels. Source and detectors are mounted on aplough shaped pad.16
  17. 17. Density Tool17
  18. 18.  The tool is run typically as a density-neutroncombination along with a gamma ray tool and acaliper. Its vertical resolution is 33 inches. Depth of investigation is 1.5 inches.The tool can be run in Open hole Cased hole. Borehole fluid of gas or air, water or water basedmud, oil or oil based mud. The logging speed of the tool is 60 feet/minute.18
  19. 19. POROSITY CALCULATION: To calculate porosity from the log derived bulkdensity it is necessary to know the density of all theindividual materials involved. By knowing the grain (matrix) density and the fluiddensity, the equation can be solved that givesFrom the summation of fluids and matrixcomponents.ρb = Ф x ρf + (1 – Ф) x ρmaWhere ρma = matrix (or grain) densityρf = fluid densityρb = bulk density(as measured by thetool hence include porosity and density of grains).19
  20. 20.  When solved for porosity this equationbecome:PorosityФ = (ρma - ρb)/(ρma - ρf) Erroneous porosities may be calculated whenthe fluid density changes. This is the casewhen a rock is saturated with gaseoushydrocarbons. In the presence of gas the fluiddensity drops dramatically. The density loggives too high porosity.20
  21. 21.  When oil is present the porosity given by thedensity log is essentially correct because thedensity of oil is quite close to that of water.Gas is more mobile and frequently occursbecause of large density difference withwater.21
  22. 22. Figure showing the effect of gas on density log. In this example gas zone reads about 35%porosity, it should be 27% 22
  23. 23. LITHOLOGY IDENTIFICATION The densities of the common lithologies arerarely diagnostic since there is too muchoverlap. Overall, oilfield densities generallymeasure between 2.0 g/cm3 and 3.0 g/cm3. The density log is itself a poor indicator oflithology, combined with the neutron log itbecomes best qualitative indicator of litholgy.23
  24. 24. Figure showing density ranges of some commonlithologies24
  25. 25.  The compaction of shales with burial is a wellknown phenomenon and it can be followedon the density log. Shale compaction involves a series of texturaland compositional changes, resulting in aprogressive increase in density. For example shallow, un-compacted clayshave densities around 2.0g/cm3, while atdepth, this figure commonly rises to2.6g/cm3.25
  26. 26. Figure showing shale compaction with depth seen on a bulk density log plotted at acompressed vertical scale 26
  27. 27.  shale density is often indicative of age. In general, older shales are denser. Paleozoic clays are rare, as are Tertiaryshales. The increase in shale density duringcompaction, although essentially due to adecrease in porosity is accompanied byirreversible diagenetic changes. In the subsurface, a change in compactiontrends will indicate a change in age, in otherwords an unconformity.27
  28. 28. Figure showing tertiary shale uncomformably overlying cretaceousshale. The abrupt change in density marks the unconformity.28
  29. 29.  Local variations in shale density are morelikely due to changes in shale composition. The increase in density is even more markedwhen iron carbonate is involved. Whenorganic matter is present, the reverse occursand the density diminishes,29
  30. 30.  Organic matter having a very low density. An increase in carbonate content is generallyaccompanied by an increase in shale density.30
  31. 31. Figure showing thin carbonate/sideritecemented horizons in shale. Theintervals may be thin continuous bands31
  32. 32.  Bulk density variations in sandstone generallyindicate porosity changes. This is not true when there are changes ingrain density. Overall grain density will change dependingon the non-quartz constituents. Sands are commonly mixed with feldspars(density 2.52 g/cm3), micas (2.65-3.1g/cm3).32
  33. 33.  Heavy minerals may also be a constituent(2.7-5.0 g/cm3). Changes in grain density in sands are gradualand of a moderate order. Abrupt changes, especially in homogenousbeds, often indicate diagenetic or secondarychanges.33
  34. 34. Figure showing the effect of muscovite on the bulkdensity log in micaceous sands. The increase indensity below15m is due to mica content34
  35. 35. Figure showing secondary calcareouscementation in sandstone.35
  36. 36.  Density becomes a criterion for lithologicalidentification when it is either abnormallyhigh or abnormally low. Coals, for example, are identified by very lowdensities, between 1.2 g/cm3 and 1.8 g/cm3 Pyrite has a very high density between 4.8g/cm3 and 5.17 g/cm3.36
  37. 37. Figure showing coal, with low density and pyritewith high density, on the bulk density log37
  38. 38.  Chemical deposits, because of theirpurity, may be identified by their densities.Most evaporates tend to give intervals ofconstant density with little variation. Whenthis occurs, along with densities near thepure mineral values, evaporates are probable.38
  39. 39. Figure showing bulk density log over asalt shale series.39
  40. 40.  The presence of organic matter in shaleslowers their density. The normal average matrix density of amixture of clay minerals is about 2.7g/cm3,while organic matter has densities between0.50 -1.80g/cm3. The presence of organic matter therefore hasa marked effect on the overall shale bulkdensity. .40
  41. 41.  This organic matter effect on the density logcan be quantified, so that the log can be usedto evaluate source rocks. Difficulties arises when organic matter ismixed with a high density mineral such aspyrite (4.8-5.17g/cm3), Since the density ofthe pyrite masks the effect of the low densityorganic matter.41
  42. 42. Figure showing effect of organic matter on thedensity log.42
  43. 43.  Schlumberger, 1998, log interpretationcharts; Schlumberger wire-line andtesting, SMP-7006, sugar land, Texas. Rider, M.H., 1986. The geologicalinterpretation of well logs, Blacky and SonLimited, Bishopbrigg, Glasgow.175P www.Wikipedia.org43
  44. 44. Any questions?44
  45. 45. Thank you45