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Basic Well Log Analysis. Reading Rocks from Wireline Logs. Wire Line Logging. Tool strings used in wireline logging operations . Core-log Integration. Important Principles You Will Need To Know To Make Any Sense of the Wiggle Traces on Wireline Log Strip Charts.

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basic well log analysis

Basic Well Log Analysis

Reading Rocks from Wireline Logs

wire line logging
Wire Line Logging

Tool strings used in

wireline logging operations

Important Principles You Will Need To KnowTo Make Any Sense of the Wiggle Traces onWireline Log Strip Charts
  • Porosity = pore volume/total volume of a rock
    • Porosity can range from 0% to in excess of 40%
  • Saturation = volume of the porosity occupied by some fluid.
    • The possible fluids are almost always water or hydrocarbons; either liquid or gas.
    • SW = water saturation in percent,
    • 1 - SW is hydrocarbon saturation in percent.
  • Lithology = rock type, including fluid filled pores, with physical characteristics of:
    • Resistivity
    • spontaneous potential; SP
    • natural radioactivity; e.g. Gamma Ray emissions
    • bulk density
    • hydrogen content of rock and fluid filled pores
    • interval transit time (sonic velocity)
basic well log analysis5
Basic Well Log Analysis
  • Logs Help Define
    • physical rock characteristics
    • Lithology/mineralogy,
    • porosity,
    • pore geometry, and
    • permeability.
  • Logging data are used to:
    • identify productive zones,
    • determine depth and thickness of zones,
    • distinguish between oil, gas, or water in a reservoir, and
    • to estimate hydrocarbon reserves
log properties of interest
Log Properties of Interest
  • The most frequently used logs are open hole logs
    • Logs are recorded in the uncased portion of the wellbore.
  • The two primary parameters determined from well log measurements are
    • Porosity, fluid composition and relative saturation
  • Log interpretations are determined by one of three general types of logs:
    • Electrical
    • Nuclear
    • Acoustic or sonic logs
bore hole environment
Bore Hole Environment
  • Where a hole is drilled into a formation, the rock plus the fluids in it are altered in the vicinity of the borehole
borehole environment
Borehole Environment
  • The formations encountered in the bore hole during drilling are invaded to some extent by drilling fluids ("mud")
  • Mud is used to
    • lubricate the bit,
    • circulate the broken rock fragments produced during drilling and most significantly to
    • maintain pressure in the hole to prevent blow out.
  • The mud invades the formation to at least some degree
    • in order to make useful physical measurements of the insitu rock properties the measurement's must be made well into the rock (if possible) or
    • mud infiltration must be accounted for.
cased holes
Cased Holes
  • Steel pipe "casing" is set in bore holes to prevent damage and caving
  • Only certain down hole tools can make useful measurements through pipe, ie.
    • gamma ray,
    • neutron porosity
lithology logs
  • Natural Gamma Ray (γ-ray) Logs
    • Decay of radioactive elements produces high energy gamma ray emissions
    • Radioactive elements (K, U, Th) are normally concentrated in shaley rocks while most sandstones are very weakly radioactive.
    • Because radioactive material is concentrated in shale, shale has high gamma ray log readings
    • Clay-free sandstone and carbonate rocks have low gamma ray log readings
neutron logs
Neutron Logs
  • Neutron logs (NL or GRN) measure the hydrogen ion concentration in a formation.
    • In clay-free formations where porosity is filled with water or hydrocarbons the neutron log measures liquid filled pores (the only significant occurrence of hydrogen).
    • The neutron log measures energy loss when neutrons emitted from the tool collide with other particles in the formation.
    • The maximum energy loss during a neutron collision occurs when
    • A neutron collides with a particle of equal mass, that is a hydrogen atom.
neutron logs13
Neutron Logs
  • A lower neutron log reading (fewer energetic back scattered neutrons) indicates abundant formation hydrogen.
    • Clay rich formations contain hydrogen in the crystal structure ofthe clay minerals and give anomalous values for liquid filled pore volume.
  • Neutron log excursions (decreasing in value from right to left) indicate higher proportions of hydrogen in the Formation
    • either increased liquid filled porosity or
    • higher shale content.
  • Neutron log excursions increasing from left to right indicate
    • less porosity and/or
    • less shale
compensated neutron logs
Compensated Neutron Logs
  • Newer “radiation” logs called CNL (for compensated neutron logs) are calibrated so that the scale is in porosity units, or neutron porosity units
  • The CNL (sometimes called the NPHI, for Neutron porosity {φ}) is almost always displayed with
  • The formation density log and these logs, in combination, can be used to infer lithology
bulk density
Bulk Density
  • Formation density (compensated; FDC) logs measure the density (grams/cm3) of the formation based on the density of electrons in the formation
  • Electron density is a function of the absolute amount of matter comprising the formation
    • measured by the back scatter of gamma rays emitted from a gamma ray source in the logging tool
bulk density17
Bulk Density
  • The absolute amount of matter in the formation is
    • inversely proportional to the degree of gamma ray penetration into the formation without back scatter to the detector
  • Since the tool averages the electron density
    • porous formations composed of dense minerals will appear similar to low porosity formations with lower density rock matrix
  • Bulk density is read on a log increasing from left to right.
lithology interpretation from fdc cnl logs
Lithology interpretation from FDC-CNL logs
  • An industry standard "quick-look" overlay methodology can be used with CNL-FDC wire-line logs
  • When Neutron porosity (CNL dashed curve) and Bulk Density (FDC, solid curve) logs are overlain on a common, limestone equivalent porosity scale changes in lithology can be inferred with depth

Hypothetical neutron-density overlay patterns for simple log-based lithofacies. The overlay uses a common calibration to an equivalent limestone porosity scale.(From Doveton, 1986).

the photoelectric index pe or pef
The Photoelectric Index (PE or PEF)
  • The photoelectric index (Pe or PEF) is a supplementary measurement by the latest generation of density logging tools
  • PEF records the absorption of low-energy gamma rays by the formation in units of barns m() per electron
  • 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.
  • The common reservoir mineral reference values are : quartz 1.81 ; dolomite 3.14 ; calcite 5.08 barns/electron.
e logs
  • Electric logs, resistivity and spontaneous potential, were the first wireline logging tools.
  • Instruments were (and still are) lowered down bore holes and physical measurements were made regarding the electrical properties of the rocks encountered.
  • Resistance of rock

R = rA/L (ohm-meter2/meter, contracted to ohm-meter or ohm-m)

    • r is the resistance (ohms)
    • A is the cross-sectional area
    • L is the length of the resistor
  • The resistivities of sedimentary rocks are determined by the rock component types and their geometry.
    • hydrocarbons, rock, and fresh water are all insulators (nonconductive, or at least very highly resistive) to electric current flow.
    • Salt water is a conductor and has a low resistivity
  • The measurement of resistivity is a measurement of the amount (and salinity) of the formation (connate) water.
spontaneous potential
Spontaneous Potential
  • Electrical current generated across
    • the boundaries between formation fluids and drilling fluids (if these fluids are of different salinity) and
    • the boundary between interbedded shale and sandstone.
  • The spontaneous potential associated with shale and sandstones is the result of higher permeability in sandstone relative to lower permeability in shale.