Petrophysical Well Log Analysis

Density Log

  • Definition: Measures the bulk density of geological formations using gamma ray scattering.
  • Formula: The relationship between electron density (Pe), bulk density (Pb), and the measured intensity of gamma rays (I) is given by: I=I0e−PeLI = I_0 e^{-PeL}
    • Where I_0 is the intensity of gamma rays emitted, and L is the distance from source to detector.
  • Applications:
    • Used to derive porosity using the formula:
      ext{Porosity ($Φ$)} = \frac{(P{ma} - Pb)}{(P{ma} - Pf)}
    • Helps in lithology interpretation.

Sonic Log

  • Function: Measures the sonic wave velocity in formations to determine porosity.
  • Basics: Involves sending sound pulses and recording the time taken for the wave to travel through the formation.
  • Key Equations:
    • The transit time (At) can be used to calculate porosity as:
      A<em>t,LOG=A</em>t,e+(1−Φ)At,maA<em>{t, LOG} = A</em>{t, e} + (1 - Φ) A_{t, ma}
    • Rearranging gives the relationship to compute porosity:
      φ=A<em>t,ma−A</em>tA<em>t,ma−A</em>t,fφ = \frac{A<em>{t, ma} - A</em>{t}}{A<em>{t, ma} - A</em>{t, f}}

Neutron Log

  • Principle: Emission of neutrons that collide with hydrogen atoms in fluids, resulting in gamma-ray emissions. This relationship is used to determine the porosity of the formation.
  • Key Properties:
    • Primarily measures hydrogen content, which is related to porosity.
    • Range: Neutron logs are sensitive to variations in borehole conditions and properties of the formation.

Corrections in Logs

  • Corrections are made to account for the effects of mud and borehole irregularities:
    • Density Tool: Uses two detectors at different distances from the source to correct for environmental effects.
    • The Gamma Ray logs must compensate for mud and mudcake effects by determining accurate transmission paths.

Cross-Plot Analysis

  • Purpose: Used to determine true porosity and lithology by comparing porosity data from density and neutron logs.
  • Interpretation:
    • Porosity cross-plots help delineate different lithologies by revealing the relationship between neutron porosity and density porosity.
    • Using standard cross-plot techniques can yield more accurate identification of fluid saturations in formations.

Equipment Evolution

  • Density and neutron tools' historical evolution has improved measurement accuracy and logging capabilities over years:
    • Examples of tools include the Powered Gamma Tool (PGT), Litho-Density Tool (LDT), and Compensated Neutron Log (CNL).
  • Modern Tools: Incorporate advanced detectors and calibration methods for precise logging in complex environments.

Summary of Log Types and Their Functions

  • Density Log
    • Tool for measuring bulk density and estimating porosity based on gamma ray interactions.
  • Sonic Log
    • Tool for measuring sonic velocity to obtain formation porosity and identify lithology.
  • Neutron Log
    • Provides measurements that are sensitive to gas and fluid content in the formation, crucial for hydrocarbon identification.