Neutron Log

1
Neutron Log
Detector bombarded by epithermal neutrons,
thermal neutrons, specific wavelengths of
gamma When used to measure porosity neutron log
is essentially measuring hydrogen density
Fast-neutron source (epithermal)
2

• 
  NEUTRON LOGS
• Formation bombarded by epithermal (fast)
  neutrons
• Bounce elastically when they collide with atoms
  except those of nearly equal
• mass e.g. hydrogen atoms
• Hydrogen atom collisions slow neutrons to
  thermal velocities
• Captured by nuclei of heavier atoms
• Release of gamma radiation specific to capturing
  atom
• Detector bombarded by epithermal neutrons,
  thermal neutrons, and specific
• wavelengths of gamma radiation
• Tool specifically designed to measure various
  properties
• When used as a porosity tool, essentially
  measures hydrogen atom density

3
Neutron Log
4
Log Worksheet 11
. Look for a zone where bulk density shows
potentially high porosity values (low bulk
density) and neutron log indicate low
porosity. Overlay neutron and bulk density logs.
Look for crossover. Mark zone in the depth track
Note upper and lower Elevations potential gas
zone.
5
Electric logs

• The dotted line is for the
• deep induction measurement
• The nearby solid line curve is
• for the short normal log
• The solid line to the right is
• a 5x short normal log
• The millimho curve (0-8000)
• is the magnified and inverse
• of the deep induction curve
• shown by the dotted line.
• .

6

• 
  RESISTIVITY LOGS
• For hydrocarbon detection
• Rely on difference in electrical conductivity
  properties between
• hydrocarbon reservoirs and water- filled
  zones - compare conductivity of
• saline water to that of petroleum
• Assumptions made about the composition of the
  water - fresh water
• at depth could be a problem because it has
  high resistivity
• Types of resistivity tools
• Those that require a conductive drilling mud
• Normal
• Lateral
• Laterolog
• Induction read in millimhos - units of
  conductivity
• (ohms 1000/millimhos)
• Deep induction produces a reading close to Rt
  (true resistivity of the
• formation)
• Measurements - ohm meters
• Resistance is measured in ohms

7

• 
  WORKSHEET 9
• What characteristics indicate that Point 1 is
  located in a clean, wet
• sandstone?
• Is the resistivity at Point 2 caused by a
  pore-filling cement or by
• hydrocarbons?
• Tabulate resistivities for Points 1-5

Location Short normal Deep Induction Deep induction (conductivity based) Petroleum potential
1.
2.
3.
4.
5.

• Petroleum potential indicate yes or no
  (non-shale
• horizons with resistivities higher than Ro
  (Formation resistivity)
• Note - porosities gt 15 are considered good
  for this exercize.

8
WORKSHEET 9 - RESISTIVITY MEASUREMENTS
Locations Short Deep Deep induction
1 0.26 0.20 0.20
2 7.50 6.00 6.70
3 1.50 0.50 0.50
4 1.08 0.80 0.98
5 0.94 0.80 0.98