Productivity from shallow resistivity logs in CBM fields

1
Productivity from shallow resistivity logs in CBM
fields

• Yuanhai Yang, Kerr McGee
• Max Peeters, Geophysics department in CSM
• Thomas A. Cloud, ConocoPhillips
• Craig W. Van Kirk, PE department in CSM

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Outline

• Introduction
• Changes in shallow resistivity observed with
  different borehole fluids
• Effect of wellbore fluid resistivity on shallow
  resistivity readings
• Derivation of the method for calculating the
  cleat volume
• Cleat volume used as permeability indicator
• Applications
• Conclusions

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Introduction

• Drunkards Wash Unit
• Cretaceous Ferron coals 1st prod. 1993
• ConocoPhillips 509 wells
• Cum prod. 497 BCF
• Current prod. 140 MMCFD
• Large Contrast in well performance
• Why?

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Changes in shallow resistivity with different
borehole fluids
Laterolog 3 same Deep Induction much lower
5
Changes in shallow resistivity with different
borehole fluids
NEW
LOW
OLD
HIGH
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Effect wellbore fluid resistivity on shallow
resistivity

• RSFL/Rmf 100-500 Bit size 8
• Correction factor 0.95-1.05
• Changes in shallow resistivity reading is caused
  by exchange of fluids within cleats

Ratio corrected and original SFL log
Courtesy Schlumberger
Ratio SFL log and mud resistivities
7
Derivation of the method for calculating cleat
volumes
Assume Sw1
Archie equations
All resistivities in Ohmm Rt formation
resistivity, Ro 100 water saturated formation
resistivity Rw formation fluid resistivity,
Ohmm f formation porosity, volume fraction Sw
water saturation, pore volume fraction m
cementation exponent n saturation exponent
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Derivation of the method for calculating cleat
volumes
Assume m1.6
Approximate by
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Application

• Criteria for selection areas
• 1. Same type of shallow resistivity tool
• 2. Similar wellbore fluid
• Similar quality coal
• Similar depth of coal

Central Study Area
Southern Study Area
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Comparison Cleat volumes and Daily Production
rates
Cleat volume
Msft/day/ft net pay
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Conclusions

• The cleat volume calculated from the ratio of
  wellbore fluid resistivity and focused electrode
  tool resistivity is a CBM reservoir productivity
  / permeability indicator.
• The shallow resistivity measured by focused
  electrode tools is reliable under the high
  resistivity environment of CBM reservoirs.
• Even though the porosity of coal is small (1-4
  ), the salinity of the fluids within the cleat
  system can significantly affect the total
  formation conductivity.
• Relation between cleat volume and productivity
  can be explained by percolation theory and a
  critical cleat density (volume).