RESISTIVITY LOGS.pdf

Chapter 8 Induction logs

Lecture notes for PET 370 Spring 2011 Prepared by: Thomas W. Engler, Ph.D., P.E.

Resistivity Logs •

Introduction

Why resistivity logs? Estimate Rt

•

Invasion

•

Resolution (vertical, horizontal) horizontal)

•

Classification - based on depth of investigation investigation - salt vs fresh mud

Resistivity Logs Flushed Zone

Classification Shallow

Medium

Deep

0.5-1.5 ft

1.5-3 ft

3+ ft

Years

 Name

Comments

1-6 in. 16” Normal

Fresh Mud

Microlog (ML)

16”Normal

Minilog

Spherically

R mf >2R w Or 

64” Normal

18’ Lateral

obsolete

1955-80

IES, IEL

Obsolete

(6FF40)

1970-85

ISF

Obsolete

1965

DIL-LL8,

Obsolete

Focused Proximity (PL)

LL8/Short Guard Spherically

DIFL, Medium

Deep

Induction

Induction

Focused Spherically Focused

Array

Mud

ES, EL

Induction

R t 12”

Resistivity Logs

Bed thickness correction

•

The bed thickness effect is a f(bed thickness, vertical resolution of tool, resistivity contrast R t/Rs)

•

Corrections necessary for:  –

thick beds w/ Rt/Rs >> 1  – thin beds with large Rt/Rs contrast

Resistivity Logs Bed thickness correction

Deep Induction Log Bed Thickness Correction (Schlumberger)

Resistivity Logs

Invasion Correction

Assume a step profile for invasion

Tornado charts to correct for invasion, determine R t, Rxo, and di •

Rt correction factor between 0.75 and 1.0

•

Depth of invasion is reflected by induction resistivity contrast: - Rim/Rid > 1.5 indicates deep invasion - Rim/Rid < 1.2 indicates shallow invasion

Resistivity Logs Invasion Correction

Example RILD = 10 Wm RILM = 14 Wm RSFL = 90 Wm

Resistivity Logs Invasion Correction

Example RILD = 10 Wm RILM = 14 Wm RSFL = 90 Wm

Resistivity Logs

Invasion Correction

Example of  Invasion Corrections

Given : Deep, medium, and shallow readings are 10, 14, 90 W - m Solution : (1).

R SFL R   9; IM  1.4 R ID R ID

(2).

R t  0.89 from chart R ID

(3).

  R t    * R ID  8.9W  m R t    R ID  

(4).

R xo  16 R xo 144W  m R t

(5).

d i 50"

Resistivity Logs Example: SFL-ILM-ILD

Big Lake (San Andres)

Resistivity Logs

Phasor Induction

Improvements (1). Thinner bed resolution (2 ft.)

(2). Comprehensive set of automatic corrections for: - shoulder effect and thin bed resolution - skin effect - borehole and cave effect - large boreholes - invasion effects

Resistivity Logs Example: High resolution or Phasor induction

Conventional induction log

Phasor induction log

Resistivity Logs

Array Induction

Array Induction Image Tool (AIT) or High Resolution Imager (HRI) Main features: 1.

full borehole corrections over a range of R t/Rm contrasts

2.

the ability to use short array information to solve for effective borehole parameters

3.

Five log curves are presented at median depths of investigation of 10, 20, 30, 60 and 90 inches. Three vertical resolutions of 1, 2 and 4 ft.

4.

improvement in invasion profiles for both oil- and water-based muds. This includes accurate Rt estimate and a quantitative description of the transition zone.

5.

Capability of producing resistivity and saturation images of the formation.

Resistivity Logs

Example: Array induction

 Ambiguous reading Correct reading

DIL log  AIT log

Resistivity Logs

Example: Array induction

Estimated invaded volumes from AIT log (Halliburton)

Resistivity Logs

References

Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series, Vol. 4, (1994) Chapter 5, Sec 5 and 6

Schlumberger, Log Interpretation Charts, Houston, TX (1995) Western Atlas, Log Interpretation Charts, Houston, TX (1992)