RESISTIVITY LOGS.pdf

Chapter 8

Chapter 8

Induction logs

Induction logs

Lecture notes for PET 370

Lecture notes for PET 370

Spring 2011

Spring 2011

Prepared by: Thomas W. Engler, Ph.D., P.E.

••

_{Why resistivity logs?}

_{Why resistivity logs?}

Estimate Rt

Estimate Rt

••

_Invasion

_Invasion

••

_{Resolution (vertical, horizo}

_{Resolution (vertical,}

_horizontal)

_ntal)

••

_{Classification}

_{Classification}

- based

- based on depth

on depth of inves

of investigation

tigation

- salt vs fresh mud

- salt vs fresh mud

Resistivity Logs

Obsolete current LL7, LL3, guard DLL-MLL DLL-MSFL 1955-80 1972 Deep Laterolog (LLD) Laterolog-7 Laterolog-3 / Guard Shallow Laterolog (LLS) Micro-laterolog (MLL) MLL MSFL Salt Mud R _mf <2R _w Or  R _t>200 Current AIT 1990 Tool Induction Array Obsolete Obsolete Obsolete Current Current IES, IEL ISF DIL-LL8, DIFL, DISG DIL-SFL Phasor  1955-80 1970-85 1965 1975 1985 Induction (6FF40) Deep Induction Deep Phasor  Induction Medium Induction Medium Phasor  Induction 16”Normal Spherically Focused LL8/Short Guard Spherically Focused Spherically Focused Microlog (ML) Minilog Proximity (PL) Fresh Mud R _mf >2R _w Or  R _t<200 obsolete ES, EL up to 1955 18’ Lateral 64” Normal 16” Normal Comments  Name Years Deep 3+ ft Medium 1.5-3 ft Shallow 0.5-1.5 ft Flushed Zone 1-6 in. Obsolete current LL7, LL3, guard DLL-MLL DLL-MSFL 1955-80 1972 Deep Laterolog (LLD) Laterolog-7 Laterolog-3 / Guard Shallow Laterolog (LLS) Micro-laterolog (MLL) MLL MSFL Salt Mud R _mf <2R _w Or  R _t>200 Current AIT 1990 Tool Induction Array Obsolete Obsolete Obsolete Current Current IES, IEL ISF DIL-LL8, DIFL, DISG DIL-SFL Phasor  1955-80 1970-85 1965 1975 1985 Induction (6FF40) Deep Induction Deep Phasor  Induction Medium Induction Medium Phasor  Induction 16”Normal Spherically Focused LL8/Short Guard Spherically Focused Spherically Focused Microlog (ML) Minilog Proximity (PL) Fresh Mud R _mf >2R _w Or  R _t<200 obsolete ES, EL up to 1955 18’ Lateral 64” Normal 16” Normal Comments  Name Years Deep 3+ ft Medium 1.5-3 ft Shallow 0.5-1.5 ft Flushed Zone 1-6 in.

Resistivity Logs

•

_{IES or IEL; short normal or spherically focused curve with a deep induction.}

(No correction for invasion!)

•

_{Replaced in the mid-1960s with a Dual induction tool (shallow, medium,}

and deep curves); DIL-LL8, DIFL, DISG

•

_{Improved DIL with the addition of the spherically focused shallow log; DISF}

_{Next improvement is known as the}

_{Phasor Induction Tool}

_{- better thin bed}

resolution and automatic corrections

•

_{Latest generation induction tool known as}

 _{Array Induction Tool}

_{or Imager}

-eliminates need for assumption of a step function for the invasion profile.

Induction Log History

Resistivity Logs

Example: Old ES

19’ Lateral

10” Normal

32” LS Lateral

Bagley Penn Field Chambers No. 2 Lea County, NM

Resistivity Logs

Resistivity Logs

Example: High resolution

or Phasor induction

Conventional induction log

Phasor

Resistivity Logs

Example: Array induction

Resistivity Logs

•

_{where fresh mud or oil-base}

mud (or air-filled holes) is

used,

•

_{where the R}

mf 

/R

 ratio is

greater than 3,

•

_{where R}

t

 is less than 200

ohm-m, and

Resistivity Logs

• _{measures conductivity C= 1000/R}

(mmho/m)

• _{current is fed to a transmitter which}

generates a magnetic field and hence a circular current in the media.

• _{This current induces a voltage at the}

receiver .

• _{Array of transmitter-receiver coils;}

 jamming the signal into the desired formation. Thus, minimizes borehole, invasion, and adjacent bed effects.

• _{Typically, six or more coil pairs, 40”}

spacing from main coil to obtain deep reading.

• _{Only resistivity tool to run in air drilled}

holes.

Resistivity Logs

Theory

Induction Equivalent Circuit

Im Ixo IT s C s G t C t G xo C xo G m C m G ILD C or  s R  s G t R  t G xo R  xo G m R  m G ILD R  1        

Resistivity Logs

Example 1

Rt = 10 ohm-m

Fresh mud, Rm = 1

Rxo = 20

di = 65”

G(ILD) = 0.2

Ra = 11 ohm-m

(10% error)

Geometric Factors

Read

apparent

 resistivity from well log,R

Correct for

 borehol

e effect (if necessary)

Correct for

bed thickness

 effect (if necessary)

Correct for

invasion

 effect (if three curves are present)

True

 formation resistivity, R

Resistivity Logs

Given

1.5” standoff in a 14.6” hole

Rm = 0.35 ohm-m

ILD = 50 ohm-m (20

mmho/m)

Solution

hole signal = 5.7 mmho/m

Ct (corr) = Ct - hole signal

Rt = 70 ohm-m

Resistivity Logs

•

_{significant when}

- mud is salty

- hole size is large and/or oval

- formation resistivity is high

•

Corrections

greatest for

- induction tool against borehole, standoff is zero.

- ILM than ILD

-

holes > 12”

Resistivity Logs

•

_{The bed thickness effect is a f(bed thickness, vertical}

resolution of tool, resistivity contrast R

/R

)

•

_{Corrections necessary for:}

 –

_{thick beds w/ R}

t

/R

 >> 1

 –

_{thin beds with large R}

t

/R

contrast

Resistivity Logs

Bed thickness correction

Deep Induction Log Bed Thickness Correction (Schlumberger)

Resistivity Logs

Assume a step profile for invasion

Tornado charts to correct for invasion, determine R_t, R_xo, and d_i

• _{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

Example

R

_ILD

 = 10

_W

m

R

_ILM

 = 14

_W

m

R

_SFL

 = 90

_W

m

Invasion Correction

Resistivity Logs

Invasion Correction

Example

R

_ILD

 = 10

_W

m

R

_ILM

 = 14

_W

m

R

_SFL

 = 90

_W

m

Resistivity Logs

Invasion Correction

"

50

i

d

(5).

m

144

xo

R 

16

t

R 

xo

R 

(4).

m

9

.

8

ID

R 

*

ID

R 

t

R 

t

R 

(3).

chart

from

89

.

0

ID

R 

t

R 

(2).

4

.

1

ID

R 

IM

R 

;

9

ID

R 

SFL

R 

(1).

:

Solution

m

-90

14,

10,

are

readings

shallow

and

medium,

Deep,

:

Given





W









W





 

 



 

 









W

s

Correction

Invasion

of 

Example

Resistivity Logs

Example:

SFL-ILM-ILD

Resistivity Logs

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

Resistivity Logs

Array Induction Image Tool (AIT) or High Resolution Imager (HRI)

Main features:

1. full borehole corrections over a range of R_t/R_m 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 R_testimate and a quantitative description of the transition zone.

5. Capability of producing resistivity and saturation images of the formation.

Resistivity Logs

Example: Array induction

Resistivity Logs

Example: Array induction

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)