4- Log Resistivitas

7/27/2019 4- Log Resistivitas

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Well LogSP Resistivity

RESISTIVITY LOG

Most abundant data for formation evaluation and

determination of fluid saturations


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TYPICAL RESISTIVITY LOG RESPONSE

RESISTIVITY LOG

With know ing of f lu ids resi t iv i ty,

we cou ld ident i fy the f lu ids

content (Gas, Oil , Water)

GR (API) 1500

Sand

Sand

Gas/Oil

Water

Gas/Oil

Res (ohm-m) 200.2

Contact

GR

Res


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IDEALIZED LOG SET

= 0.30

= 0.35

= 0.07

R = 0.4

R = 0.3

R = 4

R = 8

Sand

Shale


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FOUR COMPONENTS OF SANDSTONE

MATRIXFRAMEWORK

(QUARTZ)

FRAMEWORK(FELDSPAR)

CEMENT

PORE

Note different use of matrix

by geologists and engineers

0.25 mm

1. Framework2. Matrix

3. Cement

4. Pores

Engineering

matrix

Geologists Classification

Ayers, 2001


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FLUID SATURATIONS

Grain Water Gas Oil

Initially, water fills pores and wets the rock surface

Hydrocarbons migrate into the reservoir rock, displacing some water

Hydrocarbon distribution determined by gravity and capillary forces,

and by wettability

and matrix


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RESISTIVITY

Resistivity

The voltage required to cause one amp to pass

through a cube having a face area of one

square meter

Units are ohm-m2/m; usually ohm-m (.m)

tyConductivi

1yResistivit


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RESISTIVITY OF EARTH MATERIALS

tyConductivi

1yResistivit

Incr

easing

Res

istivity

(1) Rock

(2) Gas

(3) Oil

(4) Fresh Water

(5) Salt Water

Increasing

Conductivity


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Resistivity of water

Porosity of the formation,

Pore geometry - tortuosity Lithology of the formation

Degree of cementation, and

Type and amount of clay in the rock

FACTOR AFFECTING RESISTIVITY


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ELECTRICITY AND EARTH

MATERIALS

Conduction is by ions in water

Na+ and Cl- are very common Other monovalent ions K+ and OH-

Common bivalent ions: Ca++, Mg++


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Resistivity of NaCl Solutions

(GEN-5 H or GEN-9 S)


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ARPS FORMULA

For constant solution R1(T1 + 7) = R2(T2 + 7) (T in deg F)

R1(T1 + 21.5) = R2(T2 + 21.5) (T in deg C)

Example

Rm = 0.32 ohm-m @ surface (25 deg C)

What is Rm at 145 deg C?

R2 = R1(T1 + 21.5)/(T2 + 21.5) R2 = 0.32(25+21.5)/(145+21.5) = 0.089 ohm-m

Check this on the chart!


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ARCHIES FIRST LAW

As the salt water content increases, the formationresistivity will decrease.

A rock containing oil or gas will have a higherresistivity than the same rock completely saturatedwith salt water.

As the shale content increases, the rock matrix willbecome more conductive.

Ro = Resistivity of a rock that is 100%saturated with formation water

Rw = Resistivity of formation waterRo = F Rw

F = Formation factor (formation resistivityfactor)


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The formation factor (F) depends on:

Porosity of the formation;

Pore geometry; Lithology of the formation;

Degree of cementation; and

Type and amount of clay in the rock.

FORMATION FACTOR


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For a clean formation (no shale), the formation factor

can usually be empirically correlated with porosity.

a = constant 1.0 for most formationsm = cementation factor 2 for most formations

Common values

F = 0.8/2 (Tixier) or F = 0.62/2.15 (Humble) for

sandstones

F = 0.8/2 for carbonates

m

aF

FORMATION FACTOR


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Formation Resistivity Factor Versus Porosity


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RELATION OF FORMATION FACTOR

TO VOLUMETRIC POROSITY


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RESISTIVITY FACTOR TO SHALINESS

The formation factor (F) is constant for a clean sand;

Fdecreases for shaly sand as value ofRwincreases


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IfRt= R0, then the formation is 100%saturated with formation water. IfRt> Ro,

then the formation contains oil or gas.

General formula:

t

m

w

t

w

t

on

w

R

Ra

R

RF

R

RS

**

For clean sands, n = 2is common.

Like a and m, nis measured in the lab.

ARCHIES SECOND LAW


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LAB EVALUATION OF n


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DRILLING DISTURBS FORMATION

Drilling and rock crushing Damage zone

Mud systems and invasion

Oil based mud

Small conductivity mud

Shallow invasion

Thin cake

Water based mud

Moderate to very conductivemud

Shallow to deep invasion

Thin to thick cake

Mudcake

Invading filtrate

Damaged zone


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EFFECTS OF DRILLING AND

MUD FILTRATE INVASION


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MUD FILTRATE INVASION

Modified from J. Jensen

Wellbore

Mud

(Rm)

Mud Cake(Rmc)

UninvadedZone

(Rt)Invaded

Zone (Rxo)

UninvadedZone

(Rt)


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Resistivity of zone

Resistivity of the water in the zone

Water saturation in the zone

R1

Rs

Rs

Rw

Sw

Rm

Mud

hmc Flushedzone Zone of

transitionor

annulus

didj

Adjacent bed

rj

dh

Hole

diameter

Adjacent bed

h

dh

Uninvadedzone

(Bed

thickness)

(Invasion diameters)

Sxo

Rm1

Rxd

Rmc

Mudcake

After Schlumberger

SYMBOLS USED

IN LOGINTERPRETATION


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Borehole

Rm : Borehole mud resistivity

Rmc : Mudcake resistivity

Invaded zone

Rmf : Mud filtrate resistivityRxo : Invaded zone resistivity

Sxo : Invaded zone water saturation

Uninvaded zone

Rw : Interstitial water resistivity

Rt : Uninvaded zone resistivity

Sw : Uninvaded zone water saturation

COMMON TERMINOLOGY


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SUMMARY - RESISTIVITY

Resistivity important property Depends on ions in water

Water resistivity depends on

Concentration

Temperature

Species

Archies First Law relates rock resistivity to Rw

Archies Second Law relates Sw to Rt


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Uninvaded Zone resistivity,Rt measurement is affected by:

Mud resistivity, Rm Mudcake resistivity, Rmc

Flushed zone resistivity, Rxo

Three resistivity measurements are made to get an accurate

resistivity, Rt, since no tool can read uninvaded zone (as far

as 5-7 ft from borehole) resistivities without compromising

vertical resolution or being affected by flushed zone

resistivity, mud resistivity and mudcake resistivity.

Typical depths of investigation:

Resistivity Tool Depth of Investigation

Deep resistivity 5-7 feet.

Shallow resistivity 2-3 feet.

Micro resistivity A few inches ( 2~3 inches)

WHY 3 DIFFERENT RESISTIVITY MEASUREMENTS?


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To obtain a value for Rt, we use of correction charts The three resistivity measurements are corrected for the

presence of mud, mudcake and invasion effects and also

for other environmental factors like borehole size and tool

position in the borehole.

NOTE :When doing a Quicklook analysis on the wellsite, it

is common practice to use the deep measurement as an

approximate value for Rt and the shallow measurement anapproximation of Rxo.

CORRECTION CHARTS


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There are two kinds of Resistivity tools :

Resistivity tools - Measure resistivity, eg.

Dual LaterologConductivity tools - Measure conductivity,

eg. Array Induction

Tool

RESISTIVITY TOOLS


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Deep and shallow measurement systems worksimultaneously - this is the dual aspect of the tool

Deep and shallow measurement systems operation at

different frequencies of and have different depths of

investigation

Both measurements maintain the same vertical

resolution of 2 feet

All currents (deep and shallow) are emitted

circumferentially from the tool in a disc pattern

RESISTIVITY TOOL: THE DUAL LATEROLOG


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LLD-LLS CURRENT PATTERNS


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ttxoxommLL RJRJRJR

PRINCIPLE OF OPERATION

Where Jm, Jxo, Jt are geometric factors giving the

contribution of each zone to the resistivity read by the tool.

In the laterolog tool, the zones add up in series. The tool

responds therefore to the most resistive zone and hence

the DLL is used where:

Mud is saline or WBM (low resistivity); or

Formation is highly resistive.

A rule of thumb is to use the DLL when Rw/Rt>2.5


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These tools provide conductivity measurements

They provide both deep and a shallow measurements

A transmitter coil with an alternating current passing

through it sets up an alternating magnetic field through the

borehole and formation. Faradays Law predicts that thistime varying field will establish an EMF in the formation that

causes current flow through the formation in circular paths.

Magnitude of these currents is proportional to the formation

conductivity.

Current loops produce their own magnetic fields that cutthrough the receiver coil and induce an alternating voltage

at the receiver that is proportional to the strength of the

secondary magnetic field and, hence, the formation

conductivity.

CONDUCTIVITY TOOLS: INDUCTION TOOLS


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INDUCTION TOOL OPERATION


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There are current loops in the mud, in the mudcake, in the

invaded zone and in the uninvaded zone

Therefore, apparent conductivity read by the induction tool,

Ca is:

ttxoxomma CGCGCGC

Measurements are corrected for the effects of the borehole

and other environmental effects

Tool responds to the most conductive medium surroundingit, so we use an induction tool where we have:

Low salinity mud or OBM (low conductivity);

Low resistivity formations; or

Where Rw/Rmf> 2.5

PRINCIPLE OF OPERATION


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SELECTING THE RESISTIVITY TOOL


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Micro-resistivity has a very shallow depth of

investigation - approximately 2-3 inches.

Micro-resistivity is measured with tools like the

micro-spherically Focused Tool.

All three types of resistivity tools help determine Rt,

with the use of correction charts.

RESISTIVITY TOOL SUMMARY


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RESISTIVITY TOOL

APPLICATIONS

Well to well correlation

Sxo and Sw computation

Presence and depth of hydrocarbons

Invasion profile analysis/Imaging


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Rt

Ro

Rw

Cube of water

having resistivity,Rw

Non-shaly rock, 100% saturated

with water having resistivity,

Rw

Rock containing pores saturated

with water and hydrocarbons

Increasing

Resistivity

(1) Rock

(2) Gas

(3) Oil

(4) Fresh Water

(5) Salt Water

Increasing

Conductivity

= 100%Sw = 100%

= 20%

Sw = 100%

= 20%Sw = 20%

mw

oa

RRF

SATURATION MEASUREMENT

FROM RESISTIVITY


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xo

mf

m

n

xo

t

w

m

n

w

R

RAS

R

RAS

Uninvaded Zone water

saturation

Invaded/Flushed zone water

saturation

Archies Law


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n/1

tm

ww

R

RaS

VARIABLES DETERMINED

FROM WIRELINE LOGS


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ARCHIES EQUATION

n

t

m

w

w

R

Ra

S

Water

saturation,

fraction

wS

Resistivity of

formation water,-mw

R

Resistivity of

uninvaded

formation, -mt

R

Porosity,fraction

Empirical constant

(usually near unity)

a

Saturation

exponent(also usually

near 2)

n Cementationexponent

(usually near 2)

m


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Archies equation is based on the followingrelationships

HOW ARCHIES EQUATION WORKS

Rock type 1

Rock type 2

1000

100

10

1.01 .1 1.0

FR

Solving for Ro when

water saturation is 100%

mw

o a

R

R


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Solving for Ro when

is constant

HOW ARCHIES EQUATION WORKS

Rock type 1

Rock type 2

1000

100

10

1.01 .1 1.0

Sw

IR=

Rt

R0

nwo

t

S1

RR

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4- Log Resistivitas