resistivity logs.ppt

1 FE – OverviewFarag July 2002

Resistivity Logging


Resistivity LogResistivity Log

• Basics about the Resistivity:

• Resistivity measures the electric properties of the formation,• Resistivity is measured as, R in W per m,• Resistivity is the inverse of conductivity,• The ability to conduct electric current depends upon:

• The Volume of water, • The Temperature of the formation,• The Salinity of the formation

The Resistivity Log: Resistivity logs measure the ability of rocks toconduct electrical current and are scaled in units of ohm-meters.The Usage: Resistivity logs are electric logs which are used to:

Determine Hydrocarbon versus Water-bearing zones, Indicate Permeable zones, Determine Resisitivity Porosity.


Resistivity

• Resistivity is the opposition to flow of electrical current offered by a material of unit length and unit cross sectional area. It is a property of the material itself, not of its shape or dimensions.

Unit length, L

Current, I

Voltage, V

Unit area, A


Resistance and Resistivity

• Ohms Law: V = I R

• Resistance, R = r (L / A)

• Therefore V = I r (L / A)

• So r = (V / I) (A / L)

• Let k = A / L (Geometric Factor)

• So, r = (V / I) k


Conductance and Conductivity

• Conductance is the reciprocal of resistance. It represents the ease with which current flows through a given circuit.

• Conductivity is the reciprocal of resistivity. It represents the ease with which current flows through a certain material. It is a property of the material itself.

• Resistance unit: ohm (Ω)• Resistivity unit: ohm-meter (Ω·m2/m, or Ω ·m)• Conductance unit : siemens or mho • Conductivity unit: siemens/meter or mho/meter.


Resistivity Logging

There are two types of resistivity logging tools:

• Laterolog tools send a current from electrodes on the logging tool, through the formation, to a return electrode located either at surface or downhole.– Laterolog tools need a conductive path between the logging tool

and the formation

• Induction tools generate current loops in the formation and measure the strength of the electromagnetic signal created by these current loops. – Induction tools do not require a conductive path in the borehole.

They work in oil-base muds and air-filled holes.


Basic Resistivity tool types

Induction Logging ToolsLaterolog Logging Tools


RxoRt

Rm

Rm

Rxo

Rt

Logging ToolBorehole

Uninvaded Zone

Invaded Zone

Laterolog vs. Induction Current Path

Laterolog Response

Induction Response


Resistivity

There are two main techniques for measuring resistivity:

Laterolog : Current is injected into the formation

Induction : Current loops are induced in the formation


LatrologsPEx-HALS


Resistivity : Laterolog

Current is forced through the mud into the formation


Dual Laterolog Measurement

LLD: deep measurement, currect returns to surface.

LLS: shallow measurement, current returns to the tool itself.


Bucking Current

Measure Current

A2

A1

M2

M1

A0

280 HzCurrentSource

MonitoringLoop

Resistivity : Laterolog Shallow (LLS)


Bucking Current

Measure Current

A2

A1

M2

M1

A0

A1*

35 Hz Aux Mon.

Loop

MonitoringLoop

Bucking Current

FishLCM Module

35 Hz Current

Resistivity : Laterolog Deep (LLD)


Resistivity : Laterolog invasion effects

RLL = Vm Rm + Vmc Rmc + Vxo Rxo + (1-Vm-Vmc-Vxo) Rt


Resistivity : Laterolog



Rmc

Rm

RXO

Rt

Ra = VRm(Rm) + VRmc(Rmc) + Vrxo(Rxo) + VRt(Rt)


If Rmf < Rw then Rxo < LLS < LLD < RtIf Rmf > Rw then Rxo > LLS > LLD > Rt

Using Rxo, LLS & LLD, Rt and Di can be computed



Resistivity : Laterolog azimuthal measurements


LatrologsEnvironmental corrections


Groningen Effect

• Caused by highly resistive beds overlying the formation that is being measured.

• This forces the deep current into the mud column.

• This is caused by the voltage reference (cable-torpedo) becoming non-zero.

• LLd reads too high• More pronounced at low resistivity


Borehole & Invasion Effects

Ra = Vm*Rm + Vxo*Rxo + Vt*Rt

Resistivities measured in series


Invasion Enhances Shoulder Effects


Shoulder-Bed Squeeze Effects

Deep measurement reads too high and results in erroneous invasion

profile.

All resistivities read lower than Rt and separation is reduced.


Platform Express HRL FMI

LatrologsHRLA (High Resolution Array Laterolog)


HRLA Solutions Hardware

Mode 1 Mode 2 Mode 3

Mode 4 Mode 5

Multiple depth of investigation• Clear indication of invasion• Improved vertical resolution• No need for deep mode or bridle• No Groningen or drillpipe-

conveyed logging effects and reduced shoulder-bed effect


HRLA 1D Real-time Answer

Real-time wellsite product gives clear answers for quick decisions:

- Eliminates voltage reference effects

- Less shoulder-bed effect

- Self consistent depth matched measurements


Answer Benefits: Reduced Shoulder-bed Effect

HRLA tool HALS tool 1D-Rt comparison

1D radial model

(invasion)thick-bed

approximation


Answer Benefits: No Groningen Effect

Curve separation suggests

invasion but is due to

Groningen effect

HRLA resistivities clearly show zone is not invaded


Answer Benefits: Thin-Bed and Invasion Profiling

Curve separationshows invasion

High verticalresolution

Curve separation results from Groningen effect

Groningen effectin indicator curve

HRLA tool DLL tool


HALS - standard and hires

SHALE

SAND


HALS LQC

Groningen flag


LQC - Out of limits (example)

HIGH RESISTIVITY BED


HALS - out of limits

HIGH RESISTIVITY BED


HALS NoisePrimarily from Rig Generator

Rig Generator OFF Rig Generator ON


InductionsAIT


AIT-H Stand-Off Positioning


Resistivity : Induction


Operating Range

• 0.1 to 2000 ohmm• Moderate Rxo < Rt• Rt/Rm <100 (with hole diameter

considered)• Large diameter holes with moderate Rt/Rm

and moderate Rt


Induction response errors

• Skin effect & Mutual Inductance• Borehole effect• Cave effect• Shoulder effect• Invasion effects


Rmc

Rm

RXO

Rt


1/Ra = 1/(VRm(Rm)) + 1/(VRmc(Rmc)) + 1/(Vrxo(Rxo)) + 1/(VRt(Rt))


If Rmf < Rw, AT10 < 20 < 30 < 60 < 90If Rmf > Rw, AT10 > 20 > 30 > 60 > 90

From the 5 curves Rt can be computed



Resistivity : InductionFocusing


Resistivity : InductionHole size and shapeUncertainty in hole size or mud resistivity can cause large errors on the shallow curves but much smaller errors on the deeper measurements

Out of sequence


Resistivity : InductionInvasion

Very deep invasion can affect even the deepest measurements, non-cylindrical invasion may affect the Rt computation


Induction Resistivity in Deviated Wells

Relative dip angle

Shallow Resistivity Deep Resistivity

low relativedip angle

high relativedip angle


Inductions vs Laterologs


Comparison to AIT

BS = 8”

Rt = 200

Rxo = 30

Rm = 0.1

S/O = 1.5”


BS=8.5Rm=0.5Rxo=5.0Rt=200

Resistivity : Tool planner


BS=12Rm=0.5Rxo=5.0Rt=200



BS=12Rm=1Rxo=5.0Rt=200


resistivity logs.ppt

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