FORMATION EVALUATION PETE 663 - College of · PDF fileFORMATION EVALUATION PETE 663 ......

FORMATION EVALUATION

PETE 663

Summer 2010

RESISTIVITY TOOLS

Dr. David Schechter

• Identify permeable zones

• Discriminate HC vs. water saturated zone

• Determine Sw

• Estimate moveable hydrocarbons

• Determine resistivity porosity

• Correlate strata

RESISTIVITY DEVICES USED TO:

RESISTIVITY• Two families of resistivity devices

– Electrode tools– Induction tools

• Resistivity devices– Latero/Guard logs– Spherically focused– Microresistivity– Resistivity at bit (MWD)

• Induction– Older dual induction– Newer multi-frequency, signal enhanced

devices

Induction Logs– Should be run in non-salt saturated mud where Rmf

> 3 Rw– Run where resistivity < 200 ohm-m– Run with oil-based drilling mud

Laterolog or Dual Laterologs– Should be run in salt-saturated drilling mud where

Rmf ~ Rw– Run where resistivity > 200 ohm-m– Run where thin beds are present

RESISTIVITY LOG USES

RESISTIVITYResistivity

• The voltage required to cause current of oneamp to pass through a cube having a face area of one square meter

• Units are ohm-m / m; usually ohm-m (Ω.m)2

tyConductivi1yResistivit =

Ohm’s Law

• Ohm’s Law states that the current I, that flows through a conductor is equal to the voltage, V, applied divided by the material’s resistance, R.

• Resistivity is the specific resistance of a substance defined by the voltage required to cause 1 Amp to pass through a cube of face area of 1 meter square.

RESISTIVITY – DEFINITION OF THEOHM-METER

From Halliburton (EL 1007)

Resistance vs. Resistivity

• Why do we need resistivity vs. resistance?

• Resistance is not only a function of the resistivity measured but also the shape of the body of material on which the measurement is made

• Reistance of wire stretching across ocean is high because of distance however the resistivity of the wire is very low

We Need Resistivity

• Measurement that characterizes the rock is resistivity , not resistance.

Typical Reistivities• 0.5 Ω-m to 1000 Ω-m for typical

formations• Soft formations i.e. shaly sands

range from 0.5 Ω-m to about 50 Ω-m

• 10 Ω-m to 100 Ω-m for hard formations (carbonates)

• Evaporites (salt, anhydrites) may have several thousand Ω-m

Water Reisitivities

• Formation water will range from 0.015 Ω-m (very salty brines)

• Several Ω-m, fresh water reservoirs

• Sea water has a resistivity of 0.35 Ω-m at 75 degress Farenheit

MUD FILTRATE INVASION

WellboreMud(Rm)

Mud Cake(Rmc)

Uninvaded Zone(Rt)

Invaded Zone (Rxo)

Uninvaded Zone(Rt)

R1

Rs

Rs

Rw

Sw

Rm

Mud

hmc Flushedzone Zone of

transitionor

annulus

di

djAdjacent bed

Δrj

dhHole

diameter

Adjacent bed

h

dh

Uninvadedzone

(Bedthickness)

(Invasion diameters)

Sxo

Rm1

Rxd

Rmc

Mudcake

From NExT, 1999, after Schlumberger

SYMBOLS USEDIN LOG

INTERPRETATION

Resistivity of zoneResistivity of the water in the zoneWater saturation in the zone

A resistivity tool is affected by

• The invasion of mud filtrate

• Mud in the borehole.

• The resistivity of the shoulder

beds

• Mud cake

All resistivity readings have to be compensated for these effects.

From Halliburton, EL 1007)

BoreholeRm : Borehole mud resistivityRmc : Mudcake resistivity

Invaded zoneRmf : Mud filtrate resistivityRxo : Invaded zone resistivitySxo : Invaded zone water saturation

Uninvaded zoneRw : Interstitial water resistivityRt : Uninvaded zone resistivitySw : Uninvaded zone water saturation

COMMON TERMINOLOGY

RESISTIVITY TOOL APPLICATIONS

• Well to well correlation• Sxo and Sw computation• Presence and depth of hydrocarbons• Estimate moveable hydrocarbon• Invasion profile analysis / Imaging

RESISTIVITY TOOL FAMILIES

• Electrode tools: electrical current sent by electrodes into formation

- Requires water-base muds

• Induction tools: generate a magnetic field that induces a current in the formation

- Oil-base, air, or fresh-water muds

ELECTRODE TOOLS• Wireline

– Dual Laterolog (LLD & LLS)– Azimuthal tool (deep and shallow)– Spherically-focused– Micro-resistivity

• LWD– Resistivity at bit– Side-scanning electrodes

Schlumberger

ELECTRODE TOOL PRINCIPLE• Electrode emits current, I

– Electrode A– Green lines give current flow

• Electrodes sense voltage, V– Electrodes M and N– Red lines show equipotentials

• Formation resistivity, R– R = KV/I– K is tool constant

• Simple model ignores– Current flows up borehole– Radial changes in R (invasion)

• View animation file

Dual LaterologDeep and Shallow Current Patterns

• Multiple currents– Measure– Bucking or guard

• Objectives - to measure– Rt - deep msmt LLD– Rxo & Rt - shallow

LLS– Without borehole

effect– Without shoulder

effect

RECENT TOOLAZIMUTHAL RESISTIVITY

Features• Thin bed analysis:

VR < 1 ft• Various depths of

investigation• Azimuthal

resistivity • Applications• Fracture detection• Rt in dipping beds

LOGEXAMPLE

Rock Island 4-H, Horizontal Core

Mike Dempsey and John Lorenz Describing Core

Quantitative Fracture Analysis

0 N

90 E

180 S

270 W

360 N

BOREHOLEPlanar features are expressed by ellipses on borehole walls which are expressed as sine waves

Steepness of ellipses reflect the dip magnitude and orientation

Apparent strike and dip relate to amplitudes and inflections in sine waves

For straight hole:

•Dip magnitude is proportional to amplitude of sine wave

•Dip azimuth is located at minimum of sine waveTD: 530 / 2700 W

David Spain (1998)

HORIZONTAL CORE / FMI

FMI Log of 15,290’-15,330’ Horizontal Section

SHALE

TOP OF MARINE

SAND

FAULT CONDUCTIVENATURAL

FRACTURES

REPEATSECTION

UBI FMIUBI vs.

FMI

Courtesy of Steve Hansen, Schlumberger

UBI Shows

Topography

FMI Shows

Resistivity


Fracture Aperture

Faultson FMI

TD: 62/304

Normal fault

Striking:N25E-S25W

Down to WNW


FractureTrace

Assume Deep Resistivity Reads Rt Unless

• Rt/Rm is greater than about 10

• Rt/Rs is greater than about 10

• Hole Size is greater than about 12 inches

• The bed is thinner than about 15 ft

• Invasion diameter is greater than about 40 inches

SPHERICALLY FOCUSED RESISTIVITY MEASUREMENT

• SFL gives shallow resistivity

• Usually run with induction

• Good for thin-bed detection

• Different order of electrodes than laterolog

FLUSHED ZONE MEASUREMENTS - 1

• Pad-type tools– Pads reduce borehole

effects– Mud cake may still be

problem

• Very shallow resistivity– 2 to 5 cm typical

• Several types...

• Microlog• Two msmts, R1x1

and R2

• No currentfocusing

• Not for Rxo• High resolution• Mudcake detector

– R1x1 < R2

– Shows permeable zones



• MicroSFL• One msmt, RMSFL

• Current focusing• Rxo• Good resolution• Small mud cake

effect


• Other types of focused Rxo– Microlaterolog– Microguard– Proximity

• Electromagnetic propagation– Uses EM waves– Measures conductivity and propagation– Ultra-high frequencies

• Borehole scanners– Multi-pad– Image of borehole wall

MICRORESISTIVITY TOOLS

• Have a very high vertical resolution (~ 2 in. )

• Have a very small depth of investigation (a few inches)

• Mostly pad mounted• Measurements are in the invaded

zone only• Affected by mudcake on the borehole

wall

The microresistivity tool is the Short Guard on this log.

Most microresistivity logs are very spiky because they have a very high vertical resolution (~ 2 in.)

Good for thin-bed identification


MICRORESISTIVITY TOOLS

Microresistivity log is the Micro SFL on this log

Microresistivity tools read only flushed/invaded zones with a depth of investigation of a few inches

Note how the MSFL log reading is nearly constant even though the LLD and LLS logs have a profile inversion

This due to the fact that the MSFL reads only mud filtrate saturated formation


MICRORESISTIVITYTOOLS

INDUCTION LOGS

• Induction log was originally developed to measure formation resistivity in boreholes containing oil-based muds

• Electrode devices (conventional electric logs) do not work in non-conductive muds

• The induction log had many advantages; thus, it is now run in OBM and fresh WBM wells

INDUCTION LOGS

• Induction logging devices are focused to minimize influence of borehole and surrounding formations

• Designed for deep investigation to determine Rt

• New induction log devices are being developed using improved electronics, telemetry, and computer processing

INDUCTION LOGS• Several types

– Dual Induction - ILD and ILM

– Phasor/High Resolution Induction

• HRD, HRM (H)

• IDPH, IMPH (S)

– Dielectric induction

– Array Induction

– EWR/CDR

• All have similar physical principle...

INDUCTION PRINCIPLE - 1

RECEIVERCOILeR

ITTRANSMITTER

COIL

HL

HT

UNITGROUND LOOP

1

2

3

It

• Basic transformer• Transmitter coils

– 20 kHz fed to transmitter coil

• Generates alternating magnetic field that causes circular current that flows in formation

• creates magnetic field• Induces voltage • Receiver coils

– Induced voltage from magnetic field

– Output voltage eR

• Induced voltage proportional to conductivity

INDUCTION PRINCIPLE - 2• Voltage, eR, proportional to C• Resistivity computed as

( )( )mmmhoC

1000mohmR =−

• Similar to laterolog tools, induction tools are focused

• Depth of measurement depends on– Frequency

• Older tools, one freq – approx. 20 kHz• Newer tools, multiple frequencies

– Number, position of coils

Induction-Electrical Log Presentation-

Old Style

• Linear scales• Conductivity track 3• Resistivity track 2• Short normal

– Unfocused shallow– Bed definition

• Induction Rt

DUALINDUCTION

PRESENTATION- NEWER STYLE -

• Logarithmic scale• Resistivity track 2/3• Deep Induction Rt• Med induction Rxo & Rt

INDUCTION PRESENTATION-NEWEST STYLE

• Logarithmic scale• Five induction msmts• Allows for transition

zone

RESISTIVITY PROFILES• Separation of deep and shallow resistivity curves

suggests presence of a permeable formation

• If the formation pore fluid is more resistive than the mud filtrate, then Rdeep > Rshallow, irrespective of the type of tool (e.g., the profile in hydrocarbon bearing layers, with a saline water-based mud in the borehole)

• If the mud filtrate is more resistive, the Rshallow is greater than Rdeep (e.g., logging a fresh water bearing formation with an OBM in the wellbore)

• Across shale zones, there is no separation, because there is no permeability and hence, no invasion. All resistivity readings STACK in shales

Typical resistivity profile, resistivity pore fluid > resistivity mud filtrate


THE EFFECTS OF INVASION ON FLUID SATURATION

Difference between Sxo and Sw indicates movable hydrocarbonsFrom Halliburton, EL 1007)

Sxo

SwFiltrate(Rmf)

Formation Water (Rw)

DistanceBorehole

Wall

100

0

Oil

SHALE

SHALE

BOREHOLE

• Change in pore fluid saturations after invasion.

• A difference in Sxo and Sw indicates movable hydrocarbon.

40% moveable HC


ResidualOil

MoveableOil

ResidualOil

Connate Water

MudFiltrate

Sxo = 70%

MudFiltrate

Sw = 30%

(Sxo – Sw)=40%

0.70

0.30

1.00

0.0

OriginalConditions

AfterInvasion

SUMMARY• Two families of resistivity devices

– Electrode tools– Induction tools

• Resistivity devices– Latero/Guard logs– Spherically focused– Microresistivity– Resistivity at bit (MWD)

• Induction– Older dual induction– Newer multi-frequency, signal enhanced

devices

SUMMARYResistivity devices Used to – Determine permeable zones– Discriminate HC vs water saturated zones– Determine Sw– Estimate moveable hydrocarbons– Determine resistivity porosity– Correlate strata

Induction Logs– Should be run in non-salt saturated mud where Rmf > 3

Rw– Run where resistivity < 200 ohm-m– Run with oil-based drilling mud

Laterolog or Dual Laterologs– Should be run in salt-saturated drilling mud where Rmf ~

Rw– Run where resistivity > 200 ohm-m– Run where thin beds are present

FORMATION EVALUATION PETE 663 - College of · PDF fileFORMATION EVALUATION PETE 663 ......

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