PETROLEUM DEVELOPMENT GEOLOGY LOG INTERPRET

IS A PROCESS OF USING WELL LOGS TO EVALUATE THE CHARACTERISTIC OF FORMATION :• STORAGE CAPACITY å porosity, fluid saturations and net pay thickness • FLUID PROPERTIES å density, fluid t

1 WHAT IS WELL LOGGING:

1 WELL LOG IS A CONTINUOUS RECORD OF MEASUREMENT MADE IN BORE HOLE RESPOND TO VARIATION IN SOME PHYSICAL

PROPERTIES OF ROCKS THROUGH WHICH THE BORE HOLE IS

3 2 TYPES OF WIRELINE LOGGING :

1 OPEN HOLE LOGGING

2 CASED HOLE LOGGING

IS A PROCESS OF USING WELL LOGS TO EVALUATE THE CHARACTERISTIC OF FORMATION :

• STORAGE CAPACITY å porosity, fluid saturations and net pay

thickness

• FLUID PROPERTIES å density, fluid type, fluid contacts, API gravity, water resistivity & salinity,

temperature, GOR

• GEOLOGICAL SETTING å

structural/dip/fracture, geologic environtment, facies characteristic, top/bottom reservoir,

LITHOLOGY

LOG INTERPRETATION

Log interpretation should provide answers to questions on:

IS PART OF RESERVOIR CHARACTERIZATION PROCESS WHICH SHOULD BE INTEGRATED WITH THE FOLLOWING SURVEY AND ANALYSIS:

– DRILLING OPERATION LOGS:

• CUTTING ANALYSIS, MUD ANALYSIS, DRILLING DATA COLLECTION (PRESSURE, GAS READING, PENETRATION RATE ETC.) AND

ANALYSIS.

– CORRING & CORE ANALYSIS :

• SIDE WALL CORE & FULL HOLE CORE

• VISUAL LITHOLOGY DESCRIPTION, HYDROCARBON SHOWS, POROSITY, PERMEABILITY, FORMATION FACTOR, SATURATION ETC.

– PRODUCTIVITY TEST :

• RFT, MDT, DST, PRODUCTION TESTS

– GEOLOGY & GEOPHYSICAL :

• SURFACE GEOLOGY, SEISMIC SURVEY & INTERPRETATION ETC.

LOGGING UNIT CONTAINS:

• digital recording system

Open Hole Logging :

1 The traditional wireline logging

2 Logging While Drilling

3 Logging on drill pipe

Logging Job Sequences :

î Rig-up logging unit

î Check Tool and system

î Wellsite Geologist (WG) will

perform system & tool quality control

î Safety meeting

î Tool run in hole

î The system is on but never be

used for log interpretation

î Pull-out and logging

î WG is the witness, checks the

logging speed and quality.

î WG has authority to stop, refuse

and re-logging when necessary

î Rig-down the logging unit.

î Print the result

î WG signs the services ticket

containing type of services and charges

LOGGING UNIT

SONDE / TOOL

WIRELINE

SAMPLE :

OPEN HOLE LOG

SP, GR, AIT, SONIC, DENSITY & NEUTRON

3 ELECTRICAL LOG å INDUCTION, LATERAL, SPHERICAL FOCCUSS, MICRO LATERAL ETC

4 NEUTRON LOG å CNL, SNP

5 DENSITY LOG å LDT

6 SONIC LOG å BHC

7 OTHERS : FMI (DIPMETER &

IMAGING), NMRI (Nuclear Magnetic Resonance Immaging, TEMPERATURE LOG, CALLIPER LOG, ETC.

¬ SP results from electric

currents flowing in the

drilling mud.

¬ There are three sources of

the currents, two

electrochemical and one

• The SSP is the

quantity to be

determined.

• It is the deflection seen on the SP from the Shale Base Line (zero point) to the

Sand Line (max

deflection)

• Differentiate potentially porous and permeable reservoir rocks from

impermeable clays.

• Define bed boundaries, top &

bottom of the layer.

• For geological correlation

• Give an indication of shaliness

(maximum deflection is clean;

minimum is shale).

• Indicate vertical grain size

distribution

• Determine Rw (formation water

resistivity) in both salt and fresh muds.

we

mfe

R

R k

SSP = − log

• Baseline shifts: These can occur when there are beds

of different salinities separated by a shale which does not act as a perfect membrane.

• The SP can be affected by a number of surface effects as it relies on the fish as its reference electrode.

• Power lines, electric trains, electric welding, close radio

transmitters:

• All these create ground currents which disrupt he "fish“ reference causing a poor, sometimes useless, log.

-• The Gamma Ray log is a

measurement of the formation's

natural radioactivity.

• Gamma ray emission is produced by three radioactive series found in the Earth's crust.

density.

• Less dense formations exhibit more radioactivity than dense formations even though there may be the same quantities of radioactive material per unit volume.

• Bed definition å top, bottom, thickness

• Shalliness å content and net thickness, The minimum value gives the clean (100%) shale free zone, the maximum 100% shale zone

NEUTRON TOOLS

• The first neutron tools used a chemical neutron source and employed a single detector which measured the Gamma Rays

of capture They were non-directional The units of

measurement were API units where 1000 API units were

calibrated to read 19% in a water-filled limestone The tool was badly affected by the borehole environment.

• The second generation tool was the Sidewall Neutron Porosity (SNP) This was an epithermal device mounted on a pad.

• The current tool is the Compensated Neutron Tool (CNT) The latest tool is the Accelerator Porosity Sonde (APS), using an electronic source for the neutrons and measuring in the

epithermal region.

• The Density Tools use a chemical gamma ray source and two or three gamma ray detectors

• The number of gamma rays returning to the detector depends on the number of electrons present, the

electron density, ρe.

• The electron density can be related

to the bulk density of the minerals

by a simple equation.

• ρe = ρ( 2Z/A )

Where Z is the number of electrons per atom and A is the atomic weight.

• The density tool is extremely useful as it has high accuracy and exhibits small borehole effects.

• Major uses include:

– Porosity.

– Lithology (in combination with the neutron tool).

• Mechanical properties (in

combination with the sonic tool).

• Acoustic properties (in

combination with the sonic tool).

• Gas identification (in

combination with the neutron tool).

• There are two inputs into the porosity equation: the matrix density and the fluid density.

• The fluid density is that of the mud filtrate.

ρ φ

ρ

ρ b = f + ma 1 −

f ma

b

ma

ρ ρ

ρ

ρ φ

−

−

=

Ü The sonic tools create an

acoustic signal and measure how long it takes to pass

through a rock.

Ü By simply measuring this time

we get an indication of the

-• A simple tool that uses a pair of transmitters and four receivers

to compensate for caves and sonde tilt.

• The normal spacing between the transmitters and receivers is 3' - 5'.

• It produces a compressional slowness by measuring the first arrival transit times.

• Multi-spacing digital tool.

• First to use STC processing.

• Able to measure shear waves

and Stoneley waves in hard

– Mechanical properties (from shear and compressional).

– Fracture identification (from shear and Stoneley).

– Permeability (from Stoneley).

• It reacts to primary porosity only, i.e it does not "see“ the fractures or vugs.

• The basic equation for sonic porosity is the Wyllie Time

ma

t t

t

t

Δ

− Δ

• Raymer Gardner Hunt.

• This formula tries to take into account some irregularities seen in the field.

• The basic equation is:

• A simplified version used on the Maxis is:

C is a constant, usually taken as 0.67.

f ma

−

= Δ

φ

1 1

DETECTING

OVERPRESSURED ZONE

Determination

© Schlumberger 1999

• Most tools react to lithology - usually in conjunction with the porosity.

• Major lithology tools are:

– Neutron - reacts to fluid and matrix.

– Density - reacts to matrix and fluid.

– Sonic - reacts to a mixture of matrix and fluid, complicated

by seeing only primary porosity.

– NGT - identifies shale types and special minerals.

– Geochemical logging, identifies 10 elements; K, U, Th, Al, Si,

Ca, S, Fe, Gd, Ti

– From these the exact mineralogy can be computed.

Crossplot Solution

• The plot is a straight line from the matrix point to the 100% porosity, water point It is scaled in porosity.

Porosity and Lithology Determination

from Litho-Density* Log

and CNL*

(Compensated Neutron

Log)

Schlumberger Chart

12

Porosity 13 % 75% sand & 25% limestone

ELECTRICAL RESISTIVITY LOGS

Current can only pass through the water in the formation, hence the

resistivity depends on:

– Resistivity of the formation water.

– Amount of water present.

– Pore structure.

• The resistivity of a substance is a measure of its ability

to impede the flow of electrical current.

• Resistivity is the key to hydrocarbon saturation

determination.

• Porosity gives the volume of fluids but does not

indicate which fluid is occupying that pore space.

Smov = Sxo - Sw

• The voltage measured at M is proportional to the formation resistivity.

• This electrode configuration is the Normal tool

• The distance between the A and M electrodes.

• The spacing determines the depth of investigation and hence the resistivity being read.

• The Lateral device used the same principle

• The difference is in

electrode configuration and spacing.

• Problems came from "thin beds" when the signature

of the curve was used to try and find the true

resistivity.

• This figure shows some of the "signature curves" for the

interpretation of lateral and normal devices in thin beds.

• A library exists plus the rules to extrapolate the measured value to the true resistivity of the bed.

• Measures Rt.

environments.

LATEROLOG LIMITS :

• Cannot be used in oil-based muds.

• Cannot be used in air-filled holes.

• Poor when Rxo > Rt.

• This tool uses a set of 5 electrodes

which focus the signal into the

invaded zone just beyond the mud

cake.

• Uses:

– Rxo measurement in water-based muds.

– Correction for deep

INDUCTION LOGS

© Schlumberger 1999

Induction Principle

Induction Logs

indications & fluid contacts

shalliness

examples 3

• The AIT logs (2' vertical resolution) read correctly in this zone giving a hydrocarbon profile.

• The DIL logs are ambiguous as the SFL (electrical log) longer reading shallow because Rxo

is less than Rt

90 Inch investigation

.2 0.2

0.2

2000.0 2000.0

0.0 10000.0

(ohmm) Deep resistivity (ILD)

• The saturation of a formation represents the amount of a given fluid present in the pore space.

• The porosity logs react to the pore space.

• The resistivity logs react to the fluids in the pore space.

• The combination of the two measurements gives the saturation

• Current can only pass through the water in the formation, hence the resistivity depends on:

– Resistivity of the formation water.

– Amount of water present.

– Pore structure.

• F: Formation Resistivity Factor.

• At constant porosity F is constant.

• As porosity increases, Ro decreases and F decreases.

• Experiments have shown that F is inversely proportional to φm

• m: is called the "cementation exponent".

• a: is called the "lithology" constant.

• Saturation can be expressed as a ratio of the

• The Archie equation is hence very simple It links porosity and resistivity with the amount of water present, Sw.

• Increasing porosity, φ, will reduce the saturation for the same Rt.

• Increasing Rt for the same porosity will have the same effect.

• The same method can be applied to the invaded zone The porosity is identical, the lithology is assumed to be the same, hence the constants a, n, m are the same.

• The changes are the resistivities which are now Rxo and Rmf.

• Rmf is measured usually on surface and Rxo is

measured by the MSFL tool.

• The equation is then: S xo

• Dividing for Sxo and Sw, with n set to 2

• Rw = resistivity of connate water.

• m = "cementation factor", set to 2 in the simple case.

• n = "saturation exponent", set to 2 in the simple case.

• a = constant, set to 1 in the simple case.

All the constants have to be set.

In clastics the values are usually measured for each reservoir Values could be

An often quoted old formula, the Humble Equation uses:

• Rw is an important parameter.

• Sources include:

– Formation water analysis

– Local tables / knowledge.

• If Sw = 1, the saturation equation can become:

• Assuming simple values for a, m, n.

• Procedure is to:

• Compute an Rwa (Rw apparent) using this

relationship.

• Read the lowest value over a porous zone which

• This is the method employed by all computer based interpretation systems.

R w = φ 2

R t

• In a water zone Sw = 1, thus the alternative

saturation equation becomes:

• The value of Rmf is measured;

• Rxo and Rt are measured, the value of Rw can be calculated.

Example of variations in the Archie parameters

The following are measurements

POR = 25%, Rt = 5 ohm-m, Rw = 02 ohm-m

Assuming a simple formation with

© Schlumberger 1999

Clean formation Structural shale

Porosity

Porosity

Matrix Matrix

Porosity

Matrix

Porosity Shale Shale

Matrix

Porosity Matrix

Laminar shale Dispersed shale

• Shales have properties that have

important influences on log

readings:

• They have porosity.

• The porosity is filled with salted

water.

• They are often radioactive.

• Resistivity logs exhibit shales as

low resistivity zones.

• Neutron porosity logs exhibit

shales as high porosity.

• Density and sonic logs react to the porosity and matrix changes.

• Gamma ray logs react to shale radioactivity.

• The volume of shale must be computed to correct the tool readings.

• This is achieved using simple equations such as:

min max

min log

GR GR

min log

SP SP

• The Archie equation has to be changed to

take account of the shale effect.

• The shale looks like low resistivity so

another term is added to the equations.

• The result is an equation which will can be

used to compute water saturation in shaly sands.

• All these equations return to Archies

equation if there is no shale present.

EXAMPLE : PROCESSED LOG

OPEN HOLE LOG

PROCESSED LOG

VOLUME FLUID

ANALYSIS SATURATION

POROSITY & SATURATION

CALCULATION RESULTS

far water

bound water

dry clay

clean matrix

φwb

φwf

φhy

effective porosity

φe total

porosity

φt