geophysics short course part1

8/13/2019 Geophysics Short Course Part1

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Basic and Advanced Borehole

Geophysics Short Course: Agenda9:30 Introduction

Part 1 - Basic Geophysical Logs

9:45 Gamma Ray, Spontaneous Potential, Resistivi ty10:30 Interpretation Exercise

10:45 Break

11:00 Neutron Porosity, Litho Density, Sonic

12:00 Interpretation Exercise

12:15 Lunch Break1:15 Fluid and Flow Logging

2:15 Interpretation Exercise

Part 2 - Advanced Geophysical Logs

2:30 Dipole Shear Sonic, Magnetic Resonance

3:30 Interpretation Exercise3:45 Break

4:00 Neutron Induced Gamma Spectroscopy, Imaging

5:00 Exercise

5:15 Session Wrap-up

5:30 Adjourn


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Natural Gamma Ray, Spontaneous

Potential, Resistivity

Gamma-ray

Natural Gamma-ray

Natural Gamma-ray Spectroscopy

Spontaneous Potential Resistivity

Induction Resistivity

Electrode Resistivity


3/31

Natural Gamma-Ray Logs:

Types

Gamma-Ray Log: Measures total natural

radioactivity

Natural Gamma-Ray Spectroscopy:Measures number and energy of gamma-

rays as natural radioactivity (40K, 232Th,

238U)


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Gamma-Ray Log:

Summary Measure: count rate of total natural radioactivity

Units: API units

Open/Cased: open and cased holes Aquifer zone: saturated & unsaturated

Depth of investigation: ~10+ inches

Vertical Resolution: 12

Application: Identify shales and clays however, other rocks are also radioactive (i.e.

carbonates, feldspar-rich and volcanically-derrived sands)

Correlate between wells

Depth correlation between logging runs

Comments: the gamma ray log was the first nuclear well log andwas introduced in the late 1930s.


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Gamma-Ray Logs:

Physical Principles

Gamma-rays: bursts of high-

energy electromagnetic

waves that are emitted

spontaneously by some

radioactive elements.Gamma-ray emitted from

atomic nucleus


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Gamma-ray Log:

Preferred Response


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Natural Gamma-Ray Spectroscopy Log:

Summary

Measure: count rate and energy of gamma-rays as naturalradioactivity (40K, 232Th, 238U)

Units: K% and Th&U ppm Open/Cased: open and cased holes

Aquifer zone: unsaturated & saturated

Depth of investigation: 10 - 24 inches

Vertical Resolution:12 Application: Clay-type delineation

Well-to-well correlate

Definition of facies and depositional environment

Igneous rock recognition Estimated uranium and potassium potentials

Cation exchange capacity studies

Comments: Natural gamma ray spectroscopy logs were introducedin the early 1970s, although they had been studied from the 1950s.

Newer technology has greater depth of investigation and higherprecision detectors.


8/31

Natural Gamma-Ray Spectroscopy:

Physical Principles

Age of earth 4 109years

Look for isotopes with

t1/2 109 years

40K 1.3 109 a

232Th 1.4 1010 a

238U 4.4 109 a

2.62

1.76


9/31

Natural Gamma-Ray Spectroscopy:

Physical Principles

Sandstones Th : clay content , heavy

minerals

K : micas & feldspars

Shales U suggests sourcerock/volcanic

Th : amount of detrital

material, shaliness

K : clay type & mica

Carbonates U : phosphates, organic,

stylolites

Th : clay content (mud-

grain)

K: clay content, K-evaporites,algal lst


10/31

Limitations of Old Technologies Strongly affected by K and Barite muds

Environmental corrections cannot handle bothat the same time

Very statistical at low values

100% relative error common in sands

Cannot clay type in sands (zones of interest)

2 different detector designs in field Difficult to distinguish which was used

Very different ECs for the 2 different detectors


11/31

Advantages of New Technology

Better statistics:

2 detectors and improved crystals

Better spectral stripping 254 channels

Design minimizes barite effect Improved potassium correction

Much better spectral gamma ray

Still difficult to clay-type in sands without

additional information


12/31

Total Gamma-ray Count vs.

Spectral Energies


13/31


14/31

Spontaneous Potential Logs:

Defined

Measures natural difference in electrical potential, in

millivolts, between an electrode in the borehole anda fixed reference electrode on the surface.

The most useful component of this potential

difference is the electrochemical potential since itcan cause a significant deflection opposite

permeable beds.

The magnitude of the deflection depends mainly onthe salinity contrast between drilling mud and

formation water, and the clay content of the

permeable bed.


15/31

Spontaneous Potential:

Difficulties in Interpretation

There are other possible sources of electrical

potential not related to the electrochemicaleffect (i.e. the electrokinetic potential and

bimetallism).

The SP can measure only the potential drop inthe borehole, and not the full electrochemical

potential and can be rounded at the boundaries

between shales and permeable beds.


16/31

Spontaneous Potential:

ExampleFactors affecting SP value:

1. Aquifer thickness & trueresistivity

2. Flushed zone resistivity

and invasion diameter3. Mud & mud filterate

resistivity

4. Resistivity of adjacentshale


17/31

Resistivity Logs:

Types Induction Resistivity

Dual induction Array induction

Electrode Resistivity Normal

Lateral Guard

Laterolog

Micro Resistivity

Resistivity Imaging

Steel-cased-hole Resistivity


18/31

Resistivity Defined

The ability of a material to resist electrical

conduction.

It is the inverse of conductivity and is

measured in ohm-m.

Generally fresh water and quartz are

resistive

Generally clays and water with TDS areconductive


19/31

Array Induction Resistivity:

Summary

Measure: Formation conductivity (1/R)

Open/Cased: open, PVC-cased

Hole Condition: saturated & unsaturated

Depth of investigation: 10, 20, 30, 60 & 90

Vertical Resolution: 1-5

Comments: Schlumberger-Doll introduced the

first induction-logging technique in 1949. Newer

technology corrects for edge effects and is

highly focused.


20/31

Laterolog Resistivity:

Summary Measure: Formation resistivity

Open/Cased: open Saturated/unsaturated : saturated

Depth of investigation: Depends on manufacturer and vintage

Commonly 16 (short normal), and 64 (long normal) 8 High resolution

Vertical Resolution: 8 20

Comments: Introduced in 1949 as normal log and guard log.

Newer laterologs corrects for edge effects, is highly focused, andcan delineate thin beds.

Unfocused, normal and lateral tools are particularly susceptible

to borehole effect and can have vertical resolutions of ~10 FEET.


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Induction and Normal Resistivity:

Applications

Aquifer delineation

Correlation between wells Determination of true formation resistivity

Determination of Sw

Hydrocarbon identification and imaging

Invasion profiling

Thin-bed analysis Porosity in clay-free units


23/31

Possible large errors on shallow AIT logs

Possible large errors all AIT logs

Induction or Laterolog?

Possible large errors on shallow induction logs

Possible large errors all induction logs

Rt

(o

hmm)

Rt/Rm

1

10

100

1000

0.001 0.01 0.1 1 10 100 1000 10000

Preferred induction

operating Rangeinduction

and/or

laterolog

(Low Rm or High Rt)

laterolog


24/31

RxoRt

Rm

Rm

Rxo

Rt

Laterolog

Response

InductionR

esponse

Logging Tool

BoreholeInvaded Zone

Ideal Operating EnvironmentIdeal Operating Environment

Induction:

Rm > Rxo > Rt

Laterolog:

Rt > Rxo > Rm

Uninvaded Zone


25/31

Resistivity Difficulty Resolving

Thin Beds

Anderson, 2001


26/31

Old Log

Shoulder Effects

High contrast

withconductive

shoulders

difficult


27/31

High Quality

Log Response Laterolog tool

of choice atvery high

resistivities


28/31

Invasion

Profiling


29/31

Micro Resistivity:

Summary Measure: Flushed zone resistivity

Open/Cased: open

Saturated/unsaturated: saturated Depth of investigation: depends on manufacturer and vintage 1-3

Vertical Resolution: 2-3

Application: Groundwater salinity

Thin bed detection

Comments: Several designs: microlog, microlaterolog, proximity log, microspherical

log and microcylindrical log.

Latest technology minimizes the effect of mudcake and rugose hole,while reading as short a distance as possible into the formation, andremains unaffected by the undisturbed zone.

Combined with a laterolog or induction log to correct the latter for theeffects of invasion, compute TDS, and for saturation determination.


30/31

Applying the Processed and

Interpreted Data

Optimizing well design

Improving conceptual model

development

Characterizing contaminated sitecomplexity

Characterizing fractured media

Constraining surface geophysics


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geophysics short course part1

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