Tim Carr - West Virginia University

Modified from SPWLA Website

PETROPHYSICS

PETROPHYSICS

GEOLOGY

DRILLING RESERVOIR

ENGINEERING

GEOPHYSICS

2

Electrical

Acoustic or Sonic

Nuclear

Geometry Hole Diameter – Rugosity

Direction – Inclination

Temperature

Pressure

3

Porosity - Φ

Permeability - K, Krel

Lithology/Mineralogy

Fluid & Gas Saturations - Sw, Sg, So

Resistivity/Electrical Properties

Sonic Properties (e.g., Strength)

Bed Contacts & Thickness

Fractures (Size, Frequency, Direction)

In-situ Stress 4

“True” Rt Formation Resistivity

Fluid Saturation Sw from Archie Equation from Archie Equation

Determination of Hydrocarbon-Bearing vs. Water-Bearing Zones

Geopressure Detection

Diameter of Invasion

Porosity

Correlation

Source Rocks/Gas Shale

5

Original Log

Various Types Electrode Logs

Laterologs

Focused Electrodes

Induction

Ohms Law E = Ir

6

Pechelbronn,

Alsace, France

September 5, 1927

SPWLA

7

∆𝐸 = 𝐼𝑟

Potential difference (volts) is equal to the current (amps) times the resistance (ohms)

8

Units – ohm-meter2/meter ohm-meter or ohm-m

Conductivity is Reciprocal of Resistivity – C=1/R 9

Resistivity of Water - Rw

Resistivity of Water and Formation

Resistivity of Water, Hydrocarbons,

and Formation

Resistivity of Formation

Incr

easi

ng R

esis

tivit

y

10

11

Normal Resistivity Tool

12

Induction (Conductivity) Tool

13

14

15

16

17

Log Presentation and Scales

18

Resistivity logs :

spherically-

focused (SFL),

medium

induction (ILM),

and deep

induction (ILD)

from

KGS Jones #1

19

20

21

High deep resistivity means:

Hydrocarbons

Tight streaks (low porosity)

Low deep resistivity means:

Shale

Wet sand

Separation between resistivities means:

The formation fluid is different from the drilling fluid

The formation is permeable to the drilling fluid

Deep, Medium, and Shallow refers to how far into the formation the resistivity is reading (4 ft, 2 ft, few in)

ILD (deep)

MSFL

SFL

Formation Fluid

different from

Drilling Fluid

Formation Fluid

similar to

Drilling Fluid

We will assume that this well was drilled with an water-based mud

Gamma Ray

Caliper

ILD (deep)

MSFL

SFL RHOB)

NPHI

• Using the Gamma Ray log, define a shale base line

• Deflections far to the left are sands

• Intermediate deflections to the left are silts

ILD (deep)

MSFL

SFL RHOB)

NPHI

Gamma Ray

Caliper

Sandstone

Sandstone

Sandstone

Siltstone

Shale

Siltstone

Shale

ILD (deep)

MSFL

SFL RHOB)

NPHI

Gamma Ray

Caliper

Sandstone

Sandstone

Sandstone

Siltstone

Shale

Siltstone

Shale

• Where is the neutron porosity to the right of the density porosity?

• This indicates where gas is in the sand pores

Gas

ILD (deep)

MSFL

SFL RHOB)

NPHI

Gas

Water?

Water?

Gamma Ray

Caliper

Sandstone

Sandstone

Sandstone

Siltstone

Shale

Siltstone

Shale

• Where do the resistivity logs give different values?

• This indicates where the fluids in the rocks differ from the drilling fluid

• In this case, it confirms the gas zone

Formation

Fluid

different

from

Drilling

Fluid

Water?

Water?

Formation Fluid

similar to

Drilling Fluid

F m=a

=o

R

R

wR

R Swo

t

I == n1

Sw=

am

*

wRt

1/n

( R )27

The keystone of log analysis for the solution of water saturation

of potential oil and gas zones (Archie, 1942)

The relationship of the resistivity of a water saturated rock (Ro) to its formation water resistivity (Rw) to the fractional porosity.

The ratio of the observed formation resistivity (Rt)

to its expected resistivity (Ro) if it was completely

saturated with water, to the fractional water saturation (Sw).

Archie, G. E.: "The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics", Petroleum Transactions of the AIME 146 (1942).

28

F = 1/ m

This is the first Archie equation,

where ‘m’ is known as the ‘cementation exponent’

Archie termed the ratio of Ro/Rw as the “formation

factor” which is denoted by F.

29

A

L

ro Rw

core plug

30

Archie (1942) observed the range in value of m in sandstones:

1.3 unconsolidated sandstones

1.4 - 1.5 very slightly cemented

1.6 - 1.7 slightly cemented

1.8 - 1.9 moderately cemented

2.0 - 2.2 highly cemented

Guyod gave the name “cementation exponent” to m, but noted that the pore geometry controls on m were more complex and went beyond simple cementation

31

0 1

Ro

Resistivity (Rt)

The ‘resistivity index’ (I) is defined as the ratio Rt/Ro

I = 1/Sw

Water saturation (Sw)

n

32

For water-wet rocks, n takes a value of about 2

For oil-wet rocks, n could be 9 or higher

33

Sw = [ (a / Øm)*(Rw / Rt) ](1/n)

Sw: water saturation

Ø: porosity

Rw: formation water resistivity - Rw=0.1

Rt: observed bulk resistivity

a: a constant (often taken to be 1)

m: cementation factor (varies around 1.8 to 2.2)

n: saturation exponent (generally 2)

34

35

Rt % Sw Sh

A 39 24 0.18 0.82

B 31 10 0.45 0.55

C 36 20 0.22 0.78

D 30 20 0.25 0.75

E 19 22 0.28 0.72

F 9 18 0.49 0.51

G 2 19 1.00 0.00

H 1.5 23 0.97 0.03

I 4.0 12 1.07 -0.07

J 7 10 0.95 0.05

Sw =

Rw

Rt.

1.8

Rw = 0.10 ohm-m

1/2

m = 1.8 n = 2.0

36

Well logs give us detailed information at the location of the borehole

If there are several wells in an area, we can correlate stratigraphic units between them

The correlation is based on ‘characteristics’ of the well log responses – like a fingerprint

Often we select a datum – a correlation horizon that is registered to a common depth (flattened)

There are two main ‘philosophies’ used in well log correlation:

Correlate based on lithologic units - Lithostratigraphy

Correlate based on assume time lines – Chronostratigraphy

Which is Better? A matter of heated debate!!

Well 5

Well 4

Well 3

Well 2

Well 1

We will look at the sediments deposited above a regional unconformity

regional

unconformity

Well 5

SP for

lithology

Resistivity for

time-correlation

Resistivity Markers

Base Map

Sh

ale

Ba

se

line

On one copy of the well log cross-section, identify the sand sitting above the regional unconformity (SP deflection to the left)

Correlate the logs based on lithology

Use the resistivity markers (A, B, C, …) to correlate time-equivalent horizons (hint: markers G and H do not extend all the way to Well 1)

QUESTION: Is the lithostratigraphic correlation and the chronostratigraphic correlation different?

40

Exercise Part 1 Based on the Log on Slide 35 and the Archie

Results of Slide 36, where would you complete the well (perforate). Give a depth range?

Exercise Part 2 Undertake the well correlation exercise

Slides 38-39

Logs are Important for Evaluating the Subsurface History – Schlumberger Brothers (1927)

Borehole Environment - Resolution

Reservoir Evaluation

Geosteering

Correlation Lithostratigraphic

Chronostratigraphic

Geologic Interpretation Lithology

Facies

Sequence Stratigraphy

41

42

Assignments

Reading for this week Ch. 7, pp. 307- 356, Selley

Complete the Log Exercises by Wednesday (3/18)

Archie

Correlation

Discuss Current Energy Events Read Today in Energy for Friday (3/13) at

http://www.eia.gov/

Be Prepared to Discuss in Class - Monday

Discussion Leader – Ethan Flanigan