### R t

```BASIC WELL LOGGING ANALYSIS –
THE RESISTIVITY LOGS
1
Hsieh, Bieng-Zih
Fall 2009
GENERAL
Resistivity logs are electric logs which are used to:
 (1) determine hydrocarbon versus water-bearing zones,
 (2) indicate permeable zones, and
 (3) determine resistivity porosity.


By far the most important use of resistivity logs is the
determination of hydrocarbon versus water-bearing
zones.
2
GENERAL (CONT.)

Because the rock’s matrix or grain are non-conductive, the
ability of the rock to transmit a current （電流） is
almost entirely a function of water in the pores.

Hydrocarbons, like the rock’s matrix, are non-conductive;

therefore, as the hydrocarbon saturation of the pores
increases, the rock’s resistivity also increases.
3
CALCULATE WATER SATURATION

A geologist, by knowing a formation’s water resistivity
(Rw), its porosity (Φ), and a value for the cementation
exponent (m), can determine a formation’s water
saturation (Sw) from the Archie equation:
 a Rw 

S w   m
  Rt 
1
n
Sw = water saturation
 Rw = resistivity of formation water, Rt = true formation
resistivity as measured by a deep reading resistivity log
 a = tortuosity factor, m = cementation exponent, n =
saturation exponent (most commonly 2.0)

4
TWO BASIC TYPES OF RESISTIVITY LOGS

The two basic types of logs in use today which measure
formation resistivity are induction （感應式） and
electrode （電極式） logs.

The most common type of logging device is the induction
tool (Dresser Atlas, 1975)
5
PRINCIPLE OF THE INDUCTION LOG

6
INDUCTION LOG
An induction tool consists of one or more transmitting coils
that emit a high-frequency alternating current of constant
intensity.
 The alternating magnetic field which is created induces
secondary currents in the formation.
 These secondary currents flow as ground loop currents
perpendicular to the axis of the borehole, and create
magnetic fields that induce signals to the receiver coils.
 The receiver signals are essentially proportional to
conductivity, which is the reciprocal of resistivity
(Schlumberger, 1972).


conductivity = 1000/resistivity
7
ELECTRODE LOG

A second type of resistivity measuring device is the
electrode log.

Electrodes in the borehole are connected to a power
source (generator), and the current flows from the
electrodes through the borehole fluid into the formation,
and then to a remote reference electrode.

Examples of electrode resistivity tools include: normal,
Laterolog*, Microlog*, and spherically focused logs.
8
RESISTIVITY LOG
•

CHOOSE AN APPROPRIATE LOG SURVEY

Induction logs should be used in non-salt-saturated drilling
muds (i.e. Rmf > 3 Rw) to obtain a more accurate value of
true resistivity (Rt).

Boreholes filled with salt-saturated drilling muds (Rmf ≒ Rw)
require electrode logs, such as the Laterolog* or Dual
Laterolog* with or without a Microspherically Focused
Log*, to determine accurate Rt values.
10
USE INDUCTION LOG OR LATEROLOG
Determining when use of an
induction log is preferred
over an electrode log such
as the Laterolog*.
11





Flushed Zone (Rxo)
Microlog*
Microlaterolog*
Proxmity* Log
Microspherically Focused Log*








Long Normal
Lateral Log
Deep Induction Log
Deep Laterolog*
Laterolog-3*
Laterolog-7*
•
Short Normal##
Laterolog-8*##
Spherically focused Log*##
Medium Induction Log
Shallow Laterolog*

12
COMMON RESISTIVITY LOGS
13
INDUCTION ELECTRIC LOG
14
15
16
DUAL INDUCTION FOCUSED LOG
SFL (solid line)
ILM (dotted-and –dashed line)
ILD (dashed line)
17
18
19
20
21
22
EXERCISE

Find the true formation resistivity (Rt) and corrected
resistivity of the flushed zone (Rxo) by using the Tornado
Chart at depth 13590, 13600, 13610, 13620, 13630,
and 13640.
Depth RILD RILM
RSFL
RSFL /RILD
RILM /RILD
Rt /RILD
Rt
Rxo /Rt
Rxo
13590 70
320
4.6
1.5
0.82
57.4
7.0
401.8
105
13600
13610
13620
13630
13640
23
MICROLOG (ML)
24
25
26
```