Presentation on

Resistivity Log

PRESENTED TO: Prof. DR. SHAHID GHAZI

Presented By:Hammad Ahmad SheikhBS Hons (4th Prof)

Contents• Resistivity• Factors Affecting Resistivity• Introduction of resistivity log• Principle of Resistivity Log• Theoretical Consideration• Invasion Zone• Resistivity Tools• Log Characteristics• Principle uses• Conclusion• Refrences

Resistivity • The resistivity (specific resistance) of a substance is theresistance measured between opposite faces of a unit cube of that

substance at a specified temperature. • The meter is the unit of length and the ohm is the unit of

electrical resistance. In abbreviated form, resistivity is R = rA/L,• Where R is resistivity in ohm-meters, r is resistance in ohms,

A is area in square meters, and L is length in meters.• The units of resistivity are simply ohm-meters (ohm-m).

Conductivity is the reciprocal of resistivity

Factors Affecting Resistivity Salinity of water Porosity of the formation,

Lithology of the formation

Degree of cementation, Type and amount of clay

in the rock.

Fig 1.1 showing the response of resistivity depending on the

nature of fluid.

The resistivity log is a measurement of a formation's resistivity,

that is its resistance to the passage of an electric current. It is measured by resistivity tools.

The resistivity of a formation is a key parameter in determining

hydrocarbon saturation. Electricity can pass through a formation only because of the conductive water it contains. With a few rare exceptions, such as metallic sulfide and graphite, dry rock is a good electrical insulator.

.

Resistivity Log

Resistivity Log

Fig 1.2 showing the response of resistivity curve in the different formations.

2. Principle of Resistivity Log • The basic resistivity tool arrangement was provided

by conard schulmberger in 1927.• Currents were passed through the formation by means

of current electrodes, and voltages were measured between electrodes. These measured voltages provided the resistivity determinations for each device.

Principle of Resistivity Log

Fig 1.3 showing the different tools to measure the resstivty.

3. THEORETICAL CONSIERATIONS

• 3.1. Earth resistivity and conductivity

• 3.2. Rock resistivity

• 3.3. Resistivity of clays

3.1 EARTH RESISTIVITY AND CONDUCTIVITY

• Two tests can be applied under subsurface conditions to measure resistivity.

• The first test is a direct measurement. A current is passed between two electrodes on a logging tool and the potential drop between them provides the resistivity.

3.1 EARTH RESISTIVITY AND CONDUCTIVITY

• The second test is indirect in that it measures conductivity. A current is induced in the formation around the borehole and the capacity to carry the current is observed. This carrying capacity is the conductivity. The resistivity is simply the reciprocal of the conductivity

RESISTIVITY VS CONDUCTIVITY

Fig 1.4 showing the relationship between Resistivity and conductivity.

3.2 ROCK RESISTIVIY • It is only the formation waters that are conductive, the

conductivity of the rock in general should be that of the solution it contains. Although the rock plays no active part, it plays an important passive one .

• This passive role is basically dependent on rock texture or more specifically on the geometry of the pores and pore connections in rocks, the easier the path through the pores the more current that passes.

ROCK RESISTIVIY

Fig 1.5 showing the rock resistivity depending upon the conductance.

3.3 RESISTIVITY OF CLAYS

• Clays conduct electricity in two ways, through pore water and through the clay itself.

• The porosity in clay, like that in other rocks, encloses conductive formation water.

4. ZONE OF INVASION AND RESISTIVITY

• It is all-important to the understanding of borehole resistivity. The essential target of resistivity logging is that of the true resistivity of the formation (R,) and, especially, its saturation in hydrocarbons.

• To this effect, it is necessary to consider the invasion of mud filtrate (with a certain salinity and hence resistivity, Rmf) into a formation containing either formation water (resistivity Rw) or hydrocarbons.

ZONE OF INVASION AND RESISTIVITY

Fig 1.6 showing the zone of invasion and resistivity

Tools used in Invasion

Uninvaded Zone Transition Zone

Flushed Zone MudCake

This zone is also called as virgin Zone.

The zone where the drilling fluid and formation fluid is intermixed with each other.

This zone is totally comprises of drilling fluid.

The zone is made up of bentonite clay which is present in the drilling fluid.

SFL MSFL

MLLL

LLd

Resistivity Tool

5.RESISTIVITY TOOLS5.1.Unfocused devices

(5.1.1 ) Normal log (5.1.2) Lateral

5.2 Focused Devices

Laterolog LL3 Laterolog LL7 Dual Laterolog

SFL

5.3 Micro-Resistivity Devices

Types of Micro-Resistivity Log

Fig 1.7 showing the Focused and

Non focused electrical logs.

Unfocused Devices• Normal devices:

o In this arrangement a constant

known current is flowed from A to B (or B to A), and the potential is measured between M and N.

o Electrode B and N are kept at a long distance from electrodes A and M to provide quasi-infinite reference points for the current and potential measurements.

Fig 1.8 Showing the standard normal configuration.

Unfocused Devices• Lateral Log

o In the lateral device a constant current is passed between A and B.

o The potential difference M and N is measured

Fig 1.9 showing the electric configuration of Lateral Log.

5.2 Focused Devices

Fig1.10 showing the electrode configuration of LL3

Laterolog LL3

• The LL3 has 3 current emitting electrodes.

• The middle one, emits the main current while the either side of electrodes also emits a current.

• This helps to keep the central electrode more focused.

Focused DevicesLaterolog LL7

• The LL7 has 7 electrodes.• A constant current is emitted from the

centre electrode.• A bucking current is emitted from the

two far electrodes.• The two pairs of monitoring electrodes

are brought to the same potential difference.

• This electrode arrangement produces a thin disk of current that is confined between the two sets of measuring electrodes. Fig 1.11 showing the LL7 tool

configuration.

Focused Devices

Fig 1.12 showing the electrode configuration of Dual Laterolog LLd and LLs

Focussed devicesThe Spherically Focused Log

• The SFL device measures the conductivity of the formation near the borehole.

• In this the current is focussed quasi-spherically.

• It is useful as it is sensitive only to the resistivity of the invaded zone.

Fig 1.13 showing the electrode configuration of

SFL/

5.3 Micro-Resistivity Log

Fig 1.14 showing the electrode configuration of Microlog.

The Microlog

• The microlog(ML) is a rubber pad with three button electrodes placed in a line.

• A known current is emitted from electrode A.

• The potential differences between electrodes M1and M2 and between M2 and a surface electrode are measured.

Types of Micro-Resistivity Log

Fig 1.16 Micro- Spherically

Focused LogFig 1.15 Micro-

LaterologFig 1.17

Proximity Log

These devices have same sort of electronic configuration.These have electrode spacing of a few inches.They penetrate the formation to a very small degree.

6. Log characteristics • 6.1 Log format and scales

• 6.2 Depth of investigation

• 6.3 Bed resolution

6.1 scales • Providing scale is a common problem in all resistivity

devices that can be read accurately over the full range of response.

• There are two types of scales on which resistivity curve are recorded.

o Hybrid scaleso Logarithimic Scales

Types of Scales

Fig(1.18) showing the Hybrid Scale Fig(1.19) showing Logarithmic Scale

6.2 Depth of investigation

• Depth of investigation also has geological significance. The logs from deep-reading devices, are best used for gross formation characteristics in which individual beds are unimportant.

• Texture-related changes are best seen on the logs from tools mainly influenced by the invaded zone.

6.3 Bed resolution The resistivity tools are capable of very fine bed resolution, the

finest of all the logging tools.

The micro tool logs give too fine a resolution for practical, usable, geological bed resolutions. The logs are best used for defining bedding characteristics.

The laterologs resolve beds at the right scale for bed-boundary indications , but they should be used in conjunction with the other logs.

.

6.3 Bed resolution

Fig 1.20 showing the bed resolution determined by

MSFL.

7. Principal uses

7.1 Quantitative uses of the resistivity logs

• The quantitative use of log resistivity measurements is at the heart of the whole domain of quantitative well-log interpretation - the domain of petrophysics.

• The principal use of well logs is to detect oil: the principal use of the resistivity log is to quantify oil (and of course, gas).

7.2 Qualitative uses of resistivity7.21.Texture and facies The simplest relationship

between resistivity and texture is demonstrated by an increase in resistivity as porosity decrease.

Fig 1.21 showing the facies change marked by resistivity curve.

Lithology

Fig 1.22 showing the lithology change.

• Resistivity log can be used for the lithology identification purposes.

• Tight limestone's shows high resistivity.

Lithology

Fig 1.23 showng the difference between Shale and sand sequences

• Resistivity logs can best recognize the shale and sand sequences.

• Sand shows relatively less resistivity values as compared to shale.

Correlation

Fig 1.24 showing the correlation.

• Resistivity log can be used for correlating the subsurface data on the basis of same resistivity curve.

• The best tool to acquire the data is deep induction tool.

Correlation

Fig 1.25 showing the pay zone marked by

reisitivity curve.

Permeability

Fig 1.26 showing the permeability of a rock.

• Resistivity logs can be used to find out the permeability of a rock unit in the subsurface.

7.2.3 Source-rock investigation:

The resistivity log may be used both qualitatively and quantitatively to investigate source rock.The effect of a source rock has on the resistivity log depends on the maturity of the organic matter.

Fig 1.27 showing the matureness on the basis of resistivity curve.

Conclusions• Resistivity logs are helpful in determining the porosity of a

rock.• Used in the search of hydrocarbons.• Find out the quantity of hydrocarbons is present in the

subsurface.• Bed resolution can be noted by this log.• Used for correlating the data of different wells on the basis of

resistivity curves.• Lithology Indicator.

8. REFERENCES• Selley,R.C.(1995) Elements of Petroleum Geology.2nd

ed.London.Academic Press P 57-60• Serra, o.(1988) fundamentals of well log interpretation. 3rd ed.

New York Elsevier science Publishers P 51-76. • Rider, H.(2002) The geological interpretation of well logs. 2nd

ed. Scotland. Rider French consulting Ltd P 35-48.• Schlumberger Log Interpretation and principles