Spontaneous potential Log

Presented by;

Azhar Mahmood Roll # 09 BS Applied geology 4th prof. Geophysics University Of The Punjab Lahore

Spontaneous Potential Log

Introduction Spontaneous potential log Principle How it works Measurement tools Log presentation Factors Effecting SP Log Response Applications

Contents

What is SP ? Called self or spontaneous potential . SP is the naturally occuring electrical potential

difference in the Earth . Spontaneous potentials (SP) are usually caused by

charge separation in clay or other minerals, due to presence of semi-permeable interface impeding the diffusion of ions through the pore space of rocks, or by natural flow of a conducting fluid through the rocks.

Introduction

A log of the natural difference in electrical potential, in millivolts, between an electrode in the borehole and a fixed reference electrode on the surface.

It is very simple log .

Spontaneous Potential Log

A conductive borehole fluid (i.e., a water based mud).

A sandwich of a porous and permeable bed between low porosity and impermeable formations.

A difference in salinity between the borehole fluid and the formation fluid, which are the mud filtrate and the formation fluid in most cases.

There are three requirements for the existence of an SP current:

Spontaneous potential Electrokinetic potential

Electrochemical potential

Arise from the movement of electrically charged ions in the fluid relative to the the fixed rock.

Arise from electrical interactions between the various chemical constituents of the rocks and fluids.

Diffusion Potential The diffusion potential (sometimes called the

liquid-junction potential). This potential exists at the junction between the invaded and the non-invaded zone, and is the direct result of the difference in salinity between the mud filtrate and the formation fluid.

Electrochemical Potential

The membrane potential (sometimes called the shale potential).

This potential exists at thejunction between the non-invaded zone and the shale (or other impermeable rock) sandwiching

the permeable bed. Na+ ions penetrates through the shale from the

saline formation water to the less saline mud column, a potential is set up.

Membrane potential

The shale wall potential. This potential is the same in origin to the mud cake potential, but applies to the flow of fluids from the borehole into shale formations. It is usually very small because the flow into impermeable shale is small.

The mud cake potential. This potential is produced by the movement of charged ions through the mud cake and invaded zone in a permeable formation.

Shale wall &Mud Cake potential

The spontaneous potential produced in bore hole are actual

result of these two processes combine.

The mud filtrate is less saline than the formation water so it

becomes negetivly charge as a result of diffusion potential.

But above the sand opposite the shale because the shale

potential the saline solution (mud flitrate) becomes positivly

charge.

The excess charge is therefore negetive opposite the sand and

positive opposite the shale.

This couple works in a complementry sense and creates a SP

currents flowing between the mud filtrate the porous

formation and shale.

How it works

The tool is extremely simple. SP log consists of an electrode that is suspended

in the bore hole.

Another electrode that fixed on the surface.

A voltmeter that measure the potential difference b/w bore hole electrode and ground electrode.

Measurement tools

The flow of current is focused at the bed junction. It is only here that there is a change in potential. This is important since SP measurements are made not of absolute values, but of changes in values. It is only at the bed Junctions, then the changes take place and will be recorded.

If a bed is not permeable, ions will not be able to move. There will be no current flow and thus no potential change: that means no SP. However, even the slightest permeability will permit current low and an SP change will be recorded.

Good earth is difficult to find .

Sometimes fixed electrode is attatched to rig legs which is a big mistake.

Because

The operation of sea waves on the rig legs induces potentials in the rig legs that give rise to wavy patterns in the recorded SP log which cannot be removed.

SP log Difficult to run on offshore

Log Presentation

SP is shown in millivolts in Track 1. With negative deflections to the left and positive

ones to the right. In reading the SP log it is best to first define a

shale base line. This is the typical SP level for shales shale base line can be found by comparing the SP log with the GR log response.Permeable formation will have then variable intensity to left or right of this Shale base line .

SP Log Presentation

It is usually run with Gamma ray or Caliper Log

In sand A, Rw is less than Rmf; i.e., formation water is

saltier than the mud filtrate.

In sand B, the SP deflection is less than in sand A,

indicating a fresher formation water.

In sand C, the SP is reversed, indicating formation water

that is fresher than the mud filtrate (Rw > Rmf).

We may guess that, at about 7000 ft, Rmf and Rw are

equal.

How To read

Salinity

Resistivity

Bed thickness

Influence of clay

Factors Effecting SP Log Response

When mud filtrate salinities are lower than formation water i.e Rmf>Rw so SP deflection will be to right called normal or negative response.

When mud filtrate salinities are more than connate water i.e Rmf<Rw . SP response deflect to right this is called positive or reverse response .

When Rmf=Rw so there is no Sp response .

Salinity & Resistivity

If layer is thick ,deflection is maximum If layer is thin ,the curve is recorded as

narrow peak .

Influence of Bed thickness

Clay layer reduces the amplitude of SP deflection .

Influence of Clay

Applications of SP Log

Qualitative Quantitative

Detecting permeable

beds

Correlation from well

to well

Formation Water

Resistivity

(Rw)determination

Shale Volume

Indicator

The SP log is an extremely useful quick-look indicator of bed permeability.

It is not quantitative We can not associate very large permeabilities

necessarily with large deflections and viceversa. However, the SP log is quite sensitive, and even a

small deflection in the SP log indicates thatthe bed has reasonable permeability. It should be noted that some permeable beds might

give no deflection, such as those where there is no difference in salinity between the formation fluids and the mud filtrate.

Detection of Permeable Bed

The SP log is sometimes a useful additional log to use in correlation, but is rarely used alone.

If used then, wells should be drilled with the same mud, and the salinities in the formations should be constant between wells.

The SP log can be used to follow facies changes. However, it has been largely replaced by the GR log which has a higher resolution and is more reliable

Correlation from well to well

The relationship between the SP and the resistivities

of the mud filtrate and the formation water are

determined

SP = -K log (Rmf)e/(Rw)e

Calculation of Rw

Shale volume indicator

It is considered that the volume of shale in a water-wet, shaley sandstone can be simply calculated using the SP as follows: