Well Logging Course (1st Ed.)

1. Reading A Log

2. Examples of Curve Behavior And Log Display

3. Electrical Properties Of Rocks And Brines

1. Spontaneous PotentialA. membrane potential

B. Application

C. Log Example of The SP

Spontaneous potential

Spontaneous potential main usage:the identification of permeable zones

The origins of the spontaneous potential in wellbores involve both electrochemical potentials and

the cation selectivity of shales.

basis for the spontaneous potential is the process of diffusion –the self-diffusion of

the dissolved ions in the fluids • in the borehole and in the formation.

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The mechanism of generating the liquid-junction potentialElectrochemical potentials

of interest to the generation of the spontaneous potential are the liquid junction potential the membrane potential

Figure schematically illustrates the situation for the generation of the liquid-junction potential. To the left is a saline solution

of low NaCl concentration. To the right is one of a higher

ionic concentration.

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The liquid junction potential

As is often the case, the resistivity of the drilling mud filtrate (Rmf )

is greater than the resistivity of the formation water (Rw), so:

Where Vl−j is The liquid junction potential

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A schematic representation of the development of the SP in a boreholeThe cell marked Ed

corresponds to the liquid junction potential just discussed is sketched with the polarity

corresponding to a higher electrolyte concentration in the formation water than in the mud filtrate.

additional source of SP is associated with the shale result of the membrane

potential generated in the

presence of the shale that contains clay minerals which have large negative

surface charge

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A representation of a shale

On the left, consisting of

rock mineral grains and small platy clay particles.

On the right the

distributions of ions close to the face of one of the clay minerals is shown, which

illustrates the so-called electrical double-layer.

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How does a cation differ from an anion?A cation (s)(+)

is a positively (+) charged ion. It loses one or more negatively charged electrons when

forming ionic compounds. (are) almost always metals

An anion (s) (-)is a negatively (-) charged ion. It gains one or more electrons when forming ionic

compounds. (are) typically nonmetals

Every ionic compound must contain both a cation and an anion so that the compound as a whole has no charge.A common example: In the ionic compound table salt (NaCl),

sodium (Na+) is the cation, and chloride (Cl-) is the anion.

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electrical double layer

We assume that shale is nearly impermeable to fluid flow, but still capable of ionic transport, although considerably

altered by the presence of clay minerals.

The shale acts like a cation-selective (+) membrane. This property is related to the sheet-like structure of the

alumino-silicates that form the basic structure of clay minerals. At the surface of the clay minerals there is

a strong negative charge related to unpaired Si and O bonds. When the clay mineral particles are exposed to an ionic solution,

one containing Na+ and Cl− for example, • the anions (Cl-) will be repulsed by their surfaces while

the cations (Na+) will be attracted to the surface charge, forming the so-called electrical double layer.

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membrane potential

Close to the clay layers, the fluid will be dominated by cations since the anions are excluded by electrostatic repulsion. In this manner, in a complex mixture of clay minerals and

other small mineral particles, with pore spaces even too small to permit the hydraulic flow of water, the cations will be able to diffuse along the charged surfaces, from

high concentration to low concentration while the negative Cl ions will tend to be excluded.

Such a diffusion process will tend to accumulate a positive charge on the low ionic concentration side of the shale barrier, producing an attendant electric field. In the practical situation,

the cations from the fluid saturating the porous sand zone diffuse through the shale to the borehole with the lower cation concentration.

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evaluating the membrane potential

In this figure a semipermeable shale barrier separates the solutions of two different salinities. A schematic

representation of the mechanism responsible for the generation of the membrane potential. The diffusion process is

altered by the selective passage of Na+ through the shale membrane.

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magnitude of the membrane potential

The natural diffusion process is impeded because of the negative surface charge of the shale. The Cl ions which otherwise would diffuse more readily

are prevented from traversing the shale membrane,

whereas the less mobile Na ions can pass through it readily.

The result is that the effective mobility of the chlorine in this case is reduced to nearly zero.

magnitude of the membrane potential Vm

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SP Measurement

In the case of lower NaCl concentration in the mud, the voltages add, resulting in a more negative voltage in front of the sand

than in front of the shale zone.

The membrane potential provides about 4/5 of the SP amplitude, since the absolute value of mobilities

enters in its potential,

rather than the difference as in the liquid-junction potential.

The SP is measured, between an electrode in the borehole and a distant reference.

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natural potential vs. static spontaneous potentialThe shale baseline

represents the natural potential between the two electrodes, without electrochemical effects, and

is ideally a straight line from top to bottom.

The static spontaneous potential (SSP), is the ideal SP generated by electrochemical effects

when passing from the shale to a thick porous clean (shale-free) sand if no current flowed.

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potential drop

In practice the electrode can only measure the potential change in the borehole.

Although the mud is usually less resistive than the formation, the area for current flow is much smaller

in the borehole than in the formation, so that the borehole resistance is usually much higher

than the formation resistance.

Most of the potential drop therefore takes place in the borehole with the result that

the measured SP amplitude in the center of the bed is close to the SSP.

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The determination of Rw

In the best of cases, the measurement of the SP allows the identification of permeable zones and the determination of formation water resistivity.

Since the mud filtrate resistivity can be measured, the formation water resistivity can be calculated using factors that are well known for NaCl solutions.

A deflection indicates that a zone is porous and permeable and has water with a different ionic concentration

than the mud.

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effective water resistivities vs. actual resistivitiesIn practice the electrochemical potential is often

written in terms of effective water resistivities (Rmf e) and (Rwe) rather than actual resistivities. These are equal to Rmf and Rw

except for concentrated or dilute solutions. In concentrated solutions,

below about 0.1 ohm- m at 75◦ F, the conductivity is no longer proportional to the number density of charge carriers and their mobilities. • At high concentrations the proximity of the ions to one another is

increased; their mutual attractions begin to compete with the solvation to reduce their mobilities.

In dilute solutions of most oilfield waters, other ions than Na+ Cl− become increasingly important. Numerous charts exist for the determination of Rw from the SP,

knowing Rmf and temperature.

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Other applications of SP log

The SP is also used to indicate the amount of clay in a reservoir. The presence of clay coating the grains and throats of the

formation will impede the mobility of the Cl anions because of the negative surface charge, and thus spoil the development of the liquid-junction potential.

The ideal SP generated opposite a shaley sand when no current flows is known as the pseudo static potential (PSP).

In addition to these quantitative interpretations, elaborate connections have been established

between the shape of the SP over depth and geologically significant events.

Some examples of using the SP curve to determine patterns of sedimentation are given in Pirson [10].

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SP vs. other logging techniques

The measurement of the SP is probably the antithesis of the high-tech image of many of the other logging techniques. The sensor is simply an electrode

(often mounted on an insulated cable, known as the “bridle,” some tens of feet above any other measurement sondes) which is referenced to ground at the surface.

The measurement is essentially a dc voltage measurement in which it is assumed that unwanted sources of dc voltage are constant or only slowly varying with time and depth.

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shale and clean sand beds along with the idealized response of SP logging

deflections to the left correspond to increasingly negative values.

In the first sand zone, there is no SP deflection

since this case represents equal salinity in the formation water and in the mud filtrate.

The next two zones show a development of the SP which is

largest for the largest contrast in mud filtrate and formation water resistivity.

In the last zone, the deflection is seen to be to the right

of the shale baseline and corresponds to the case of a mud filtrate which is saltier than the original formation fluid.

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several cases of SP log for a given contrast in Rmf & Rw

It illustrates several cases, for a given contrast in mud filtrate salinity and formation water salinity, where the SP deflection

will not attain the full value seen in a thick, clean sand.

The first point is that the deflection will be reduced

if the sand bed is not thick enough because not enough of the potential drop

occurs in the borehole. The transition at the bed boundary is much

slower for the same reason.

Depending mainly on the depth of invasion and the contrast between invaded zone and mud resistivity, the bed thickness needs to be

more than 20 times the borehole diameter to attain its full value.

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several cases of SP log for a given contrast in Rmf & Rw

The second point is the effect of clay in reducing the SP.

The third point is the effect of oil or gas. In a clean sand the electrochemical

potentials are not affected by oil or gas, but the formation resistivities are higher so that the transition at bed boundaries may be slower and a thicker bed may be needed for full SP development.

However, the effect of oil or gas is stronger in a shaley sand.

The electrochemical potentials are reduced compared to a water-bearing sand because there is less water in the pore space, so that the effect of the surface-charged clay particles is proportionately higher.

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Other effects which can also upset the SPelectrical noise, and bimetallic currents between

the different metal parts of a logging tool that can create an unwanted potential at the SP electrode.

the electrokinetic, or streaming, potential caused by the higher pressure in the borehole moving

cations through a cation-selective membrane.

The membrane may be a shale that has some very small permeability (Esh 3.8),

or the mudcake which contains a large percentage of clay particles and also has some very small permeability (Emc).

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Other effects which can also upset the SP (Cont.)Normally these effects are small and balance each

other out. However, when the pressure differential is high,

or the mud and other resistivities are high enough that even a small current produces a large potential, the electrokinetic effect can be comparable to the electrochemical effect.

The baseline often drifts slowly with time and depth.

Sharper shifts occur when the membrane potential at the top of a sand is different to that at the bottom. This happens when the top and bottom shales have different

cation selection properties, and also when the formation water or hydrocarbon saturation changes within the sand.

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summary SP curve behavior under a variety of logging circumstancesFinally, the symmetric

responses of SP logs can be upset by vertical movement of mud filtrate in high permeability sands: upwards in the

presence of heavier saline formation water, and

downwards in the presence of gas and light oil.

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1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007. Chapter 3