sp logging

8/10/2019 Sp Logging

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SP Logging


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In sand A- Rw is less than Rmf; which means

that formation water is saltier than the mud

filtrate. In sand B- the SP deflection is less than in sand

A and thus a fresher formation water is

indicated.

In sand C- the SP is reversed, indicating thatformation water is fresher than the mud

filtrate and thus Rw is greater than Rmf.

Somewhere in the region of 7000 feet it may

be guessed that Rmf and Rw are equal.


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SPLiquid Junction Potential

Two mechanisms cause SP

Electrokinetic (very small)

Electrochemical

i. Liquid Junction

ii. Membrane Potential


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SPMembrane Potential


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Static SP (SSP)


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SP as a permeability or shale indicator

Since invasion can only occur in

permeable formations,

deflections of SP can be used toidentify permeable formations.

The vertical resolution of SP is

poor, and often the permeable

bed must be 30 ft or more to

achieve a static (flat baseline) SP


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Rw from SSP

Under certain circumstances Rw can

be estimated from SP.

The SP value remains constant for

at least 30 feet.

The area where the SP is constant

must correspond to a very clean

sandstone. The value of Rmf must remain

constant across this same interval.

These conditions are rare, and

large errors in the Rw estimatemay occur.

2._:,

24.065

133.061

10

log

SPChartRR

CTcK

FTcK

k

SSP

mfeq

weq

weq

mfeqc

mfeqweq

c

R

R

RRKSSP


10/27

Rmfeq from Rmf or Rw from Rweq

If Rmf @ 75degF > 0.1

Ohmm then Rmfeq=0.85

Rmf @ BHT

If Rmf @ 75 degF < 0.1

Ohmm then Rmfeq from

chart sp2,

Same with Rw

Chart SP-2


11/27

SP as Rw indicator

Rmf < Rw

Saline mud

Rmf > Rw

Fresh mudRmf = Rw

SP is more often used

qualitatively to predict

whether Rw > Rmf or not.


12/27

SP for correlation

-ve SPdeflectio

n

+ve SPdeflectio

n


13/27

SP for correlation

Keep in mind that SP deflection is Rmf dependent and never an absolute value


14/27

SP log

Rmf > Rw?Where is Sand?

Where is Shale?

What is Vsh?


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The Electrochemical Component

The electro-chemical component Ec consists of the liquid junction

potential (Ej) and the membrane potential (Em). These potentials

create a current that flows at the shale / reservoir interface. When a

reference electrode is moved across this interface a difference in

potential is measured.

Liquid Junction Potential

SP is affected by formation water salinity. When solutions of

differing concentration are brought into contact, ions from the

solution with a higher concentration tend to migrate toward the

solution of lower concentration until equilibrium occurs


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However, with sodium chloride (NaCl) solutions, the Cl- anions move faster

than Na+ cations, so a conventional current (or potential) flows from the less

concentrated solution to the more concentrated solution. The electrical

potential that results from the combined sodium and chlorine ion movement is

known as the liquid junction potential(Elj). In terms of the solutions present in a formation, mud filtrate can be substituted

for the less concentrated solution and formation water will be the more

concentrated solution. Borehole mud-weight is usually higher than the

formation fluid pressure. This produces an over-pressure at the face of the

reservoir exposed to the borehole, and causes mud filtrate to invade thereservoir. A mud cake is subsequently formed and the invasion process slows

down. An invasion profile as shown, Liquid Junction Potential is formed which

separates, in this case, a high saline formation water and the low salinity mud

filtrate


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The liquid junction potential Ejis created at the interface between the invaded

zone and the uncontaminated zone due to a salinity difference between mud

filtrate and formation water. Since the negative Cl-anions (assuming an NaCl

solution) have a greater mobility than the positive Na+cations, the net result is a

flow of negative charges (Cl-ions) from the moreconcentrated solution to the less

concentrated solution. This mechanism, which is driven by the conductivity

difference the mud filtrate and formation water is also shown by the above Liquid

Junction Potentialgraphic. The greater the contrast in salinitybetween mud

filtrate and formation water, the largerthis potential

M b P i l


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Membrane Potential

Another "battery" found in the formation arises from the molecular constructionof shale beds. Shale can act as an ionic sieveor membrane. This means that shalecan be permeable for one type of ion while acting as a barrier for another type.This property is called ionic perm-selectivity, and the result is that the shale-

membrane can preferentially prevent the movement of negative ions. In thiscase, shales are permeable to Na+ ions, but not so permeable to C1- ions.

Shales are cation exchangers; they are electro-negative, and therefore repelanions. This phenomenon occurs as a result of the crystalline structure of clayminerals. Their exterior surfaces exchange sites where positively charged cationscling temporarily. In most instances, the shales are 100% effective and thereforerepel all chlorine (negatively charged) ions. The positive sodium ions move towardthe lower salinity mud in the borehole, but the chlorine ions cannot follow thismovement.

Since Na+ ions effectively manage to penetrate through the shale from the salineformation water to the less saline mud column, a positive potential is generatedtoward the low-concentration NaCl solution of the mud column. This potential isknown as themembrane potential(Em). Figure 8 indicates the process.


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The currents created by this series of potentials flow through

5 different media, each with its own resistivity:

borehole filled with mud (Rm),

mudcake (Rmc),

invaded zone filled with mud filtrate (Rxo),

virgin zone filled with uncontaminated fluids (Rt)

surrounding shales (Rsh).

In each medium the potential along a line of current flow (I)

drops in proportion to the resistance that is encountered.

Etotal = I. Rm

mud

+ I. Rmc

mudcake

+ I. Rxo

flushed zone

+ I.R t

virgin zone

+ I.

Rshadjacent shale

The motor providing the potential (Etotal) can therefore be expressed as:

[E-6]

kshkmcjmtotal EEEEE


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SHALE VOLUME CALCULATION

The presence of shale in art otherwise "clean" sand tends to supress the SP. This effect can beused in estimating the shale content of a formation. If SPlog is the value of the SP curve at thedesired measurement point on the log, and SPsand is the value observed in a clean, water-bearing sand and SPshale is the value observed in a shale, then any intermediate value of the SP

may be converted into a value for the shale volume (Vsh) by the relationship

sandshale

sand

SPsh

SPSP

SPSPV

)()(

)()(

log

This concept is illustrated in Figure 3:Shale Volume Calculation Example Using theSP.


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GEOLOGICAL INFORMATION

FACTORS AFFECTING THE SP


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FACTORS AFFECTING THE SP

SP readings are usually accurately and easily measured. However, there are some

circumstances where SP readings need careful consideration.

Oil-base muds completely lack an electrical path through the mud column,

hence no SP can be generated. Shaly formations suppresses the measured SP. This phenomenon permits

the formation shaliness to be determined if a clean sand with the same water

salinity is available for a legitimate comparison.

Hydrocarbon saturation suppresses SP deflections. Thus, only water-bearing

sands should be selected for Rwdetermination from the SP. Unbalanced mud columns, with differential pressure into the formation, can

cause "streaming" potentials that augment the SP. This effect, known as

electrokinetic SP, is noticeable in depleted reservoirs, and is impossible to

compensate quantitatively.

Resistivities may be very high in hard formations, except in permeable zonesor shales. These high resistivities affect the distribution of the SP currents, hence

the shape of the SP curve. As illustrated in Figure 1: Schematic Representation of

the SP in Highly Resistive Formations, the SP currents flowing from shale bed Shl

toward permeable bed P2 are largely confined to the borehole between Shl and

P2 because of the very high resistivity of the formation in this interval.


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Accordingly, the intensity of the SP current in the borehole in this interval

remains constant. Assuming the hole diameter is constant, the potential drop

per foot is constant and the SP curve is a straight line.

In high resistivity formations, SP current can leave or enter the borehole only

opposite permeable beds or shales. This causes the SP curve to show a

succession of straight portions with a change of slope opposite every

permeable interval (with the concave side of the SP curve toward the shale

line) and opposite every shale bed (with the convex side of the SP curve toward

the shale line). The boundaries of the permeable beds cannot be located with

accuracy by use of the SP in such highly resistive formations.

Bed thickness can affect the SP measurement quite dramatically. In thin beds,

the SP does not fully develop. Figure 2: Factors Affecting SP Reduction

illustrates the factors which produce this effect.

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