6.-log interpretation methods

8/10/2019 6.-Log Interpretation Methods

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September 2014

A K S Kakani

Foundations Basic Well Log Tool Physics andFormation Evaluation

Log Interpretation Methods


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QUALITATIVE INTERPRETATION - QUICKLOOK


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Qualitative Interpretation -Quicklook

Three quick look (qualitative) methods exist for rapidly locating pay (oilor gas) from logs without performing calculations (next section).

The three quick look methods are: Side by side technique

Overlay technique

Neutron-Density Crossover

All are effective in experienced hands and are widely used. It is

recommended to use them to have a general idea of the presence of

hydrocarbons in the well


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Side by Side Technique (1)

Logs are scaled so both the resistivity and porosity curves move in thesame direction (left or right) in water filled zones.

Resistivity increases to the right.

Porosity decreases to the right (decreasing porosity causes increasingresistivity).

As the porosity varies, both the porosity and resistivity curves will movein the same direction (right or left) as long as the rock is water filled.

If the rock contains hydrocarbons, the resistivity and porosity curves willgo in opposite directions.

These last two statements are the basis for qualitative hydrocarbondetection.

Shale reduces the amount of separation, but the effect still holds true.


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Lay the resistivity and porosity logs side by side with depths aligned.

Only three things cause the resistivity to go to high values:

Low porosity Hydrocarbons

Fresh Water (discarded by local area knowledge)

Look for any place where the resistivity increases. Check to see if the

porosity decreases there. If the porosity decreases, the zone is mostlikely water bearing (resistivity increase due to decreasing porosity). Ifthe porosity increases (or remains the same), this is a potentialhydrocarbon bearing zone.

The zone one wants to find has high resistivity AND high porosity

(hydrocarbon bearing).

This side by side technique is a good, first, fast-look method.

Side by Side Technique (2)


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Exercise - Side by Side Technique

Find the pay zone,

what kind of

hydrocarbon is

there? Is there a

water zone, where

is the OWC?


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Other Techniques

Overlay Technique

The overlay technique consists of laying the resistivity log on top the porositylog on a light table, with depths aligned. Slide the resistivity log left or right(sideward) to align the deep resistivity curve on top the porosity curves in aclean high porosity zone.

Maintain this same relative position and examine the overlaid logs over theentire log.

The logs (deep resistivity and porosity) should track each other fairly well,except in hydrocarbon bearing zones. In hydrocarbon bearing zones the

resistivity will lie significantly to the right of the porosity curves. Look for thisseparation of resistivity curve to the right of the porosity curves, making surethe two curves remain on top each other in water sands.

Neutron-Density Crossover

It consists of looking at only the neutron-density curves for crossover and

mirror imaging. Such crossover with mirror imaging means gas is present.Nothing else causes such a response. Be sure the mirror imaging ispresent, as washouts or lithology can cause mere crossover .


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Low Resistivity Pay

A certain type of pay sand exists fairly commonly in certain typeenvironments that is not as obvious on the resistivity logs asconventional pay sands.

This type sand is called a Low Resistivity Pay sand. It is marked by amuch lower resistivity than would be expected for a pay sand.

Basically, shales or very fine pores reduce the resistivity to much lowerthan normal values for hydrocarbon bearing sands.

Low resistivity Pay sands can be quite prolific producers.

The key to spotting them is a very careful study for even a smallresistivity increase over what should be there for a water sand. The useof image logs and cores are also a good help to identify these sanda.Detecting these sands requires experience, and preferable specificexperience in the area of question


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SATURATION


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Saturation

The saturation of a formation represents the amount of a given fluid

present in the pore space.

The porosity logs react to the pore space.

The resistivity logs react to the fluids in the pore space.

The combination of the two measurements gives the saturation

Matrix

water

oil

Sw= Swirr + Sw"free"

So= Soresidual+ So"free"


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Saturation Basics (1)

Rw = resistivity of water in the pore space.

Define Ro = resistivity of a rock totally filled with water.

F: Formation Factor.

At constant porosity F is constant.

As porosity increases, Ro decreases and F decreases.

Experiments have shown that F is inversely proportional to m.

m: is called the "cementation exponent".

a: is called the "lithology" constant.

F = R

0

Rw

F = a

m


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Saturation Basics (2)

Saturation can be expressed as a ratio of the resistivities:

Sw

n=

R0

R t

Sw

n=

FRw

Rt

where n is the "saturation exponent", an empirical constant.

Substituting for Ro:

Substituting for F:

w

n

S ==== a

mR

w

R t


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Archies Saturation Equation

The Archie equation is hence very simple. It links porosity and

resistivity with the amount of water present, Sw.

Increasing porosity,, will reduce the saturation for the same Rt.

Increasing Rt for the same porosity will have the same effect.

w

n

S ==== a

mR

w

R t


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Invaded Zone The same method can be applied to the invaded zone. The

porosity is identical, the lithology is assumed to be the same,

hence the constants a, n, m are the same.

The changes are the resistivities which are now Rxo and Rmf

measured by the MSFL tool.

The equation is then:

Sxon

=

aRmf

mRxo


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Other Relationships

Dividing for Sxo and Sw, with n set to 2

Observations suggest:

Hence:

providing a quick look saturation answer when porosity is not available.

Sw

Sxo

= R

xo R

t

R mf R w

1

2

Sxo

Sw

1

5

Sw=

Rxo

Rt

Rmf

Rw

5

8


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Archie parameters Rw = resistivity of connate water.

m = "cementation factor", set to 2 in the simple case.

n = "saturation exponent", set to 2 in the simple case.

a = constant, set to 1 in the simple case.

Two common sets of numbers for these constants are:

In a simple carbonate, the parameters are simplified to:

m = 2,

n = 2,

a = 1

In a sandstone the following values are often quoted:

m = 2.15,

n = 2, a = 0.62

w

n

S ==== a

mR

w

R t


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Rw determination

Rw is an important parameter.

Sources include:

Client.

Local tables / knowledge.

SP.

Resistivity plus porosity in water zone.

RFT sample.

From Rxo and Rt tools.


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Rw from Rwa

If Sw = 1, the saturation equation can become:

Assuming simple values for a, m, n.

Procedure is to:

Compute an Rwa (Rw apparent) using this relationship.

Read the lowest value over a porous zone which contains water

This is the method employed by most computer based interpretation

systems.

Rw

= 2

Rt


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Rw from resistivity

In a water zone Sw = 1, thus the alternative saturation equation

becomes:

The value of Rmf is measured; Rxo and Rt are measured, thevalue of Rw can be calculated.

Sw

Sxo

= R

xo R

t

R mf R w

1

2


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Other Archie Parameters

The constants a, m, n are an integral part of Archie's

saturation equation. They can, and do, vary. They areusually taken from local knowledge if at all possible.

n is dependent on the wettability of the rocks; in

the common water wet case it is usually close to 2.

a and m are dependent on the lithology and pore

systems of the rock.


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F Relation chart


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Computing Saturation

The standard saturation equation can be used with specialattention taken to obtain the correct value for the cementexponent m:

In vuggy formations this will be greater than 2. Theresistivity logs see read higher as the pathway is more

tortuous. Saturations calculated with an m of 2 will showtoo much hydrocarbon

In fractured formations m will be less than one as theresistivity pathways are straight. In this case saturationscomputed with m = 2 will show too much water.


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Variation of m

m reflects the tortuosityof the formation, thepathway for electricalcurrent flow Carbonateshave complex porositiesand hence currentpathways an values of m


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Variable m

Hence in a carbonate the major problem is the determinationof m

A good method of determining m is as follows:

In a water zone, rearranging Archies formula

Log Rt = - m log + log (aRw)

Plotting on a log-log scale, slope will give m, and theintercept a . The assumption is that m is constant through theentire reservoir.


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M relationship to secondary porosity

This chart gives the value of the fracture or vug porosityas a function of the total porosity and the cementationfactor, m.


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DUAL WATER MODEL


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Shale and Saturation

The Archie equation has to be changed to take

account of the shale effect.

The shale looks like low resistivity so another term is

added to the equations.

The result is an equation which will can be used to

compute water saturation in shaly sands.

All these equations return to Archies equation if thereis no shale present.


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Saturation Equations (1)

Indonesia Equation

Nigeria Equation

Waxman-Smits Equation

Dual Water Equation

1

R t=

S w2

F *R w

+ BQ v S w

F *

C t = tm S wtn

aC w +

S wb

S wtC wb C w( )

S w = 1

V cl

1 V cl

2

R cl+

eR w

* 1

R t

1

R t=

V cl1 . 4

R cl+

em

2

aR w

2

S wn


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Saturation Equations (2)

One of the difficulties is the number of equations

available for shaly sands.

They are often country oriented, Nigeria, Venuzeula..

The choice of equation is dictated by local practice.

Waxman-Smits (WS) and Dual Water (DW) approach

the problem from experiments on the clay properties and

are thus more realistic and universal.


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Dual water

The Dual Water Model takes the basic work of Waxman

Smits and expands it for use with logged information

It divides the formation into solids and fluids.

It splits the clay into dry clay and its associated water,

called bound water

The standard definitions for porosity and saturation to

describe the fractions of fluids in the formation areexpanded to include the new model.


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Clean to Shale t

t

t

t

Matrix

Matrix

Matrix

Dry Colloid

Dry Colloid

Bound water

Far Water


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Dual Water definitions

the porosities are combined to give the saturations of the fluids present

S wb =

wb

t

S wf = wf

t

S hy = hy

t

S wt = S wf + S wb

t = e + wb = t 1 S wb( ) + tS wb

S wt + S hy = 1

V cl = V dcl + tS wb

saturation of bound water

saturation of free water (this is Sw)

Hydrocarbon saturation

Total water saturation is the sumof the saturations of the two waters

total water saturation plus hydrocarbon saturationmust be one

wet clay volume includes the

volume of bound water

The total porosity is given by


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Simplified DWM (1)

Swt2

==== Rf

t2

Rt

Archie Equation can be generalized into the following form;

where;

Swt - total water saturation

ft - total porosity

Rt - true formation resistivity

Rf - resistivity of the water(s)

The equation can be solved if Rf is known.


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Simplified DWM (2)

1) Clean water bearing zone

Swt = 1

t2Rt = Rf

This is Rwf, the resistivity of Free water

2) Clean 100% shale zone

Swt = 1

t2Rt = Rf

This is Rwb, the resistivity of Bound water

These are the two end points. To give a universal solution they are

combined linearly using the volume of shale.


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Practical DWM

Ct

=

t

m

Swt

n

aC

wf+

Swb

Swt

Cwb

Cwf

( )

The standard equation for the water saturation is expressed in terms of theconductivity, as it is linear.

This equation is in terms of measured quantities, porosity andresistivity and parameters that can be found, the far and boundwater conductivities.


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DWM Saturation solution

S wt = x + x 2 + C tF 0

C w

x =S wb C w C wb( )

2 C w

F o = a

m

The solution to the equation is

where

and


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Practical outputs

The equations give total water saturation Swt and total porosityt. These have to be transformed into effective saturation, Swand effective porosity, wf (or e)

S w =

S wt S wb

1 S wb

wf

= t

Swt

Swb

( )


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Dual water equation solution

This derivation of the Dual Water equations is valid for any rock withany mixture of fluids

It is possible to use the Dual Water Model to make a manualcomputation of a shaly zone.

However computer programs are best equipped to handle the

calculations.

The selection of key parameters is essential to obtain the correct

answers,

Cwf - free water conductivity

Cwb - bound water conductivity

Swb - bound water saturation

t - total porosity


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SHALES


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Shale Deposition Types

Clean formation Structural shale

Porosity

Porosity

Matrix

Matrix

Porosity

Matrix

Porosity Shale

Shale

Matrix

Porosity

Matrix

Laminar shale Dispersed shale

Shale

Shale


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Clay Minerals

b N (thermal) Pe

Kaolinite 2.54 59.6 1.85

Illite 2.52 47.9 3.97

Smectite 2.02 87 1.70

Chlorite 2.73 59.6 4.07

Most shales are comprised of these clay minerals.

Clay minerals frequently occur together in "mixed layers", e.g. Illite -Montmorillonite.

Kaolinite Al, Si, little K

Illite K, Fe, Mg, Si

Smectite Very high porosity.

Chlorite Fe, Mg, no K


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Shale and Logs (1)

Shales have properties that have importantinfluences on log readings:

They have porosity.

The porosity is filled with salted water.

They are often radioactive.

Resistivity logs exhibit shales as low resistivity zones.

Sh l d (2)


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Shale and Logs (2)

Neutron porosity logs exhibit shales as high porosity.

Density and sonic logs react to the porosity and matrix changes.

Gamma ray logs react to shale radioactivity.


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Shale Corrections

The electrical properties of shales greatly influence thecalculation of fluid saturations.

A layer of water close to the clay surface is electricallycharged.

Archie's equation assumes that the formation water is the onlyelectrically-conductive material in the formation.

The clay layer requires an additional term in the saturationequation.

Porosity tools can be corrected for the shale effect. An"effective porosity" can be computed as compared to a "totalporosity" which includes the shale effect.


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Sh l V l (2)


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Shale Volume (2)

However, as every tool reacts to shale, each tool is a shaleindicator. For example:

Shale volume can be computed from different sources andfrom crossplots of different kinds of log data.

The ideal method of computing shale volume is to use theNeutron Density plot.

(((( )))) (((( )))) clclclmawhwwb VVSS ++++++++++++==== 11


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LITHOLOGY AND POROSITY

Lith l d P it


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Lithology and Porosity

The next major step in the procedure is lithology identification. Lithology data gives information onporosity and other parameters.

Lithology of a formation can be:

Simple

Dirty

Complex

Lithology Determination


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Lithology Determination

The lithology can be obtained in several ways:

From the cuttings (depth problems).

From local knowledge (good during development).

From the known depositional environment (good in general basis).

From a log Quicklook (good starting point).

From individual log readings (difficult if there are no areas of zeroporosity).

From crossplots (the best method).

Lithology and Porosity Tools


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Lithology and Porosity Tools

All tools react to lithology - usually in conjunction with theporosity.

Major lithology tools are: Neutron - reacts to fluid and matrix.

Density - reacts to matrix and fluid.

Sonic - reacts to a mixture of matrix and fluid,

complicated by seeing only primary porosity. SGT - identifies shale types and special minerals.

NMR - magnetic resonance reacts to the porosity witha small element if lithology.


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Volume


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Volume Formation model:

Water-bearing, mono-mineral.

This formation can be described by the density tool and the neutron tool.

2 equations for 1 unknown:

system is over-determined.

for limestone: Nma = 0

for sand: Nma = 0.04

(((( )))) ++++==== 1mamfb(((( )))) ++++==== 1mamfn

Crossplot Solution


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Crossplot Solution The plot is a straight line from the matrix point to the 100%

porosity, water point. It is scaled in porosity.

Neutron-density X-plot (1)


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This crossplot

has b plottedagainst the

corrected

neutronporosity. Fluiddensity in this

plot is1.0g/cm3.



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Neutron density X plot (2)

This plot is the

same as theprevious one

except that the

fluid densityhere is 1.19g/cm3.

Dual Mineral model


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Dual Mineral model

= + V + V B mf m1 m1 m2 m2

= + V + V N N mf m1 N m1 m2 N m2

1 = + V + Vm1 m2

3 unknown : , V , V , 3 equations

system is just determined

m1 m2

(Material Balance Equation)

Dual Mineral plot


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p

The plot now has two lines, one from each matrix point. The equi-porosity lines join the lines, anypoint falling between can be assigned its porosity the zero porosity line is scaled in ratio (or percent)of the two minerals. This can be extended to the water point. Points falling inside the lines can besubdivided in mineral percent


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Z-axis Plot


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Z axis Plot

Other Crossplots


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p

There are numerous other crossplots to identify minerals

using combinations of tools.

ma - Uma b - Pe

MID plot (n, b, t)

MN plot (n, b, t)

The z -axis is used for clarification.

Pe - b Crossplot


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p

This plot is ideal toidentify the lithologyin conjunction withthe neutron density

plot.

ma - Uma (1)


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( )

ma - Uma


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(2)Umadetermination

Matrix Identification Plot


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The Matrix Identification Plotuses neutron, density andsonic data as inputs. Anapparent crossplot porosity is

found on a density-neutronand a sonic neutroncrossplot. The values areentered into the relevantsection of the following chartand the values of tmaa and

maa read;

MN plot


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The MN plot usesdata from the

neutron, density andsonic logs to solve

complex lithology.Used when Pef is not

available or as extrainformation.

Hydrocarbon Effect


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The presence of light hydrocarbons especially gas, in the invaded zone seriouslyaffects the main porosity tools, the density and neutron.

Both tools are calibrated to read correctly in water-filled rock.

Light hydrocarbon has a lower hydrogen index, hence the neutron reads low andthe low density of the fluid makes the density low.

Points exhibiting this problem plot above and to the right of the lithology line on thecrossplot.

Hydrocarbon Effect Correction


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Complete Well Evaluation


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Perform a complete well evaluation, determining VSH, f, RT, RW, SW, lithology type, fluidcontents on the attached log, assume all environmental corrections have already beenmade


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6.-log interpretation methods

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