basic analysis conceps ohla section1 pag 1 30

Halliburton1 Open Hole Log Analysis Notes

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Section 1

Basic Analysis Concepts

Table of ContentsIntroduction.............................................................................................................................................2Objectives................................................................................................................................................2Formation Evaluation and Log Analysis...................................................................................................4The Basis for Log Analysis......................................................................................................................5Water Saturation of Clean Formations......................................................................................................6Archie's Equation Dissected...................................................................................................................10Essential Calculations............................................................................................................................11

Determining Geothermal Gradient......................................................................................................11Determining Formation Temperature (Tf)...........................................................................................12Determining Rmf from Rm...................................................................................................................12Correcting Resistivity for Temperature...............................................................................................12

Determining Formation Water Resistivity (Rw) by the Inverse Archie Method........................................13Example Application of Archie's Equation.............................................................................................14

Rw Calculation by Inverse-Archie Method..........................................................................................16Sw Calculations..................................................................................................................................17

Permeability Indicators..........................................................................................................................18Determining Formation Water Resistivity (Rw) by the SP Method..........................................................20

Detailed Procedure of SP Method.......................................................................................................21Additional Notes about Formation Water Resistivity..............................................................................22Additional Rw Calculation Example.......................................................................................................23"Quick-Look" Methods in Log Analysis................................................................................................27References.............................................................................................................................................30



2001, Halliburton

Introduction

This section presents an overview of the basic concepts of open hole log analysis,and provides practical examples of the techniques and methods discussed. Aworking knowledge of each of these concepts is fundamental to performing abasic well-site analysis. For further information pertaining to tool specifics anddiscussions on theory, the participant is referred to the Halliburton EnergyServices Open Hole Log Analysis and Formation Evaluation manual as well aspertinent tool theory and operations manuals.

Objectives

After completing this section, the participant should be able to

clearly identify and mark on a log potential water-bearing zones.

clearly identify and mark on a log potential hydrocarbon-bearing zones.

recognize potential water-bearing zones that are amenable to formation waterresistivity (Rw) derivation by judging their cleanliness, porosity, andqualitative permeability.

estimate lithology of potential water-bearing and hydrocarbon-bearing zones.

calculate the cross-plot porosity of a zone of interest.

select appropriate values for tortuosity factor (a) and cementation exponent(m) required for calculating formation water resistivity (Rw) and watersaturation (Sw) in zones of different lithology and/or porosity.

calculate geothermal gradient (gG) for a particular well location by equationand by chart.

calculate formation temperature (Tf) for any depth of interest by equation andby chart.

determine values for mud filtrate resistivity (Rmf) and mudcake resistivity(Rmc) from mud resistivity (Rm) by chart and by equation.

convert measured and/or derived resistivity values (Rm, Rmf, Rmc) to formationtemperature (Tf) for any depth of interest by equation and by chart.

calculate value for formation water resistivity (Rw) in a selected clean water-bearing zone by the inverse-Archie method.



2001, Halliburton

determine value for formation water resistivity (Rw) in a selected clean water-bearing zone by the SP method.

determine a reasonable and optimistic value for formation water resistivity(Rw) by comparing values derived from the inverse-Archie and SP methods.

convert derived values of formation water resistivity (Rw) to formationtemperature (Tf) for any depth of interest by equation and by chart.

calculate water saturation (Sw) for a clean hydrocarbon-bearing zone by theArchie equation.

calculate hydrocarbon saturation (Shc) for a clean hydrocarbon-bearing zoneby equation.

clearly identify and mark on a log potential perforated intervals based onwater saturation (Sw) calculations.



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Formation Evaluation and Log Analysis

Formation evaluation can be generally defined as the practice of determining boththe physical and chemical properties of rocks and the fluids that they contain.The objective of formation evaluation is to locate, define, and produce a givenreservoir by drilling as few wells as possible. To this end, oil companies utilize avariety of formation evaluation methods, some of which are outlined in Figure1.1.

Figure 1.1. Formation evaluation methods.

Exploration Define structure Seismic, gravity mapping, magneticmapping

Drilling Drill well Mud logging, whole coring, MWD

Logging Log well Open hole logs

Primary Evaluation Log analysis and testing Sidewall cores, vertical seismicprofile (VSP), wireline formationtesting, drillstem testing

Analysis Core analysis Laboratory studies

Feedback Refinement of seismic model and Log calibration via core analysislog analysis results, seismic calibration from log

analysis results

Exploitation Producing hydrocarbons Material balance analysis

Secondary Recovery Water or gas injection and Production log analysis, floodproduction logging efficiency analysis, micro-rock

property analysis

Abandonment Economic decisions

PHASE ACTIVITY EVALUATION METHOD

Wireline logs are just one of the many different sources of data used in formationevaluation. However, through accurate depth determination, wireline logs are theone medium that is used to bring together all methods of formation evaluation.Logging is a very small, but very important, piece of the larger puzzle. Thedecision to plug or complete a well is often based upon the logs, and a proper andaccurate analysis of these data is a must.



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The Basis for Log Analysis

Resistivity is, perhaps, the most fundamental of all measurements in logging. Allgeological materials possess some amount of resistance, or the inherent ability toresist the flow of an electrical current. Resistivity (R) is the physical measurementof resistance, and is defined as the reciprocal of the material's electricalconductivity (C).

yonductivitC1000

yResistivit=

Rock matrix, oil, and gas are electrical insulators. They will not conduct the flowof an electrical current, and therefore their resistivities are said to be infinite.Water, however, will conduct electricity depending upon its salinity. This impliesthat any current flow through a formation is taking place in the formation water,and not hydrocarbons or the rock matrix. Salt water, with high concentrations ofdissolved solids (e.g., NaCl, etc.), will conduct electricity much more readily thanwill fresh water. Therefore, salt water has a much lower resistivity than freshwater. In most instances, the water present in a formation at depth will bemoderately saline. Water-bearing zones, therefore, have higher conductivity--orlower resistivity--than hydrocarbon-bearing zones.

Because oil and gas will not conduct electrical current, it is impossible todistinguish them from rock matrix on the basis of resistivity. These fluids do,however, fill the pore space of a formation, leaving less room for conductiveformation water. The electrical current that does flow through a hydrocarbon-bearing formation is forced to take a more tortuous path, weaving around thehydrocarbon that occupies part of the pore space. The overall effect of thepresence of hydrocarbons is an increase in resistivity.

The basis for log analysis is to compare the measured resistivity of a formationwith the calculated resistivity of that formation assuming its porosity is 100%water-filled. The resistivity of a rock at 100% water saturation is referred to aswet resistivity (Ro). If, for a given porosity, the measured resistivity issignificantly higher than the wet resistivity, then the presence of hydrocarbons isindicated. This relationship is the basis for determining the percentage of porositythat is filled with formation water (water saturation), and therefore the percentageof porosity that is filled with hydrocarbon (hydrocarbon saturation). Watersaturation (Sw) for a clean formation may be calculated using the Archie equation.

Archie Water Saturation nt

wmw R

RaS

F=

Hydrocarbon Saturation whc S0.1S -=



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Water Saturation of Clean Formations

Consider a formation with a given amount of porosity, and assume that porosity iscompletely filled with saline formation water of a given resistivity (Figure 1.2).The formation water resistivity (Rw), because the saline water is capable ofconducting electrical current, is quite low. The resistivity of the formation itself(Ro, or wet resistivity, where porosity is 100% filled with water) will depend uponthe formation water resistivity and some other factor referred to as the formationresistivity factor (Fr).

Figure 1.2. Model formation: 100% water saturated.

wro RFR =

By rearranging this equation, formation resistivity factor (Fr) can be quantified asthe ratio of the formation's wet resistivity to the resistivity of the water (Rw)present in that formation.

w

or R

RF =

In this example, formation water resistivity (Rw) is defined as constant, thereforechanges in formation resistivity factor (Fr) will occur only with changes in theoverall formation resistivity (Ro). The one way in which Ro can change in aformation of constant Rw is by changing the amount of fluid available to conductan electrical current. This is accomplished through changes in porosity. Asporosity decreases, the amount of water available to conduct electrical current isdecreased, resulting in an increase in formation resistivity (Ro). Therefore,formation resistivity factor (Fr) is inversely proportional to porosity (F).

F=

1Fr



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This relationship between formation resistivity and porosity was researched by G.E. Archie of Shell Oil while working on limestones in France. Archie had electric(resistivity) logs from several wells, and core porosity from productive zoneswithin these wells. He noticed that there was some relation between resistivityand porosity, and thus was able to identify zones of interest through the use ofelectric logs alone. What he wanted to know was if there was some relationshipthat made it possible to determine whether a zone would be productive on thebasis of measured resistivity and core porosity.

Changes in the porosity of a formation may have effects other than simplyincreasing or decreasing the amount of fluid available to conduct electricalcurrent. With a change in porosity, there may be concomitant changes in thecomplexity of the pore network that affect the conductive nature of the fluidspresent, and formation resistivity factor (Fr) can therefore vary with the type ofreservoir. These changes are expressed by the tortuosity factor (a) andcementation exponent (m).

mr

aF

F=

For the limestones of Archie's experiments, the tortuosity factors and cementationexponents were always constant (a = 1.0, m = 2.0). However, this may not be thecase for all reservoirs. Although both parameters can be determinedexperimentally for a specific reservoir, log analysts commonly use set values fortortuosity factor (a) and cementation exponent (m), depending upon lithology andporosity. These standard values are presented in Figure 1.3.

Figure 1.3. Standard values for tortuosity factor and cementationexponent.

Porosity > 16% Porosity < 16%(Humble) (Tixier)

SANDSTONESCARBONATES

0.81a

2.0m

1.0

2.0

0.62

2.15

Consider now that the porous formation discussed previously is filled with somecombination of conductive formation water of constant resistivity (Rw) and oil(Figure 1.4). Oil is an insulator and will not conduct electrical current.Furthermore, because the formation is filled with both water and oil, theresistivity of the formation can no longer be referred to as wet resistivity (Ro).



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The measure of formation resistivity in this instance--taking into account theresistivity of the rock matrix and the fluids contained--is called true resistivity(Rt).

Figure 1.4. Model formation containing both water and oil.

True resistivity of a formation will only be equal to wet resistivity (Rt = Ro) whenthe porosity of that formation is completely filled with conductive water.However, because some of the available porosity may be filled with non-conductive oil or gas, the wet resistivity (Ro) of that formation can now be relatedto the measured true resistivity (Rt) by some additional factor, referred to as F'.

to R'FR =

The factor F' can therefore be expressed as a ratio of the theoretical wet resistivityof that formation (Ro) to the actual measured resistivity of the formation (Rt).

t

o

RR

'F =

In the example formation, because both porosity and formation water resistivity(Rw) are considered to be constant, the resulting wet resistivity (Ro) will beconstant. Therefore, changes in the factor F' will occur with changes in measuredtrue resistivity (Rt). Under the given conditions, the only way in which measuredtrue resistivity (Rt) of the formation can change is through the addition orsubtraction of conductive fluid. For example, the addition of oil to the reservoirwould result in the increase of that formation's measured resistivity (Rt) becausesome amount of conductive formation water would be displaced by the oil.Therefore, the factor F' is dependent upon the relative proportion of conductivefluids (water) and non-conductive fluids (hydrocarbons) in the formation.

The factor F' in the above equation represents water saturation (usually expressedas Sw) which is the percentage of pore space within a formation that is occupied



2001, Halliburton

by conductive formation water. By substitution of equations, water saturation canbe related to the physical properties of the formation and the conductiveproperties of the fluids it contains.

t

wm

t

wr

t

onw R

RaR

RFRR

S F

=

==

Water saturation is related to these properties by the exponent n (saturationexponent). Saturation exponent may have a range of values dependent uponspecific reservoir conditions, but generally is assumed to be equal to 2.0. Withknowledge of the production characteristics of the formation in question, it ispossible to determine more accurate values for saturation exponent.

The equation for water saturation (Sw), an expanded version of that presented as afootnote in Archie's 1942 publication and commonly referred to as "Archie'sequation," has become the foundation of the entire industry of well logging. In itssimplest form, Archie's equation is often expressed as:

n

t

wmw R

RaS

F=

where:

n = saturation exponent

a = tortuosity factor

F = porosity

m = cementation exponent

Rw = formation water resistivity

Rt = true formation resistivity

It is important to realize that while water saturation represents the percentage ofwater present in the pores of a formation, it does not represent the ratio of waterto hydrocarbons that will be produced from a reservoir. Shaly sandstonereservoirs with clay minerals that trap a large amount of formation water mayhave high water saturations, yet produce only hydrocarbons. Water saturationsimply reflects the relative proportions of these fluids contained in the reservoir.Nonetheless, obtaining accurate values for water saturation is the primary goal ofopen hole log analysis. With knowledge of water saturation, it is possible todetermine what percentage of porosity is filled with a fluid other than water (i.e.,hydrocarbons), and therefore hydrocarbon reserves.



2001, Halliburton

Archie's Equation Dissected

n

t

wmw R

RaS

F=

Sw = water saturation

n = saturation exponent

Obtained through lithology assumptions or data manipulation and coreanalysis.

a = tortuosity factor


F = porosity

Obtained from logs (density, neutron, sonic, MRIL) or core analysis.

m = cementation exponent


Rt = formation resistivity

Obtained from logs (induction, laterolog). Assumed to reflect resistivityof the uninvaded zone, and taken as the resistivity measured by thedeepest reading device.

Rw = formation water resistivity

Among the most difficult variables to determine, but one which has atremendous impact upon calculated values of water saturation (Sw). Oftenbest obtained from the customer, but can be obtained from logs underideal conditions. Other sources include measured formation watersamples (DST or SFT), produced water samples, or simply local reservoirhistory.



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Essential Calculations

Log analysis calculations require values of resistivity, in particular mud filtrateresistivity (Rmf) and formation water resistivity (Rw). A single measured orcalculated value of Rmf and/or Rw may need to be applied over a wide range ofdepths. Because resistivity varies with temperature, this practice requires thatresistivities be corrected for the appropriate temperatures at depth. Bear in mindthat Rmf and/or Rw must be corrected to the temperature at a certain depth if thosevalues are to be used in calculations.

Determining Geothermal Gradient

The first step involved in determining temperature at a particular depth is todetermine the geothermal gradient (gG) of the region. Temperature increases withdepth, and the temperature gradient of a particular region depends upon thegeologic, or tectonic, activity within that region. The more activity, the higher thegeothermal gradient. Geothermal gradients are commonly expressed in degreesFahrenheit per 100 feet (F/100').

If the geothermal gradient of an area is not known, then it can be determined bychart or by formula. If using a chart, it is important to use the correct chart,depending upon your location. Instructions and an example for using these chartsaccompany charts GEN-2a (international locations) and GEN-2b (North Americalocations).

Geothermal gradient may also be determined by taking pertinent information fromthe header and using the following equation:

100TD

TBHTg msG

-=

where:

BHT = bottom hole temperature (from header)

TD = total depth (Depth-Logger from header)

Tms = mean surface temperature

Note that both the chart method and the formula method require a value for meansurface temperature (Tms). This refers to the average annual temperature of aregion, and not the temperature at which resistivity measurements were madeduring the logging job (e.g., mud press resistivities). Mean surface temperaturesfor international and North America locations are presented on charts GEN-2aand GEN-2b, respectively. If the mean surface temperature for a region is not



2001, Halliburton

known, then it is standard practice to assume 75F as a value for Tms, and realizethe potential calculation errors that may result from this assumption.

Determining Formation Temperature (Tf)

Once the geothermal gradient (gG) has been established, it is possible to determinethe temperature for a particular depth. This is often referred to as formationtemperature (Tf). As with geothermal gradient, Tf may be determined through theuse of charts GEN-2a or GEN-2b. It may also be calculated using the followingequation.

+=100D

gTT Gmsf

where:

Tms = mean surface temperature

gG = geothermal gradient

D = depth at which temperature is desired

Determining Rmf from Rm

In some cases, a value of mud filtrate resistivity (Rmf) may not be available fromthe header, or there may be a question about the validity or accuracy of themeasurement. A value of Rmf may be obtained from the mud resistivity (Rm)through the use of chart GEN-3. This chart requires only mud density (or mudweight) as input, and allows the determination of both Rmf and mudcakeresistivity (Rmc) from Rm. It should be remembered that values of Rmf obtainedfrom this chart also require correction to formation temperature before their use.

Correcting Resistivity for Temperature

Resistivity decreases with increasing temperature, and therefore any value of Rmfand/or Rw determined at one depth must be corrected for the appropriateformation temperature (Tf) where those values will be used to calculate watersaturation (Sw). It is vital that formation water resistivity (Rw) be corrected fortemperature. Failing to correct Rw to a higher temperature will result inerroneously high values of water saturation (Sw). Therefore, it is possible tocalculate a hydrocarbon-bearing zone as a wet zone if the temperature correctionis not applied.



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Correction may be applied through the use of a chart (GEN-5) or an equation(Arp's equation). Chart GEN-5 may be used to determine the resistivity of asolution (such as Rm, Rmf, Rw, etc.) at a given temperature when the NaClconcentration of that solution is known, and vice versa. It may also be used todetermine the resistivity of a solution at a given temperature when the resistivityof this same solution at another temperature is known. Instructions and examplesfor these particular uses accompany chart GEN-5.

A more straightforward method of correcting resistivity for temperature is throughthe use of Arp's equation:

++

=kTkT

RR2

112

where:

R2 = resistivity value corrected for temperature

R1 = resistivity value at known reference temperature (T1)

T1 = known reference temperature

T2 = temperature to which resistivity is to be corrected

k = temperature constant

k = 6.77 when temperature expressed in F

k = 21.5 when temperature expressed in C

Determining Formation Water Resistivity (Rw) by theInverse Archie Method

Determining a value for formation water resistivity (Rw) from logs may notalways provide reliable results; however, in many cases logs provide the onlymeans of determining Rw. Two of the most common methods of determining Rwfrom logs are the inverse-Archie method and the SP method.

The inverse-Archie method of determining Rw works under the assumption thatwater saturation (Sw) is 100%. It is necessary, therefore, that the inverse-Archiemethod be employed in a zone that is obviously wet. Furthermore, it is desirableto calculate Rw from the inverse-Archie method in a clean formation withrelatively high porosity.

aR

R tm

wa

F=



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Once a clean and porous wet zone is located, lithological assumptions must bemade about that formation in order to select the appropriate values of cementationexponent (m) and tortuosity factor (a) to use in the equation. This estimate shouldbe accomplished by quick-look means using a combination of the gamma ray,porosity, and Pe curves. Formation water resistivity calculated by the inverse-Archie method (Rwa) depends upon lithology; however, Rwa calculated in onelithology can be used for water saturation (Sw) calculations in a zone of differentlithology. For example, Rwa may be determined in a sandstone, and this valuemay then be used in the Archie equation to calculate water saturation (Sw) in alimestone, provided that the necessary temperature corrections have been made.This is one of the many assumptions that must be made in log analysisapplications.

Example Application of Archie's Equation

The following examples are worked with respect to the log presented in Figure1.5. It is assumed that any zones of interest are limestone.

By first observing the resistivity log, one can infer that the areas of high resistivity(8515 and 8610) indicate zones containing hydrocarbons. Areas with lowresistivity (8535 and 8710) are more likely to contain conductive formation water.These axioms are not always correct because high resistivity in a formation mayalso be caused by a lack of porosity. Therefore, sections of higher porosity (8515and 8710) should be of more interest than those with lower porosity (8610). Theflat-line areas, falling between the zones of interest, are assumed to be non-productive shale zones.

For optimistic values of Rw to be obtained, a zone most likely to produce 100%water should be chosen for calculations. This zone should have low resistivityand relatively high porosity. There are two obvious zones fitting these criteria(8535 and 8710). The zone at 8710 has higher porosity; however, the zone at8535 is in close proximity to the hydrocarbon zone just above it at 8515. The Rwvalue of this wet zone probably closely matches the Rw value of the hydrocarbonzone because they occur at virtually the same depth. On a more pessimistic note,however, this upper wet zone (8535) may contain some hydrocarbons becauseboth the wet zone and hydrocarbon zone occur in the same porous lithologic unit.Because two wet zones are present, values of Rwa should be calculated for both,and the lesser of these two values should be used in order to obtain moreoptimistic water saturation (Sw) results.

Lithology of the zones of interest has been given as limestone. Therefore, for allcalculations, the appropriate values of cementation exponent (m) and tortuosityfactor (a) must be assumed. In this case, for limestone, a = 1.0 and m = 2.0.



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Figure 1.5. Example log.



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Rw Calculation by Inverse-Archie Method

at 8535

m7.0R 0.28;or %28 t -W==F

( )0.1

7.028.0R

0.2

wa

=

m0549.0Rwa -W=

at 8710

m4.0R 0.31;or %31 t -W==F

( )0.1

4.031.0R

2

wa

=

m0384.0Rwa -W=

There are several possible explanations for the variance in calculated values forRwa. The lesser of the two values (at 8710) may possibly be the result of a cleanerwet zone. It could also be the result of the water at 8710 having a completelydifferent salinity than the water at 8535. More than likely, the higher value (at8535) results from the fact that the wet zone probably contains residualhydrocarbons from the overlying zone.

The decision of which value of Rwa to use in water saturation calculations shouldbe based on experience, common sense, and logical deductions. All of theconditions discussed above should be considered.

In any case where Rw may be calculated in different zones or bydifferent methods, the lowest calculated value of Rw (within reason)should be used in order to obtain more optimistic (lower) calculated

values of water saturation. This is a critical assumption!

For the purposes of this example, the lowest value of formation water resistivityfrom 8710 (Rw = 0.038 W-m) will be used. This value, because it is the lesser ofthe two, will produce more optimistic values of water saturation.

Once a reasonable value for Rw is established for a zone or groups of zones, itshould be temperature corrected for depth, depending upon the differences indepth between its origin and its implementation. This is accomplished by using



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either GEN-5 or Arp's equation. In this particular example, the temperaturevariation between the top and bottom of the log is only 2F, therefore notemperature correction is necessary.

Sw Calculations

Potential hydrocarbon-bearing zones may now be evaluated using the value forRw that was previously established. High resistivity and high porosity typicallycharacterize hydrocarbon-bearing formations, again because of the non-conductive behavior of oil and gas. There are two zones illustrated in Figure 1.5that fit these criteria--8515 and 8610. The zone at 8610 has very low porosity; itshigh resistivity results from the fact that there is little pore water available toconduct current. The zone at 8515 has good porosity (~28%), and warrantsfurther investigation.

When taking measurement values from a log for use in the Archie equation, it isdesirable to select a single depth rather than averaging values across a zone.Through the course of actual interpretation there may be many appealingformations. In any single formation, an analyst may choose several depths atwhich to calculate water saturation (Sw). Because the zones in the example logare so well defined, only two calculations are required--one in each zone.

at 8515

m0.5R ;28.0 t -W==F

( )2 2w 0.5

038.0

28.0

0.1S =

= 0.3113 or 31.1% water saturation

at 8610

m4.8R ;09.0 t -W==F

( )2 2w 4.8

038.0

09.0

0.1S =

= 0.7473 or 74.7% water saturation



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Permeability Indicators

Scanning a log in search of zones with high porosity and high resistivity mayyield a number of appealing formations. However, the presence of high porosityand high resistivity does not necessarily mean that a formation that containshydrocarbons will actually produce those hydrocarbons (especially withoutstimulation or hydraulic fracturing). Without data from a Formation Tester orMagnetic Resonance Imaging log, quantitative estimates of permeability arelacking. Permeability refers to the ability of a formation to transmit the fluids itcontains through the existing pore network, and is a fundamental requirement ofa productive reservoir.

Some standard open hole logging services provide several means of getting aqualitative estimate of a formation's permeability. The most commonly usedpermeability indicators are the Micro Electric (or Microlog) and the SpontaneousPotential (SP) tools. The Microlog indicates permeability when there isseparation between the Micronormal (or Normal) and Microinverse (or Lateral)curves. The Micronormal curve will read a higher resistivity than theMicroinverse curve because of the effects of mudcake (Rmc) on the resistivitymeasurements. Mudcake can only be present opposite a permeable formation,therefore the presence of this separation is used as a qualitative indicator ofpermeability. The Spontaneous Potential, apart from providing a qualitativeestimate of permeability, may also be used to determine a value of formationwater resistivity (Rw).

A permeability indicator (in this case the SP response) for the log presented inFigure 1.5 might appear as the curve presented in Track 1 of Figure 1.6. The SPwill often respond in such a way that it reflects the same trend as the porositydevice; however, this is not always the case. Negative deflections of the SP curveare used as qualitative indicators of permeability. Permeable zones in thisexample log (Figure 1.6) are indicated at 8500 to 8535, 8595 to 8610, and 8680 to8720. The zone responsible for the most SP deflection (8700) is not necessarilythe zone with the most permeability. Likewise, because the zone at 8500 exhibitsless SP deflection than the zone at 8700, this does not mean that it has lesspermeability than the deeper of the two formations. Whereas the presence ofnegative SP deflection may be an indicator of permeability in a particular zone,the absence of any deflection does not indicate an absence of permeability.

If permeability is not evident on a log, evaluation of the porosity and resistivitycurves can still result in low water saturation calculations. Depending upon thegeology and the type of tool used to indicate permeability, hydraulic fracturing orother formation treatment methods may be necessary to produce hydrocarbons.

Locating permeable zones using SP response is an important firststep in any "quick-look" analysis program.



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Figure 1.6. Example log illustrating permeability indicator (SP curve) inTrack 1.



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Determining Formation Water Resistivity (Rw) by theSP Method

Once zones of interest are located by observing trends in their resistivity, porosity,and permeability indicator responses, determination of formation water resistivity(Rw) is in order. As discussed previously, Rw can be calculated by rearranging theArchie equation and assuming a water saturation (Sw) of 100%. An additionalmethod of assessing Rw is through the use of an SP versus Rmf chart (SP-4), and isreferred to as the SP method. As with the inverse-Archie method, the SP methodgives best results in clean and relative porous formations. However, becausevirtually anything and everything affects the SP measurement it sometimes doesnot yield reliable results. The SP method may be advantageous in certaincircumstances where porosity data are not available.

Several steps are involved in determining Rw from the SP response. Theseprocedures are outlined in Figure 1.7.

Figure 1.7. Steps involved in determining Rw by the SP method.

PROCEDURE STEPS INVOLVED

Determine formation temperature (Tf) of the zone of interest

(GEN-2b)Determine mud filtrate resistivity at Tf of the zone of interest

(GEN-3 and GEN-5, or Arp's equation)Extend horizontal line from intersection of Rmf and Tf values

to the Y-axis of SP-4, and read the SSP valueFrom the log, determine the amount of SP deflection in thezone of interestSubtract the logged SP deflection from the plotted SSP value(derived from Rmf) to get a value for DSP

Re-enter the Y-axis of SP-4 at the proper value of DSP, andextend a horizontal line to intersect the proper Tf curve

Project vertical line from intersection of DSP and Tf to the

X-axis, and read a value for Rw

Differentiate between SSP and SP

Plot DSP

Determine Rw

Determine Tf

Determine Rmf

Determine SSP

Determine SP deflection

Using the example log in Figure 1.6 and the following header information,determine a value for formation water resistivity (Rw) at 8710 by the SP method.

Location: Mid-Continent North AmericaT.D.: 10,000 feetB.H.T.: 175 deg. FRm: 0.88 W-m at 70 deg. FMud weight: 12 lbs/gal



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Detailed Procedure of SP Method

Determine Formation Temperature (Tf)

From chart GEN-2b, locate the mean surface temperature (Tms = 60F) for theMid-Continent. Using this value, determine the geothermal gradient (gG =1.14F/100') and formation temperature (Tf = 159F) from the chart or by theappropriate equation.

Determine Rmf

Plot Rm = 0.88 W-m versus Rm reference temperature (70F) on GEN-5. Thisresults in a salinity value of 7,000ppm NaCl. Following this salinity curve to theformation temperature of the zone of interest (Tf = 159F) results in a mudresistivity (Rm) value of 0.40 W-m at 159F.

With the value of the mud resistivity (Rm = 0.40 W-m) at the proper formationtemperature (Tf = 159F), use GEN-3 to determine Rmf = 0.22 W-m and Rmc =0.75 W-m at 8710.

Plot Rmf and Determine SSP

Plot Rmf = 0.22 W-m on the X-axis of SP-4. Project a vertical line upward to aninterpolated imaginary line representing Tf = 159F (slightly less than half-waybetween 150F and 175F). From this point, extend a horizontal line to the Y-axis to find SSP = -132mV.

Determine SP Deflection

Assuming the SP base line to be the second division from the right of Track 1, thedeflection at 8710 is -70mV.

Differentiate Between SSP and SP

SPYSPSSP D==-

mV62mV70mV132 -=---

Re-enter SP-4 on the Y-axis at 62mV. Project a horizontal line to intersect theinterpolated imaginary line representing Tf = 159F.



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Determine Rw

From the intersection determined in the previous step, project a vertical linedownward to the X-axis. This plot should fall on a value of Rw = 0.037 W-m.

There is a 0.001 W-m difference between the Rw values determined by theinverse-Archie method and the SP method at 8710 (Rwa = 0.038 W-m and RwSP =0.037 W-m). This minor difference is in support of the fact that bothmeasurements likely represent accurate values of formation water resistivity (Rw).Water saturation (Sw) calculations using these two values would result indifferences of less than 1%.

Additional Notes on Formation Water Resistivity

Determining an accurate value of formation water resistivity (Rw) from logs isoften quite difficult, and usually not as straightforward as presented in theseexamples. A zone that is assumed to be 100% water saturated may, in actuality,not be. The presence of hydrocarbons may suppress any SP deflections, resultingin erroneous calculations. Furthermore, in a slightly shaly formation, clayminerals may result in abnormally low resistivities. Perhaps the most dangeroussituation is assuming that a particular zone is wet when it actually containshydrocarbons. This misinterpretation will result in compounded errors in theprocess of log analysis.

When possible, it is best to calculate formation water resistivity (Rw) using avariety of methods at several different depths. The results can then be ranked andcompared to reveal a "best pick" for the reservoir. In an effort to be optimistic inwater saturation (Sw) calculations, it is usually beneficial to pick the lowest value(within reason) of formation water resistivity (Rw). The worldwide average forformation water resistivity without correcting for temperature is 0.05 W-m.Additional methods of evaluating formation water resistivity will be discussed inlater sections of this text.



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Additional Rw Calculation Example

The log for this example calculation is illustrated in Figure 1.8. The objective isto determine an appropriate value for Rw from the log. It may be assumed thatany zones of interest are sandstone.

Given

Location: Santa Cruz, BoliviaT.D.: 3,600 metersB.H.T.: 60 deg. CMud weight: 13 lbs/galDrilling Fluid Constituents: Sodium 3,000 ppm Chloride 4,000 ppm Magnesium 2,900 ppm Calcium 2,500 ppm

Define Zones of Interest

The only worthwhile SP deflection occurs from 2775m to 2830m. Within theselimits there are two definite zones of interest. The upper zone (2790m) has lowresistivity and high porosity, and is an ideal choice for Rw calculations assuming100% water saturation. The lower zone (2815m) has high resistivity and highporosity, making it a likely candidate for a hydrocarbon-bearing zone.

The zone at 2900m exhibits no indication of permeability, and has both lowerresistivity and lower porosity than the zone at 2815m. Because the SP responsemay be suppressed by the ratio Rmf/Rw, a zone of this nature may still be ofinterest to the client, and should be evaluated.

Determine Formation Temperature (Tf)

From chart GEN-2a, determine the mean surface temperature (Tms = 15C) ofSanta Cruz. After establishing a base line, project a vertical line upward fromBHT = 60C on the X-axis, and project a horizontal line from the right of the TD(3600m) on the Y-axis. The intersection of these two lines should fall on a linerepresenting the geothermal gradient (gG = 1.25C/100m). Following thegeothermal gradient line upward to the depth of the zone of interest anddescending from that intersection to the X-axis yields a formation temperature(Tf) of 50C at 2790m (wet zone).



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Figure 1.8. Example log from Santa Cruz, Bolivia, region.



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Determine Equivalent NaCl Concentration

The equivalent NaCl concentration can lead to an estimated value of mudresistivity (Rm) at the zone of interest. To determine this concentration, chartGEN-4 must be used.

Total Solution 12,596

3,000

2,000

4,000

3,596

3,000 1.00

2,500

4,000

2,900

0.80

1.00

1.24

Calcium

Chloride

Magnesium

Sodium

ION CONCENTRATION MULTIPLIER CNaCl Equ, X

Add the concentrations of the four ionic constituents to obtain a total ionconcentration. Enter GEN-4 on the X-axis at a value equal to this totalconcentration. Project a vertical line upward to intersect with the linescorresponding to each of the particular constituents (Ca, Cl, Mg, Na). From theprojected intersections, extend horizontal lines to intersect the Y-axis. The Y-axisvalues represent corrective multipliers for each constituent.

Determine Rm at Zone of Interest

With the estimated total solution of NaCl = 12,596ppm, use chart GEN-5 toobtain a mud resistivity (Rm = 0.29 W-m) at 2790m.

Determine Rmf

Using GEN-3, determine Rmf = 0.13 W-m at 2790m.

Plot Rmf and Determine SSP

Using SP-4, plot Rmf = 0.13 W-m on the X-axis and extend a vertical line upwardto the proper formation temperature line (Tf = 122F). To convert between F andC, use the top and bottom scales of GEN-5.

Project a horizontal line from this intersection to the Y-axis and obtain an SSPvalue of -98mV.

Determine SP Deflection

From the log, the SP deflection at 2790m is roughly -62mV from the baseline.



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Differentiate between SSP and SP

SPSPSSP D=-

mV36mV62mV98 -=---

Plot DDSP

Re-enter chart SP-4 on the Y-axis with a value of 36mV. Project a horizontal lineto the interpolated 122F line representing formation temperature (Tf).

Determine Rw

From the intersection established in the previous step, extend a vertical linedownward to the X-axis. This plot should fall on a value of Rw = 0.035 W-m.

Determine Rw from the Inverse-Archie Method

Because the lithology of formations of interest is given to be sandstone and theporosity of the zone at 2790m is greater than 16%, the Humble values oftortuosity factor (a) and cementation exponent (m) may be assumed.

a = 0.62, m = 2.15

m4.1R ;26.0 t -W==F

( )2790mat Rm125.0

62.00773.0

62.04.126.0

R wa15.2

wa =-W==

=

Comparison of Rw Results

The values of Rw calculated by different methods for the zone at 2790m differ by0.091 W-m. This is a major difference, and will have detrimental effects oncalculated values of water saturation (Sw). The decision as to which value to useshould be based on experience as well as information taken from the log. The SPmethod has yielded a more reasonable and optimistic value of formation waterresistivity (Rw = 0.034 W-m), and should be used in future calculations to obtainmore optimistic values of water saturation (Sw).



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"Quick-Look" Methods in Log Analysis

Before water saturation is calculated for any zone, it is necessary to scan a log andlocate favorable zones that warrant further investigation. This is true not only forpotential hydrocarbon-bearing zones, but water-bearing zones as well. This isoften referred to as scanilizing" a log. There are certain responses that should belooked for, and these responses may indicate whether a zone is water-bearing orhydrocarbon-bearing.

"Quick-look" log analysis employs scanilizing to locate potential zones ofinterest, and also employs the basic concepts and procedures thus far consideredin this text. The objective in performing a "quick-look" analysis is to quicklyproduce values of water saturation for zones that appear interesting on a log. It isimportant to remember that in "quick-look" analysis environmental correctionsare not applied. Therefore, the water saturation values obtained during "quick-look" analysis may not be as accurate as those determined through in-depth anddetailed log analysis and interpretation.

When performing a "quick-look" analysis--which should be the first step of anydetailed investigation--six questions must be asked when considering whether azone is potentially productive.

What value will be used for Rw?

What are the lithologies of the zones of interest?

Are the hydrocarbon-bearing zones "clean" (shale-free)?

Is there sufficient porosity in the zones?

Is there satisfactory resistivity in the zones?

Are the zones permeable?

The particular methodology by which an individual approaches the "quick-look"analysis may vary, yet should address all of the questions posed above. Thereshould be some order and consistency to the method. A suggested "quick-look"approach is outlined in the following paragraphs.

Identify Permeability Indicators

Scan the appropriate permeability indicators presented with the log. These mayinclude the SP, Microlog, Caliper, and even resistivity invasion profiles. Mark onthe log all zones that exhibit potential permeability, regardless of whether theyappear water-bearing or hydrocarbon-bearing. This should always be the first



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step of a "quick-look" analysis, particularly with High Resolution Induction (HRI)logging suites.

Determine Formation Water Resistivity (Rw)

If the customer provides this data, then the source is defined. If not, then it maybe necessary to calculate Rw from the logs. Locate a relatively clean water-bearing zone of sufficient porosity and determine Rw using the inverse-Archieand/or SP methods. If more than one water-bearing zone is located, then Rwshould be calculated for all zones. Tabulate the results and select the lowest valueof Rw for future calculations, remembering that lower values of Rw (withinreason) produce more optimistic values of water saturation (Sw).

Determine Porosity and Resistivity of Zones

Once a permeable zone is located, porosity and resistivity curves should bechecked to see if the relationship between them indicates the possible presence ofhydrocarbons. These curves should be considered together, and not withoutrespect to one another. Recall that it is entirely possible for a zone to exhibit anincrease in resistivity because of a decrease in porosity. Therefore, withoutconsidering all the data, it is possible to misidentify a tight zone as beingpotentially productive.

Most porosity logs will present two porosity curves--density porosity (FD) andneutron porosity (FN). Both of these curves reflect formation porosity, but thedifferences in their values depend upon the different ways in which the respectivemeasurements are made.

The Archie equation provides for only one value of porosity, therefore it isnecessary to calculate cross-plot porosity before calculating water saturation.Cross-plot porosity is a weighted average of the two values, and is calculated bythe equation below. Additional discussion of cross-plot porosity is included inlater sections of this text.

Cross-Plot Porosity2

2N

2D

XPLOT

F+F=F

A quick determination of cross-plot porosity may be made by estimating "two-thirds" porosity. This is done by visually estimating two-thirds the distancebetween the minimum-porosity curve and the maximum-porosity curve. For"quick-look" purposes, the use of visually estimated "two-thirds" porosity issufficient for making water saturation calculations.



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Determine Formation Lithology

Lithology identification can be accomplished in several different ways, the mostbasic of which is to examine the responses of various curves. For "quick-look"purposes, the curves most useful for lithology determination are gamma ray, Pe,resistivity, and a combination of neutron porosity and density porosity. Oncelithology of the zone is determined, the necessary parameters (a & m) may beselected for water saturation calculations.

Determine Formation "Cleanliness"

An additional concern is the "cleanliness" of the formation which refers to theamount of shale present. All types of formations--sandstone, limestone, anddolomite--may contain clay minerals ("shale"). The presence of these clayminerals effects the responses of certain tools--namely, resistivity and porositytools--and may result in a productive formation being overlooked as water-bearing. The degree of shaliness of a formation can be judged from the gammaray response. In general, the lower the gamma ray response of a porous zone, thelesser the amount of shale ("clean formation"). This judgement requires someamount of experience and knowledge in the area, and a later section of this textaddresses more detailed methods of shaly sand analysis.

Calculate Water Saturation

Water saturation may now be calculated for those zones that appear to behydrocarbon-bearing. Remember that this value is not a reflection of the ratio ofwater to hydrocarbons that will be produced from the reservoir. It is simply therelative proportion of water to hydrocarbons in the porosity of that formation.There are no safe guidelines for determining what constitutes "good" and "bad"values for water saturation. This judgement calls upon experience and localknowledge.



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References

Archie, G. E., 1942, The electrical resistivity log as an aid in determining somereservoir characteristics: SPE-AIME Transactions, v. 146, p. 64-62.

Asquith, G. B., 1982, Basic well log analysis for geologists: AmericanAssociation of Petroleum Geologists, Tulsa, OK, 216 p.

Bateman, R. M., 1985, Open-hole log analysis and formation evaluation: IHRDCPublishers, Boston MA, 647 p.

Dewan, J. T., 1983, Essentials of modern open-hole log interpretation: PennWellPublishing, Tulsa, OK, 361 p.

Halliburton Energy Services, 1994, Log Interpretation Charts.

basic analysis conceps ohla section1 pag 1 30

Documents

formation water resistivity

inverse archie method

inversearchie method

rw calculation example

procedure of sp method

formation temperature

saturation of clean

geothermal gradient