Clean Formation Evaluation

7/29/2019 Clean Formation Evaluation

1/11

Halliburton1 Open Hole Log Analysis Notes

2001, Halliburton

Sect ion 4

Clean Formation Evaluation

Table of Contents

Introduction.............................................................................................................................................2

Objectives................................................................................................................................................2

Clean Formation Evaluation.....................................................................................................................3Typical Approach.................................................................................................................................3

Selecting the Appropriate Logs.............................................................................................................4Exploratory Wells.............................................................................................................................4Development Wells...........................................................................................................................5

Infill Wells ......................... ........................... ........................... ........................... ........................... ..6

Log Quality Assessment.......................................................................................................................7Potential Water-Bearing Zones and Calculations .......................... ........................... ........................... ..8

Potential Hydrocarbon-Bearing Zones and Calculations...................................... ........................... .......9Decisions on Productive Capability ........................ .......................... ........................... ....................... 10

References............................. ........................... .......................... ........................... ........................... ..... 11


2/11



2001, Halliburton

Introduction

The idea of compiling all of the information necessary for a complete analysis

may seem a bit overwhelming at first glance. This is especially true when onerealizes that calculations are typically performed at intervals of one to one-half

foot throughout the zone or zones of interest. This complex task can result inliterally hundreds of data points, all needing environmental corrections, invasion

corrections, cross-plot determinations, and lithology identification. If evaluatedone depth at a time, then this process could result in maddening hours of tedious

calculations or shoddy interpretations based on misread data points and flailingapproximations. The purpose of this section is to provide the participant with a

basic outline of the steps and procedures of clean formation evaluation, and theorder in which these steps should be accomplished.

Objectives

After completing this section, the participant should be able to

recognize the importance of an orderly analysis.

formulate a reasonable and efficient approach to evaluating a clean formation.

recognize the importance of having the most available data when making adecision to set pipe and perforate versus abandon a well.


3/11



2001, Halliburton


A complete evaluation of a clean (i.e., shale-free) formation requires many steps

and involves many different and complex calculations and techniques.Additionally, there are a variety of assumptions that must be made during this

analysis. The number of steps involved makes it difficult to remember at timesthe order in which the steps should be performed. This section provides certain

guidelines that should be followed when analyzing a clean formation, andpresents an orderly sequence by which such an analysis should be accomplished.

Typical App roach

1. Select the appropriate logs.

2. Perform detailed log quality assessment.

3. Locate potential water-bearing zones and determine their lithology.

4. Select depth(s) at which formation water resistivity (Rw) is to be determined,and perform environmental corrections on these data.

5. Determine formation water resistivity (Rw) by available means.

6. Locate potential hydrocarbon-bearing zones and determine their lithology.

7. Select depth(s) at which water saturation (Sw) is to be calculated, and perform

environmental corrections on these data.

8. Correct formation water resistivity (Rw) to formation temperature of zone(s)

of interest.

9. Calculate water saturation (Sw) of the potential hydrocarbon-bearing zone(s).

10.Make a decision on the productive capability of the potential hydrocarbon-bearing zone(s) based on all of the available information.

When making a decision on the productive capability of a potential hydrocarbon-

bearing zone, all of the available information should be considered. Values ofwater saturation (Sw) should notbe the sole determining factor. Remember that

water saturation is nota reflection of the ratio of water to hydrocarbons that willbe produced from the reservoirs. It is simply the relative proportion of water to

hydrocarbons that exists in the pore space of that reservoir. There are no safeguidelines for determining what constitutes "good" and "bad" values for water

saturation. Consider the log responses and any other information that might beavailable.


4/11



2001, Halliburton

Select ing the Appropr iate Log s

The choice of logging combinations will depend upon a variety of factors,

including: mud system, formation type, previous knowledge of the reservoir, holesize and deviation, rig time and cost, equipment availability, and the type of

information desired. The types of log run also strongly depend upon the welltype. Exploratory wells typically require a comprehensive logging program,

whereas infill and development wells may only require basic services.

Additional logs may be required in cases where geologists, reservoir engineers,

completion engineers, and geophysicists desire additional information for the

evaluation and completion of the well. The use of computers in formationevaluation and the availability of logging data in a variety of formats (i.e., LIS,

LAS, ACSII) has vastly increased the utilization of data recorded withcomprehensive logging programs.

Exploratory Wells

With exploratory (or "wildcat") wells, very little information, if any at all, is

known about the reservoir. These situations typically demand a comprehensivelogging program to gain information about subsurface structure, reservoir

porosity, and fluid saturations. In many cases a sonic log may be necessary forcorrelation to seismic sections. Formation tester and sidewall cores may also be

necessary to gain better insight into the formation. All of this information is usednot only to streamline the approach to further exploration in the area, but also to

develop the drilling and logging programs.

Typical Logging Suites for Medium-to-Soft Rock,Fresh Mud Exploratory Wells

High Resolution Array Induction, High Resolution

Induction/DFL, or Dual Induction/Short Guard

Spectral Density/Dual Spaced Neutron/Compensated SpectralGamma Ray/Microlog

Full Wave Sonic

Magnetic Resonance Imaging

Six Electrode Dipmeter, Electrical Micro Imaging, orCircumferential Acoustic Scanning Tool-Visualization

Formation Tester

Sidewall Coring


5/11



2001, Halliburton

Typical Logging Suites for Hard Rock or Salt MudExploratory Wells

Dual Laterolog/Micro-Spherically Focused Log (or equivalentinduction survey if mud salinity marginal)

Spectral Density/Dual Spaced Neutron/Compensated SpectralGamma Ray

Full Wave Sonic

Magnetic Resonance Imaging (for optimal borehole conditions)

Six Electrode Dipmeter, Electrical Micro Imaging, or

Circumferential Acoustic Scanning Tool-Visualization

Formation Tester

Sidewall Coring

Development Wells

Development wells are those that immediately follow exploratory wells; their

purpose being to "develop" a field that has recently been discovered, and to

identify the limits of the field. Most wells drilled can be classified asdevelopment. Although acquisition of data pertaining to the characteristics of the

formation is still a priority, logging suites for development wells typically aremore limited than those for exploratory wells. The information that is gained may

be "correlated" back to the data acquired on the associated exploratory wells for a

better picture of the overall field.

Typical Logging Suites for Medium-to-Soft RockFresh Mud Development Wells

High Resolution Array Induction, High ResolutionInduction/DFL, or Dual Induction/Short Guard

Spectral Density/Dual Spaced Neutron

Magnetic Resonance Imaging (with increasing development ofa discovered field, MRIL may become the log of choice for

gaining information about porosity and fluid types within areservoir)

Sonic Porosity, Formation Tester, Six Electrode Dipmeter, and

Sidewall Coring as required


6/11



2001, Halliburton

Typical Logging Suite for Hard Rock or Salt MudDevelopment Wells

Dual Laterolog/Micro-Spherically Focused Log

Spectral Density/Dual Spaced Neutron/Compensated Spectral

Gamma Ray Magnetic Resonance Imaging (for optimal borehole conditions)

Sonic Porosity, Formation Tester, Six Electrode Dipmeter, andSidewall Coring as required

Infill Wells

In situations where a reservoir has been very well defined, perhaps by the drilling

of numerous wells, the typical logging suite becomes even smaller. Infill wells,

or those drilled to "fill in" the areas between previously drilled developmentwells, are typically logged with only very basic services. Magnetic ResonanceImaging has a tremendous application here because of its ability to gain insight on

fluid types and porosity with a single tool; something that required multiple toolsand possibly multiple runs in the past. It should be realized that the limited

amounts of data typically gathered during logging of infill wells is generallyinsufficient for any type of post-processing analysis applications.

Typical Logging Suite for Medium-to-Soft RockFresh Mud Infill Wells

High Resolution Array Induction, High ResolutionInduction/DFL, or Dual Induction/Short Guard

Magnetic Resonance Imaging

Typical Logging Suite for Hard Rock Salt Mud Infill Wells

Dual Laterolog/Micro-Spherically Focused Log

Magnetic Resonance Imaging (for optimal borehole conditions)

Sonic Porosity

As is the case with any logging program, the types of logs run must be tailored tothe conditions that exist and the types of information sought. The decision about

which logs to run is typically made well before the field engineer becomesinvolved; however, situations may arise during a job in which additional services

should be offered to the customer for their consideration.


7/11



2001, Halliburton

Log Qual i ty Assessment

Quality of the recorded data should be of the utmost concern to both the field

engineer and the customer. Very expensive decisions about the future of a wellare based on log data, and accurate data are vital to the decision making process

and future success/failure of a well. The first step in any analysis problem shouldbe to scan the logs, searching for any anomalous or otherwise strange looking log

responses. All service companies and many customers have very detailed logquality assurance programs in place. There are four main areas of concern that

should be addressed with any log quality assurance program.

Depth Control

Depth control is only one of the many vital components of quality data. However,

it also is one of the most difficult to assess. In exploratory situations, someassurance can be maintained from comparisons of log depth to driller's depth and

casing depth, and to general knowledge of the region's geological structure. Keepin mind, however, that these are by no means accurate references. In

development and infill situations there is typically sufficient well control to assessthe correctness of depth data for a particular well. Every effort should be made toinsure that proper depth control is practiced on every well.

Overall Technical Quality

Many conditions beyond human control may adversely affect the technical quality

of logging data. The most obvious of these is equipment malfunction.

Preventative maintenance programs are the best way to minimize equipmentmalfunctions and the possibility of poor quality logs. Other possible causes of

poor data may include: rugose boreholes, sticking tools, tool rotation, excessivelogging speed, deviated wells, poor centralization or eccentralization, and

engineer error. Each of these possibilities should be kept in mind when assessingthe quality of log data. In some instances, it may be necessary to make another

run, perhaps with a different toolstring.

Repeatability

Many of the previously mentioned factors affecting technical quality of a log

might also apply to repeatability. In addition, a repeat may be affected by suchtime-based phenomena as changing degree of invasion. Comparing repeated log

sections is an important step in assessing the quality of log data; however, itshould not be the only method of quality control.


8/11



2001, Halliburton

Absolute Log Values ("Markers")

Comparison of log readings with known absolute values is seldom possible;

however, this positive check should be performed where it is possible. Knownformations consisting of pure, non-porous lithologies such as halite, anhydrite, or

limestone can be used to verify the accuracy of log readings. Casing may also beused to check the accuracy of caliper and sonic measurements. Furthermore, logs

of offset wells may provide a ballpark figure of expected values, but these valuescan vary dramatically between wells.

Log quality control is the responsibility of the service company performing the

logging job. However, log acceptance should always be determined from thepoint of view of the customer. Will he or she be able to obtain accurate and

reliable information from the log? If an affirmative answer to this question is everin doubt, then making another run with a different toolstring or pursuing some

other alternative should be considered.

Potent ial Water-Bearing Zon es and Calculat ions

Locating a potential water-bearing zone should be approached by qualitatively

assessing intervals in terms of their porosity and resistivity, and considering any

permeability indicators presented with the logs. This "visual sifting" of data isusually accomplished by first considering porosity. If a zone is porous, then there

are fluids present within that zone. Next, resistivity of the zone must beconsidered. Because hydrocarbons are electrical insulators, porous zones

containing them will have relatively high resistivities. Porous water-bearingzones, on the other hand, will have relatively low resistivities. This process is

also aided by recognition of the various resistivity invasion profiles associatedwith different types of resistivity logs.

Do not hesitate to mark logs or highlight intervals to make them more noticeable.

A practical method of doing this is to use a yellow highlighter pen to color fromthe middle of Track 1 leftto the Gamma Ray curve. This provides a good visual

image ofpotentially porous formations; those possibly containing water orhydrocarbons. Where a Spontaneous Potential curve is present, the process of

locatingpotentiallypermeable formations (again, regardless of fluid typescontained) is much faster. Those impermeable zones that lack any SP deflection

will be of less interest than those with deflection. Keep in mind, however, that the

SP response is only a qualitative indicator of formation permeability.

Once a potential water-bearing zone is located, several necessary calculations are

in order. The formation temperature (Tf) of the interval should be determined.Furthermore, resistivity measures such as Rm and Rmfshould be corrected to

formation temperature for the purpose of determining formation water resistivity(Rw).


9/11



2001, Halliburton

Before determining formation water resistivity (Rw), the lithology of the

formation of interest should be determined. This may be done by quick-look, orby use of one of the lithology charts. Determination of lithology will assist the

analyst in determining the appropriate values of tortuosity factor (a) andcementation exponent (m) for inverse-Archie Rw calculations.

In a quick-look analysis, environmental corrections are typically not performed onany log measurement. However, to be more precise in an analysis, the variousinfluences of borehole and invasion should be corrected for before using any log

measurement to determine formation water resistivity (Rw).

Every reasonable effort should be made to obtain an accurate and valid value of

formation water resistivity (Rw) from the logs. If the required data is available,

then both the SP method and inverse-Archie method of determining Rw should bepursued. Keep in mind that determining Rw from log data does not always yield

accurate results. When analyzing any log, the potential for error created by usingan impractical value of Rw should always be considered. Always use the lowest

value of determined Rw, within reason, for obtaining more optimistic values ofwater saturation (Sw).

Potent ial Hydro carbon -Bearing Zones and Calculat ions

Locating a potential hydrocarbon-bearing zone should also be approached by

qualitatively assessing the porosity and resistivity of zones, and consideringpermeability indicators. Again, if a zone is porous, then there are fluids present

within that zone. Porous zones containing hydrocarbons will have relatively highresistivities because of the poor electrical conductivity of those hydrocarbons. As

was the case with water-bearing zones, permeability indicators should also beconsidered to determine the priority with which a certain zone will be evaluated.

The most important thing to consider is that the value for formation water

resistivity (Rw) determined in a water-bearing zone must be corrected to theformation temperature (Tf) of the zone in which it is to be used to calculate water

saturation (Sw). Failing to correct Rw for temperature at greater depths will resultin water saturation values being too pessimistic (too high). It is therefore

possible, and in many cases likely, that a potential hydrocarbon-bearing zone willbe overlooked as being wet if Rw is not corrected to formation temperature. This

will, of course, require that formation temperature (Tf) be determined for eachpotential hydrocarbon-bearing zone.

Before calculating water saturation (Sw), the lithology of the formation of interest

should be determined. Again, this may be done by quick-look, or by use of one ofthe lithology charts. Knowledge of lithology will determine the appropriate

values of tortuosity factor (a) and cementation exponent (m) for inverse-ArchieRw calculations.


10/11



2001, Halliburton

Again, in a quick-look analysis, environmental corrections are typically not

performed. To be more precise, environmental corrections should be applied toany log measurement before calculating water saturation (Sw).

For clean formations, it is assumed that the Archie equation is applicable. Bear in

mind, however, that there are certain instances (such as when clay minerals are

present in a shaly sand) that alternative methods of calculating water saturationwill be more appropriate. Some of these methods will be discussed in the ShalySand Analysis sections of this text.

Decisions on Prod uct iv e Capabi l i ty

The most difficult process in the basic evaluation of a clean formation has now

been reached; the decision of whether to set pipe and perforate or consider

abandonment. Calculated values of water saturation (Sw) only provide the analystwith information about what fluids are present in the formation of interest. In

many cases, water saturation is nota reflection of the relative proportions of fluidsthat may be produced. Therefore, when making the decision to set pipe or

abandon, all available information should be taken into account.

Water saturation (Sw) should be the basis for this important decision, but other

factors also enter into the decision making process. These factors include:volume of shale (Vsh) of the reservoir, irreducible water saturation (Swirr) and bulk

volume water (BVW), moveable hydrocarbons, etc.. In many instances, much ofthe decision revolves around a "gut feeling"; however, in allcases, there is no

substitute for experience in a particular region when making the choice. Someadditional methods that may be used during the decision making process will be

addressed in the Additional Log Interpretation Techniques section of this text.


11/11



2001, Halliburton

References

Asquith, G. B., 1982, Basic well log analysis for geologists: American

Association 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: PennWell

Publishing, Tulsa, OK, 361 p.

Clean Formation Evaluation

Documents

Transcript of Clean Formation Evaluation