Notes

11

Basic Measurement

Basic Measurements

JJ Consulting 1999

Notes

The objective of zoning is to eliminate (or put aside for later study) zoneswhich are not of prime interest, i.e. non reservoir or poor data quality. Thebest tools to use are the simple ones, the SP and GR which react to simplephenomena. The caliper is good as it often shows shale as bad hole andclean zones as having mud cake, in addition to showing bad hole wherethe log response is poor.The neutron-density-Pef are good but the first two also react to the fluidtype and the Pef may be affected by barite.The resistivity is the last tool to use as it is affected mainly by fluids.

22

Basic Measurement

Zoning

Zoning is the first step in any interpretationprocedure. During zoning, the logs are split intointervals of:

1) Porous and non-porous rock.2) Permeable and non-permeable rock.3) Shaly and clean rock.

Zoning Tools:SP.GR.Caliper.Neutron Density-Pef.Resistivity.

Notes

33

Basic Measurement

Zoning Continued

In addition the logged is examined for:

1) Good hole conditions and bad hole conditions.2) Good logs and bad logs.

The first condition will adversely affect a lot ofmeasurements.The second, bad logs are often caused byenvironmental conditions, e.g. barite in the mudaffected the Pe curve.

Notes

44

Basic Measurement

SP Theory 1

SP results from electric currents flowing in thedrilling mud.

There are three sources of the currents, two electrochemical and one electrokinetic.

Membrane potential - largest.

Liquid - junction potential.

Streaming potential - smallest.

Notes

The convention is that the SP deflection is negative when the Rmf>Rwand positive when the opposite is true. The Magnitude of the deflectiondepends on the difference between the two and the temperature.The shale base line may shift over a long interval. Hence, before anycomputations the base line is adjusted back in agreement for all the shalesin the interval.The maximum deflection indicates the cleanest zone. Smaller values ofSSP mean increasing shaliness.Note the SSP in this example is -60mV (3 divisions at 20mV perdivision).

55

Basic Measurement

Example-1

The SSP is thequantity to bedetermined.

It is the deflectionseen on the SPfrom the ShaleBase Line (zeropoint) to the SandLine (max.deflection)

Notes

The log depends on invasion, if there is no invasion there is no SP. Hencethe SP shows where there is permeability. If there is permeability theremust also be porosity.The vertical resolution of the tool is poor, hence while it will show theboundaries they will not be precise.The volume of shale can be computed from the SP using a simple linearequation.

66

Basic Measurement

SP uses

Differentiate potentially porous and permeablereservoir rocks from impermeable clays.

Define bed boundaries.

Give an indication of shaliness (maximumdeflection is clean; minimum is shale).

Determine Rw in both salt and fresh muds.

Notes

The SP is an excellent method of computing the vital Rw parameter. TheRmf value is usually measured on a sample, if not it can be computedfrom charts knowing the mud salinity. The constant, K, is a complexfigure that is incorporated into the charts.

77

Basic Measurement

Rw from the SP

Rw is often known from client information orlocal knowledge.

The SP can be used to check the value or computeit when it is unavailable.

It is especially useful when there are variationsalong the borehole.

K is a constant - depending on the temperature.

we

mfe

RR

kSSP log=

Notes

The first chart gives the ratio of Rmfe/Rwe knowing the SSP and thetemperature.Rmfe and Rwe are used instead of Rmf and Rw as the complete equationrelates the chemical Activities of the two solutions. These equivalentresistivities take this transform into account.The entire computation must be carried at the relevant boreholetemperature.

88

Basic Measurement

Rw from the SP

Knowing the SSP (the maximum deflection) fromthe log and the temperature, the ratio ofresistivities is obtained from Log InterpretationChart SP-1.output = Rmfe

Rwe

Notes

99

Basic Measurement

Rw from SP 2

Rmf is measured, using the mud cell.Rmfe is computed from Log Interpretation ChartSP-2.Rwe is computed, from the ratio above and Rmfe.Chart SP-2 is used to determine Rw.

Notes

In some situations there can be a number of different salinities along thelogged interval. In this case the SP deflection will show these beds asdistinct from each other.This is the only measurement that will show the changes in salinity.Resistivity tool will simply show more or less resistivity which could besalinity changes or changes in water saturation i.e. hydrocarbon presence.

1010

Basic Measurement

SP borehole Effects - 1

Baseline shifts:

These can occur when there are beds of differentsalinities separated by impermeable shales.

Notes

Unit effects such as magnetized cable drum or measure wheel willsuperimpose a sine wave on the SP curve. The source is simple to recognise from the period of the effect.In both these cases, it may be possible to eliminate the problem.

1111

Basic Measurement

SP surface Effects

The SP can be affected by a number of surfaceeffects as it relies on the fish as its referenceelectrode.Power lines, electric trains, electric welding, closeradio transmitters:All these create ground currents which disrupt he"fish" reference causing a poor, sometimesuseless, log.

Notes

The Gamma ray is a unique measurement in that it passive. The formationis not altered in any way either by a measurement method (for exampleneutron bombardment) or by invasion.The level of gamma ray emission is very small.The log has to be corrected for borehole effects, specifically hole size andmud weight. The more material in the mud (heavy muds) the moregamma rays are absorbed before the reach the detector leading to a lowerthan expected reading.Barite, a very heavy material, has a considerable effect on themeasurement.

1212

Basic Measurement

Gamma Ray Principles

The Gamma Ray log is a measurement of the formation's natural radioactivity.

Gamma ray emission is produced by three radioactive series found in the Earth's crust.

Potassium (K40) series.

Uranium series.

Thorium series.

Gamma rays passing through rocks are slowedand absorbed at a rate which depends on theformation density.

Notes

The commonest uses for the measurement is for zoning , picking cleanbeds from shaly ones. the GR has a reasonable vertical resolution andhence it is a good tool to identify the bedding.The use of the gamma ray for shale volume is very common. It is perhapsthe tool most used for this application. Care has to be taken when there isradioactive material in the cleans zones. A typical example would befeldspar in a sandstone.

1313

Basic Measurement

Basic Gamma Ray Uses

Bed definition:The tool reacts if the shaleis radioactive (usually thecase), hence show thesands and shales, thepermeable zones and thenon-permeable zones.

Computation of theamount of shale:

The minimum value givesthe clean (100%) shale freezone, the maximum 100%shale zone. All other pointscan then be calibrated inthe amount of shale.

Notes

The total GR is made up of these three series in varying proportions. Theactual amounts of each depends on many geological factors such asdepositional environment. As this leads to a greater understanding of thereservoir the measurement brings a lot more information than the simpleGamma Ray.

1414

Basic Measurement

Gamma Ray Spectroscopy

The tool measures a spectrum that is the result ofthe three naturally occurring radioactive series.

The Potassium has a sharper shape than theother two as it decays through a single reaction toa stable element. The other two decay through anumber of daughter elements each with somecontribution to the final picture.

Notes

The sharp peaks of the previous slide are the ideal case. In practice thespectrum of the gamma rays is smoothed to give the picture in this slide.The typical tool puts a number of windows around the energy levelswhere each element is expected. Extra windows help with the statisticalprecision of the measurement.

The measurement outputs are always filtered using a special type ofalgorithm which keeps the real information while eliminating the noiseassociated with such a type of operation.

1515

Basic Measurement

Spectroscopy Principle

The measurement in the standard tool is made bya measurement in a number of fixed energywindows. Three of these at the highest levels areset over a characteristic peak of each of theelements.The statistical nature of the measurement ispartly improved by using another two windowsset at a lower energy which has a higher rate.

Notes

The outputs are in the relative proportion of their abundance, Uraniumand Thorium in ppm and Potassium in percentage.The CGR curve is especially important in shale volume calculations. Themineral Uranium has nothing to do with the shaliness of the rock and canbe present in both clean and shaly formations. This can lead to a situationwhere a clean uranium-bearing formation is seen by the total gamma rayas a shaly formation.

If available the CGR curve is always used to compute the amount ofshale.

1616

Basic Measurement

Example Log

Outputs are the relative amounts of Thorium,Uranium and Potassium in the formation.With:

Thorium in ppm.Uranium in ppm.Potassium in %.

Additional curves are the total gamma ray (SGR)and a Uranium-corrected gamma ray (CGR).

Notes

The tool has a number of important uses in formation evaluation. Thecorrected gamma ray curve (CGR) is essential to correctly compute shalevolumes. In addition, minerals such as mica which contain potassium alsoconfuse the standard gamma ray. The three outputs of this tool canidentify this type of lithology and the appropriate corrections made.

Depositional environments and the rocks diagenesis are studied bylooking at the relationships between the various elements.

1717

Basic Measurement

Spectroscopy GR uses

This tool has many applications:

Lithology identification. Study of depositional environments. Investigation of shale types. Correction of the GR for clay content

evaluation. Identification of organic material and

source rocks. Fracture identification. Geochemical logging. Study of a rock's diagenetic history.

A major application was to solve North Sea loginterpretation problems in micaceous sands.

Notes

The distribution of the 3 elements in a carbonate formation helps inanalysing these complex lithologies, especially identifying some of theadditional minerals associated with these rocks.The radioactive evaporite is Sylvite, KCl, a very radioactive rock.

1818

Basic Measurement

Spectroscopy GR uses

The three radioactive elements measured by thetool occur in different parts of the reservoir. Ifwe know the lithology, we can obtain furtherinformation.

In Carbonates:

U - indicates phosphates, organic matter and stylolites.

Th - indicates clay content.

K - indicates clay content, radioactive evaporites.

Notes

Clastic rocks range from sandstones to shales and always contain anumber of additional minor minerals. These frequently affect log readingsand sometimes formation characteristics. Thus it is useful to identify themif possible. The gamma ray spectroscopy tools gives considerableinformation of use in this situiation.

The most useful curve form the tool in shale quantification is Thorium asit is unaffected by other secondary minerals.

1919

Basic Measurement

Spectroscopy GR in Clastics

In Sandstones:

Th - indicates clay content, heavy minerals.

K - indicates micas, micaceous clays and feldspars.

In Shales:

U - in shale, suggest a source rock.

Th - indicates the amount of detrital material or degree of shaliness.

K - indicates clay type and mica.

Notes

The three plots increase in complexity from simple Th v K to Pe v Th/Kratio. The first plot can be difficult to use as slightly different variations inthe Th or K content have large effects on where the data fall on the plot.

The second of the plots Pe v K is useful but superceded by the final plot inpracticality.

2020

Basic Measurement

Spectroscopy GR CrossplotsThe data is interpreted using threemajor crossplots. In order ofcomplexity:

Thorium versus Potassium:

Gives clay type

Photoelectric factor, Pe, versusPotassium:

Gives clay type and micas.

Pe versus Thorium/Potassiumratio:

Gives clay type and micas.

Notes

These are some typical values for the gamma ray tools in a variety offormations. Anhydrite and salt are normally very clean, and have lowvalues.

2121

Basic Measurement

GR parameters

Vertical resolution 18"

Depth of investigation 6"-8"

Readings in: API units

Limestone

Notes

The simple gamma ray records a total of all the radioactivity in theformation, hence it is confused by the presence of organic materials orany other radioactive materials such as micas.The NGT tool has no problem in identifying the components of the totalgamma ray. However this tool does have to record parts of the totalspectrum hence the signal level is low and statistical. Errors will beincreased by the presence of anything likely to reduce the total signal suchas barite in the mud or large borehole.Modern tools have less effect than older versions.

2222

Basic Measurement

GR Limits

GR

- Organic materials (see the uranium as "shale").- Micas (sees micaceous sands as shaly).

Spectroscopy Tools

- Barite in the mud (reduces the count ratebut can be partially corrected for).- KCI mud (Potassium in the mud masks the formation response but can be partially corrected for).- Large boreholes decrease the count rate hence increase the statistics.- Statistical errors.

Notes

2323

Basic Measurement

GR Correction 1

GR logs require correction for the effects of themud.

Notes

2424

Basic Measurement

GR Correction 2

An additional correction is needed if there isbarite in the borehole.