rdöls - Organische Geochemie

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Kapitel 7 - Well Logging

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There are many different types ofwell logs. Some logs are used tointerpret rocks matrix propertiesin a well, others are designed toanalyse fluid composition or wellinstallations.

Various types of logs measuretemperatures, the flow rate of oiland gas that is being produced inthe well, and the quality ofcement used to bond the produc-tion pipe (called casing) to thesurrounding rock. Today, thereare even cameras that can belowered into wells to take videosof the inside of the casing anddetermine what types of fluidsare flowing out of perforationholes shot into the casing.

General Overview

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Principle ways to acquire well logs.

General Overview

Wireline LoggingLogging while Drilling

Cable side-entry sub

Horizontal Well Logging

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GR (gamma ray) logs measure natural radioactivity to determine what types of rocks are present in the well. Because shales contain radioactive elements, they emit large quantities of gamma rays (photon radiation >200keV). On the other hand, clean sand-stones emit very few gamma rays.

SP (spontaneous potential) logs indicate the permemabilities of rocks in the well by measuring the amount of electrical potential generated between the drilling fluid and the formation water that is held in pore spaces of the reservoir rock. Porous sandstones with high permeabilities tend to generate higher electrical current than impermeable shales. Thus, SP logs are often used to tell sandstones from shales.

Resistivity logs determine what types of fluids are present in the reservoirrocks by measuring how effective these rocks are at conducting electricity. Because fresh water and oil are poor conductors of electricity they have high resistivities. By contrast, most formation waters are salty enough that they conduct electricity with ease. Thus, formation waters generally have low resistivities. There are many different types of resistivity logs, which results in a confusing array of acronyms.

Sonic - BHC (borehole compensated) logs, also called sonic logs, determine porosity by measuring how fast sound waves travel through rocks in the well. In general, sound waves travel faster through high-density shales than through lower-density sandstones.

General Overview

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FDC (formation density compensated) logs, also called density logs,determine porosity by measuring the density of the rocks. Because these logs overestimate the porosity of rocks that contain gas they result in "crossover" of the log curves when paired with Neutron logs (described under CNL logs below).

CNL (compensated neutron) logs, also called neutron logs, determine porosityby assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. Because these logs underestimate the porosity of rocks that contain gas they result in "crossover" of the log curves when paired with FDC logs (described above).

NMR (nuclear magnetic resonance) logs may be the well logs of the future. These logs measure the magnetic response of fluids present in the pore spaces of the reservoir rocks. In doing so, the logs measure both porosity and permeability, and the types of fluids present in the pore spaces.

Dipmeter logs determine the orientations of sandstone and shale beds in the well, as well as the orientations of faults and fractures in these rocks. The original dipmeters did this by measuring the resisitivity of rocks on at least four sides of the well hole. Modern dipmeters actually make a detailed image of the rocks on all sides of the well hole.

Borehole scanners do this with sonic (sound) waves, whereas FMS (formation microscanner) and FMI (formation micro-imager) logs do this by measuring the resistivity. Photographic cameras are in increasing use for wellbore imaging.

General Overview

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GR 1000

Shale

Shale

Shale

Sand

Shaley Sand

Sand

Depth

Formation

Log

Bo

reh

ole

Depth of

investigation

Resolution

The Resolution Problem:Depth of Investigationand Resolution

Deeper penetration requires more energy, larger sondes and space between emitter/recorder and reduces spatial resolution.

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Types of Logs with different penetration

AIT (Array Induction Tool) - the resistivity log of the future. It measures five depths of investigation.

DIL (Dual Induction Log) - a frequently used log with deep and medium depths of investigation.

DLL (Dual Laterolog) - a frequently used log with deep and medium depths of investigation.

LAT (Lateral Log)- an obsolete log with a deep depth of investigation.

LN (Long Normal) - an obsolete log with a deep depth of investigation.

SN (Short Normal) - an obsolete log with a shallow depth of investigation.

SFL (Spherically Focused Log) - a frequently used log with a shallow depth of investigation.

SGR (Shallow Guard Log) - a frequently used log with a shallow depth of investigation.

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Caliper logs are required to assist in the quantitative interpretation of many otherlogs that are sensitive to borehole diameterand wall roughness (rugosity).Compensated logs such as density (FDC) and neutron (CNL) are corrected for thesefactors.The caliper shows where deviations occur from the nominal drill bit diameter. The deflections are towards smaller radius where mud cake has accumulated in porous formations and the oversize excursions where caving has taken place.Shales and coals are lithologies that tend to cave. The absence of mud cake adjacent to a porous bed may indicate a tight sand orpossible overpressure.

Caliper Logs

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Electrical Logs

There are a large number of electrical sondes.They are used to measure electrical properties in three different frequency ranges:

1. DC voltages that appear spontaneously in wells and boreholes (SP).2. Strata and fluid resistivity at low to medium frequencies 10 Hz to 20 kHz (LF).3. Dielectric (capacity to store charge from an applied electric field) constants, at high frequencies >10 MHz and up to 1 GHz (VHF, UHF).

Dielectric logs give good results in low or variable salinity formation waters where resistivity methods have poor performance.Dielectric logs can also be used where low gravity oils have not been displaced. UHF logs have a very shallow investigation range; VHF logsinvestigate deeper.The advantage of the dielectric propagation log is that the lower frequency permits a larger depth of investigation and therefore an analysis of the undisturbed zone.

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SP Logs

Spontaneous potentials (SPs) are naturally occurring potentials within the earth. In wellbores, they are measured between a single downhole electrode and a reference electrode in a mud pit on the surface. SPs arise because of differences in salinity between mud filtrate and formation fluids.

The potential read for shales normally varies very little with depth. SP is measured relative to this base line zero called the shale line.Negative deflections, measured in millivolts (mV), to the left of the shale line occur opposite sands and reach a maximum in clean porous brine-saturated sands called the clean sand line.

SP is positive if the mud filtrate is saltier than formation water, and negative if the formation water is more saline than the mud filtrate. Negative SPs occur in brine sands near the oil reservoir. Positive SPs occur in freshwater sands near the surface.

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SP Interpretation

Main use of SP log is differentiating between tight, electrically conductive beds (shales) and more permeable, electrically resistive beds (sandstone and carbonate reservoirs).Magnitude of SP deflection depends on the resistivity contrast between the mud filtrate and the formation brine.Permeable beds typically deflect the SP curve to the left (more negative SP).The rightmost deflection is characteris-tic of shale. The leftmost deflectionsindicate clean sands (no fines) or carbonates. Baseline shifts in SP curves are caused by changes in formation fluid salinity. SP curves are sensitive to bed thickness, the deflection being attenuated in thin beds.

shale line

sandline

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RESISTIVITY LOGS

Conventional resistivity logs were made by means of electrodes in contact with the formation through the drilling mud. Several sondes were capable of measuring to different distances (Short Normal, Long Normal and Lateral). Conven-tional logs gave good results in soft formations with fresh mud but the quality of results declines in hard formations and carbonates. They have been largely replaced by modern vertically or spherically focused logs and induction tools.

Guard logs or Laterologs produce results that are much less depend-ent on mud resistivity than conven-tional logs. These sondes have excellent vertical resolution to iden-tify thin porous layers. Often the short guard or Laterolog 8 (LL8) is combined with a dual induction log.

http://www.oilfield.slb.com/content/services/evaluation/petrophysics/hrla.asp?

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Mud or Rock Penetration Depth

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Mud cake penetration

Modern Rt-logs offer 1D and 2Dsimulation techniques based onvariable penetration depth. Thismay lead to strongly differentassessment of reservoir proper-ties, which in turn will affecteconomic evaluation of drillingtargets.

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MICRORESISTIVITY LOGS

Microresistivity logs are recorded on a small volume near a well filled with conductive mud. The aim is to determine the flushed zone resistivity, Rxo, and the exact thickness of beds.

The measuring device is mounted on a pad held against the well wall. The Microlog (ML), Microlaterolog (MLL), Proximity Log (PL) and Micro Spherically Focused Log (MSFL) are microresistivity sondes. The sondes are variously affected by factors such as mud cake thickness and the extent of the invaded zone.

Microresistivity logs are not used for correlation but because they focus on very small volumes, they provide a means for the very precise delineation of lithological boundaries.

Microresistivity is used to estimate porosity assuming the flushed zone is saturated with mud filtrate.

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RESISTIVITY LOG INTERPRETATION

The set of three focused logs shownare produced by the DIL system (Direct Induction Log). All three sondes show a similar response in shales.

The ILd (Iduction Log deep) log tends to approximate the formation resistivity Rt. Where the three logs deflect to the right, severe mud filtrate invasion of a reservoir pay zone is indicated.

The shallow guard (LL8) is the mostaffected by mud filtrate invasion anddeflects right in all porous formations.Conglomerates with oil saturation mayhave lower Rt than sands due to lowerirreducible water.

reservoirmudinvasion

reservoirmudinvasion

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ELECTRICAL LOG INTERPRETATION

Lithological information isobtained via the g-Ray-Log, SP-Log and the Caliper-Log.

Pore fluid composition anddegree of mud infiltration(dependent on porosity andpermeability) derives fromresistivity logs employingdifferent depths ofpenetration / investigation.

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RADIATION

Nuclear logs record radioactivity that may be either naturally emitted or induced by particle bombardment.Radioactive materials emit a (He2+), b (electron) and g (photon) radiation. Of these only gamma radiation has sufficient penetrating power to be used in well logging. Only neutrons are used to excite atoms by bombardment in well logging. They have high penetrating power and are only significantly absorbed by hydrogen atoms. This is an important property for well logging since the hydrogen atoms in formation fluids (hydrocarbons and water) are very effective in slowing neutrons.

Various types of nuclear well logs exist, common basic logs are:1. Conventional Natural Gamma Ray (GR)2. Spectral Gamma Ray (SGR)3. Formation Density Compensated (FDC)4. Photoelectric Effect or Litholog (PE)5. Compensated Neutron (CNL)6. Sidewall Neutron Porosity (SNP)

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Nuclear logs detect element compositions for C, O, Cl, H, Si, Ca, Fe, S. They include neutron or g-spectroscopy or nuclear magnetic resonance (NML) logs. Examples are natural gamma ray (GR), photoelectric effect (PE), compensated density (FDC) and neutron (CNL).

NATURAL GAMMA-RAY LOG

Natural radiation is due to disintegration of nuclei in the subsurface. Potassium, Thorium and Uranium are the major decay series that contribute to natural radiation.In conventional g-sondes, a simple scintillation counter indiscriminately detects total disintegrations from all sources in a radial region close to the hole (150-250 mm).Because K, Th, U tend to be concentrated in shales and are low or absent in clean sandstones and carbonates, the g-response is similar to the SP log.Openhole and cased-hole gamma logs can also be correlated and used to precisely locate pay zones for perforation.Gamma-ray logs can yield an approximate quantitative estimate of clay content or shaliness.

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GAMMA LOG INTERPRETATION

The GR and SP logs show strong correlation. Both deflect to the right for shales and to the left for clean sands.Notice that the GR response is much less sensitive to bed thickness and that coals producealmost no gamma response.

Because GR is not sensitive to bed thickness and can be run as a cased hole log, it is used to delineate zones for perforation.

SP is ineffective in salt mud and nonconductive mud.GR is unaffected and is valuable in these situations.

coal

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SPECTRAL GAMMA LOGSpectral gamma logs record individual responses for K-, Th- and U-bearing minerals. The detectors record radiation in several energy windows as GR-K, GR-U, GR-TH. The main applications are:

1. Clay content evaluation - spectral logs will distinguish between clays and other radioactive minerals such as phosphate.

2. Clay type identification - ratios such as Th:K are used to distinguish particular clay minerals.

3. Source rock potential - there is an empirical relationship between U:K ratios and organic carbon in shales.

DIFFUSED GAMMA-RAY LOGSA g-source is used to bombard the formation and the scattered energy returning to the wellbore is measured. Detectors are used at different distances from the source to allow for correction of mud cake effects. g-rays react with matter in three ways:

1. Photoelectric absorption occurs for low energy g-rays (photons). The absorption depends on the atomic number of the nucleus and is the basis for the litholog (PE).

2. Compton scattering occurs over the entire energy spectrum and is the basis of the density log (FDC).The intensity of the diffused energy at the borehole wall is proportional to the bulk density.

3. Electron-positron pairs are produced at relatively high energy.

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LITHOLOGS

The litholog sonde records low energy gamma radiation arriving at the detector. The photoelectric absorption factor depends on the atomic number of the atoms in the formation and the PE log issensitive to the composition of mineral phases.

Because photoelectric effect is only slightly affected by porosity and variations in fluid content, the PE log thus provides a direct indication of lithology. Table showing average photoelectric absorption factors (Pe) for commonsedimentary minerals.

Mineral Pe

Quartz 1.81

Kaolinite 1.83

Montmorillonite 2.04

Dolomite 3.14

Illite 3.45

Halite 4.65

Calcite 5.08

Anhydrite 5.05

Chlorite 6.30

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LITHOLOG INTERPRETATION

The photoelectric absorption factor is typically about 2 for sandstones and 4 for shales.In the simple clastic section, coal shows a very low PE factor since carbon has arelatively low atomic number.Notice the increase in the PE factor in the calcareous units where the log values are influenced by calcite (Pe ≈5)and lower deflection for the dolomitic bed (Pe ≈ 3).In common with the GR log, PE is not strongly affected by bed thickness so the thin coals and sandstone units arerelatively well resolved.Small quantities of siderite, pyrite and especially barite can produced very high PE log values.

carbonate

coal

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SONIC LOG

Formation travel time of p-waves. Run in open hole filled with mud or

water.

Note that travel time DT (msec/ft) scale is inverse.

Low velocity indicates high

porosity.

Shales at 1130-1150 ft possess high porosity.

Sands at 1090 to 1130 ft are tight.

tight

porous

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SONIC LOG

Acoustic log fordetermination of

travel times(inverse acoustic

impendance) combined with

Gamma Ray andRLL Laterolog.

Sands at 1290-1340 ft are tight.

Those below1340 m possesshigh porosity.

Fining up-ward sequences bet-ween 1350 to1500 ft show

variable porosity.

tight

porous

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DENSITY LOG

If the grain density and the density of the mud filtrate are known, density logs give direct estimates of porosity (n). Mud filtrate has a density from 1.0 to 1.1 g/cm3.

n = (ρg - ρb) / (ρg - ρf) g=grain, b=bulk, f=fluid

It is usual to calculate two porosities, one using the grain density of quartz (2.65 g/cm3) and another using the density of calcite (2.71 g/cm3). Dolomite has an even higher density (2.85 g/cm3). Shalegrain densities are in the range 2.4 to 2.6 g/cm3.

Assume the density log (FDC) indicates a bulk density of 2.2 g/cm3

with a mud filtrate density of 1.1 g/cm3, then for sand and lime:

nsand = (2.65 - 2.2) / (2.65 - 1.1) = 0.290nlime = (2.71 - 2.2) / (2.71 - 1.1) = 0.317

If the formation is gas-saturated (porosity calculated from density logs give anomalously high values since ρf for gas is 0.1 to 0.3 g/cm3

and 1.0 to 1.1 was assumed).

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DENSITY LOG

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NEUTRON LOGS

Fast neutrons are emitted by a source in the sonde and travel through the formation where they are slowed mainly by collisionwith hydrogen atoms. Slow neutrons are captured by atoms whereby g-rays are emitted.

Various logs detect:

1. Captured g-rays,2. Slow (thermal) neutrons,3. Partly slowed (epithermal) neutrons.

The Compensated Neutron Log (CNL) tool has two detector spacings and is sensitive to slow neutrons. The Dual Porosity CNL tool has two sets of detectors for both thermal and epithermal neutrons. CNL logs can be run in liquid-filled open holes and cased-holes.In addition, there are several single-detector, pad-type neutron tools that use epithermal detectors. These include the Sidewall Neutron Porosity (SNP).

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POROSITY LOGS

Neutron logs respond to hydrogen ion (proton) content andhence to the fluids occupying porosity. Since both oil and waterhave roughly the same hydrogen ion content per unit volume, calibrations for oil and water saturation are very similar.

Hydrocarbon gas has a much lower H+ content per unit volumeand neutron porosity logs underestimate gas filled porosity.

However, porosities derived from density logs (electrons) overestimate gas filled porosity. Density and neutron porosityexhibit diagnostic cross-overs in gas-saturated formations.

Because neutron porosity logs respond to H+ mineralscontaining H in the structure (e.g. gypsum, clays) can beresponsible for anomalously high apparent porosities.

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POROSITY LOGS

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POROSITY LOG INTERPRETATION

The porosity logs are calibrated for sandstone based on density (FDC) and neutron (CNL) logs. An SP log indicates lithology.

At 1020m, the porosity is over-estimated where the shale "wash out" has occurred.

Between 1040 and 1058m, the density and neutron logs separate as shaliness increases.

At about 1105m, the two porosity logs crossover indicating a clean gas zone in a sandstone reservoir.

At 1130m, a very low porosity is given for a tight sandstone layer.

Where the two porosity logs agree at 1135 to 1142m, a clean sand-stone is indicated.

tight

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PORE FILL INTERPRETATION with DENSITY/NEUTRON

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Application of Quick Multi-Log Interpretation

Evaluate net

sand from GR

Evaluate

from Density in water

bearing zone

Calculate Rw

Calculate

in HC zones

Calculate Sw

Identify water

zone

Evaluate net

sand from GR

Evaluate

from Density in water

bearing zone

Calculate Rw

Calculate

in HC zones

Calculate Sw

Identify water

zone

1

3

2

4

5

6

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OWC EXAMPLE

The SP logs show the Tuscaloosa Sand interval as a strong left excursion of between -80 and -100 mV.

In the upper part of the log, sand is oil saturated and the magnitudes of SP deflections are attenuated due to the relatively high resistivity of oil.

In the lower part of the log, the sand is brine saturated and shows a deflection of -120 mV with an almost flat response with depth.

The OWC is picked at the start of the flat section of the log following the section where the SP becomes more negativewith depth.

Log type Purpose Log reading Interpretation

Gamma Ray LithologyRight high Shale

Left low Sands, carbonates, coal

Porosity density

Porosity

Density left higher porosity Porous

Density right lower porosity Tight, very low porosity

Neutron density / neutron

Gas indicatorNeutron more right than density

Lower density value

Gas (neutron density cross-over)

Coal indicatorNeutron left

super high density valueCoal

Sonic porosityPorosity

Left slow Porous rock

Right fast Tight, non-porous rock

Gas indicator Very left super slow Think gas or coal

SPIndirect

permeability

Left negative current Permeable rock

Right positive current Impermeable rock

Resistivity

Hydrocarbon indicator

Tight rocks

(in porous rocks) left low (< 5 W) Water

(in porous rock) right high (> 5 W) Hydrocarbons

(tight rocks) (SP to right) High resistivity

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Logging summary - interpretations

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A Standard

Log used in

Petroleum

Exploration

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hour

A Mudlog from the Wellsite

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A Mudlog

from the

Wellsite

Combined

with a

Litholog

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Kapitel 7 -Well Logging

A Mudlog

from the

Wellsite

Combined

with a

Litholog

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A Log

Assembly

common in

Petroleum

Exploration

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A Log Assembly common in Petroleum Exploration

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Interpretation of Sedimentary Features from Wireline Logs

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Channel Fill

Interpretation of Sedimentary Features from Wireline Logs

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REGRESSIVE SEQUENCES

In regressive sequences, grain size increases upwards. From bottom to top, a sequence will show increasing wave energy with an eolian or beach environment at the top of the sequence.

This kind of regressive sequence pro-duces highly characteristic SP and SN (short-normal) resistivity logs. A basal shale unit is deposited in a deep-sea low energy reduced environment followedby progressively more neritic, less reduced sediments with coarser grain sizes.

Well-sorted sand units develop as the shoreline retreats in anoscillating or cyclic fashion producing interlaminated silts andshales. Eventually, clean sands predominate. Fingers on the SP and SN curves indicate sedimentary laminations that get longer (larger amplitude) as the sands get cleaner towards the top of the sequence. A transgressive episode terminates the regressive sequence. In the example, the transgression brings the littoral sand unit in contact with marine shales. This change may be abrupt (asshown) or more gradual.

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REGRESSIVE SEQUENCE – LONG DISTANCE

The Albian (Lower Cre-taceous) Viking sand in Alberta (Excelsior Field)is a typical regressive sequence. The overlying Cenomanian (UpperCretaceous) Fish Scale marker bed makes the sequence very easy tolocate on logs.

Well #1 shows an accele-rating rate of regression, well #2 shows a morelinear regression rate and in well #3 the regression rate is decelerating. Inwell #4, the Viking sand has pinched out to the North, the Viking strand-line being between well #3 and #4.

TRANSGRESSIVE SEQUENCES

A transgression results from an advance of the strandline overland as a result of a rise in sea level. Again, this kind of transgressivesequence produces highly characteristic SP and SN (shortnormal)resistivity logs.At the base of the sequence are coarse, well-sorted sediments followed by an abrupt grain size break indicated on the SN log. The rising sea level may be oscillatory or cyclic leading to inter-bedded finer and coarser units in an upward fining sequence.The fingers on the SP curves are of decreasing amplitude towards the top of the sequence. As with regressive sequences, it is possible to estimate the relative rate of movement of the strandline from the SP-SN logs and hence estimate the lateral extent of pinchout stratigraphic traps.The transgressive process involves only small volumes of preserved sand relative to regressive sequences. The fundamental reason is that transgression involves erosion and redeposition of a finite sand volume at the strandline.

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REGRESSIVE SEQUENCE – LONG DISTANCE

The example shows "cartoons" of SP logs over a regressive/ transgressive sequence in Mesaverde Group Cretaceous sediments in the San Juan Basin, New Mexico.

The shaly Mancos Fm is overlain by the regres-sive Point Lookout Fm. This is followed by the Menefee Fm (shale) and the transgressive Cliff House Fm. The final unit in the package is the Lewis Fm (shale).Notice the variation in the rate of regression or transgression with respect to the position of the strandline.

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CHANNEL FILLS

Two types of channel-fill can be distinguished:

1. Distributary channel-fill

2. Point-bar alluvial build up

In the case of distributary channels, the SP and SN logs have a characteristic "barrelshape"with a marked increase in SP-SN deflections at the channel centre indicating coarser cleansediments. The edges of the channels show variable-length SP fingers that dip towards thecentreline.

In point-bar channels, the SP-SN logs show upward fining and almost horizontal layering. Logs near the centreline have a transgressive or "barrel shape", as illustrated. Logs near the margins have characteristics that are moreregressive. Dipmeter logs are a useful addition aid in the interpretation of channel-fill deposits.

Channel-fill sand bars are generated by the fluvial process of riverchannel-cut and then channel-fill or valley-fill. This is associated with lateral migration of stream channels.

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TURBIDITIES

As the successive failures occur, thin sand stringers are deposited within massive silty shales.

The layers are laterally extensive and represent periodic major failures of soft sediment and subsequent widespread deposition of the suspended material.

Turbidites are unmistakable on SP logs where they display a linear transgressive appearance within each individual gravity flow.

Turbidite sequences, generated by subsea gravitational slope failures, are readily recognized on SP-SN logs.

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rdöls - Organische Geochemie