4 petrobasic
DESCRIPTION
slb log interpretationTRANSCRIPT
Notes
11
Life of a well
Life of a well
© Schlumberger 1999
Notes
22
Life of a well
Life of a well-1
Drilled well Cased Well Perforated Well
Need to find:
Saturation casing integrity
Porosity cement quality
Zones
The information requirements in a wells life depend on the stage. The firststage of the well is short, a few months. Once the well is drilled thequestion is “where is the hydrocarbon?” The logs are run for this purpose.Once the well is cased and cemented, the question is “how good is thecement”. Then the zone(s) are perforated.
Once cased it is difficult to make measurements, especially of theimportant resistivity.
Notes
33
Life of a well
Objective of a wellThe Objective of most wells is to findhydrocarbons.The volume of hydrocarbons in place isgiven by:
H=Constant x φ(1−Sw)hΑ
where
H = initial oil in place
φ = effective porosity
Sw= initial water saturation
h = productive interval
A = drainage area
This is the formula giving the amount of oil in place, vital for theexploitation of the reservoir.
Logs give
porosity
saturation
height (from the depth)
This means they are vital to the operator.
Area comes from surface seismic and/or well testing
Notes
44
Life of a well
Hydrocarbon in PlaceThis is simple to visualise
A - area of the reservoirh - the thickness of the reservoirtogether the product gives the total volume ofrock
φ - percentage of pore space in that volume ofrock. i.e. the volume that contains fluids
Sw = percentage of the pore space containingwater so(1-Sw) = percentage of pore space containinghydrocarbons
Hence the equations for the hydrocarbons inplace
The constant in the equation is used to put the result into the requiredunits, for example in oilfield units it is acre-ft.
Logging measurements form a major part of the input to this equation,hence their importance. Errors in reading or interpreting the logs isreflected in the results of the hydrocarbon in place.
Notes
The standard logging tools cover a wider range than LWD tools. thelatter are limited (at present) to the basic measurements. The advantageof LWD is that it is real time (while drilling), this allows decisions to bemade rapidly. This is especially important in the case of horizontal wells.
Highly deviated wells need to be logged using drillpipe because the toolswould not go down the hole by themselves. A special technique is used inthis case.
55
Life of a well
Open Hole Measurements
Open Hole Measurements are made by threemethods;
1) The traditional wireline logginghere the tools are lowered into the well onthe end of an electrical line. Measurementis usually made pulling out of the hole.
2) Logging While Drillingthe tools are built into drill pipe. Logging is made while drilling the hole and data is stored in downhole memory as well as being transmitted up hole.
3) Logging on drill pipehere the standard wireline tools are attached to drill pipe. A cable is still used for data transmission. Logging is made both down and up.
Notes
All the porosity tool use linear scales. The scales are set to fit the mostcommon values met in the formations. The bulk density and neutronporosity are usually run together and have “compatible scales”. Thismeans that the log track covers the same amount of porosity. There arevariations on these scales to suit local conditions. For example theneutron porosity is sometimes run on a scale of 0 - 60 p.u.
In all scales a “backup” curve is used to handle times when the maincurve goes out of the track. This curve only appears at these times.
66
Life of a well
Porosity Log Scales
45.0 -15.0Neutron Porosity (p.u.)
01530
Bulk Density (g/cm 3 )1.95 2.95
2.702.452.20
Sonic Slowness ( µsec/ft )140 40
7590105
The porosity scale is linear in Porosity Units(p.u.)
The density scale is linear in grams/cm3
The sonic slowness is linear and displayed inmicroseconds/ft
Notes
All resistivity logs have the same scale whether they are deep, medium orvery shallow reading. The standard scale of four logarithmic decades isshown. The scale could also cover only two decades, for example from0.2-20 ohmm.
The use of this type of scale facilitates the reading of the log curves.
At times the resistivity and porosity logs are displayed on the same log.In this case they each are restricted to half the width, i.e one track each.
77
Life of a well
Resistivity Logs
All resistivity logs are displayed over 2 tracks.The scale is always logarithmic to cover the widerange of possible values.Most scales start at 0.2 ohm-m and go to 2000ohm-m
0.2
1 10 100 1000
2000Resistivity (ohm-m)
Notes
These logs have much more variable scales. The caliper scale is chosento fit the bit size and is often presented with a constant companion curveshwoing the nominal value of the bit size. Differences indicate thecondition of the hole more clearly.
The scale of the GR depends on the natural radioactivity of the formationbeing studied. It is good policy to have the majority of the log inside thetrack without the need for back-ups.
The same philosophy applies to the SP curve, although this is ofetn moredifficult to predict. It is adjusted during the repeat section to fit inside thetrack. Its scale is usually a number of millivolts per division. An absolutescale is impossible for this sort of measurement.
88
Life of a well
Other Log Scales
Tracks 2/3Gamma Ray (G API)0 150
Tracks 2/3Caliper (inches)6 16
Gamma is on a linear scale. The valuedepends on the well.
SP is on a linear scale with a given numberof millivolts per division, negative to the left.
Caliper is on a linear scale depending on thebit size.
Tracks 2/3Spontaneous Potential (mV)
->10mV< +
Notes
In addition to these basic formation parameters a number of other datacan be found. These include,
- lithology, not only the major component but also the minor constituentsas well. These is important as they may affect the wells production.
- shale content and type. This gives the total pay zones as well as theproduction properties.
- fracture indications, which affect the wells production
- permeability
- fluid type, especially in gas zones.
99
Life of a well
Use of Open Hole logs
The basic parameters needed are:
Thickness - measured by the tool depths
Porosity - measured by porosity tools
Saturation - computed from a combination of porosity and resistivity
Notes
1010
Life of a well
Life of a well-2
Well Produced Workover activity Recompleted
Need to know:
Production Perforation efficiency Flow rates
fluid mix new zones Zone Production
Pressures Flow rates Pressures
In the second “half” of a wells life the questions are different. Here theemphasis is on production , fluids and pressures. Different techniques areemployed. Well testing and reservoir monitoring tools are used to answermost of the questions. Some specialist devices such as corrosionmonitoring tools may be required. The phase of the wells life lasts for amuch longer time, often years; hence there will be a number of surveysduring this time.
Notes
Perforation is the most popular method of reservoir completion. Theobjective is to create a path for flow from the formation to the wellthrough the casing and cement. The requirement is thus for a hole to bemade in the casing, cement and into the formation for a short distance.Standard perforations have an entrance hole of about 0.4” and apenetration of around 20”.
The perforation “gun” contains these three components. The detonator tostart the reaction, the prime cord to propagate it and the shaped charge tomake the holes.
1111
Life of a well
Perforation
Gun systems use three components:
- detonator - primary high explosive ignited byheat or shock
- primacord - secondary high explosive ignited bythe detonator, burns at 8400 m/sec
- shaped charges - create the perforations,detonated by the primacord.
Notes
1212
Life of a well
Shaped charge
The explosion forces the liner to flow inwards and out.
It forms into a characteristic shape, the jet.
This is moving rapidly and has extremely high pressuresat the tip.
The dimensions of the perforation, length of the tunnel,and diameter of the entrance hole are linked and dependon the geometry of the shaped charge.
Case
ExplosiveCharge
Liner
Primacord
PrimerCharge
Jet
Tip
Slug
7000 m/s
500 m/sp=100GPa
Shaped Charges were developed shortly after World War II from themilitary bazooka weapon.
Three basic elements of a shaped charge
1. CASE (Steel or Aluminium).
2. CYLINDER OF HIGH EXPLOSIVE & A PRIMER.
3. CONICAL METALLIC LINER.
It was found that the conical shape produced a depression / hole in a metaltarget. The addition of the liner increased the efficiency of the system.Modern liners are made of powdered metal and leave a powder residue atthe end of the perforation. A Typical charge has only about 20 grams ofexplosive material.
The pressure causes the material in the path of the jet of metal to moveout of the way creating the perforation.
If the liner opening is widened the entrance hole size increases but thepenetration decreases. These type of charges are used for applicationssuch as gravel pack.
Notes
The advantage of a casing gun completion is that all perforation materialis carried inside the carrier hence it is protected from the well fluids. Theresulting debris is also brought out of the well in the same carrier. Thecarrier can be either re-usable or not depending on the type of operationbeing performed. The more complex gun types are all “ throw-away”type carriers. The disadvantage of overbalanced perforation is that themud in the well bore will enter the well as it is at a higher pressure.Through tubing perforation eliminates the invasion problem and gives theformation the chance to flow immediately. The disadvantage is thatsmaller guns have to be used, which means either smaller charges in asmall carrier, or larger charges exposed to well fluids and debris left inthe well. The choice depends on the type of well being perforated.
1313
Life of a well
Types of Perforation
Three Types of perforated completion
a) Wireline - Carried on an electric line
1) Casing Gun Completion
Well Pressure > Formation Pressure
Overbalanced perforating
Large diameter carrier gun
2) Through Tubing Perforation
Well Pressure < Formation Pressure.
Completion and final surface production equipment, or a temporary completion andtesting facilities are in place
Underbalanced perforating, with pressurecontrol equipment
Through tubing gun (small guns)
Gauges can be run with the string
Notes
Tubing conveyed perforation ( TCP ) connects a carrier gun to the end ofthe drill pipe or tubing. The gun can be fired by a number different typesof detonators such as drop bar, pressure firing heads or inductivecoupling. The choice depends on the conditions and type of well.
The advantages of this method are mainly the long interval (s) possibleand the possibility of a simultaneous well test using downhole gauges.
1414
Life of a well
Tubing Conveyed Perforating
b)Carried on Drill Pipe or Tubing
3) Tubing Conveyed Perforating
Perforation gun is carried on either the drill pipeor on tubing.
Well Pressure < or > Formation Pressure
Large interval of perforation in one run - in - hole
High explosive content, perforation spacing
Gauges can be run at the same time
Notes
The number of shots per foot depends on the application and the reservoirparameters. The objective is to obtain the best flow efficiency mosteconomically. Computer program exists which allow the reservoirengineer to select the best combination of shots per foot and phasing.The most common number of shots per foot is four or six.
1515
Life of a well
Perforation Characteristics
Shaped chargesshotsperfoot
Guns are classified by thenumber of shots per foot,spf.
The current maximum is21 spf.
PerforationDirections
90 ÞphasingGuns are also describedby their Phasing- thedirections of theperforations. This rangesfrom 0Þ to 30Þ/60Þ
The example shows 90Þ.
Notes
Cementing of the casing in place is one of the most vital operations in thedrilling phase. It is necessary to have a perfect seal between zones toavoid unwanted fluid production or reservoir contamination. The cementquality has to be evaluated before the completion and any repairs made atthat time.
One of the major difficulties in cementing is the presence of gas zones.These will cause problems if precautions are not taken during the cementjob.
1616
Life of a well
Cement Evaluation
Cementation ofwells is of vitalimportance for thewells performanceas it seals one zonefrom another,
To properlyevaluate anymeasurement incased hole thequality of thecement has to beevaluated
Unwanted fluidflow
Bad Cement
Notes
The cement band log-variable density tool uses a standard sonic tool tomake the measurement. This is the traditional tool and serves well toidentify the quality of the cement job. It has an added advantage in seeingthe bind from cement to formation which the other tools cannot.
The pulse-echo tools use either an array of ultrasonic transducers or asingle rotating transducer. Both methods produce a “map” of cementquality around the borehole. Combining both types of tool provides thebest possible picture of the cement quality.
1717
Life of a well
Cement Bond Logging ToolsThere are three types of tool in current use
1) Cement Bond Log (CBL)- Variable DensityLog (VDL)
a) CBL measures the amplitude ofsignal reflected from the casingwall. The higher the amplitudethe lower the amount of cement.
b)VDL image of the recorded wavetrain. The only log to seebeyond the first casing into theformation.
2) Pulse Echo type tool
measures the acoustic impedance of the casing-cement interface using ultrasonics.
The latter tool is either segmented usingindividual transducers or rotating covering theentire casing
Notes
Fluid flow in the casing tubing depends on the fluids flowing from thereservoir. An oil with a high gas-oil ratio will produce a lot of gassomewhere on its journey to the surface, a low GOR oil will produce lessgas. If there is water production as well, three phase flow will exist in thetubing and two phase (diphasic) flow in the casing before the gas hascome out of solution.
These flow regimes cause problems for measurements.
1818
Life of a well
Fluid Flow
Fluid flow in the borehole is complex. The fluidmay start as oil but then bubbles of gas come outof solution.
There may also be mixed flow of water and oil.
Notes
1919
Life of a well
Flow Regimes
1 10 102 103
1
10
102
10-1
BUBBLE FLOW
PLUG FLOWSLUG FLOW
MIST FLOW
REGION IIIREGION II
REGION I
GAS VELOCITY
TRAN
SITI
ON
LIQ
UID
VE
LO
CIT
Y
FLOW REGIMES
The actual flow regime depends on a number offactors, such as gas-oil-ratio.
Flow in the casing and/ or tubing is broken into different regimes fromBubble flow, gas bubbles in oil, to mist flow, oil droplets in gas. Theactual flow regime encountered in the well depends on the flow velocitiesand gas-oil ratio.
More than type of flow will be present in the well as the pressure changeand more gas come out of solution.
Notes
2020
Life of a well
Production Tools
Current tools consist of a number of sensors. Themajor ones are
Flowmeter flow measurement
Gradiomanometer fluid density/ hold up, fluid mix
Pressure reservoir, borehole pressures, fluid density
Temperature production ,temperature, flow measurement, cement evaluation, channel identification
Caliper borehole size, Flowmeter correction
The measurement of fluid production requires a number of sensors due tothe complexity of the problem. For example the spinner which measuresflow rate will change if the casing diameter changes hence the need for acaliper tool.
A combination of flow rate and fluid density will give the flow rates ofeach fluid present. The temperature is most often used in the analysis ofproblems.
There a number of additional sensors which have specific uses. Thenuclear fluid density tool is used in horizontal or highly deviated wellswhere the conventional tool does not work.
Notes
2121
Life of a well
Production Log InterpretationStep 1
The first thing that is done is to look at the logs.
Questions asked:
• do the spinners change at the perforationdepths
• is the fluid density "sensible"
• does the shut in pass show anything e.g.crossflow
• does the temperature have any anomalies
• are there any stable zones for calibration
• what's the problem
The interpretation of production logs always involves the collection of asmuch information as possible about the well itself, especially thecompletion and the fluids expected. The logs have then to be examinedclosely to identify any anomalies and also calibration zones for thespinners. (the spinners use a downhole calibration technique). As the log isalways run in a number of passes with the well flowing and shut-in , allmust be examined.
It is normal for a number of answers to be possible for a given set of logdata. However knowledge of the well and fluids eliminates all but thecorrect one.
Notes
2222
Life of a well
Example 1
What happens here?
Spinner RPS
WellSketch
The example shows a well sketch of a casing perforated in one place andthe spinner (flowmeter) logged in the well. The log shows an increaseacross the perforated zone but also another increase further up the well.This could be a casing leak at that point. If this was the case the repairwould involve an expensive workover.
Notes
2323
Life of a well
example-1 solution
Spinner RPS
Casing/ hole sizechange
The true answer is much simpler. A re-examination of the well data showsa change in casing weight at this depth. The increased weight decreasedthe internal diameter, causing a spinner increase. This would also havebeen seen on a caliper log, if one had been available.
This example highlights the need for complete information.
Notes
2424
Life of a well
Flowmeter Interpretationmonophasic flow
0 10 20
2.5 rps
3 . 0 r p s
1 .5 r p s
7.0 rps
cps
35.7% oftotalflow
42.9% oftotalflow
21.4% oftotalflow
A
B
C
A simple phase (monophasic) flow interpretation is straight forward. Theflowmeter response can be calibrated downhole to show as a percentage ofthe total flow.
Calibration is made in the zones between the perforation where the flowhas stabilised. Zero flow is usually assumed below the lowest perforation.The percentages can be translated into quantities knowing the wellproduction.
Notes
2525
Life of a well
Saturation and Time
Matrix
oil
S w- original
water
water
∆S w
Over time the saturation undergoes anotherchange with the oil being displaced by theinvading water
The water could be of a different salinity to thatoriginally in place
Saturation,as well as having a “radial” component in the form of invasionhas a time component. As the reservoir is produced the water moves in tovacate the space left by the producing oil. This process continues until theoil saturation equals the residual value.
Notes
2626
Life of a well
Saturation Monitoring
The objective is tomonitor the depletion ofthe producing zone, i.e.the difference betweenthe original and thecurrent oil saturation.
It is also to detectproblem zones such aswater fingering orconing.
Remaininghydrocarbon
displaced hydrocarbon
original waterin placeP
erforations
Porosity %50 0
Many reservoirs are bounded on a portion or all of their peripheries byaquifers. The aquifers may also be so large compared with the reservoirsthey adjoin as to appear infinite for all practical purposes, and range downto those so small as to be negligible in the effect on reservoirperformance. When pressure decreases due to oil production, the aquiferreacts to offset or retard pressure decline providing a source of waterinflux or encroachment.
Water may be injected to supply external energy to improve the recoveryof hydrocarbons. The injected water may advance evenly or may channelthrough the streaks of better permeability leaving hydrocarbons behindthe water front. To achieve optimum hydrocarbon recovery , themonitoring of water saturation at regular intervals is essential;. Thisachieved by measuring the water saturation in different portions of thefield and then drawing contour maps of ISO-saturation curves. This helpsto detect and measure the rise of the water/oil contact, locate water fingersor bypassed hydrocarbons, estimate the residual oil saturation andevaluate the efficiency of water-flooding projects. Proper monitoringallows to take the necessary steps to maximise the final recovery.
Notes
Reservoir evaluation and saturation monitoring through casing aregenerally performed in two ways. One measures the decay of thermalneutron populations ( TDT-P, pulsed neutron capture) and the otherdetermines the relative amounts of carbon and oxygen in the formation ofinelastic gamma ray spectroscopy, as used in the GST or RST ( inducedgamma ray spectroscopy ). Because chlorine has a large neutron capturecross section, the PNC technique provides good results in areas withhighly saline formation waters.
2727
Life of a well
Monitoring Tools
Two measurement methods exist:
Pulsed neutron capture logging
Carbon Oxygen logging
Both use an electronic source and pairs ofdetectors measuring gamma rays
PNC measures the capture cross section of theformation. This is related to the amount ofchlorine and hence the water
C/O measures the relative amounts of carbon andoxygen. This is related to the amounts ofhydrocarbon and water.
Notes
The equation linking the log reading and the formation is linear. Theunknowns are the capture cross sections for the water, hydrocarbon andmatrix plus the porosity. The latter can be measured with the tool but it ispreferable to use open hole value.
2828
Life of a well
PNC Method and Analysis
The log reading is a linear mixture of the matrixand the fluid:
The fluid term can be expanded to:
Hence if Σw, Σma, Σh and the porosity, φ are knownthe saturation Sw can be obtained.
Σ log = Σ f φ + 1 − φ( )Σ ma
Σ f = Σw S w + 1 − Sw( )Σ h
Notes
The equation can be solved graphically. At f=0 the tool output is equal tothe matrix capture cross section Sma. If this is known it serves as a pivotpoint. At f =100% the output depends on Sw. If this is 1 Sl = Sw. if itequals 0 then Sl = Sh.The values for Sh and Sw can be found in advancefrom charts, hence the plot can be sealed in Sw.
The method provides a simple method of analysing the results.
2929
Life of a well
Pulsed Neutron Capture Interpretation
LΣ = maΣ 1− φ( )+ fΣ φ( )
fΣ φ( ) = hΣ φ 1− WS( )+ WΣ φ WS
Σ
Σ Σ
Σ
POROSITY
log
ma
w
H
Sw = 100%
Sw = 0%
Notes
The slide presents an example of time-lapse monitoring. The open holecomputed log is displayed with three computed TDT logs that were runover several years. The rise in the oil/water contact between logs runs isobvious. Water fingering has also developed in all upper high-permeability zone.
This type of survey are normally performed in several wells of the samereservoir. This allows to map the water saturation and monitor the waterfront advances
3030
Life of a well
Time Lapse Example
Notes
The first stage of the measurement computes the individual elementsfrom the spectra. This is very statistical. The next step take large windowsover the expected carbon and oxygen peaks to give a statistically goodmeasurement. The combination of these two gives an accurate carbon-oxygen ratio which can then be transformed into saturation
3131
Life of a well
Carbon Oxygen Analysis
The tool measures carbon (in thehydrocarbons) and oxygen (in the water)
Combining these gives the saturation
Notes
This plot is for the RST-B tool, which has the ability to compute both theformation and borehole percentages. The shape of the plot depends on thelithology.
The smaller tools have a plot which has less spread the near and fardetectors “see” almost the same thing, hence it can only distinguish theformation percentage. The borehole fluid must be known in this case.
3232
Life of a well
C/O Saturation
The plot is of the Far C/O ratio against theNear
The combination gives both the formationwater percentage Sw and the boreholepercentage Yo
F a r C / O r a t i o
Ne ar C/ O Rat io
Sw= 0 , Yo =1 0 0
Sw=0 , Yo=0
Sw=1 0 0 , Yo =0
Sw =1 0 0 , Yo =1 0 0
Notes
3333
Life of a well
Vertical Wells
Wells can be split into three categories
1) Vertical
• drilled to a specific target
• measured depth = true depth
Vertical wells are most common in exploration situations. The well isdrilled to its target without the complications of deviation.
Notes
3434
Life of a well
Deviated well
2) Deviated
• usually from a platform or
• from land to near offshore
• measured depth has to beconverted to true vertical depth
possible well tracks
Target formation
Deviated wells are very common in a lot of situations. The well track canbe almost anything; starting vertical and then deviating, starting vertical,deviating and then vertical again, starting deviated and then going vertical.The change in direction is called a dog-leg. Severe doglegs can causeproblems for logging as it makes it difficult for the tool to go down andsometimes to come out. The deviation angle is measured with respect tothe vertical. The true depth has to be computed, knowing this angle andhow it has changed.
Notes
3535
Life of a well
Horizontal well
3) Horizontal
• drilled to maximise production orminimise problems such as coning
• well is precisely guided along apredetermined track
Vertical section
Curvature
Ramp
The ultimate deviated well is a horizontal well. Here the well is drilled inthree sections, the vertical section, the curved section and finally theramp. The curved section is typically a couple of hundred metres but canbe less for specific cases. The ramp is as long as required, severalkilometres is common. Guiding the well is done from surface usingsensors mounted near the drill bit. These give information on directionand deviation as well as logging data such as gamma ray which helps inguiding the well paths.
Notes
Measurements are made in the borehole using tools based on a set ofphysical principles. The standards for the measurements are fixed atnominal values, for example an 8” borehole. The borehole environmentis different from the standard hence there are deviations from the“perfect” measurement. These deviations are known and can ( in mostcases) be corrected. It is important to recognise the differences and havea good knowledge of the environment.
3636
Life of a well
Measurements in Open Hole
The measurements made in the borehole areaffected by the environment.
The major effects are:
Borehole size and shape
Borehole Fluids
Borehole Temperature
Notes
The addition of casing and cement plus tubing adds complications to themeasurement of formation properties. Each of the elements presentaffects the measurement, sometimes they are unknown and have to beinferred. All this makes cased hole analysis more difficult than open hole.
3737
Life of a well
Measurements in Cased HoleThe cased hole environment is more complex thanopen hole because of
casing and tubing • steel affects measurements
cement • to be measured but unknown in most cases
unknown fluids • to be measured, affects tools
limited tools • slim hole restrictions plus casing etc limits tools which
can be run
lot of unknowns • have to take into account reservoir behaviour, fluid dynamics as well as rock mechanics and formation properties
Notes
The tool contains a sensor package and the electronics for processing thedata and communication with the surface. The formation to be measuredid “separated” from the tool by the borehole and its constituents, mud andmudcake. Thus the borehole is a filter through which the formation isseen. Borehole corrections are the method used to eliminate thisenvironmental effect.
3838
Life of a well
Measuring in the borehole
Sensors+Electronics
Borehole
Formationto beMeasured
The formation to bemeasured is maskedby the borehole.
The boreholecontains fluids and isof an irregular shape.
The sensor has to beable to measure theformation propertyaccurately and sendthe information tosurface.
Notes
The first problem for measurement is the borehole shape. This dependson the formation being drilled, regional stresses and the drilling practiceused.
The best case is the perfectly circular hole. This will only cause problemsif it is very large. Ovalised boreholes are often caused by local tectonicstress imbalance. A lot of tools will lie along the long axis and the calipermeasuring a large hole size. This may cause too much correction to beapplied hence two caliper measurements at 90Þ to each other is preferredas it gives an indication of the borehole shape.
Irregular or rugous borehole causes problems for most tools butespecially when the sensor is carried on a pad applied to the boreholewall. In this case correction may be impossible.
3939
Life of a well
Borehole -Size and Shape
Perfect shape no problemsexcept if very large.
Ovalised hole; will giveproblems for some tools.Best to run two calipers.
Irregular borehole, givesproblems for most tools.
Notes
The position, of the tool in the hole depends on the type of measurement.All tools need either a specific position or at least their position to beknown for the relevant correction to be applied. Centralising a toolinvolves putting a set of centralisers at specific points on the tool. Thesedevices have symmetrical spring arms which adjust to changes in theborehole size keeping the tool in the centre of the borehole.
4040
Life of a well
Tool Positioning - 1
Formationto beMeasured
CentralisedTool
Some tools are runcentralised in theborehole in order tomeasure properly.
These include laterologand sonic devices.
Special centralisers areput on the tool.
Notes
The opposite of the centralisation / eccentralisation pushes the toolagainst the borehole wall. This is accomplished with either a springeccentraliser or, in the case of most pad tools, with a powered back uparm. The objective here is to keep the sensor in as close a contact aspossible with the wall minimising interference by the mud.
4141
Life of a well
Tool Positionning - 2
Some tools are runeccentred, pushed,against the boreholewall.
In some cases this isdone with aneccentraliser.
In other cases a caliperarm does this job.
Formationto beMeasured
EccentralisedTool
Notes
Stand offs are physical devices placed on the tool to keep it a fixeddistance from the wall. Their use is to keep the tool away from the wallbut still in a known position. In some cases (the induction family) this isdone to optimise the tools functioning.
In a long combination tool strings, some tools may have stand-offs whileothers are eccentralised. This conflicting requirement is possible using“knuckle-joints” which act as a crossover between the two systems.
4242
Life of a well
Tool Positionning - 3
Formationto beMeasured
Tool withStand-offs
Stand-Offs
Some tools are runwith “stand-offs” toposition them at afixed distance fromthe wall.
The induction familyare usually run in thismanner.
Notes
The fluid in the borehole can have as enormous effect on the tool andhence its type, properties and additives must be known. The differentfluids are used to drill different rock types. Oil based mud is oftenemployed to drill shales which would swell on contact with water. Airand foam drilling are used in cases where there is a weak formationwhich will crack if mud is used.
4343
Life of a well
Borehole - Fluids
Sensors+Electronics
BoreholeFluid
Formationto beMeasured
The borehole fluid can be
- water based mud
fresh or
salt saturated
- oil based mud
varying quantitiesof water
- air
- foam
In addition there are anumber of additives toincrease weight, viscosityand so on.
Notes
The variations in mud type are large. Salinity is measured (usually by thelogging engineer) on samples of mud, mud filtrate and mudcake. Theadditives are obtained from the mud engineer and should be knownaccurately even in small quantities, will render the Pe curve uselessalthough some modern tools may be able to make sufficient correction.
4444
Life of a well
Borehole fluids 2
Oil based mud will not allow current to pass soelectrical logs will not work.
Foam and air muds will not transmit sonicssignals. Neutron tools are also affected.
Mud salinity affects electrical and induction toolsin different manners.
Additives such as barite affect density, gammaray and photoelectric effect measurements.
The mud type, salinity and additives must beknown so that the appropriate corrections can bemade.
Notes
Temperature measurement is made with maximum reading thermometersattached to the logging head during each run. These are normallyemployed for redundancy. There are some tools which measuretemperature on a continuous basis .These are extremely useful whenlooking at profiles. Most logging tools are rated at 350ÞF. They usuallyhave “high temperature” versions where the electronics are put in aDewar flask.
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Life of a well
Borehole - Temperature
Increasing temperature affects the measurementsin some tools. The most affected is the thermalneutron devices.
High temperature also affect the performance ofthe electronics in the tools.
Temperature affects the mud resistivity (itdecreases with increasing temperature).
Temperature is measured during each loggingrun.
Notes
Tools are constructed to measure a certain volume of formation. Thisvolume depends on the physics of the measurement being made and thetype of sensor.
The first type of measurement is omni-directional, i.e.. all directions atonce. The tool reads a circular volume which includes some of theborehole and some formation. The depth of investigation, how much ofthe formation is actually measured, depends on the specific tool. Mostread a few inches in the invaded zone.
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Life of a well
Volume of Investigation
The tool shown here measures all around theborehole. It is omni-directional.
An example of this type of tool is the GammaRay.
Some of the “signal” is in the borehole. Mostcomes from the invaded zone.
Formationto beMeasured
Volume investigatedby the tool
Invaded Zone
VirginZone Virgin
Zone
Notes
The pattern here is similar to the last slide, being, once again omnidirectional. However these tools (deep resistivity) are different in thatthey are “focused” to and as much as possible beyond the invaded zone.They are still affected by both borehole and the invaded zone, hence needcorrections.
These tools see a few feet into the formation.
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Life of a well
Volume of Investigation 2
Formationto beMeasured
Volume investigatedby the tool
Invaded Zone
VirginZone
VirginZone
This type of investigation is also omni-directionalbut it reads mainly in the virgin zone.
This pattern is that of the deep resistivity tools.
Notes
This pattern is in a single direction. hence the tool sees a volume of theformation just in front of its sensor. This type of tool is eccentered as anyother borehole position would make it read too much of the borehole.These tools see a few inches into the formation, again measuring theinvaded zone.
If the formation is very heterogeneous it may be difficult to reconcile thereadings from the three different volumes. This situation is often seen inhighly deviated or horizontal wells.
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Life of a well
Volume of Investigation 3
Formationto beMeasured
Volume investigatedby the tool
Invaded Zone
VirginZone Virgin
Zone
This type of measurement has the sensor facingin one direction only.
Examples of this are the neutron porosity andbulk density measurements.
Notes
The tools are built to read correctly in an infinite homogeneousformation. This situation applies reasonably well with vertical wells. Inthe horizontal case the focusing of the deep resistivity tools may makethem read beyond the layer seen by the shallower tools. This causesconfusion when trying to use combinations of both types ofmeasurement.
Despite the problems involved, valuable information can be obtainedfrom the data on the geometry of the bedding.
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Life of a well
Vertical Wells
In vertical wells, with homogeneous layers alltypes of tool are reading in the same formation.
In horizontal (or highly deviated) wells the deepreading resistivity tools may read a differentlayer to the shallow reading tools.
In addition the omni-directional tools (e.g. GR)may read different layers from the singledirection devices.
Notes
The depth is different depending on whether the wells are vertical orhorizontal. In addition there are differences if the drilling is on land oroffshore. However the reservoir is at a constant depth irrespective of thesurface topography. Hence a reference is used to give a preciserepeatable depth. The reference is mean sea level.
5050
Life of a well
Depth Overview
The depths and position of all wells has to be wellknown. This is important in mapping andevaluation.
Notes
There are a number of depths associated with a well, however the loggingdepth is always used as the reference as it is repeatable. (Logs can be runin the future which can be correlated with those originally run).
Differences between drillers depth and logging depth is usually quitesmall, a few feet. Anything greater than this need checking. The mostcommon error is one of around 30 feet, the length of a joint of pipe.which has been forgotten in the count.
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Life of a well
Reason for Depth Control
The depth of a well is first measured by thedriller.
A reference such as the drill floor is used for themeasurement.
All depths are referred to the field reference,usually mean sea level, m.s.l.
Logging depths are used for all subsequentoperations on the well, perforations, monitoring,test depths, pressures and so on.
They are measured during the logging process.
Major differences with the drillers depth have tobe investigated
Notes
As noted the logging depth is the reference hence it is necessary to haveit as correct as possible. As the cable used for the measurement of thedepth, it has to be in good condition; regular checks are made. Thesurface acquisition system is also controlled as is the measuring systemitself.
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Life of a well
Depth Control
Logging Depth Control is of vital importance.
The System used has a number of safeguardsand crosschecks against error.
The controls include cable stretch corrections.
The accuracy is
±2.5 foot in 5000 feet
The consistency between logging runs is
<2 feet in 10000 feet.
Notes
Depth measurement is made in the following way :
1) The tool is put at reference zero, for example the drill floor.
2) The distance from this to the unit is measured, its called L1
3) The tool is run in the hole. At logging depth the logging tension ismeasured and the stretch correction made.
4) The distance from the unit to the tool zero point is checked, L2. L2should equal L1 if nothing has changed with the set up.
5) The log is made; the surface measuring system computes depth.
6) Zero is checked again on surface.
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Life of a well
Depth Measurement
Notes
The stretch correction is part of the depth control measurement. It can beeither computed using the chart, entering tension and depth to get thestretch or using the formula
² L = L * G * [Tup - Tdown]
Where L is the depth , and T is the logging tension, up and down
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Life of a well
Stretch Correction
Notes
5555
Life of a well
Depth Measurement Summary
Depth measurement starts with setting the toolzero.
The depth is measured using two independentwheels.
The depth at the bottom of the well is correctedfor cable stretch.
Magnetic marks on the cable are used to monitorthe system.
Notes
The tool zero in this example (and usually in practise) is set at the bottomof the tool. The various sensors in the tool string are at different andknown distances above this. Thus when the tool is at the bottom of thewell, in the example at 10219 feet. This sensor will never see the bottom15 feet of this well.
Once logging has started the log is recorded with reference to the toolzero. This means that the other sensors readings have to be held inmemory until the zero passes the depth before being output
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Life of a well
Tool depths
Notes
The log shows the three measurements made. Measurement starts and allthree show straight line static values. After two feet of travel the zerocrosses the tension measurements first reading, this curve moves, after 8.5 feet sensor moves and after 15 feet sensor. All the curves are on depth.
As a corollary of this total depth can be found using first reading of thesensors and the distance of the sensor from the bottom of the tool (zeropoint)
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Life of a well
Log Depths
The distance from the sensor to the measure zerois known hence all the logs are “on depth”.
Notes
The quality of logging jobs can be evaluated by the usefulness of the dataacquired in the interpreting them to answer the problem (s) or obtainparameters. The process begins before the job in a pre-job briefingidentifying the objectives and recognising any potential difficulties.During the job data monitoring gives the real time control, A final checkat the end of the operation should ensure that all the data is good enoughhas been collected.
The next section details some of the possible errors in loggingmeasurements and how to minimise them.
5858
Life of a well
Log Quality Control
Log Quality Control - LQC is an important partof every logging job.
Log data can be affected by the boreholeenvironment or by tool problems.
If this is not recognised in time the data may beuseless for further interpretation.
LQC starts before the operation by identifying thepossible problems in the well.
It continues during the job, monitoring the logcurves.
It ends after the job with an evaluation of the dataacquired.
Notes
A quality standard can be set for every tool/measurement made. Thelisting for these is given in a Log Quality Control Reference Manual.This sets out the responses of the tool and any quality control curves.
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Life of a well
Standards General
The tool quality standards vary from tool to tooland depend not only on the tool but also theenvironment in which it is run.
For example the induction and laterolog familiesprefer the opposite types of mud in the borehole.
Each tool has a set of standards which are basedon the physical principles of the measurement.
The performance of the tool can be checked by:
Response in known conditions
Specific Quality control curves
Notes
A quality control curve shows some aspect of the tools functioning to theengineer. These curves can be seen as curves on a standard log display orby their numerical value on a monitor. They are most useful whilelogging to see how the tool is behaving if incorrect to allow the fault tobe fixed if possible or at least the data acquired with a back up tool. Theycan also be used after the job to explain spurious readings and variations.
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Life of a well
Tool Quality Control Curves
Some tools (the most modern varieties) haveoutputs which monitor the proper functionning ofthe electronics and sensors.
These are in the form of curves which the loggingengineer can monitor during the job.
Each has known limits indicating goodmeasurements. Failure or, in some cases, badborehole conditions, cause the reading to beoutside the acceptable zone.
The limits are laid out on the relevant page of theLQC Manual.
Notes
The three categories of well have different objectives and hence differentlogging problems. Exploration wells, in unknown conditions, pose thegreatest questions while development wells are usually the simplest tolog and evaluate.
Appraisal wells often allow excellent data acquisition as the early drillingproblems are solved and the evaluation questions known, hence can beanswered with a well planned survey.
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Life of a well
Drilling Objective
A well is drilled to a pre-determined objective:
An exploration well targets a suspected reservoir.
An appraisal well evaluates a discovery.
A development well is used for production.
Notes
The exploration well is often in very unknown territory. Th surfaceseismic will give structure, outcrops will give some idea of the geology.Depths, fluids porosity, saturation etc. are all unknowns. The loggingsuite has to cover all eventualities, a switch in mud type or higher thanexpected resistivities may require a change of resistivity tool.
In addition the hole condition may be bad leading to poor dataacquisition.
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Life of a well
Pre-Drilling KnowledgeExploration
Structural information obtained from surfaceseismic data.Rough geological information can be provided bynearby wells or outcrops.Approximate depths estimated from surfaceseismic data.
Notes
In the appraisal well the initial information has been obtained from theexploration well. The evaluation problem(s) is now known and a surveycan be tailored to maximise the information. The data from this type ofwell is often the best and most complete in the field.
6363
Life of a well
Pre-Drilling KnowledgeAppraisal
Detailed structure from logs taken in previouswells.
Time-to-depth conversion for surface seismicfrom logs taken in previous wells.
Notes
6464
Life of a well
Appraisal Wells - Continued
Geological data from cuttings, cores and logsfrom previous wells.
Notes
Development well logging is concerned with completion, where toperforate. The rest of the information about the reservoir should beknown but there can be surprises, for example unexpected faults.
6565
Life of a well
Pre-Drilling KnowledgeDevelopment
Notes
Tools and acquisition systems have continued to be developed since thefirst log was recorded in 1927 by the Schlumberger brothers Marcel andConrad. Some development has improved existing measurements, thesimple electrical log has become the Azimuthal Resistivity Imaging Tool.Other are new measurements added to the battery of existing techniquessuch as nuclear magnetic imaging.
Recent advances have used the explosion in computing power to increasethe density of data recorded and hence create images of the borehole andformation properties. At the same time these tools have become more andmore reliable. Surface systems have become more sophisticated whilebecoming easier for the user (both the engineer and customer).
6666
Life of a well
Tool History1927 - First electrical log recorded.
1930s - SP, Short Normal, Long Normal and Long Lateralcombined, Core Sample Taker.
1940s - Gamma Ray and Neutron, 3-arm Dipmeter using SP, thenelectrical measurements, Induction tool.
1950s - Microlog tool, Laterolog tool, Sonic tool, FormationTester.
1960s - Formation Density tool.
1970s - Dual Spacing Neutrons, Advanced Dipmeters,Computerised Surface Systems, Repeat Tester tools,Electromagnetic Propagation tool.
1980s - Resistivity Imaging tool, Advanced Sonic tools
1990s - Advanced testing tools, Induction imaging tools,Azimuthal Laterolog tools, Ultrasonic imaging tools,Epithermal porosity tools, Magnetic resonance tools
Notes
The early interpretation used a combination of resistivity and the SP topick zones. The SP was labelled as a “porosity” curve.
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Life of a well
Early Interpretation
Early resistivity logs were used to findpossible producing zones.
high resistivity = hydrocarbon
SP was used to define permeable beds,compute Rw and determine shaliness.
Resistivity was also used to determine"porosity".
Archie developed the relationship betweenresistivity, porosity and saturation.
Notes
This is a fast quicklook technique to recognise hydrocarbon zones. In awater zone the porosity and resistivity will track each other, as theporosity decreases there is less water hence the resistivity increases andvice versa. In shale the resistivity usually reads low and the porosityreads high.
In hydrocarbon the resistivity increases while the porosity is the same orincreases.
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Life of a well
Interpretation Procedure
Resistivity PorosityGamma Ray
Water
Water
Shale
Hydrocarbon
The simplest evaluation technique consists ofrecognising the hydrocarbon zone using theporosity and resistivity curves
Notes
This interpretation procedure follows some simple guidelines to arrive ata final answer. The input is the environmentally corrected and qualitychecked log data. This is an important step which cannot be avoided if aproper answer is required. Additional information such as core data mayalso be used. This information is zoned, broken into sections of interest(the reservoir) and other (such as shale and bad hole).
Lithology selection takes the flowchart into two paths. In carbonates theproblem is porosity and porosity type before computing saturation. Inclastics it is the shale, shale type and possible other minerals that have tobe evaluated first.
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Life of a well
Interpretation Flowchart
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.The best tools to use are the simple ones, the SP and GR which react tosimple phenomena. The caliper is good as it often shows shale as badhole and clean zones as having mud cake, in addition to showing badhole where the 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.
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Life of a well
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.
Additionally; Good hole conditions and bad hole conditions.Good logs and bad logs.
Zoning Tools:SP.GR.Caliper.Neutron Density-Pef.Resistivity.