coring & core analysis

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FORMATION EVALUATIONSeries 3 Rock and Fluid Sampling and Analysis

! "oring and "ore Analysis

B.1. Introduction

In#roduc#ion

Reservoir rocks saturated with hydrocarbons are complex on both a macroscopic and microscopicscale. The complexity of both rock and fluid properties controls the initial quantity and distribution ofhydrocarbons and the rate of flow of these fluids within the reservoir, as well as the volume ofhydrocarbons recovered. A sample (core) can be taken to recover a portion of the reservoir rock, sothat it may be examined firsthand and tested in the laboratory. uch direct physical measurementsfurnish both !eolo!ical and en!ineerin! information and !uide the decisions affectin! both the coredwell and subsequent wells in that area.

Reservoir rock characteristics vary both vertically and areally. "nder the best of circumstances, thevolume of rock recovered throu!h corin!, even when all wells at a location are cored, is small whencompared to the rock volume of the reservoir. Althou!h wells should be cored and data !athered todefine reservoir rock properties in all dimensions, in many cases insufficient core data are taken to

do a complete evaluation. #ell lo!s then become helpful. A syner!ism exists between directphysical measurements on core samples and what can be learned from downhole well lo!s andother well$test data. %ood en!ineerin! reco!ni&es this fact and utili&es the stren!ths of allevaluation tools to make the best possible development decisions.

'ore analysis is the name !iven to the test procedures and data collected on core samples. Avariety of information and data may be obtained via measurements of physical and chemicalproperties, visual observations, and photo$!raphs. The two maor cate!ories of core analysis areconventional core analysis, with associated complementary data, and special core analysis. therspeciali&ed studies are often made that do not fall neatly within either of these cate!ories but areimportant to both en!ineers and !eolo!ists.

Conventional core analysis yields the most basic data about a reservoir, such as*

the presence or absence of hydrocarbons+

the stora!e capacity (porosity), the flow capacity and its distribution (ma!nitude and

profile of permeability)+

the litholo!y and texture of the formation.

These data, and complementary measurements made on request, can be available for use withinhours or days after a core is recovered, since laboratories are normally close to the area where thecores are cut.

Special core analysis tests are more complex and the data furnished are of wider diversity.Typically, they will require lon!er core preparation and testin! times and more speciali&ed andexpensive equipment. ar!e quantities of data are captured on the more sophisticated tests, andcomputer assistance is routinely used to calculate results. The increased time factor should bereco!ni&ed and accounted for when plannin! a proect. pecial core analysis tests may be dividedinto static and dynamic measurements.

#hen cores are removed from the reservoir environment they are subected to alterations ofpressure and temperature. These alterations cause chan!es in bulk and pore volume, reservoir fluid

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saturations, and, in some cases, reservoir wettability (preference of the rock for water or oil). Theeffect of these chan!es may be ne!li!ible or substantial, dependin! upon the rock and reservoirfluid characteristics, as well as the rock property bein! investi!ated. -n conventional core analysis(with the exception of unconsolidated rock where overburden pressure effects are included) theseeffects are normally i!nored. -n many of the special core analysis test sequences, both pressureand temperature are important, and laboratory equipment and techniques are desi!ned to simulate

reservoir conditions.

O$%ec#i&es o' a "oring (rogram

'orin! has both en!ineerin! and !eolo!ic obectives, and these should be carefully defined beforecorin! commences. -n some cases the obectives conflict, and it is impossible to satisfy allrequirements on a !iven well. The obectives that are established will affect the selection of both thecorin! fluid and the corin! device to be used. The decision will also affect the choice of a suitablecore handlin! and preservation technique and will define most measurements required.

Engineering O$%ec#i&es

The en!ineerin! obectives of a corin! pro!ram include*

definin! areal chan!es in porosity, permeability, and litholo!y$the data needed for

estimates of reserves and mathematical models+

definin! reservoir water saturation (this requires the use of corin! fluids that are oil base

or that do not invade the rock, and that the core be taken from above the water transition&one)+

assistin! in the definition of reservoir net pay+

providin! information for calibratin! downhole lo!s as well as the measured values of

electrical properties that will be used to improve lo!$calculated water saturations+

acquirin! data on the ma!nitude and distribution of reservoir residual oil saturation (this is

normally needed in enhanced oil recovery studies and utili&es either a pressure or spon!ecore barrel)+

providin! core material from which petro!raphic studies can be made to define clay type

and distribution+ these yield subsequent !uidance in selection of nondama!in! drillin!,corin!, and completion fluids+

acquirin! rock samples for special core analysis studies, includin! relative permeability,

capillary pressure, and formation wettability tests+

providin! data on porosity, as well as on hori&ontal and vertical permeability distributions,

for use in the desi!n of well$completion pro!rams to ensure that oil is not isolated and leftbehind the pipe.

(ne company su!!ests that an oil$base mud, with physical properties close to those of thereservoir oil and with no surfactants and low A/- filter loss, be used to determine residual oilsaturation and to preserve wettability.)



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)eologic O$%ec#i&es

The !eolo!ic obectives include*

definin! !as$oil and oil$water contacts, formation limits, and type of production expected+

providin! core data from which the depositional environment can be deduced, includin!

!rain si&e and !rain si&e sequences+ vertical sequence of facies+ sedimentary structures(ripples, cross beddin!)+ bio!enic structures (root &ones, burrows)+ dia!enetic alterations(cementin!, secondary porosity, secondary minerali&ation)+

permittin! a visual study of the frequency, si&e, strike, and dip of fractures. This requires

that fracture studies be undertaken and may require the availability of an oriented core+

retrieval of oriented cores so that directional permeability trends can be ascertained+ this

applies to both fractured and to nonfractured samples+ acquisition of samples ofnonreservoir rock in exploration areas so that source bed studies can be made.

B.2. Borehole Environment

"ore Al#era#ion *uring Reco&ery

There are a number of causes of core alteration durin! recovery, three of which are discussedbelow.

Fil#ra#e In&asion

-n most cases, filtrate from the corin! fluid will invade the reservoir rock as the core is cut. Thiscauses an increase in the filtrate saturation within the core and a chan!e of in situ fluid saturations.

The quantity of filtrate invadin! the core is dependent on both controllable and noncontrollablefactors. The noncontrollable factors are related to reservoir rock and fluid properties. 0or example, alow pressure differential between the drillin! fluid pressure and reservoir pressure at the sandface,increased corin! speed, hi!h reservoir fluid viscosity, and low rock permeability impede coreflushin!. 0luids that exhibit low filtrate loss durin! standard static$type filtrate tests can still causeextensive flushin! under the dynamics of corin!. 1valuation of fluid invasion can best be made byaddin! a suitable tracer, such as nitrate or tritiated water, to the corin! fluid, and then checkin! itsconcentration in the fluids extracted from the recovered core.

Fluid E+pansion and E+pulsion

2urin! most corin! processes, the core and fluids contained therein are subected to a continuousreduction in pressure and temperature as the core is retrieved to the surface. 3inor chan!es occur

in the physical dimensions of the core, but reservoir fluids under!o substantial chan!es in volume.il releases !as from solution and the oil shrinks. The !as expands and escapes from the core,dra!!in! oil and water with it. This process can be seen in some cores at retrieval, when both !asbubbles and oil bleedin! can be observed on the surface of the core. (2espite the seemin!favorability of this phenomenon, oil bleedin! at this uncture usually denotes poor quality rocks oflow permeability.) The saturations seen at the surface are different from those downhole. 0i!ure 4(Saturation changes that occur during coring and recovery with water-based coring luid) illustratesthe ma!nitude of saturation chan!es that occur from reservoir conditions to surface conditions whena core is taken with a water$base mud.

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#hen oil or oil$base mud is used to core a wellpenetratin! a homo!eneous reservoir that ishi!h enou!h upstructure to contain immobileinterstitial water, no water will be added by thecorin! fluid. -f excessive pressure differentialsare avoided, the interstitial water will remain in

place durin! both the corin! and core recoveryoperations. -n such a case the measuredvalues of water saturation will approximatereservoir saturations. 0i!ure 5 (Saturationchanges that occur during coring and recoverywith oil-based coring luid) illustrates thisphenomenon.

The pressure core barrel maintains reservoir pressure in the core durin! the trip to the surface, sothat fluids that would otherwise be lost are maintained in the pore space and can subsequently berecovered in the laboratory test apparatus. 'onsiderable research effort has been directed towardreducin! core flushin! durin! the corin! process, and pro!ress has been made. 6owever, evenpressure corin! is likely to flush some of the fluids from the core durin! this process.

*amage #o #,e Rock

ne of the maor obectives of corin! is to recover representative, nondama!ed samples of the

reservoir. 2ama!e to the core must be minimi&ed. This is difficult to achieve with the percussionsidewall corin! procedure where the core is subected to hi!h$impact stress as the hollow proectileis fired into the formation from an explosive char!e.

Selection of Coring Fluid

'orin! fluids can be divided into two maor cate!ories that relate to the filtrate lost* water$basemuds (which tend to flush the core with water) and oil$base muds (which tend to flush the core withoil). ther less frequently used corin! fluids include both water$in$oil or oil$in$water emulsions, !as


0i!ure 4.

0i!ure 5.


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or air, and foam. The latter is now used successfully in special applications. 'ommon fluids usedand filtrate loss are presented in Ta$le -!, below.

"ompa#i$ili#y .i#, "oring O$%ec#i&e

'orin! fluids must be compatible with the obectives of the core analysis pro!ram. 0or example, ifthe specific value of reservoir water saturation is sou!ht, no water should be added to the core.ocatin! !as$oil or oil$water contacts requires that no oil be added to the core. il$base mud filtrateflushes the !as &one, the oil &one, and any water &one present, and produces similar core residualoil saturations for all three &ones.

Coring fluids Filtrate Effect on core saturations

Water Hydrocarbons

Water loss fluids

Water-base Water Increased Decreased1

Oil emulsion Water Increased Decreased

Foam* * * *

Oil loss fluids

Oil base Oil No change2 Replaced

Inerted oil Oil No change2! 3 Replaced

emulsion

Gas loss fluids

"as #h$drocarbon% "as No change2!4 Replaced

&ir 'ncertain 'ncertain( Decreased

2ata indicate that foam that is properly formulated may exhibit what is essentially a &ero waterloss.

4. aturations are decreased in hydrocarbon$productive &ones. -n water$flushed &ones thatcontain residual hydrocarbons, the hydrocarbon is at residual and should remain so.

5. #ater saturation should be unchan!ed if it is at irreducible (immobile) saturation+otherwise, water saturation.

7. These muds may contain water. oss of whole mud in hi!h permeability rock canincrease water saturation.

8. 0rictional heat may evaporate water from core.

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9. #ater saturations are erratic, dependin! on heat, condition of the hole and the a!ent thatis mixed with air.

Ta$le -!Coring luids, iltrates, and saturation alteration eects

#ater$:ase and il$:ase 0luids

0i!ure 4 (!ater saturation: oil-base and water-base coring luids) illustrates core analysis watersaturations as determined from cores cut with oil$base and water$base muds. #ater in excess ofthe reservoir value exists in cores cut with water$base mud. A saturation approachin! the reservoirvalue is observed in cores that come from abovethe transition &one and that have been cut withan oil$base fluid. ;ote that in the transition &one< the &one where the water saturation chan!esrapidly from 4===>= to the minimum interstitialwater saturation < the water phase is mobile. -nmost cases the water saturation is reduced byflushin! with oil filtrate durin! the corin! process.-n some cases water within the transition &onewill be flushed to the minimum saturation value+under such conditions, when the core extendsinto the water le!, it will be impossible to pick theoil$water contact.

Foam

0oam corin! fluids are now receivin! considerable attention. They offer the advanta!es of allowin!a low corin! fluid pressure at the formation face* in addition, preliminary data su!!est that minimalor no invasion of the foam into the core occurs. "nlike other corin! fluids, which remain incirculation durin! the operation, corin! with foam is a once$throu!h process. 0oam is !enerated,then vented at the surface after a one$time use. This fluid has been used in both conventionalcorin! and in the recovery of pressure cores, as discussed by parks (4?@5). The technolo!y offoam corin! requires that the process be monitored by computer and variables adusted to assureproper quality foam durin! corin!.

B.3. Bottomhole Coring (Coring While Drilling)

"oring *e&ices

Bottomhole Goring: An Overview

The core bit and core barrel used in bottomhole corin! are installed below a conventional, butsli!htly shorter and li!hter, strin! of drill collars and stabili&ers. 'onnectin! the core barrel andbottom collar is a safety oint or back$off sub ( 0i!ure 4, Conventional core barrel and dia"ond

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0i!ure 4.
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core bit). This is a coarse$threaded connector that allows the drillstem to be unscrewed (backed$off)and retrieved, should the core barrel become stuck in the hole.

0i!ure 4."on&en#ional "ore Assem$ly

'ommonly, only a sin!le si&e corin! assembly and only one or two core bit types (based onformation hardness) will be available at the wellsite. This is mainly because the hi!h cost andintermittent usa!e of corin! equipment economically precludes maintainin! a full ran!e ofequipment. A core bit havin! the same diameter as the smallest drill bit that will be used can be runinto any hole of this, or lar!er, si&e (the cored hole may later be enlar!ed usin! a hole opener).

-n order to achieve !ood core recovery, it is necessary to use a bit$barrel combination that cuts arelatively thin kerf, i.e., a lar!e diameter core relative to the diameter of the bit. -f a ran!e of core bitsi&es is used, then a compatible ran!e of core barrels, capable of acceptin! various core si&es,must also be available. 'orin! costs may be reduced by standardi&in! on a sin!le core bit$barrelcombination. The most commonly used is an @ 4>5$inch nominal diameter bit (actual diameter is,in fact, @ 49>75$inch, since the bit cuts a sli!htly over!au!e hole) and a 7 4>5$inch (inside diameter)core barrel. This results in an optimal kerf of 5 4>5$inch annular radius.

The core barrel consists of two main parts* a core$retainin! inner barrel, and a protective outerbarrel. :oth are approximately 7= ft lon!. "p to three barrel oints may be combined to allow a ?=

ft lon! core to be cut. The outer barrel providesconnection between the drill collars and the corebit, and has stabili&er blades to prevent tiltin!,

bendin!, or flexin!, which may break or am thecore.

#hen the corin! assembly is run into theborehole, drillin! fluid is free to flow throu!h theinner barrel ( 0i!ure 5 (a), Core barrel runninginto the hole). #hen the core bit reachesbottom, the kelly is attached and fluid iscirculated throu!h the inner barrel for a short

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0i!ure 5.


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time in order to flush from it any debris that may have accumulated durin! the trip to bottom ( 0i!ure5(b), Core barrel circulating on botto").

A steel ball is then introduced into the drillstem and pumped down to the top of the core barrel,where it seats in a check valve in the inner barrel (0i!ure 5(c), Core barrel coring).

'irculation is now diverted throu!h circulation ports into the interbarrel annulus (the space betweeninner and outer barrels), and to dischar!e ports in the face of the bit. 'orin! is commenced by thecareful application and pro!ressive increase of wei!ht on bit as rotation proceeds.

As the core enters the inner barrel, it pushes the rabbit upward, displacin! drillin! fluid and wipin!the walls of the barrel. The core catcher has sprin!$loaded fin!ers that prevent the core frommovin! downward and out of the barrel. ;evertheless, the !reatest care is required when liftin!,lowerin!, stoppin!, or startin! rotation. Any arrin! may result in the core slippin! and bein! lostfrom the barrel.

pecial care is needed both when a connection is made and when the drill$stem is finally picked upoff bottom at the completion of corin!. ome overpull (resistance to liftin!) will occur initially+ but if

the core catcher operates correctly, the core will break cleanly and remain in the barrel. ometimes,slippa!e may occur and the core catcher fail to latch. The barrel must then be lowered back tobottom and the pick$up attempted a!ain.

'orin! is necessarily complete, in that the rate of penetration falls to &ero, either when the corebarrel is full or it becomes ammed with core debris. The latter occurs most commonly in extremelyhard, brittle rocks, which may split sub$vertically inside the core barrel to form wed!e$shapedfra!ments that am in the barrel and block further core entry.

ofter, less well$consolidated rocks do not break up inside the barrel this way. They may, however,be eroded by circulatin! drillin! fluid enterin! the core catcher. This erosion within the inner barrelcan reduce the diameter of the core, allowin! it to slip throu!h the core catcher and fall. nce corin!is completed, the drillstem should be tripped out of the hole as quickly as possible $but also !ently,

to avoid olts or sudden accelerations that could cause the core to be lost.

Re#rie&a$le "ore arrel

ccasionally, it may be necessary to core a lar!er formation interval than the ?= ft maximumallowed by a conventional core barrel. -n this case, it will be necessary to make several trips out ofthe borehole to recover core whenever the barrel is full.

An alternative method utili&es a retrievable inner core barrel. This can be pulled back throu!h thedrillstem to surface on a wireline, or pumped back up by reverse circulation of the drillin! fluid (downthe annulus and up the drillstem). A replacement inner barrel can then be lowered down to bottom,and corin! continued with minimum delay.

The retrievable core barrel offers a substantial time savin! over conventional corin! in the casewhere a lon!er core section is required. 6owever, because the inner barrel must be free to movethrou!h all drillstem components, the cores obtained by this method are substantially smaller indiameter (44>5 inch or less) than re!ular samples.

Ru$$er/Slee&ed "ore arrel

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Althou!h a hi!h rate of drillin! penetration is an encoura!in! si!n of !ood porosity and permeability,it can be very worrisome to the !eolo!ist durin! corin!. The only formations that core quickly arethose that are extremely weak, and almost totally unconsolidated. 'ores of such sediments,therefore, can be readily lost from the core barrel. 1ven if successfully recovered to surface, theymay collapse into loose debris when removed from the barrel.

The rubber$sleeved core barrel offers a solution to this problem. -t is similar in desi!n to aconventional core barrel, but incorporates a shrink$fit rubber tube. This is drawn into the inner barrelby the rabbit as the core enters. The whole core is, therefore, contained in a ti!ht rubber sheath.

n recovery to surface, the whole sheathed core may be removed in a sin!le piece, and cut intoconvenient len!ths for shippin! or analysis. ne resultin! disadvanta!e, however, is that in order tovisually inspect the core, a wellsite !eolo!ist only has access to the cut ends of the core len!ths.ater, the core may be fro&en or artificially consolidated by inection of plastic !el. The rubbersheath can then be removed to allow complete core evaluation.

-n recent years, the use of fiber!lass or aluminum inner barrels has effectively replaced rubber$sleeve corin! methods in fractured or unconsolidated rock (kopec, 4??8)

"nconsolidated samples can be recovered in a rubber$sleeve core barrel. nce encapsulated inthis special, tou!h, rubber$sleeved tool durin! the corin! process, the core remains there durin!removal from the corin! device and on the trip to the laboratory. 'are must be taken when removin!the sleeve from the core barrel+ this is done by pullin! the sleeve onto a B$shaped tray to preventtwistin! or bendin! of the core.

Foam/Lined "ore arrel

ne of the purposes of corin! and core analysis is to obtain an estimate of the type and relativesaturations of water, oil, and !as in the formation. "nfortunately, the decrease in confinin! pressureas the core is brou!ht to surface will chan!e ori!inal saturations. As the core barrel is pulled fromthe hole, the confinin! pressure on the fluids in the pore space of the core will drop from bottomhole

hydrostatic pressure down to atmospheric pressure. 2ependin! on the cored depth, this drop canbe substantial. As confinin! pressure declines, dissolved !as will escape from solution, while free!as will expand, flushin! oil and water from the core. Therefore, measured saturations at surfacewill be hi!her in !as and lower in oil and water than the ori!inal values.

A simple solution is provided by the foam$lined core barrel. 0luids expelled from the core onrecovery are absorbed into the porous plastic foam linin! of the inner core barrel. After the core hasbeen removed, this foam linin! is stripped from the barrel and analy&ed separately for oil and watercontent.

(ressuri0ed "ore arrel

A more complete, but also more expensive, solution to the problem of !as expansion is provided bythe pressuri&ed core barrel. "sin! automatic valves, this apparatus seals the upper and lower endsof the inner barrel after corin! is completed, so that ori!inal pressures are maintained.

At surface, the mud is removed from between the inner and outer core barrels by displacement witha !elled diesel oil and the barrels are immersed in dry ice for several hours to free&e the core. ncethe core is fro&en, the inner barrel may be disconnected, removed, cut into four$ or five$foot len!thsand shipped to the laboratory for analysis (one disadvanta!e of this procedure is that the wellsite!eolo!ist has little chance, if any, to examine the core.

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2urin! shipment and until analysis, the barrel len!th must be kept fro&en. This samples are thenprepared and thawed in the laboratory where the oil that would normally escape with pressurereduction can be measured. #hen #ilhirt& and 'harlson (4?C@) compared the results ofconventional versus pressure corin!, they found that approximately 9=D of the residual oil in a an

Andres core was lost durin! the pressure drop associated with non$pressured core recovery.

The primary advanta!e of the pressuri&ed core barrel is that fluids do not escape as the core isbrou!ht to surface. Thus, it provides additional information on residual oil saturation valves(6a!edorn and :lackwell, 4?C5). Althou!h the barrel has been available for years, it achievedspecial prominence when enhanced oil recovery became a matter of special interest. #hile thisdevice does not prevent flushin! of the core, improvements in corin! fluids and bits have assisted inreducin! flushin!.

-t is often desirable to know the quantity of oil remainin! after waterflood or in a natural water$encroached &one. At the time a core is taken in a water$flushed &one, the oil exists as an immobile,trapped phase. :y usin! a corin! fluid with a water filtrate and maintainin! a low sandface pressuredifferential, the immobile residual oil is not displaced and, in this case, flushin! is not detrimental.

The pressure corin! process is complex and relatively expensive, but is considered by some to beone of the better techniques for definin! reservoir residual oil saturation (6ensel, Er. 4?@8+ parks,4?@5).

Orien#ed "ore arrel

The maor advanta!es of the core sample over well cuttin!s is that it offers a coherent Fmini$exposure of the selected interval, and therefore offers nearly the same basic ran!e of crucial!eolo!ic information as a fresh road$cut outcrop. "nfortunately, a conventional core is not oriented,and important data re!ardin! directional features can not be determined. 1ven if the orientation ofthe inner barrel is known, this may drift durin! the corin! operation. 1ven dip amounts may beuncertain, if the borehole itself is deviated from the vertical.

-n an extreme example, consider a core cut in a borehole that deviates at 89G from the vertical. -nthis example, beddin! planes are observed to dip at 89G to the lon! (vertical) axis of the core.Biewed from one borehole orientation, these beds are hori&ontal (89 $ 89 H =), but from the oppositeorientation they are vertical (89 I 89 H ?=). "nless the orientation of the core, relative to theborehole, is known, one can only say that true dip is 89G, plus or minus 89G

A conventional core barrel may be adapted for oriented corin! by the addition of two devices thatmark the core with respect to the barrel, and the core barrel with respect to the outside world.

rientation of the core is performed by addin! an orientin! shoe to the core catcher. This hasblades that scribe !rooves onto the core as it passes into the barrel. :y ali!nin! these !rooves, it ispossible to orient the entire se!mented core relative to itself and to the core barrel.

The latter, meanwhile, is oriented by ri!idly attachin! a multishot survey tool to the top of the innercore barrel. This tool records a series of measurements of inclination and a&imuth (dip and strike) atfixed time intervals. :y comparin! these timed measurements with the drillin! record, it is possibleto determine the core barrel orientation foot by foot as the core was cut. 'omparison with measureddips at measured depths on the oriented core !ives the true dip and strike of structures and !raintextures.

"ore E%ec#or i#



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This is not a real alternative to conventional bottom hole corin!, but rather a means of obtainin!usable si&e cuttin!s when drillin! with a diamond bit.

The core eector bit is similar to a conventional diamond bit, but has a central orifice like that in acore bit (althou!h much smaller). #hen the bit drills, a small diameter core is cut and passesthrou!h this orifice into the drillstem $there is no core barrel or catcher. -mmediately above the bit is

a core catcher sub$a short section of drill collar$with an eccentric inside diameter. As this subrotates, its inner wall will periodically strike the core and break it.

ome fra!ments of core will be retained inside the sub and recovered on its return to surface+others will fall out of the bit orifice when the bit is picked up off bottom and will be carried to surfaceby drillin! fluid circulation.

(repara#ion

#hen extensive corin! and core analysis are planned, a speciali&ed crew of core technicians maybe assi!ned to the wellsite from a core analysis service company. -n most other circumstances, themudlo!!in! crew and the wellsite !eolo!ist will be responsible for core handlin! and evaluation.

-n either case, the wellsite !eolo!ist must determine what procedures are to be followed in handlin!and processin! the core, and what supplies will be required to accomplish this. After takin!inventory at the wellsite, he or she should order sufficient quantities of the necessary supplies,leavin! sufficient time for delivery.

As supplies arrive, they should be checked a!ainst the inventory and stored in a clean, dry, safeplace. #ood is a precious commodity on an offshore ri!. "nless locked away, wooden core boxesand lids will tend to disappear.

-f the drillin! crew are unfamiliar with corin!, a meetin! should be arran!ed so that the !eolo!ist ormud lo!!er can brief them on what will be expected of them durin! core retrieval. -t should beexplained where they can be of help and where they cannot. 0or example, warn them a!ainst

washin! the core with a hosepipeJ This may dama!e the core and will render it useless forsaturation measurements.

"ore (oin# Selec#ion

/ickin! a core point is a normal ob of strati!raphic correlation. The approximate depth of the &oneof interest will often be already known from seismic profiles. "sin! lo!s from other wells, it ispossible to identify marker hori&ons immediately above the &one. The sonic lo! is usually best forthis, since it is stron!ly responsive to chan!es in formation rock stren!th and porosity. These arethe same characteristics that affect the rate at which a bit will drill the formation. 0or this reason, thesonic lo! from a previous nearby well will show !ood correlation with a rate of penetration lo! from

the present well. 1ven better correlation is !iven by Fnormali&ed drillin! exponents (dx, ;K, ,

etc.). These are mathematical treatments used to standardi&e and cancel the influence of certainmechanical variables (e.!., wei!ht on bit, rotary speed, cuttin! structure desi!n) on rate ofpenetration, in order to derive a parameter that is predominantly controlled by rock stren!th andporosity ( 0i!ure 4 , Correlation o sonic and drilling e#ponent logs in order to select a coring point).



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-f a core barrel is run into the holetoo early, before the selectedinterval has been reached, ri! timeand operatin! costs are wasted. -fthe &one of interest is reco!ni&edtoo late, the core can no lon!er be

cut and a maor obective of the wellmay be lost. -deally, a clear,prominent marker hori&on should befound that occurs immediatelyabove the core point. This, ofcourse, is not always possible, andon remote exploration wells (rankwildcats) it may be necessary, whenapproachin! the &one of interest, toadopt a more time$consumin!, butfailsafe, procedure.

This involves haltin! the drillin!

periodically, or when a drillin! breakoccurs+ continuin! to circulate forthe la! time, inspectin! the wellcuttin!s from the drilled interval, andthen decidin! whether to drill further

or commence corin!.

"u##ing #,e "ore

There is little that the !eolo!ist can do to obtain useful information durin! the corin! process itself.Rate of penetration is slow and unresponsive to litholo!ic chan!es. #ell cuttin!s will be of the samepoor quality as those from a diamond drill bit. There will also be a much smaller volume of cuttin!s,since only an annular kerf of formation is bein! cut.

Althou!h visual examination of these cuttin!s is virtually useless, the !eolo!ist should have themudlo!!er or sample catcher catch as much of them as possible for the archival sample sacks.These may be dumped later when successful core recovery is confirmed. -f, on the other hand, allor part of the core is lost, these archival samples will be the only sample material available, forbetter or worseJ

#hile corin!, even the depth of the well becomes uncertain, due to the low wei!ht on bit used. Thetotal depth of a well, durin! drillin!, is determined as the simple sum of the measured len!ths of allcomponents of the drillstem below the rotary kelly bushin! (RL:), which is used as the datum of&ero$point for depth measurement. Thus, the depth at any time is taken to be the len!th of the drillbit, drillstrin!, and as much of the kelly as has been drilled down below the RL:.

#hen drillin!, the drill collar will be in compression, i.e., shortened by wei!ht on bit, and the drillpipewill be in tension, i.e., stretched. These two effects will introduce small but opposin! errors into thedepth calculation* the net error will be very small$a few feet in several thousands of feet$and willremain consistent as drillin! proceeds.

#hen a core barrel is run, the bottom$hole assembly will contain fewer drill collars, and the len!th ofthe drillstem will be compensated by the addition of extra drillpipe. ess wei!ht on bit will be appliedand, therefore, the relative len!ths of shortened or stretched pipe will be chan!ed. The net deptherror will remain small, relative to total depth, but will differ from that measured durin! normal


0i!ure 4.


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drillin!. The effect is that total depth will appear to have chan!ed after the core barrel is run into thehole. Althou!h the difference of a few feet is relatively insi!nificant in terms of the total depth, it isimportant with respect to the core for two reasons. 0irst, it affects how the coreFs len!th isestimated. econd, a few feet can become extremely important when the &one of interest is itselfshort, say 59 ft or less.

;othin! can be done to solve this problem entirely. 6owever, the !eolo!ist, en!ineer, and drillin!foreman can work to!ether to have the drillstrin! accurately remeasured before it !oes into the hole.The kelly is measured when wei!ht is first applied to the core bit on bottom. A consensus startdepth for the core should be decided on$one that can be used for all further reports andcalculations.

Retrieving and Handling the Core

Se#/Up

As soon as the core barrel is picked up off bottom, and the trip out of the hole be!un, !eolo!icalpersonnel should be!in their preparations to receive, process, and packa!e the core. This willrequire a period of nonstop, hectic activity$especially if the core barrel is to be returned to bottom fora second or third corin! run. #ork areas should be prepared, and materials made ready, in order tofacilitate these activities.

1ork Area

'ores will be collected, and eventually shipped, in boxes that are approximately M inches in hei!htand width, and 7 feet lon!. An area must be found where the whole core can be laid out, walkedaround, and worked on by one or two people. -deally, a bench should be used for layin! down thecore boxes so that the !eolo!ist may work in a comfortable standin! or sittin! position. The areashould be under cover and well lit (cores mi!ht be recovered at ni!ht or in bad weather). -t shouldhave electrical outlets for the microscope illuminator, ultraviolet inspection lamp, and other devices.Runnin! water is a convenience, but not essential, since the core should not be washed.

-n addition to the equipment required for microscopic examination of cuttin!s, the followin! specialsupplies should be available in the work area*

N sufficient clean core boxes to receive the expected core foota!e (plus a few spares)+

supply of clean ra!s for cleanin! the core and stuffin! core boxes+

measurin! tape+

!eolo!ical hammer (or hammer and cold chisel)+

hand lens+

waterproof marker pens in red, black, and other colors (with spares)+

spare work !loves+



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worksheets, note pads, and pencils+ wrappin!, sealin!, and cannin! supplies+

labeled archival and dried sample sacks+

copy of the core samplin! and shippin! instructions.

Rig Floor

About one hour before the core barrel is due on surface, the !eolo!ist should be!in preparin! thearea of the ri! floor that has been desi!nated as the core$retrieval area. The materials neededinclude*

N catchin! set of core boxes, labeled and stacked as shown in 0i!ure 4(Labeling and stackorder o core bo#es)*

!eolo!ical hammer+

soft metal hammer+

broom+

work !loves+

supply of clean !loves+

notepad and pencils on clipboard.

The catchin! set is used to keep the core incorrect order, both while it is bein! retrievedand later, after it has been removed to thework area. After use, the boxes may bewashed and reused. They should be clearlylabeled with top, bottom, and box number,and stacked in an orderly manner to avoidconfusion even on a crowded, poorly lit ri!floor at ni!ht.

;otice that boxes are numbered accordin!to the order of the core as it comes out ofthe barrel* box O4 holds the first (bottom)section of core, box O5, the second section,etc. This numberin! scheme also reflectsthe a!e pro!ression of the rocks sampled,box O4 holdin! the oldest sediments and soon.

1nou!h boxes should be available toaccommodate the whole len!th of core.

1ach will hold about two feet of core (it will, in fact, hold almost three feet, but the core cannot beexpected to come from the barrel in convenient len!ths, and should not be broken until after initialinspection).


0i!ure 4.


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A nearby area should be cleared for stackin! the filled core boxes and drillin! crew membersdele!ated to help transport them to the work area. -f the filled boxes are to be lifted down from theri! floor by crane, then a pallet should be laid down upon which the filled boxes can be stacked.

Reco&ery 'rom #,e arrel

#hen the core reaches surface, the outer core barrel is suspended in the rotary table. The innerbarrel is lifted out and suspended with the travelin! block over the core retrieval area. The corecatcher is removed, and replaced with a barrel clamp and core ton!s, which allow the core to beslipped from the barrel at a safe and controlled rate ( 0i!ure 5 , Conventional retrieval o the corero" the inner barrel).

The first two or three feet of the core should beslipped from the barrel, briefly inspected, wipeddry of drillin! fluid, and placed in box O4. Thenext section of core should not be slipped untilthis is completed. -f loose debris falls from thebarrel, it should be swept clear with the broom

and placed into the appropriate position in thecore box. -t may be possible later to pieceto!ether this broken material.

At no time should anyone reach under the corebarrel or block the view of the driller operatin!the drawworks brake. o!!ers should removecore pieces from beneath the barrel with abroom or hammer and stand clear when notdoin! so.

-f the core ceases to slide from the barrel, it maybe ammed (the barrel is not empty until the

rabbit slides out). i!ht tappin! with a li!ht metal hammer or mallet should free the core. 2o not usethe !eolo!ical hammer, which can dama!e the barrel. -f hammerin! cannot free the core, it must bepumped out ( 0i!ure 7 , $etrieval by pu"ping in the case where the core is %a""ed). As corepieces are pumped free of the barrel, they will fall to the !round and may be pushed away with thebroom to be dried and boxed. 2o not walk in front or handle the end of the core barrel, or any corethat is protrudin! from it.

Dicky Haris Hidayat Library: Coring and Core Analysis 1(

0i!ure 5.
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0i!ure 7.o+ing #,e "ore

n arrival at the work area, the core must be transferred to clean core boxes, fitted to!ether wherepossible, and measured. 3ost oil companies require core boxes to be numbered in the mannermentioned, i.e., from the bottom upward, but some 1uropean companies prefer a reverse order, so

that the core is reboxed with the top of the corein box O4. #hatever method is to be usedshould be decided on beforehand.

1ach piece of core should be taken from thecatchin! box and wiped clean with a dry ra!+ itsobvious features (such as fractures, beddin!planes, oil or !as bleedin! from porosity) shouldthen be noted on a worksheet. The piece is thentransferred to its restin! place in a clean, newcore box. oose rubble should be put in a cleansample sack and labeled with well and coreidentification information, as well as anestimation of the number of feet it represents (0i!ur

e8, Labeling o the core or shipping).

#hen each box is about full, the core piecesshould be rotated and pushed to!ether to obtainthe best possible fit between broken ends. 1ndsshould then be marked with appropriate symbolsto indicate the quality of fit* double chevrons forno fit, sin!le chevrons for a poor fit, and no

Dicky Haris Hidayat Library: Coring and Core Analysis 1)

0i!ure 8.

0i!ure 9.
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symbol for a perfect fit. The core is then oriented by markin! two lines on it in felt tip pen from top tobottom (red to the left, black to the ri!ht), with the pieces pushed to!ether for best measured fit. Atally sheet should be kept with the measured len!th of core and estimated rubble content of eachbox. #hen the total len!th of core is known, these len!th measurements are translated into depths (0i!ure 9, a"ple core tally sheet) $$missin! core is always assumed to have been lost from thebottom of the barrel.

0inally, the pieces are moved sli!htly apart and separated by ra!s, which are also stuffed into thespace between pieces and at the ends of the box. This prevents dama!e to the core durin!transport. The process is repeated for each box until the whole core is reboxed. The catchin! set ofboxes may now be removed, and the !eolo!ist can proceed with samplin! and !eolo!icalevaluation.

O#,er "oring *e&ices

R"::1R, /AT-', A;2 0-:1R%A 'R1* Rubber sleeve, plastic sleeve, and fiber!lasscores should be marked with waterproof materials for top, bottom, and depth. and then sealed withcaps (when available) furnished by the corin! company. Tape should be wrapped around the

sleeve$cap oint. This in turn can be dipped in strippable plastic to ensure air ti!htness.

The sleeves are sometimes transported fro&en in full$len!th wooden boxes. ften they have firstbeen cut into 7 to 9 ft (4 to 4.9 m) len!ths. The shippin! boxes should also have top, bottom, anddepth identified. 'are should be exercised to prevent the sleeves from bendin!, since this woulddestroy the !rain$to$!rain inte!rity of the core.

/R1"R1 'R1* 'ore handlin! of the pressure core barrel is mana!ed by the corin! company.The samples remain in the inner barrel for transport to the laboratory. Typically, the inner barrel isfro&en at the wellsite, cut into 7 to 9 ft (4 to 4.9 m) len!ths while in a fro&en state, and then shippedin a free&e box. amples must remain fro&en until analysis in the laboratory.

/;%1 'R1* Thirty ft (? m) of formation is normally cored with the spon!e barrel, after which

the core is hoisted to the surface. The inner core barrel is removed from the outer barrel and laiddown. This inner barrel contains the spon!e liner, which in turn contains the core. 6ydraulicpressure is used to force the 7= ft (? m) of spon!e liner from the inner barrel. As 9 ft (4.9 m)sections are exposed, the spon!e$liner$enclosed core is broken off and placed in /B' handlin!tubes for transport to a core analysis laboratory. -f desired, the handlin! tubes can be filled withdrillin! mud or formation salt water to eliminate exposure of the cores to air.

"ore (reser&a#ion Tec,ni2ues

'ores are packa!ed to prevent fluid loss between the time of recovery and analysis. 6eavy$dutyplastic ba!s are commonly used for short$term stora!e. 0or lon!$term stora!e, samples are oftensealed by first wrappin! the core in layers of aranT3 wrap followed by several layers of aluminumfoil, and then dippin! the wrapped core in strippable plastic or wax. 0ree&in!, cannin!, and

submer!in! a core under fluid have all been successfully employed as means of preservin! cores.'ores cut to evaluate interstitial water, measure fluid levels, or to interpret !as, oil, or waterproduction can be packa!ed by any technique other than submersion, since exposure of the core toliquid would result in imbibition of that liquid and alteration of residual saturations.

1xposure of a core to the atmosphere should be minimi&ed when preservation of core wettability isdesired. -mmediate submersion under deaerated water is suitable for cores taken with water$basemud. 'ores cut with oil filtrate muds can be stored under nonoxidi&ed oil. The aranT3 wrap$foil$

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strippable plastic preservation technique is suitable for all corin! fluids and is !enerally accepted asthe best.

ome corin! techniques have a built$in preservation ability that can be used to !ood advanta!e, atleast for short$term stora!e. Rubber sleeve, pressure, plastic$sleeve, and fiber!lass core barrelsneed only be cut into suitable len!ths and then capped to properly preserve the core.

Sealing in (las#ic ags

ealin! a core in plastic ba!s is an excellent short$term preservation technique that should berestricted to no lon!er than several days. The sample should be placed within the plastic ba! andair space squee&ed from between the core and the ba! wall. The top can then be twisted to seal thesample and taped a!ainst the plastic or sealed with a rubber band. -f the samples are to beshipped, the core should be double$ba!!ed with insulatin! material placed between the cores. 1achplastic ba! should be labeled for depth interval, top, and bottom. The plastic ba!s normally used aresuitable for cores of up to approximately 4 ft (7= cm) in len!th. A newer, 7 ft (4 m) ba! with a &ip$lock liner exists for fast preservation within cardboard boxes. This lon! ba! can be opened andfolded over the side of the box, and then samples may be put in depth sequence within it. This also

allows the top to be opened so that the !eolo!ist may secure small chips of the rock for descriptionor view the core without the cover of a plastic film.

Sealing in S#rippa$le (las#ic or 1a+

This technique is the best for lon!$term stora!e. -ndividual pieces of core should be wrapped withseveral layers of aranT3 wrap or 2ow 6andiwrapT3 (other plastic products affect samplewettability). The plastic should then be wrapped with several layers of aluminum foil, makin! surethe foil is pressed to the sample to eliminate sharp corners where subsequent sealin! materialsmay run off and not coat. 1ach sample should be marked clearly for well name, depth, and top orbottom with a permanent marker or an attached label.

6eavy wire or twine should then be tied to each piece of wrapped core to allow immersion of the

sample into molten wax or plastic, or the samples can be dipped from opposite ends with a sealantoverlap. The samples should be immersed and removed rapidly so as not to melt the inner plasticwrap. amples should be dipped at least twice, and then hun! on a rack to cool. The wire or twineshould be clipped near the core. The clipped end must be sealed to prevent a wick effect that willallow moisture to escape. -f paraffin wax is used as the preservative material, the wax should beheated only sli!htly above the meltin! point+ otherwise, the temperatures may be too hi!h, anddama!e may occur to the inner layer of aranT3

/lastic materials used for coatin! cores must be stable over lon! periods of time, must not reactwith water or oil, and should not exude materials when set. Recent research has resulted in thedevelopment of a product with superior sealin! properties known as 'oreealT3 The 'oreealT3product is new+ history on its performance is available only for a two$year period.

#hen tests are to be made on cores that have been stored for lon! periods, selected samplesshould be analy&ed for water saturation. The results should be compared to saturations present inthe core at the time the rock was preserved in order to assess the amount of fluids that mi!ht havebeen lost.

Sampling 'or "ore Analysis

nce the core is cleaned, boxed, and measured, the core analysis samples should be selected andpreserved. ;ormally, samples are taken at re!ular intervals in massive formations considered to

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have reservoir potential (porosity and permeability), but must be more carefully selected in thin,inhomo!eneous beds.

Thick, impermeable formations, meanwhile, need to be sampled only at the top, bottom, and oncein the middle (or every 9= feet, whichever is less). All permeable &ones more than two inches thickwithin such formation should also be sampled. Thicker permeable intervals should be sampled

every foot, or every two feet if litholo!ically uniform. -n less homo!eneous formations, the samplin!interval should be reduced to obtain reasonably representative samples for the whole core.

0or analysis, a coherent piece, about 7 inches lon! and showin! no splits or cracks, should betaken. At each sample point, the minimum amount of core that will ensure 7 inches ofhomo!eneous, unbroken rock should be selected. 'ore samples must be sealed in order topreserve contained fluids until analysis. There are a number of methods used for this. 'annin! is acommon method, but one that is not recommended. Althou!h it provides a ti!ht seal, the air spacein the can allows evaporation and expansion of core fluids. -mprovement is !ained by wrappin! thesample in a lar!e quantity of nonabsorbent material prior to cannin!.

2ippin! the core sample in wax or a thermoplastic !el is an excellent method of sealin! and is verylon!$lastin!. To prevent surface porosity dama!e to the core, it should be wrapped in aran #rapPor aluminum foil before dippin!.

A modern method, widely used to preserve pressuri&ed cores, involves free&in! in dry ice. -trequires special facilities at the wellsite, however, and either rapid or refri!erated transportation to acore laboratory.

#here core analysis is to be performed at the wellsite, and preservation of the core sample isrequired for only a few hours, double wrappin! is quite successful. The sample is first wrapped inaran #rapP (aran #rapP onlyJ ther forms of food wrap are !as permeable), and then inaluminum foil. -f lon!er preservation is required, it is recommended that the wrapped sample befurther enclosed in a heat$sealed polyethylene ba! or sleeve.

-f core analysis is to be performed at the wellsite, the sample need only be labeled by depth andreturned to its appropriate location in the core box. -f the sample is to be shipped to a corelaboratory, then it must be labeled in more detail*

oil company name and division+

well name and location+

core number+

sample number+

sample depth interval.

amples should be packed in a wooden or metal container with ra!, straw, or newspaper stuffin!between the samples. A sample inventory should be prepared, and copies of it made. ne copyshould be enclosed in the sample box. The other should be retained at the wellsite until receipt ofthe samples has been acknowled!ed. The sample inventory should contain the followin!*

name of the laboratory and responsible person+

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oil company name and division+

well name and location+

core number and depth interval+

total number of samples+

total number of boxes+

list of samples by number, depth, and box+

type of analysis required+

drillin! fluid type and special peculiarities+

drillin! fluid filtrate loss, salinity, nitrate concentration, and any other special tests thatwere run+

name and location of person to whom results are to be reported.

%aps in the core box from removal of a core analysis sample should be stuffed with ra!s and thenotation 'R1 A;AQ- A3/1* -;'61 made on the inside of the box at that point.

)eological E&alua#ion 4and Samples

0ollowin! the brief examination made while boxin! the core, a more thorou!h evaluation should bemade to describe in detail all si!nificant macroscopic features of the core. #hile doin! this, the

!eolo!ist should also be extractin! small chips of core that will be used both for microscopicexamination and to take the place of washed and dried cuttin!s samples in the well sample set.

ample description should include*

litholo!y and thickness of maor formation units+

dip (apparent or true$if known) and strike of boundaries, beddin!s, fractures, and other

structures+

nature of litholo!ical boundaries+ nature of beddin! planes, and of sedimentary and

dia!enetic structures+

!radational features in !rain si&e, sortin!, etc.+

spacin! and surface texture of fractures and oints.

Microscopic E+amina#ion

'ollected core chips should be subected to the full litholo!ical and hydrocarbon examination that isstandard for cuttin!s examination. amples may be broken with a hammer and crushed in the



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blender to obtain fine$!rained material for examination of !rain texture and to liberate !as foranalysis by the mudlo!!er. ievin! or point countin! of this disa!!re!ated material can !ive aquantitative estimate of !rain si&e and sortin!.

A sample lo!, consistin! of a !raphical representation of litholo!y and structure, and a writtendescription should be prepared from the notes made durin! all sta!es of sample examination.

"ore Sla$$ing

The core slabber is a table mounted, diamond circular saw ( 0i!ure 4 , Dia"ond saw or cuttingcore slabs). -t may be used to cut flat hori&ontal or flat vertical slabs of core. The faces on theseslabs !ive very fresh exposure of !eolo!ic features and improve visual inspection at the microscope( 0i!ure 5 , 'rientation o etched core slab or e#a"ination). After slabbin!, the core may bepolished with carborundum and etched with dilute hydrochloric acid.

0i!ure 4. 0i!ure 5.T,in Sec#ions

-nspection of porosity, texture, mineralo!y, and microfossils can be much improved by theproduction of thin sections from slabbed samples of core. These can be prepared by most coreanalysis service companies.

ome mud lo!!in! contractors will also be able to provide thin sections at the wellsite. This can beespecially useful for cores cut in carbonate sections, where determinin! the specific !enetic ori!inand distribution of porosity can be more important than measurin! its quantitative value.

Ace#a#e (eels

An alternative to thin sections is the acetate peel, which can also be prepared at the wellsite.oftened plastic is applied to the slabbed and etched surface of the core and allowed to set. Theplastic is then peeled off the surface and, when mounted between !lass sheets, used as aphoto!raphic ne!ative and enlar!ed.

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The traditional method requires the mixin! of unpleasant chemicals in order to prepare a solution ofcellulose acetate. An improved method uses precut sheets of acetate film that may be softened withacetone and applied to the core surface.

La$ora#ory Sample

"ore Sampling

-n some cases wellsite samplin! is performed, but it is preferable to preserve and transport theentire core to the laboratory for samplin! under more controlled conditions. #hen the entire corereaches the laboratory, it should be placed in depth order on a layout table. After the core is refitted,the natural !amma activity of the core can be lo!!ed and photos of the native rock can be taken. Adetailed core description should then be made.

Two basic approaches exist in sample selection. ne is a statistical approach, in which cores aresampled from the top or middle of each foot of rock, independent of the litholo!y. The secondapproach requires that the analyst secure a representative sample, re!ardless of its location, fromeach foot. The actual approach will depend upon company philosophy and the formation

characteristics.

"on&en#ional 5(lug6 Analysis

-n homo!eneous formations a core se!ment of approximately 8 inches (4= cm) in len!th or lesstaken from every foot of core is sufficient for core plu! analysis. -f the core has !reat litholo!icalvariations, however, samples should be obtained more frequently. -t is important that the samplesbe representative < core data have been skewed by improper sample selection.

Full *iame#er Analysis

-f a full$diameter analysis is to be performed, samples M inches (49 cm) or lon!er are prepared in

the form of ri!ht cylinders from each foot of core, usin! a diamond saw. The core ends aresometimes used for saturation determinations. -t is important that the lubricant selected for the sawblade corresponds to the filtrate of the corin! fluid, so that additional extraneous fluids are notadded.

Ru$$er7 (las#ic7 and Fi$erglass "ores

Rubber$sleeve cores arrive in the laboratory within the rubber sleeve, and a core !amma lo! is runon the core to assist in selectin! samplin! points. -n thinly laminated sands, the core is sometimesK$rayed to locate beddin! planes and sand strin!ers suitable for samplin!. -n some cases the coresare fro&en and then slabbed. -n other cases the rubber sleeve is placed in a wooden or cardboardbox and a stabili&in! foam or plaster of /aris is used to surround the core. The uppermost part ofthe rubber sleeve is removed, the mud cake is cut off, and the core is then available for samplin!.

This technique exposes the full len!th of the sample, and the core can be photo!raphed prior to orafter the samples have been taken.

Another technique is to cut a window in the sleeve, leavin! one side hin!ed so that it may be foldedback while the sample is taken. The window may subsequently be closed and taped. The rock issometimes fro&en and plu!s are drilled with liquid nitro!en to assure that !rain$to$!rain contacts aremaintained until tests commence.



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/lastic sleeve and fiber!lass core barrels containin! unconsolidated reservoir rock are often fro&enand then slabbed. /lu!s can be drilled from the fro&en core at desired intervals. -n other cases ahole is drilled into the sleeve and samples are either drilled from the encased rock or removed by apunch when the rock is very unconsolidated. -n other applications twin cuts are made down thesides of the barrel parallel to the lon! axis of the core. The sleeve material is lifted away, exposin!the core for examination and sample selection.

(ressure "ores

'ores recovered in a pressure core barrel arrive in the laboratory fro&en within the inner metalbarrel, havin! been transported in insulated chests filled with dry ice. elected len!ths are placedon a millin! machine and !rooves are cut alon! the opposite sides of the metal barrel almost to thecore. A tissue$thin layer of metal is allowed to remain to preserve the inte!rity of the barrel. At thetime the analysis is to start, a screwdriver is wed!ed into the !roove and turned, which removes theupper half of the metal barrel and exposes the core for subsequent samplin!. #hile a plu! analysiscan be performed, it is common to use full diameter samples of up to @ inches (5= cm) in len!th.

Sponge "ores

The spon!e core barrel arrives at the laboratory in a /B' handlin! tube. The aluminum shell andthe spon!e are split open to expose the core. 0ull diameter analysis techniques are employed.amples are selected and marked on both the core and the spon!e before the core is removedfrom the spon!e. pon!e samples for analysis are taken directly adacent to the correspondin! coresamples. This is to insure that the oil recovered from the spon!e is attributed to the pore space fromwhich it was expulsed. The spon!e samples are subsequently extracted and residual fluid volumesare determined.

B.4. SideWall Coring (Coring Ater Drilling)

Side.all "oring

1ireline "ore )un

The chronolo!ical sample taker or core sample tool ('T) is a multishot !un that is lowered into theborehole on a wireline lo!!in! cable.

At the appropriate sample depth, a 4$inch hollow bullet is fired hori&ontallyinto the borehole wall, and a core upto 5 inches lon! recovered ( 0i!ure 4 ,Sa"ple taking operation o thesidewall gun). n a sin!le run into the

hole, combined !uns can shoot up toC5 sidewall cores.

-n a very irre!ular, washed$out,over!au!e hole, cores may be muchshorter, or may even consist entirelyof filter cake or drillin! f luid$contaminated sediments. -n very hard


0i!ure 4.


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rock, the bullets may break on impact, fail to penetrate, or, if recovered, may contain only fracturedor crushed material.

Thou!h, for a number of obvious reasons, sidewall cores are inferior to bottomhole cores, they arefar more commonly cut. ne reason is that they are cheap to obtain, requirin! minimum ri! time.3ore importantly, they are taken after the well has been drilled and lo!!ed and, therefore, are

planned with the benefit of hindsi!ht.

Ro#ary Side.all "orer

This tool is an attempt to combine the advanta!es of sidewall corin! with conventional corin!technolo!y. The inner barrel is forced out at an an!le throu!h the side of the outer barrel, and intothe borehole wall by the hydraulic action of pumped drillin! fluid. /umpin! is then stopped, and theinner barrel is retrieved on a wireline+ the outer barrel is moved to a new location in the hole, and anew inner barrel pumped into place for the next core.

The rotary sidewall cores are 4 inch in diameter and up to 4 foot lon!. They are of only sli!htlybetter quality than wireline !un cores and require more expensive equipment and ri! time to obtain.This type of corin! is not commonly performed.

Side.all "ore Slicer

The tricone tool that is used is run on a wireline and can cut lon! (up to 7 ft) trian!ular cores fromthe borehole (0i!ure 5 , (he sidewall core slicer )tricone* tool).

The tool is lowered to the &one of interest, at which point a pad is extended a!ainst one side of theborehole wall, forcin! the cuttin! toola!ainst the opposite wall. Two diamond$

tipped, circular saw blades mounted atM== to each other move out of the tool andup its len!th, cuttin! a trian!ular slice offormation, approximately 4 inch on eachside. After cuttin!, the pad is retracted intothe tool and the core falls into a corecatcher below. "p to four cores can be cuton a sin!le run in the hole. Any irre!ularityin the borehole will prevent tool contact,resultin! in a fra!mentary core.

uch core slices are very useful!eolo!ically. They provide extensivestrati!raphic information rapidly and at arelatively low cost. "nfortunately, tricorescan only be obtained successfully in verywell$consolidated formations. ofter rockstend to disinte!rate with saw vibration.

"ore (oin# Selec#ion


0i!ure 5.
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idewall core points are selected at intervals that have been indicated to be important by cuttin!s orwireline lo!s. A second crucial selection criteria is the likelihood of successful recovery of the core.

-f the borehole is deeply washed out andover!au!e, the normal sidewall bullet will failto penetrate the borehole wall ( 0i!ure 4 ,

Selection o the correct sidewall bullets andasteners or or"ation type and hole si+e).#here the borehole is this much oversi&ed,extra lon! wire fasteners are used to extendthe bulletFs penetration ran!e. imilarly,special hardened bullets are used to penetrateand core unusually hard formations. 6owever,lon! fasteners and hardened bullets used insoft formations and in$!au!e holes can resultin excessively deep bullet penetration andfailure to retrieve the core. /reviousexperience with both the interval of interestand the success rate of the wireline company

bein! used should be a factor in decidin! thenumber and type of shots required, includin!some accountin! for misfires.

"ore Reco&ery

n retrieval from the borehole, the sidewall core !un is taken to the wireline service company workarea. :efore any other work is attempted, the lo!!in! en!ineer should remove the explosivechar!es from all bullets that failed to fire.

The successful bullets are then removed from the !un by cuttin! the wire fasteners. 1xtra careshould be taken to keep the bullets in correct order after they have been removed from the !unJ1ach core is then extracted from the bullet into a labeled !lass ar ( 0i!ure 4 , traction o thesidewall core ro" the hollow bullet ater re"oval ro" the gun).

After the core is cut, the key to successful recovery is preparation. This means efficient and speedycompletion of the various sta!es of recovery, boxin!, samplin!, !eolo!ical evaluation, and shippin!.-f two !eolo!ists are present at the wellsite, e.!., an oil company !eolo!ist and a mud lo!!in!!eolo!ist, duties should be allocated between them so that they can work sequentially on eachaspect of the ob without !ettin! in each otherFs way ( 0i!ure 5 , Seuential core processing by twogeologists working together). -t is critical that the whole process be carried out in a disciplinedmanner.

Dicky Haris Hidayat Library: Coring and Core Analysis 2(

0i!ure 4.
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0i!ure 4. 0i!ure 5.

B.!. "a#or $%&e' o Core Anal%'i'

(lug 5"on&en#ional6 Analysis

"onsolida#ed Forma#ions

This technique is normally restricted to homo!eneous formations that can be characteri&ed withplu!$si&e samples. Typical plu! si&e is 4 inch (5.9 m) in diameter, and 4 inch (5.9 cm) lon!.'ylindrical samples (0i!ure 4, Core sa"ple or conventional analysis with properly and i"properly

selected hori+ontal and vertical plugs) are normally cut with a diamond core bit parallel to beddin!planes and trimmed to yield a plu! from the center of the core where minimal filtrate flushin! andinvasion of mud solids is to be expected. Bertical permeability samples are drilled at ri!ht an!les tobeddin! planes. Althou!h !enerally used for sandstones, this technique is also satisfactory for themore homo!enous, nonfractured, and nonvu!!y carbonates.

Unconsolida#ed Forma#ions

"nconsolidated sand recovered within arubber sleeve core barrel, a plastic inner

barrel liner, or a fiber!lass barrel is oftenstabili&ed by free&in! prior to samplin!.0ro&en interstitial water present at the !raincontacts immobili&es the rock particles./lu!s are drilled usin! liquid nitro!en as thebit lubricant.

-n other cases a rubber sleeve core is firstimmobili&ed by surroundin! it with wax,plaster of /aris, foam or other suitable

Dicky Haris Hidayat Library: Coring and Core Analysis 2)

0i!ure 4.


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materials, followin! which the sample may be fro&en and drilled. #hen a core is co"pletelyunconsolidated, plu! samples can be removed by insertion of a hollow punch into nonfro&en core.0riable cores, however, should not be punched, as porosity and permeability will be created in thecore. -nstead, such plu! samples should be confined in a metal, plastic, or rubber sleeve, and besubected to simulated overburden pressure durin! analysis. 0ailure to treat unconsolidated cores inthis fashion will yield much hi!her porosity and permeability values than those actually present in

the reservoir.

The plastic inner liner has been a successful solution to recovery of unconsolidated 'anadian tarsands. These formations are subsequently mined, and it is essential that tar content be accuratelydefined. A modified evaluation technique is used that does not rely on plu!s cut at selectedintervals, but uses a small representative portion of the full diameter core. This process requiresthat the plastic sleeve be cut into 4 ft (7= cm) len!ths, which are then cut in half vertically. Threeadditional cuts down the full len!th of the sample are made on one of these core halves. Thisresults in three continuous wed!e sections of rock approximately 4 ft (7= cm) lon! and 4 squareinch (M.C9 sq cm) in area. The center portion of the half is used for determination of tar saturationand can be related to a !iven volume or wei!ht of reservoir rock. /lu!s are taken from the one$halffull diameter slice resultin! from the ori!inal cut. These are confined in ackets, and are thenanaly&ed for porosity and permeability, usin! standard techniques.

Full Diameter Analsis

Rou#ine Analysis

0ull diameter analysis was introduced to allow testin! of rocks with complex litholo!y, such ashetero!eneous carbonates (0i!ure 4 , Heterogeneous carbonate reuiring ull dia"eter analysis)and fissured, vu!ular formations unsuitable for plu! analysis. Analysis of these rocks requiressamples that are as lar!e as can be obtained, so that pore spaces are small compared to the bulkvolume of the samples. itholo!y and pore space in carbonates may be hi!hly variable, and theporosity can exist as micro$porosity, inter!ranular, vu!!y, fracture, or a combination of all four. The

full diameter technique does not differentiate between the contribution made by each of the varioustypes of porosity, but yields a sin!le porosity value that includes all pore type combinations.

amples in the form of a ri!ht cylinder up to 4= inches (59 cm) lon! and approximately 9 inches(45.9 cm) in diameter are often used. 2ata !enerated include :oyleFs law porosities, utili&in! heliumas the saturatin! medium. Two hori&ontal permeability values are determined. #hen fractures orvu!s are present, one of the permeability measurements is visually oriented throu!h the more

permeable section, and the secondpermeability is at ri!ht an!les to thismeasurement. -n this manner, the effectof vu!s or fractures on hori&ontalpermeability is indicated. Berticalpermeabilities are also frequently

determined.

!ro"ert !lug data #hole core data

&ir permeabilit$! md. .1 ),

/orosit$! 0 1.3 11.3

Residual oil!0 pore space 14. 1.1


0i!ure 4.


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otal ater! 0 pore space 24.) 3.

Ta$le -! Co"parison o plug and whole-core data on "icro ractured oil-productive sandstonesa"ples

A method for differentiatin! between matrix properties and full diameter data affected by fracture orvu! porosity is to drill and test plu!s selected from the more uniform matrix. A comparison of suchdata usin! this type of test is shown in Table 4. The difference is si!nificant. 3atrix properties areimportant because they control initial water content and, hence, matrix hydrocarbon saturation.

(ressure "ore Analysis

The analysis of full diameter pressure cores follows, in a modified form, the procedures normallyemployed in more routine analysis. 0ull diameter samples are cut in the form of a ri!ht cylinder andthen placed in speciali&ed, airti!ht containers where they thaw, so that fluids expulsed from the corecan be collected and measured. The cores are subsequently moved throu!h a 2ean$tark device(ection M.8.5) for measurement of water saturation in each sample. /ressure core samples should

be further cleaned in the toluene$'5 pressure fluxer after removal from the 2ean$tark device.This requires that the samples be placed in a sur!ical stockin! so that any rock fra!ments thatcome loose from the core durin! cleanin! are retained. This is necessary because the residual oilsaturation value that is obtained from the analysis is at least partially dependent upon wei!hts takendurin! the analytical process.

The airti!ht vessel in which each fro&en core is placed is evacuated for a short time interval toremove air surroundin! the core. As the rock thaws, the !as that evolves from the residual oilsaturation escapes from the core and is retained in the vessel surroundin! the core. The volume ofthis !as is measured and its composition determined by chromato!raph. The latter is helpful ifexotic !ases have been inected into the formation and you must know what portions of thereservoir have been swept by this inected !as. The surface volume equivalent of the residual oilsaturation present in the core at reservoir conditions is determined by summin! the oil that is

expulsed durin! the thawin! process with the oil that is subsequently removed durin! the 2ean$tark and toluene$'5 cleanin!.

To summari&e the handlin! process for pressure cores*

N The metal barrel is milled down its len!th and the core is removed.

The drillin! mud is chipped from the core surface.

The core is cut into full diameter ri!ht cylinders.

The core is wei!hed and then thawed in evacuated !lass chambers.

The oil, water, and !as expulsed are collected and the !as volume is determined. The

composition of the collected !as can also be measured.

The core samples are cleaned in a 2ean$tark apparatus. This furnishes water saturation

data and partial data for the determination of residual oil.

The core is cleaned in a toluene$'5 extractor.

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A :oyleFs law porosity value is determined, as well as hori&ontal and vertical permeability.

The water and residual oil saturations are calculated and a correction for oil shrinka!e isapplied.

At selected intervals, plu!s from sections of the fro&en rock not used in full$diameter

analysis are drilled vertically down the center line of the core. The water that is present inthe centermost plu! and in the surroundin! dou!hnut is analy&ed for the presence oftracers previously added to the filtrate. This yields insi!ht into core flushin!.

S"onge Core Analsis

0ull diameter analysis of samples recovered within the spon!e barrel proceeds alon! the usual linesonce the core has been removed from the barrel. The spon!e itself is cut from the core barrel andthe fluids it contains are extracted usin! a vacuum retort technique. :oth oil and water volumeswithin the spon!e are measured. Ta$le 8!, below shows residual oil saturation data for the corealone and for the core plus spon!e for a specific field example. ;ote that the contribution of thespon!e is variable and may be si!nificant.

De"thFeet Core residual oil $

pore space

S"onge residual oil $

pore space

%otal &s"onge "luscore' residual oil$

pore space

4)3) 23.1 .+ 23.,

4)3 23.1 .+ 23.,

4)3+ 21. +.3 3.

4)3, 2.4 1. 3.4

4)4 2+. . 3(.

4)41 22.1 ).2 2+.3

Ta$le 8!Core, sponge, and core-plus-sponge residual oil saturation data

Side.all "ores

idewall core analysis is made on all non$shale samples sent to the laboratory. The samplin!,therefore, is at the option of the operator selectin! depth points at which to recover a core. Resultsof these analyses are often used to define the !as, oil, and water &ones+ hence, samples should bespaced at re!ular intervals throu!hout the vertical section to be evaluated. -t is important that theanalyst receive these samples in correct vertical depth sequence, as this assists in theinterpretation of the probable production. -n areas where sidewall core$conventional core

correlations are not available, it is important to take a conventional core in a reservoir and then tofollow this with samples of sidewall cores. 0rom this, a sidewall$conventional core data relationshipcan be developed for use in subsequent wells.

Side.all "ore Analysis

idewall samples are used extensively in softer sand areas. (;ote, however, that a sidewall$drilledplu! from a new sidewall corin! device can be used for harder formations and can be analy&ed inthe same manner as a standard plu!$si&ed core.) /ercussion sidewalls are often smaller and

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demand additional attention. 1quipment for them is miniaturi&ed to reduce dead volume in thetestin! apparatus, althou!h techniques used for analysis are similar to those utili&ed in conventionalplu! analysis. The sidewall cores are normally coated with drillin! mud, which is removed prior toanalysis. -n areas of hi!h A/-$!ravity (li!ht) oils, sidewall cores are often smaller than 4 inch (5.9cm) in diameter. -n areas of low A/-$!ravity (heavy) oils, samples are often lar!er.

er"eabilities measured on percussion samples rarely yield true in situ values. 0or hard, low$permeability formations, permeability values are too hi!h due to impact fracturin!, whileunconsolidated sands are usually compacted and yield erroneously low values. This conclusion wasdocumented by Reudelhuber and 0uren (4?9C), as well as Loepf and %ranberry (4?M=).

2ata show thatporosities measured on sidewall samples approach conventional analysis values informations havin! true porosities ran!in! from 75D to 78D. -n hard formations, sidewall porosityvalues are normally hi!her than conventional plu! values, as shown by #ebster and 2awson!rove(4?9?).

/orosity in hard, well$cemented rock is increased by !rain shatterin! durin! bullet impact, and thesealterations in properties limit sidewall sample usefulness in reservoir en!ineerin! evaluations.6owever, sidewall cores are excellent indicators of litholo!y, furnish data on the presence orabsence of oil and !as, and are valuable for interpretation of probable production. They also furnishsamples suitable for petro!raphic work.

:ecause of the small sample si&e, techniques employed in some areas require that all the samplebe used for porosity and saturation determinations. -n this circumstance, a visual assessment of the!rain si&e, the shaliness of the sample, a measured porosity, and the natural density of the freshcore is used with correlation charts appropriate to the area to arrive at an empirical value ofpermeability. -n the hands of an experienced and competent analyst, such estimated values ofpermeability are suitable for formation evaluation.

An improvement in the visual assessment of !rain si&e and sortin! was recently developed and isnow used in selected laboratories. The instrument is referred to as a particle si+e analy+er. Thisprocedure utili&es tokesF law and rapidly furnishes the distribution of !rain si&es for each sidewallcore sample, usin! a tokesF law device. A small portion of the sample is disa!!re!ated andallowed to settle in a water bath. 3aterial settlin! to the bottom of the tube is retained on a balancepan and the increasin! wei!ht is transmitted electronically to a computer. The settlin! time within atube of known hei!ht is related to the !rain diameter. -nterpretation is made by a computer, whichyields both tabular and !raphical histo!ram reports. The !rain si&e distribution and the median !raindiameter are then used to assess the quality of the rock and, with correlations, to furnish estimatedvalues of permeability.

idewall samples from heavy oil formations are sometimes encapsulated in metal or plastic acketsprior to analysis. This maintains the inte!rity of the core as the heavy oil is extracted durin!analysis. A common method of analy&in! encapsulated samples utili&es a 2ean$tark cleanin!process, followed by a :oyleFs law porosity test.

B.. Core Sam&le re&aration

"leaning

The measurement of permeability

coring & core analysis

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