117132024 Coring and Core Analysis

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CORING AND CORE ANALYSIS

CORING:

Definition: A core sample is a cylindrical section of (usually) a naturally occurringsubstance. Most core samples are obtained by drilling with special drills into thesubstance, for example sediment or rock, with a hollow steel tube called a core drill.

The hole made for the core sample is called the "core hole". A ariety of coresamplers exist to sample different media under different conditions. More continueto be inented on a regular basis. !n the coring process, the sample is pushed moreor less intact into the tube. emoed from the tube in the laboratory, it is inspectedand analy#ed by different techni$ues and e$uipment depending on the type of datadesired.

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Introduction:

&ince most coring operations are conducted for geological information, the oil

company's geology department will put forth their expectations on the type of data

they want from the core. As a result, coring operations are undertaken for a ariety

of reasons. ue to the larger si#e of the sample, recoered cores allow more

detailed assessment of rock properties. rimarily a core allows $uantitatiemeasurements of the following*

+ orosity The olume of oids within a unit olume of rock.

+ ermeability The $uality of the connections between the oids.

+ &aturation The composition of the fluids filling the oids.

-f a secondary importance is the additional information relating to formation

boundaries, large scale sedimentary structures, undisturbed paleontological data,

and the opportunity for uncontaminated geochemical sampling.

Most of the basics behind coring operations is presented in the Advanced Logging

Procedures Workbook , howeer, it is best to reiew the two main reasons why

coring takes place at the ell site

%. Stratigraphically the oil company will core a formation (generally

accomplished on deelopment wells)

/. Hydrocaron Sho!" the oil company will core any formation based upon

unexpected hydrocarbon shows (generally done on wildcat wells).

#Y$ES O% CORING:

Two types of coring operations are used*

%. 0onentional (at the time of drilling)

/. &idewall (while wireline logging)

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CON&EN#IONAL CORING:0ores can be cut as lithological confirmation (primarily at T or in reseroirs), eitheras company policy or as a goernment re$uirement. This is especially true ifbasement is encountered ery high in relation to the well1s prognosis. The ell site2eologist may be inoled with decisions to include coring in the well program, andthey should be aware that the decision to cut a core is a costly one, inoling rigtime, extra contractors and laboratory analysis. !n order for coring to be successful,it must be recogni#ed from the beginning that the ob3ectie of the coring operationis not to make hole rapidly, but to successfully obtain the core. Therefore, successis measured by core recoery. To achiee this, the ell site 2eologist's duties willinole seeing that the core is cut at the correct depth, that it is retrieed properly,described, packed and dispatched in an expedient and safe manner. Make nomistake about it, coring operations are complex, and expertise is re$uired tosuccessfully cut and process a core.

#he follo!ing are reco''endation" !hich apply to con(entional coring:%. 4ery precaution should be taken to insure that the hole is 3unk free5 smallpieces of steel (bit teeth, tong dies, etc.) will $uickly ruin a core bit, whetherdiamond or conentional. unning a subtype 3unk basket with the last' two orthree bits will usually proide safe conditions./. 6ust as in normal drilling, sufficient dry collars should be run to furnish the bitweight. &tabili#ers hae been successfully applied in some cases to preent drillstring wobbling. The core barrel itself should be inspected for straightness. Acrooked inner barrel will cause eccentric action on bottom.7. 0ore heads should be run into the hole at a safe speed to aoid plugging or

damage from hitting a bridge or dogleg.8. uring coring the weight should be fed off smoothly and uniformly 9 not inbunches. This re$uires the full attention of a crew member at the brake, unlessan automatic feed control is aailable.:. 0oring should begin at light bit weight and low rotary speed5 these may beincreased as soon as cutting action is established. ;ormally the applied bitweights and table speeds should be held within the limits, unless specificexperience in the area dictates otherwise. 0irculating olumes for conentional

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core bits approach those of regular bits of the same si#e. iamond bits re$uireless fluid olume, and may actually be pumped and bounced off bottom byexcessie circulating rates. Also, seere erosion of the water courses and bitmatrix may occur. includes recommended circulating rates for diamond coring.<. ump pressure should be closely watched during diamond coring as anindication of whether drilling fluid is passing oer the face of the bit. ith the biton bottom, pressure should be higher than when the bit is offbottom. This isessential to bit cleaning and performance. A sudden pump pressure increase notalleiated by raising the bit off bottom, may mean that the core barrel is pluggedby trash in the mud5 if this happens, it should be pulled for inspection.

SIDE)ALL CORING:&idewall coring is a supplementary coring method used in #ones where corerecoery by conentional methods was less than expected or where cores were notobtained as drilling progressed. &idewall coring is useful in paleontological work, forit is possible to get shale samples for micropaleo analysis at definite depths. Thesidewall coring deice is lowered into the hole on a =wire line cable> and a sample

of the formation is taken at the desired depth. This is done by shooting a hollow=bullet> into the borehole wall, then pulling it out of the wall and up to the surface. There are as many as thirty bullets per gun, and since two guns can be used, up tosixty cores can be obtained during one run. !f electric logs hae been runpreiously, a spontaneous potential (&) or gammaray (2) cure is used todetermine gun position by direct log correlation.

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CORE $OIN# SELEC#ION:

+ 0oring points are usually selected through correlation with known marker

hori#ons, if a database exists. This practice is more common in deelopment or

delineation wells. icking the core point is thus a matter of stratigraphiccorrelation. ?rom seismic data and correlation wire line and mud logs, the

approximate top of the reseroir will be known, and at the ell site, correlation

with offset logs is used to pick a point as close as possible to the top of the

selected formation.

+ -ne drilling parameter that can be used at the ell site is the drilling

exponent. &ince exponents hae lithology and porosity dependent

characteristics, when plotted on suitable scales, they can illustrate minor, but

distinctie ariations (related to lithology), which can be correlated with other

logs from offset wells.

+ !f drilling exponents are not being calculated, correlation between the drill

rate and offset logs is possible. This re$uires the geologist to monitor the well

ery closely, possibly re$uesting a slower - when approaching the potential

core point.

+ 4en when drilling exponents and drill rate are used for correlation,

confirmation of the core point generally re$uires gas or lithology data. This

means fre$uent circulation of bottomsup.

+ hen the criterion for coring is met, then the decision to core can be made. !f

not, drilling is resumed until the correct depth is reached.

+ !n any eent, it is best to clarify what constitutes a good show in terms ofpercentages of fluorescence, types of fluorescence, gas shows in terms of

percentages, before the possibility of coring occurs. The decision can then be

left to the discretion of the ell site 2eologist.

+ hen the core point is reached, the usual routine is to stop drilling, flow

check, circulate bottoms up and ealuate all the data aailable (cuttings for

lithology, porosity, oil shows, gas shows, drilling exponents, -, tor$ue, etc.)

prior to making the decision to trip out of hole.

+ The actual coring depth is always confirmed by the ell site 2eologist.

&eeral criteria that will assist in selecting the coring depth are*

%. eiew of the prognosis concerning the formation to be cored and

comparison with data from the present well.

/. eiew of the arious correlation plots and logs.

7. 0onfirmation of the core point from circulation data, hydrocarbons and

lithology from drill cuttings.

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+ The correct depth must be confirmed. Any discrepancies must be resoled

before the trip out begins.

+ uring coring, a careful watch must be maintained on the pit leel, all coring

parameters and gas alues.

CORING $ROCED*RES:rior to the coring process, there should be a ell site meeting with all thoseinoled in the coring operations. The drilling superisor must ensure that*+ All drillingrelated items in the prognosis are satisfactory,+ !ncluding the rig e$uipment, gauges and indicators+ The correct drilling fluid properties hae been obtained+ The borehole is cleaned+ The core barrel has been assembled correctlyArriing at the optimum coring parameters is based on many factors*

%. The type of core bit being used,/. The coring parameters,7. ositioning the core catcher for the best recoery,8. The length of the core to be cut, and:.@A design. All hae to be taken into consideration.

+ hen the core barrel is nearing the bottom of the hole, if contact is made withcaings, it will be necessary to rotate and circulate (wash) ten feet at a time untilthe hole is clean.

+ @e sure that all measurements are correct to determine the bottom has been

reached.+ Abnormally high pump pressures can indicate that there is debris in the core

barrel or core catcher, which must be pumped clear before coring can commence.+ -nce bottom is reached, pick up off bottom one to two feet and circulate with

sufficient annular elocity to condition the mud and keep the hole clean.+ This is done with the ball out, for %: minutes to one hour, or longer if necessary.+ After the bottom is cleaned, the drill string should be raised seeral feet off

bottom while circulating to ensure the inner barrel is clean.+ The kelly is then raised to the first 3oint of drillpipe.+ 0irculation is stopped, the kelly is remoed, and the ball is dropped into the drill

pipe.+ -nce the ball has been dropped, the kelly is replaced and circulation started to

pump the ball down at a good rate.+ hile the ball is falling, record the pump rate and standpipe pressure.+ As the ball nears the setting position, the pump rate is reduced to allow the ball to

seat properly.+ As soon as the ball is seated, the drill string is returned to bottom.

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+ -nce the ball is seated in the check ale, the circulating fluid is dierted throughthe circulating ports between the inner and outer barrels and out the dischargeports in the bit face, in the conentional manner.

+ !t is important to know the type of bit that has drilled the preious section of hole.+ The first few inches of actual coring are the most important because*

%. it will determine the optimum coring parameters re$uired to cut and recoerthe core

/. it will signify if problems are going to occur during the coring process7. it will erify that all the planning and precautions taken hae proen

worthwhile.

+ hile the core is being cut, careful monitoring of the coring ariables is importantto detect problems which may cause a halt to the coring operations.

+ 0oring parameters (weightonbit, rotary speed, flow rate, and standpipepressure) must be closely monitored and held as constant as possible. !t isadisable to perform tests on the -@, M and 2M until optimum coring ratesand conditions are found.

+ -nce found, these parameters should be held constant until there is a definitechange in the coring rate or the entire core is cut.

CORE RE#RIE&AL:+ The type of core being cut will hae a direct bearing on the handling procedures

once the core is on the surface.+ &ecurity precautions should also be taken into consideration.+ Most conentional cores are handled on the rig floor. The inner core barrel is

suspended in the derrick and raised periodically to allow the core to slide onto therig floor to be collected. &ome wire line retrieal core are caught this way, andbroken into lengths for easy handling.

+ !f there are problems with the core sticking in the core barrel, or if speciali#edcoring has been done (sponge coring, pressure coring, rubber sleee, specialliners), the core barrel will hae to be laid down on the cat walk. ire line coresare often laid down this way to facilitate remoal. To preent small diameter coresfrom breaking, they are placed into a rigid container before being laid down.

+ -nce on the catwalk, lifting subs are remoed and a rubber plug or core pushermay be inserted in the top of the barrel. A pumpout connector may be made upon the inner core barrel and high pressure air or water is used to pumpout thecore.

+ !t is the ell site 2eologist1s responsibility to retriee the core correctly and allowthese operations to continue as soon as possible without 3eopardi#ing the $uality

of the core. A methodical approach is re$uired5 do not rush the remoal or initialinspection of the core.+ !n offshore locations, space constraints may make this difficult, but the ell site

2eologist should strie for as large an area as possible.+ A supply of core boxes, marked Top and @ottom and numbered should be present

near the drill floor prior to the core reaching surface.+ &ome sample bags, a hammer and a note book should also be present on the rig

floor.

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+ hen the core reaches the surface the ell site 2eologist should superise thecatching of the core.

+ At no ti'e "hould you place a hand under the core arrel+ !f the coreleaes the barrel in a continuous piece it will hae to be broken using a hammer inlengths of less than 7 ft, so it will fit into the core boxes etrieal rate is goernedby the rate at which you can catch and box the core.

+ The core must be retrieed so that no confusion occurs as to the orientation offragments.

+ hile recoering the core, the ell site 2eologist should be making anassessment of the lithology.

+ &mall samples may be taken from the base of the core for examination under theCD lamp and microscope.

+ !f the decision to resume drilling or coring is in doubt, or the -perations 2eologisthas re$uested notification, the following information should be relayed*

%. epth interal of core/. ecoery of core7. Eithology of core

8. ydrocarbon shows obsered+ The core is then cleaned with dry rags, unless specifically instructed to use damprags (to remoe the filter cake).

+ The core is then realigned, closing any gaps or fractures so that an accuratelength measurement can be made.

$AC,ING #HE CORE:+ -nce the core has been described and wrapped, it is placed back into it's

respectie boxes and the boxes filled with either paper or rags to ensure that thecore does not shift within the box during shipment.

The outside of the core boxes should be marked with the following information*

%. ell ;ame/. Top and bottom number (i.e. %T, %@, /T, /@ etc.)7. The address of the recipient.8. The core number (i.e. core F%, core F/):. The box number and total number of boxes (i.e. @ox % of B, @ox / of B)

+ !f a fiberglass or metal liner is used, the recoery process and packing proceduresare somewhat different.

+ The liner is laid down on the catwalk and cut into 7 feet (% m) lengths, aftermeasurement and marking. 0ore chips are remoed from the cut sections for=$uick look> inspection.

+ -nce the chips are taken, caps are fitted to the ends of the liner and taped on.

+ The liner lengths are then packed in core boxes or placed into core crates.

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Laeling and $ac-ing a Core

CORING ANALYSIS

CON&EN#IONAL CORE ANALYSIS:• -f all commonly aailable coring methods, this is the most important source of

information source of information in that it furnishes measured alues of basic

rock properties.• orosity, permeability, residual fluids, lithology and texture are some of the

parameters that characteri#e a core ertically, and representatie samples arecommonly taken eery foot (and more fre$uently when core examinationindicates the need).

• 2rain si#e, an indication of sorting, coloring of the rock, presence of laminationsand other important structures are described in the following table*

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• ?ractures, ugs and color, intensity and distribution of oil fluorescence are also

reported.

• 0ore photography offers a permanent and ob3ectie record of both the core1s

appearance and fluorescence.

• This is of particular alue needs that may occur years after the core is cut (i.e.,

net pay determination) or when well participants are located at prohibited trael

distances from the point of analysis.

• 0olor ideo with audio lithologic description is another new and innoatie means

of recording and presenting core data.

SIDE )ALL CORE ANALYSIS:

0ore Eab offers the innoatie ealuations needed to ealuate the most

difficult reseroirs 0ore Eab was the first company to introduce laser optics particlesi#e analysis as a commercial, routine serice. ith a range of measurement which

coers sand, silt and clay si#e particles, the applications of the data hae grown to

include enhanced sidewall permeability determination, grael pack design and

capillary pressure simulation.

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$ERC*SSION SIDE)ALL CORES:

•

ata proided by the analysis of percussion cores has been in use in the industryfor more than half a century.

• The analysts at 0ore Eaboratories are trained in methods of analysis, which will

proide to the client a reliable, fast and accurate data set that can aid in

completion decisions. ata normally proided include porosity (summation of

fluids method), permeability (enhanced by Easer article &i#e Analysis), fluid

saturations, description of probable production, sample $uality index and

lithological description.

• igital photography, both white and ultraiolet, and Easer article &i#e Analysis

are often added for a more complete ealuation.

• All data presentations are aailable in a digital format.

RO#ARY SIDE)ALL CORES:

• otary tools hae proided samples, which are suitable (sub3ect to good recoery)

for all tests, which can be proided on plug samples, drilled from a conentional

core.

• ?luid saturations may be determined by either the ean&tark method or the

summation of fluids techni$ue.

• orosity and permeability may be measured in a core holder, at stressed

conditions, as with any conentional plug sample.

• The fact that one point may hae to be chosen to represent many feet of reseroir

rock makes this method of sampling less desirable than a full section of

conentional core, but reliable data may certainly be ac$uired for the sample

recoered.

• otary cores are most often $uite suitable for more adanced testing when

re$uired.

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ENGINEERING DA#A O.#AINED .Y CORE ANALYSIS: The following table shows the engineering data obtained by core analysis*

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S$ECIAL CORE ANALYSIS:• This field consists of seeral more complex and timeconsuming measurements

that extend and supplement the more commonly aailable information.%8



• A number of these tests are utili#ed in the reseroir engineering applications, asthey furnish information to $uantify oilinplace (capillary pressures), fluid flowcharacteristics (relatie permeability) and recoery anticipated with ariousimproed recoery schemes (water flood and enhanced oil recoery).

• Eogrelated parameters such as formation factorporosity and acousticalproperties are also measures, and these reliee the need to use published,

aerage alues that may be inappropriate.• rilling, completion, work oer and in3ection fluid reactions with the reseroir rock

can be ealuated with special core analysis so that suitable fluids can be selectedthat will not damage the formation or reduce productiity.

• These special tests are generally made on fewer samples than used for routinemeasurements, yet the ariation noted in the routine data forms the basis ofsubse$uent special core analysis sample section.

$E#ROLOGY:• The study of rock composition, characteristics and origin of sediments, adds depth

to information generated by both conventional and special core analysis.

• !ts use has expanded as instrumentation has improed and been reduced in cost,and as operator awareness of the cost benefits from utili#ing the information hasincreased.

• etailed core descriptions are aided by microscopic examination that includes thinsection analysis, Jray diffraction analysis (J) and scanning electronmicroscope (&4M).

• These data yield information on depositional enironment, diagenesis, reseroirpotential, porosity type and control, potential completion damage productionproblems and authigenic (formedinplace) minerals.

IN%OR/A#ION %RO/ CORE S#*DIES AND SA/$LES:

• !nformation gained from core study and physical measurements on samples isdiided into three ma3or categories designed to satisfy arious needs andob3ecties*

%. !ncluding core data for geological parameters,/. 0ompletion data, and7. eseroir engineering data

• -erlapping use of these areas of data defy sharp demarcation lines, as some areappropriate for all three categories.

• !n many instances, the depth and detailed knowledge re$uired to use a particulargeological property increases as use of that property is focused on a particularproblem.

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CONCL*SION:• 0ore data furnish much information aailable from other sources.

• hysical and chemical analysis of the rock and its contained fluids suppliesaluable geological and engineering input and enhances understanding of currentwell or reseroir response and potential difficulties to be aoided.

• irectly measured data that $uantify the presence and character of the formation

of interest reduce uncertainty, and allow interpretation, deduction and predictionof well and reseroir performance as well as basin characteristics.

• hile certain core data stand alone, some support, strengthen and improeunderstanding of other formation ealuation tools.

• 0oring should be considered early in a pro3ect to furnish information forsubse$uent wells, and to coer aerial and ertical diersity of the rock.

• 0oring tools, coring fluids and appropriate analyses should be selected to furnishdata that best meet the operator1s current and future needs for geological,completion and engineering purposes.

RE%ERENCES:

%. )ell Site Geology0Reference Guide 12134 Re(5 . April 6778: .a-erHughe"5

/. Coring y Dare ,eelan9 &ice $re"ident of Core Anlay"i"9 Core

Laoratorie" Inc59 Dalla"5

7. http*KKwww.corelab.comKrdKpetroleumsericesKoutineKsidewall.aspx

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http://www.corelab.com/rd/petroleumservices/Routine/sidewall.aspx

http://www.corelab.com/rd/petroleumservices/Routine/sidewall.aspx

117132024 Coring and Core Analysis

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