Coring.doc

#/2).'�'5)$%,).%3

&/2

5.#/.3/,)$!4%$�3%$)-%.43

*!. -/24%.�(/,-"/%��$#33

2/26

Coring.doc

#/.4%.4�,)34

1. INTRODUCTION

2. PLANNING

3. CORING EQUIPMENT

3.1 Core bits

3.2 Inner barrels

3.2.1 Fiberglass

3.2.2 Aluminium

3.2.3 PVC

3.3 Core jamming indicators

3.4 Coring systems

3.4.1 Conventional systems

3.4.2 Full closure systems

3.4.3 Gel coring system

3.4.4 Glider coring system

4. CORING OPERATIONS

4.1 Pre-coring well site activities

4.1.1 Communication

4.1.2 Selection and set-up of core processing area

4.1.3 Drilling fluid

4.1.3.1 Selection of drilling/coring fluid

4.1.3.2 Tracing of the mud

4.2 Pre-coring drilling precautions

4.3 Barrel length and diameter

4.4 Coring

4.4.1 Trip in

4.4.2 Circulation

4.4.3 Coring conditions / parameters

4.4.4 Coring termination

4.4.5 Core retrieval

5. CORE HANDLING & PROCESSING

5.1 Inner barrel separation

5.2 Core lay-down

5.3 Core processing

5.3.1 Core logging

5.3.2 Extraction of the PVC sleeve

5.3.3 Cutting of the core

5.3.4 Geological sampling

3/26

Coring.doc

5.3.5 Plastic caps & clips

6. CORE STABILISATION & PRESERVATION

6.1 Resination

6.2 Injection of foam (mousse)

6.3 Injection of gypsum

7. PACKING & TRANSPORTATION

8. PERSONNEL

4/26

Coring.doc

,)34%�$%�&)'52%3

&IGURE�� ,OW�INVASION�CORE�BIT� �"()

&IGURE�� 3TANDARD�COREHEAD��$"3

&IGURE�� #ONVENTIONAL�FACE�DISCHARGE�COREHEAD��$"3

&IGURE�� #$���COREHEAD��LOW�INVASION��$"3

&IGURE�� &LUTED�ALUMINIUM�INNER�TUBE��$"3

&IGURE�� 3PRING�CATCHER�ASSEMBLY��$"3

&IGURE�� 0/3)#,/3%��$"3

&IGURE�� (9$2/,)&4��"()

&IGURE�� 3HEAR�BOOT�PLATE��"()

&IGURE��� #ORE�LAY DOWN�CRADLE��"()

&IGURE��� #ORE�LAY DOWN�CRADLES��"()

&IGURE��� 3AW�MODULE�ON�CORE�CRADLE��"()

&IGURE��� /BSERVATION�OF�THE�ANNULUS

&IGURE��� 4RANSFERRING�PUNCH�PLUGS�TO�RUBBER�TRANSPORT�ANALYSIS�SLEEVES

&IGURE��� #ORE�CONSOLIDATION�BY�FOAM�INJECTION�ON�ANGLED�RACKS

&IGURE��� 0REPARATION�FOR�TRANSPORT� �PLASTIC�BOXES�AND�FOAM�PROTECTION

5/26

Coring.doc

#/2).'�).�5.#/.3/,)$!4%$�3%$)-%.43��'5)$%,).%3

�������).42/$5#4)/.

The purpose of this handbook is to present a set of guidelines on how to approach coring inunconsolidated sediments. Coring is an expensive operation, mainly in terms of rig-time, butnevertheless it is still a type of data that is absolutely necessary, as a core represents the only really« hard fact » from the reservoir.

It is not physically possible to obtain a core which represents 100 % the same rock in the subsurface,simply because the environment has changed completely, in terms of stresses, pressure, temperatureetc. However, the core data can vary from being very representative to being almost useless, and thisdepends to a large extent on the core processing on the well site .

All cores must be handled with caution, but this is particularly important when coring unconsolidatedreservoirs. It is known that significant structural damage can occur to unconsolidated sedimentsduring coring, trip-out, surface handling and transportation, and that this will lead to unrepresentativecore data.

Standardised procedures are therefore required in order to provide for high quality, intact core fromwhich accurate and reproducible geological, petrophysical and reservoir engineering measurementscan be made.

The aim of this handbook is to describe the equipment and procedures which are considered bestsuited in order to obtain the highest possible quality core sample. Laboratory techniques andprocedures are not within the scope of this book. The coring parameters, corebit specifications, etc.are discussed, but not greatly elaborated, as more engineering directed publications on this subjectalready exist.

This handbook is not intended to replace the necessary dialogue and planning between driller,geologist and reservoir engineer, but act as an aid. Both the availability of some of the equipmentdescribed here, and various operational circumstances will in some instances necessitatecompromises, but hopefully this handbook can act as a guide to establish the best adapted coringprogram in each specific case.

�������0,!..).'

Good planning of each specific coring job is very important in order to obtain the best possible result.

Ideally there ought to be a « coring team » which consists of a representative from all the parties thatare involved in the coring job. That means :

• Well Geology

• Reservoir Department

• Drilling Department

• Geologist from the district/licence

• Core analysis Department/contractor

• Coring contractor

• Other departments/service companies (mud, tracer, etc.)

6/26

Coring.doc

The team should have a « core project leader », who should co-ordinate the work and also act as acontact person. This will frequently be the operations geologist.

This team must clearly define the following :

• The various objectives for the coring job

• Which equipment, service company and procedures that are best suited to give a core that willmeet all the objectives

• A sampling / core processing program that satisfies all the various users. The geologist and thereservoir engineer may not necessarily have the same priorities, and such different interests arebest resolved before the beginning of the job. It is also essential to decide exactly in which orderthe various actions shall be carried out.

• A program that, as far as possible, contains alternative plans in case of unexpected events.

• The core project leader should then communicate the coring objectives and the selected technicalsolutions to the "coring team" at the well site.

�������#/2).'�%15)0-%.4

������#ORE�BITS

The kind of bit that should be used in unconsolidated sediments (or in any other type of sediments) isa� LOW� INVASION� CORE� BIT�WITH� FACE� DISCHARGE. Both Baker Hughes Inteq (BHI) and Security DBSproduce these bits. In a low invasion bit little or no mud flows along the core being cut (Figs. 1 - 4)

If shaly interbedding is expected, then it is an advantage that the bit is not only « low invasion », butalso an « anti-whirl » bit type. For the Hydrolift system (BHI), this kind of bit is not an « off the shelf »product and therefore has to be specially ordered.

!DVANTAGES��

1. The face discharge ports will limit invasion of drilling fluid into the core sample.

2. The traditional catcher is the transfer point of shocks and vibrations from the core barrel to thecore. The anti-whirl design will minimise these vibrations, and thereby decrease the risk of corecollapse and jamming.

3. Both the face discharge ports which gives an effective removal of cuttings, and the anti-whirldesign will maximise the rate of penetration.

All these effects will improve core physical integrity.

KNIFE SHOE : NOT�RECOMMENDED�� Will only have a destructive effect in unconsolidated sediments.

Low invasion bits generally have a flat cutting face. If the formation contains pebbles the bit may spinon the pebbles. In such cases a bit with a parabolic cutting face could be more efficient in cutting thepebbles or more probably pushing them into the borehole walls.

������)NNER�BARRELS

3.2.1 FIBERGLASS

7/26

Coring.doc

This is the best solution for coring unconsolidated sediments, as the fiberglass has the lowest frictioncoefficient among the materials used for inner barrels. It is also very easy to cut through, a fact thatsimplifies core processing.

The only limitation is when temperatures are too high, > 175 °C (unlikely in this case, mostunconsolidated sediments being at shallow depths)

6ENTED�FIBERGLASS�SLEEVES is an option that is very useful if there is a risk of trapped gas inside thesleeve, for instance if you have a gas bearing sand in between two shale layers that could have atendency to swell and thereby trap the gas. This is the safety aspect, but there is also a core integrityaspect : When pulling out of hole the expanding reservoir fluids / gas will normally travel along thecore towards the bit and the valve at the top of the inner barrel. If this passage is partly blocked, thegas and fluids can escape through the valves instead of forcing its way along the core, which cancause core damage.

The only disadvantage with this system is that the valves will be a weak point in the inner barrel. It isnot totally uncommon to see that the valves have been washed out, and it has also occurred in somerare instances that this washing out has continued until completely splitting the fiberglass sleeve.

However it should be noted that some of our deep water unconsolidated sand objectives are veryshallow and therefore the pressures concerned may be insufficient to action the venting effect.

3.2.2 ALUMINIUM

Aluminium has a higher friction coefficient than fiberglass and is therefore only a preferred option incase of high temperatures, > 175 °C.

The aluminium inner-barrel is of course more difficult to cut than the fiberglass, but with the propersaw blade the cutting does not represent a problem. As for fiberglass, it is not necessary to use wateras a cooling agent.

The aluminium sleeves can, as the fiberglass, be made with valves. The advantages are of course thesame, and as the steel valves are set in aluminium instead of fiberglass, they do not in this caserepresent a particularly weak point.

DBS has also a FLUTED�ALUMINIUM�SLEEVE�(Fig.5), which has two objectives :

1. As the valves, the flutes will normally make it easier for the gas and fluids to escape.

2. The surface contact area between the core and the inner barrel is reduced because of the flutes,and this will normally give a lower overall friction.

Both principles work fine in consolidated sediments, but the method is NOT� RECOMMENDED� INUNCONSOLIDATED� SEDIMENTS. The reason being that the unconsolidated sediments will have atendency to fill the flutes, and thereby increasing �instead of decreasing the friction. This phenomenawill naturally also be very unfortunate for the core integrity.

The fluted aluminium sleeve has an additional disadvantage which is similar for unconsolidated andconsolidated sediments : it makes the interpretation of the X ray scanner images quite difficult.

3.2.3 PVC

PVC as material in a conventional inner barrel is too weak to be a good alternative.

In the « Full closure system » - Hydrolift however, it is used together with an aluminium, fiberglass orsteel inner barrel. See Ch. 3.4.2

8/26

Coring.doc

������#ORE�JAMMING�INDICATORS

Being able to know whether you are coring or drilling / milling the formation because of jamming orcore collapse, is one of the most important aspects of the coring operation. A jamming / core collapseMAY�affect the drilling parameters in the following way :

1. TORQUE : will often decrease and become very stable, but it might also increase or be erratic (voirexemple Fig.6)

2. PUMP PRESSURE : will depend on the circumstances.

• It will in theory DECREASE if the core has entered into the inner barrel before the jamming occurs.The reason is that as the core no longer enters into the inner barrel, the bit will just wash away thecuttings on bottom plus the formation just below the bit, and this will give a drop in pump pressure.

• If the jamming / core collapse happens before the core has entered into the sleeve and a blockingbetween the inner- and outer barrel and/or blocking of the nozzles occurs, there should be anINCREASE in the pump pressure.

• The result may also simply be a very ERRATIC pump pressure, which in many cases will mean nochange at all.

3. RATE OF PENETRATION : In hard formations the ROP will be strongly reduced, but inunconsolidated sediments the drop in ROP will most likely be small, or maybe not noticeable at all.

CONCLUSION : It is often very difficult, or impossible to detect jamming / core collapse when coringunconsolidated sediments.

-ECHANICAL� CORE� JAMMING� DETECTORS exist or are under development, that should be able to tellwhen the core has jammed or collapsed inside the inner barrel.

The « Core Jam Indicator » of "() works in the following manner : The inner barrel is floating freely inthe outer barrel, being able to move independently of the outer barrel. When a jamming occurs theinner barrel will be lifted up, pushing up a pressure relief plug which will restrict (but not seal) the portsin the inner tube plug. This produces an increase in standpipe pressure.

!DVANTAGES��

• Reduces shocks and vibrations.

• Coring can be stopped before core is damaged.

• Avoids drilling / washing of reservoir rock.

• Fits standard core barrel without modification. No special procedures are required. Standard dropball.

• Pressure signal can be adjusted to suit mud weight, pump rate, and pump capability.

$ISADVANTAGES���

• Does not work with Hydrolift !

• Requires modification for oriented coring.

9/26

Coring.doc

• Requires modification for 6.25” barrel.

• This device will only work when the core has entered into the core barrel. That means if the corejams or collapses just inside the bit or the core catcher (which can often be the case), the increasein standpipe pressure will not take place.

• False jam indications may happen if the equipment is not set up correctly, and this is quitedifficult. « Some trial and error will have to be included in selection »,quoting BHI.

• If the formation is quite different than expected and/or the selection of equipment has beenincorrect, the result can also be that the core has collapsed but without triggering the Core JamIndicator to give a pressure increase.

$"3�has a core jamming indicator that is very similar to BHI’s, but they do not recommend its use,because of the above mentioned disadvantages of the system.

The idea of this mechanical / hydraulic core jamming indicator is good, but today there seem to be tomany uncertainties linked to the system.

To our knowledge Elf Aquitaine has no direct experience of the above core jamming indicators.

The only core jamming indicator principle that can give a clear answer to when a core jamming /collapse has taken place, is one that can tell when the core stops entering into the core barrel. DBS,and maybe BHI as well, are working on a prototype based on sonic principle, similar to a pit levelindicator. They have however not succeeded yet.

BHI have recently taken over a coring system which means a completely different approach to theproblem of jamming. « Jam buster » consists of a telescopic inner barrel, which allows three jammingincidents to occur before coring must be terminated. When a jamming occurs it will lock the core to theinnermost telescopic tube. This tube can then move with very little friction free within the nexttelescopic tube once a set of shear pins are sheared. The operator can then keep on coring and copewith one more jamming, being obliged to pull out of hole if a third jamming should occur. When thecore barrel is full or 3 jams have occurred, the jam detector valve will close and a pressure increasewill be noted on the surface. This coring system could be considered for example in a wellconsolidated, fractured reservoir where the risk of jamming is considered very high, but it is NOTrecommended in poorly consolidated or unconsolidated formations. The reason is that, instead of« lifting » the first inner barrel and continue coring, there is a great probability of washing/millingthrough the formation. The system is not combinable with HYDROLIFT.

#ONCLUSION�: As the existing core jamming indicators are quite uncertain and potentially misleading,and the ideal core jamming indicator is still not operational, we still have to rely on the drillingparameters.

Even though they are uncertain, they often DO� give an indication of when a core jamming/corecollapse has taken place, but because the signals are often inconclusive they are much too oftenignored. If good core recovery/quality is the aim, this practise must be changed. )F� A� REASONABLEDOUBT� EXISTS� TO� WHETHER� ONE� IS� CORING� OR� MILLING�WASHING� THE� FORMATION�� CORING� SHOULD� BETERMINATED���Better safe than sorry, and the coring operator and drilling supervisor must be clearlymade aware of these objectives.

������#ORING�3YSTEMS

3.4.1 CONVENTIONAL SYSTEMS

The conventional coring system with an inner barrel of fiberglass / aluminium and a standard springcore catcher (Fig.6)has three DISADVANTAGES�:

1. It represents an obstacle for the entering of the core into the inner barrel, and the core catcher mayscrape off some of the protecting mudcake, which could thereby lead to further invasion.

10/26

Coring.doc

2. It acts as the permanent transfer point of shocks and vibrations from the core barrel to the core.This may potentially lead to core jamming / collapse.

3. As the traditional core catcher does not provide a full closure at the bottom of the core, there is arisk, particularly in very unconsolidated sediments, of loosing parts of the core or the complete corewhen pulling out of hole .

The conventional coring system has nevertheless proved to be a very good system in poorlyconsolidated sediments, for instance in Norway, where the traditional core catcher does not appear tobe a weak point. The anti-whirl core bit and proper coring procedures are probably especiallyimportant when using a conventional system (see Ch. 3.1 & 4.4).

When encountering extremely poorly consolidated or totally unconsolidated sediments however, theabove mentioned disadvantages will be even more difficult to overcome, and a Full Closure Systemmay be the best solution.

&OR�BOTH�CONVENTIONAL�CORING�SYSTEMS�AND�THE�SPECIAL�SOLUTIONS�HEREUNDER�IT�HAS�BEEN�SHOWNSTATISTICALLY�THAT�IN�UNCONSOLIDATED�FORMATIONS�RECOVERY�RATES�INCREASE�WITH�CORE�DIAMETER��)T�ISRECOMMENDED� WHEREVER� POSSIBLE� TO� CORE� IN� ������� DIAMETER� WITH� THE� LARGEST� POSSIBLE� COREDIAMETER�FOR�THE�CORING�EQUIPMENT�USED�

3.4.2 FULL CLOSURE SYSTEMS

The Full Closure Systems offer unrestricted core entry to the inner core barrel and a full closurecatcher for maximum recovery. There are two different systems :

��� 0OSICLOSE���3ECURITY�$"3�(Fig.7)

The system consists of :

• POSICLOSE UPPER SECTION : It consists of a « Travel joint » which is the mechanism thatprovides the means to close the full closure catcher, and the « Travel release » which locks the slipjoint closed during coring. After the core run a valve at surface, available for Kelly or Top drive,releases the second ball while circulating. This activates the travel release which unlocks the traveljoint. By extending the travel joint, the inner tube is lifted in relation to the outer barrel. Thisexposes the two catchers and closes the clam shells across the bottom of the inner tube, cuttingthe core.

• POSICLOSE LOWER SECTION : An aluminium inner sleeve conceals the core catchers andprovides an unrestricted bore for the core into the inner tube. The first 9” of extension on the traveljoint raises the inner sleeve which exposes the core catchers. The next 3” of travel joint extensionraises the lower part of the shoe assembly, including the clam shells. As they move, the clamshells come into contact with a closure cone which mechanically forces them to shut. If the clamshells are unable to cut through the core, the conventional spring catcher will catch the core in theusual manner.

!DVANTAGES��

• Full closure.

• The catcher is masked, giving reduced core disturbance when entering the inner barrel(SLICK ENTRY)

• Surface indication of unlocking. The hydraulic unlocking produces an recognisable pressure signalon surface to identify the system unlocking prior to catching the core.

• Simplicity of the system.

11/26

Coring.doc

• Conversion of the conventional core system to POSICLOSE can be done at the well site.

• Can use any kind of inner barrel, and core bit.

• Capable of coring 27m with Top drive drilling systems.

• May be combined with the Glider Coring System.

$ISADVANTAGES��

• In order to expose the clam shells, the travel joint must successfully raise the inner barrel inrelation to the outer barrel. The operation depends on the outer barrel staying on bottom due togravity / friction, and this could possibly cause problems in deviated wells. According to DBS thesystem can be used in wells with inclinations up to approximately 60°.

• The lowermost 1.5m of the inner barrel is made of aluminium, and this material has a higher frictioncoefficient than fiberglass.

• Only available in 6 3/4” hole, core size 4”. Coring in 12 1/4” not possible !

• The length of the core barrel is limited to 9m for kelly drilling systems since, during coring, the corebarrel can not be picked off bottom because the core catchers are concealed.

• The old Posiclose system, with a 1 1/2” diameter second ball, had some unlocking problems. Thelatest version however uses a 2” diameter ball and does not appear to have the same problems.

�

����(YDROLIFT��"()�(Fig.8)

OPERATION

The Hydrolift is positioned at the top of the inner tube. After flushing out the core barrel, a ball isdropped, allowing free circulation through the inner tube before coring begins.

Both the clam shells and the traditional spring-type catcher are hidden during coring, permitting asmooth unrestricted entry of the core into the inner barrel. The Posiclose and Hydrolift systems aresimilar in this phase of the operation, but from this point the Hydrolift system is different and slightlymore complicated.

When coring is complete a 1 1/4” ball is dropped through a wireline access port in the swivel andpumped down hole. The ball seats in the Hydrolift assembly diverting the flow of drilling mud into asmaller chamber at the top of the system.

Pressure from the drilling mud provides force to lift the inner barrel several inches. This action pullsthe smooth, core-protecting sleeve out of the catcher assembly allowing a heavy spring and cam toclose in force the two full-closure shells and expose the spring-type catcher which provides a backup(as for Posiclose) if the clam shells are unable to close.

INNER BARRELS / CORE SIZE / CORE LENGTH

While the Posiclose system is only available in 8 1/2" hole, the Hydrolift system can be used in both 81/2" and 12 1/4" hole. The composition of the Hydrolift system in the two hole sizes are quite different.

���|���HOLE :

• In 12 ¼ ” hole there is an inner PVC sleeve in addition to the inner barrel which is made offiberglass, aluminium or steel.

12/26

Coring.doc

• The size of the core is 4 3/4”.

• It is physically possible to rig up 2 barrels to obtain 18m, but BHI do not guarantee properfunctioning of the closure system in this case. The maximum recommended length is therefore 9m.

• There is no spring catcher as back-up system.

��}���HOLE��

• In 8 ½ " hole there is only the inner barrel, made of aluminium of fiberglass.

• The size of the core is 4".

• Maximum length of core barrel is 27 metres.

• A conventional spring catcher exists as back-up system if the clam shells do not manage to cut thecore, or if they for one reason or another do not function.

Under certain circumstances aluminium / fiberglass will have to be replaced by steel : because of thetool design, it is not possible to space out the inner barrel, and as a consequence there is only ¼ "margin for expansion when using a 9m barrel. If the expansion is larger, the Hydrolift closure systemcan be triggered unintentionally. That means there is a temperature limit for the use of thiscombination, which for a 9m barrel is a bottom-hole temperature of 110°C. With higher bottom holetemperatures one MUST use the combination PVC and steel !

The extraction of the PVC sleeve from the inner barrel is quite time consuming and must be performedvery carefully, but by using the right procedures it works OK. See Ch. 5.3, « Core processing ».

!DVANTAGES��

• Full closure.

• The catcher is masked.

• Field proven for a long time, since 1985.

• Two available sizes :

• 8” x 4 3/4”

• 6 3/4” x 4”

• In 8 ½ " it is possible to core 27m with a Top Drive system, but this is normally not recommendedbecause of the risk of compaction of the core / sandstone.

• Can be used in holes with any inclination.

$ISADVANTAGES��

• For the 8" x 4 3/4" size there is no spring type core catcher as a backup system. If the formation isvery hard, it can be a problem for the Hydrolift clam shells to close properly.

• Maintenance more complicated (than Posiclose), two sets required.

• In 12 ¼ " hole one can only core 9m. In most cases however this does in reality not represent anydrawback, as 9m is very often the recommended core-length, because of the risk of compaction /collapse of the core / sandstone when running 18 or 27m core barrels.

13/26

Coring.doc

• The way the Hydrolift system is constructed, the clam shells close ~30cm above the bottom of thecore, thereby loosing this part of the core.

• The lowermost 70-75cm of the recovered core is below the protective PVC sleeve, which meansthis part of the core must be emptied out of the steel shoe. This does not cause any problems if thecore is claystone or consolidated / cemented sandstone, but for poorly consolidated sandstone it isimpossible to do this operation without seriously affecting the core integrity.

• The Hydrolift system can accidentally close before coring begins, and this is a serious weakness inthe system. What can happen is the following : There are two ball seats in the Hydrolift system.The lower one is for the 1" ball that is dropped before the coring is started, to divert the mud-flow,just like in conventional coring. The upper ball seat is for the second, 1 ¼" ball, that activates theclosing of the clam shells. Under certain circumstances the first ball may activate this closingmechanism :

1. If there are lots of cuttings inside the core barrel at the time when the 1" ball arrives at the upperball seat (for the 1 ¼ " ball), these cuttings might have partly blocked the ball seat enough for the1" ball to seat in this position, and thereby doing the job of the 1 ¼ " ball and activating the closingof the clam shells.

This can be avoided by ALWAYS�USING�A�FLOAT���FLAPPER�VALVE�IN�THE�STRING, in this way ensuringthat only clean mud, without cuttings, enters the drill string.

2. Flow / Pump pressure : while pumping down the first ball, care must be taken regarding the flow / pump pressure used, as an excessive pump pressure might activate the Hydrolift closing system.BHI has procedures for this, depending on the weight and viscosity of the mud, but high pump pressure, combined with cuttings in the mud as described in point 1, could still trigger the closing mechanism.

3. Human error : The Hydrolift is a relatively complex system, that needs to be made up exactlyaccording to procedure. If this is not done properly, an accidental closure of the clam shells mightbe the result.

But by using a float / flapper valve in the string, and carefully follow the procedures, the system should normally have an acceptable reliability.

It may seem that there more emphasis has been put on the Hydrolift system than the Posiclosesystem. This does not mean that one system is necessarily better than the other, but it simply reflectsthe fact that 1) Elf has some more experience with Hydrolift, and 2) that the Hydrolift is a slightly morecomplex tool than Posiclose.

It is not possible to state which of the two coring systems is the superior. The various advantages anddisadvantages of the two systems must be seen in relation to the objectives of each specific coringprogram, and earlier experience with the two coring companies in the specific subsidiary might alsohave to be taken into consideration before making the decision.

3.4.3 GEL CORING SYSTEM

The Gel coring system by BHI will not be described in great detail here simply because it is NOTRECOMMENDED in unconsolidated sediments.

Throughout the coring process, the core is encapsulated by a pre-loaded, viscous and non-invasivegel material. The technique has proved to be quite effective both in giving good recovery and a highquality core-sample, for example in limestones.

14/26

Coring.doc

In very poorly consolidated / unconsolidated sediments however, the forces that the core is exposedto when entering this gel-filled inner barrel, are too great and could by themselves cause core damageor even prevent the core entering the barrel.

In the case of deepwater shallow objectives it will be very difficult to prepare a gel capable of being ina virtually solid state at surface and virtually liquid down hole because of the low formationtemperature.

3.4.4 GLIDER CORING SYSTEM

Statistical analysis shows an increase in coring efficiency of over 150 % when coring in an oil/pseudo-oil based mud, clearly showing the importance of the lubricating properties of this kind of drilling fluid.This fact is what forms the basis of the DBS Glider coring system.

OPERATION

It consists of a Heavy Duty core barrel with a sealed pre-filled inner tube. The filling fluid is normally amixture of environmentally safe oil based fluid and solid lubricant. The inner tube assembly isunsealed downhole at the start of coring. The coring fluid is expelled by the action of the coringassembly moving downward over the core. This motion provides a high lubricity between the core andthe surface of the inner tube. Once coring is completed, the core remains in a known, « friendly »environment provided by the selected coring fluid.

The top part of the inner tube is closed, no ball seat. Pressure equilibrium is ensured by a speciallydesigned piston.

GLIDER CORING FLUID

The pre-filled fluid that DBS recommends is a Petrofree (Ester) based drilling fluid formulated toachieve the following properties :

• Very low friction between the entering core and the inner tube.

• Zero spurtloss.

• Environmentally approved fluid.

• Provide mechanical support to the core surface.

The Petrofree based fluid is DBS’s recommended fluid, but the operator may choose an other kind offluid if preferred.

LUBRA BEADS

Co-polymer plastic beads have been used in conventional drilling operations for many years to reducetorque and drag, and during the last 6 years also to reduce friction in horizontal drilling.

DBS have now introduced the plastic beads in the coring technique, by using a drilling mud with a highconcentration of lubra beads in the pre-filled fluid. The system is being used for the first time inNorway (Heidrun field) as we go to press.

It appears to be a good method to reduce the friction between the inner barrel and the core as far asconsolidated cores are concerned, but it is not believed to have the desired effect for unconsolidatedmaterial.

Note: Neither the Gel coring nor the Glider system has been tested by Elf.

15/26

Coring.doc

�������#/2).'�/0%2!4)/.3

�������0RE CORING�WELL�SITE�ACTIVITIES

�������#OMMUNICATION

The use of the right equipment is very important, but what is equally significant is that ALL�THE�PEOPLEinvolved in the coring, core processing, packing, etc. at the well site, are well informed about thefollowing :

• the objectives of the coring job.

• who is doing what.

• how to properly handle this very fragile material.

• that a material inventory significantly prior to the job is essential - no last minute calls forequipment !

A pre-job meeting should be held at the wellsite prior to each coring run. It is essential that everyonebe in agreement with the core termination criteria. Because of the fast penetration rates in this type offormation it will be usually too late to start discussing what to do when the coring is under way.

It will be especially out of the question to start calling the base - the core will be finished before adecision is made.

The following personnel should be present at the pre-job meeting:

- company man

- toolpusher

- wellsite geologist

- driller (if possible)

- coring engineer

- mud engineer

- mud logger

Written instructions should result from this meeting and will be displayed on the rig floor and in themud logging unit.

��������3ELECTION�AND�SET UP�OF�CORE�PROCESSING�AREA

It is essential to correctly set up the core processing area to optimise operations on the core andminimise interference with normal rig activities. The coring engineer should set up and test all hisprocessing equipment (saw, pneumatic screw driver, core gamma logger, etc.) prior to coring.

Note : When using the Hydrolift system in 12 ¼" hole, and extraction of the PVC sleeve is necessary,the processing area must be sufficiently long to allow two core cradles to be laid down in line.

��������$RILLING�FLUID

��������3ELECTION�OF�DRILLING�CORING�FLUID

16/26

Coring.doc

General coring mud recommendations :

1. Minimise differential pressure.

2. Minimise water loss (invasion).

3. Use of filtrate that contains minimal/no surfactant that could effect wettability.

4. Use of solids that will quickly bridge, forming a stable filter cake (low spurt loss mud).

Which of these recommendations that can be carried out, and to what extent, is a matter that must besolved between drilling, subsurface and reservoir engineering. In most cases the above mentionedrequirements must at the same time, for economical reasons, be compatible with the associateddrilling phase and other acquisition processes (wireline logs).

In certain cases where residual saturation measurements are of fundamental importance the use of aspecific coring mud may be considered. The multidisciplinary team will weigh the evaluationadvantages against the significant extra cost involved.

In all cases the composition of the drilling fluid should be discussed before and during the pre-spudmeeting.

Note : For the petrophysical laboratory, remember to collect one mud sample (about 1 litre) from themud « IN » and one sample from the mud « OUT ». This sampling must be done at the beginning andat the end of the coring job, and also intermediate sampling must be done if considerable changes aredone to the mud system during the coring operation.

���������4RACING�OF�THE�MUD

Despite the precautions taken to minimise invasion, it may be necessary to estimate the amount ofinvasion into the cores, and thanks to tracing techniques this objective can be achieved. This is, ingeneral, only carried out when Sw measurements are a priority.

A tracer is a substance which is introduced into a process in order to follow the process developmentand describe its mechanisms.

A good tracer has the following properties :

• The tracer should follow and behave like the substance being traced, under all conditions.

• If its a chemical reactive system, the tracer must have the same reactivity.

• If its a chemical inert system, the tracer must be inert.

• The tracer should be easily detectable.

• The tracer should not represent any health hazards to the rig personnel.

1. WATER-BASED MUD

Tritiated water is an ideal tracer in water based muds as it is water. This is a weakly radioactive tracer,tritium (T) in the form of tritiated water (HTO). Tritium is a radioactive isotope of hydrogen, emitting aweak beta particle which may be detected by liquid scintillation.(The half-life of tritium is 12.3 years).

The HTO is added to the mud before the start of the coring, and adjusted to a pre-selected level andkept constant during the coring operation.

17/26

Coring.doc

After the coring, in the laboratory or even on the site, the invasion of mud filtrate can be quantified.

2. OIL-BASED MUD

If the oil-based mud is an emulsion mud with a water phase, the tritiated water or a similar tracer canbe used to trace the water phase.

The oil phase can also be traced, but this is a new method and there exists very little or no experienceof this kind of tracing (IFE, which is among the leaders in the industry, have not yet tried this methodin the field).

However, tracers exists that are supposed to be suitable for tracing the oil phase. They are all organicmolecules with substituted halogens, mainly bromine (Br) and fluor (F) and they are not radioactive.

)-0/24!.4��

What is very important to keep in mind during the planning of the well / core program, whatever kind oftracing one intends to perform, is that this kind of operation can not be a last minute decision. Onemust also be aware of the fact that this is NOT a job that can be done by either the mud-engineer orby the mudlogging company, but ONLY by highly specialised companies, such as IFE in Norway orsimilar enterprises.

It needs thorough planning, and the selected company needs all the specifications concerning the joband preferably also a sample of the mud, AT�LEAST�THREE�WEEKS�BEFORE�THE�START�OF�THE�CORING�JOB.This is the case for a regular tracing operation in the North Sea. If it is a complex job, like tracing ofthe oil phase, and/or the operation is to be done in some exotic area far away from the base of thetracing company, even more time is required.

�������0RE CORING�DRILLING�PRECAUTIONS

Before running in hole with a core barrel there are a few things that should be kept in mind whiledrilling the hole section above the coring point (mostly applicable in appraisal/development wellswhen the coring point is more or less known).

• The bore hole should be as smooth as possible and free of junk and fill

• In order to reduce vibrations and risk of sticking when coring, the last drilling assembly shouldideally be as stiff as the coring assembly that is going to be used afterwards.

• It is not recommended to core in a section with a dogleg of > 2.5°/30m. The dogleg createsvibrations and high torque which increases the risk of core jamming.

�������"ARREL�LENGTH�AND�DIAMETER

In poorly consolidated / unconsolidated sediments it is recommended to start with a 9m barrel. Ifserious coring problems are expected or have been experienced before, it might even be a good ideato stop coring after approximately 5 meters. If the result is successful using a 9m barrel, the questionof running a 18m core barrel will clearly arise. Whether this should be recommended or not willdepend on the priorities defined by the coring objectives.

If core quality is the only concern, it is safer to continue with 9 meter barrels. Even if the previous coreappears to be of good quality (and this is not necessarily easy to determine on the well site afterhaving observed only the cut faces of the core) it does not mean that a 18 meters core barrel can berun without having core compaction or possibly core collapse.

18/26

Coring.doc

But then again ; 18m and even 27m cores have been taken in relatively poorly consolidatedsandstones with success before, and of course there is always the economics aspect. This decisionmust therefore be taken if / when the situation arises. As a general rule, the more the formation isconsolidated, the longer the core that can be taken. If petrophysics are not a priority and the core ismainly destined to geological descriptions then coring 27m could be acceptable.

Recommended core size is 5 1/4” diameter for conventional systems, and for the Full Closuresystems 4 3/4”; maximum size available (Hydrolift). The greater the core diameter the less theopportunity for structural damage and invasion. If, in 12 1/4” hole one chooses to take a 4” coreinstead of 5 1/4” or 4 3/4”, one must be aware of the following extra operations that such a choiceimplies :

1. Before running the core barrel, it is necessary to drill a few meters of 8 1/2” hole in order tostabilise the coring assembly. This means an extra trip and in most cases it also means drilling ofreservoir rock that one would prefer to core.

2. After coring the cored section must be reamed before drilling can continue.

Conclusion : Such a choice should be avoided if possible.

������#ORING

������4RIP�IN

The trip in rate should be slowed down 2 or 3 stands before bottom to avoid surge pressure damage.Periodic circulation will prevent blockage of the face discharge ports.

When using the Hydrolift system it is strongly recommended to use a flapper / float valve to avoid anaccidental activation of the closing system when dropping the 1" ball. See Ch. 3.4.2, Hydrolift -Disadvantages.

�������#IRCULATION

It is recommended to clean the hole thoroughly before coring, as a smooth well bore free of junk andfill is very important in order to obtain the best result possible. At the same time however, AVOIDEXCESSIVE� CIRCULATION� BOTH� IN� TERMS� OF� DURATION� AND� FLOW RATE�� AS� THERE� IS� ALWAYS� THE� RISK� OFMAKING�WASH OUTS�IN�THE�SANDY�FORMATIONS.

The ball must be pumped down at a moderate flow rate. If high flowrates are used the pressure pulseas the ball seats may cause structural damage to the inner barrel, and it might also result in anunintended activation of the Hydrolift system.

�������#ORING�CONDITIONS���0ARAMETERS

• 7EIGHT�ON�BIT : Should be kept constant and relatively low.

• 2OTARY�SPEED�: Moderate and constant rotary speed are recommended to minimise vibrations thatcould damage the core.

• 2/0�: Should if possible be high. The faster the core can enter into the core barrel, the less it willbe exposed to flushing / invasion and erosion. At the same time the ROP should not be tooexcessive. It is important that a good mudcake can form on the core. This mudcake has twofunctions : 1) It prevents further invasion and 2) it helps to hold the core together.

19/26

Coring.doc

• -UD� FLOW� RATE�: The minimum safe mud flow for the selected core head should be applied inunconsolidated sand, to reduce washing / flushing of the core, but at the same time it must besufficiently high to clean the hole properly and avoid bit-balling.

• -UD�WEIGHT�: An overbalance in the order of 200-300 psi on the core will, together with a wellsuited mud, normally serve to build a good mudcake and keep the core together.

It is unfortunately not possible to put any exact numbers on these parameters, simply because it is theproperties of the formation, strength, grain size, sorting, etc, that will determine which parameters willgive the best result in terms of recovery and core quality.

������#ORING�TERMINATION

• By any clear signs of jamming/sand collapse (see Ch. 3.3)

• Approximately ½m before the core barrel is filled completely. The coring engineer will often try to fillit completely or even core an extra ½ meter. The intention might be good, but this will causecompression of the core and must be avoided. This technique could also diminish the efficiency ofthe full closure system.

• If a certain length of core is pre-determined, then of course this is also a termination criteria, aslong as no clear signs of jamming have been observed before reaching that depth.

There are operators who use some special procedures while coring the last 50cm :

1. Coring without circulation « to ensure full gauge core in the catcher », and

2. Spinning the barrel at a higher RPM to « burn the core in ».

The second method is not recommended, whereas the first method is debatable. In theory this is agood idea when coring with a conventional system in formations where one has doubt if the springcatcher will manage to hold the sandstone. The intention of the method is to provoke a jamming, andin this way make sure the core stays inside the barrel. The method can however, if not done correctly,result in damage to the equipment. And if the formation is such that serious doubt exists whether thespring catcher will work or not, then it is most likely a better alternative to use a full closure system.

It is recommended to avoid circulating bottoms up which has earlier been a common practise. Thereason is that such a practise increases the risk of washing the core in the catcher causing coreslippage and loss of core or of creating washouts.

�������#ORE�RETRIEVAL

Field studies have indicated that reducing the trip-out rate yields core of improved quality, whilelaboratory studies have shown that the majority of core dilation occurs over the latter stages of the trip,in particular over the last 100 meters (gas-decompression / expanding pore fluids).

The following tripping speed is therefore recommended :

��� Up to 350m : 1.5 minutes per stand (or at a normal controlled rate)

��� 350 - 100m : 3 minutes per stand

��� 100m to the drill floor : 10 minutes per stand

20/26

Coring.doc

PS : The time specified here is only the time when actually pulling the stand, meaning the time fromremoving the slips, pulling the stand to setting the slips.

Care should also be shown when setting the slips, in order to avoid unnecessary vibrations andvertical shocks, which could lead to sand compaction / core damage and possible loss of core. This isincreasingly important as the core barrel is approaching the surface.

�������#/2%��(!.$,).'���02/#%33).'

When the core reaches the drill floor it is important that everybody knows what to do and how tohandle this fragile material. From the moment the core reaches the drill floor until it arrives in thelaboratory, a lot of core damage can and often does occur. But most, if not all, can be avoided byfollowing some simple procedures.

During the complete core handling process it makes everything much easier if only the peopleconcerned with the coring job and processing are present, meaning the coring engineer, well sitegeologist, drilling supervisor, mudlogger and of course the drill crew when they are physically handlingthe corebarrel.

�������)NNER�BARREL�SEPARATION

If a 18m core barrel is used it is preferable to use a « Shear plate boot » /guillotine when splitting theinner barrel at the drill floor (Fig.9).

When the upper sleeve has been unscrewed, a « shear plate boot » is clamped around theconnection. This shear plate should preferably be driven through the core by a mechanical (orhydraulic) jack in a controlled manner in order to avoid shocks.

It has been shown by visual and X-ray examination that the use of a hammer-in shear plate damagesthe core up to a meter from the joint, and should therefore be excluded.

������#ORE�LAY DOWN

When the core barrels have been separated, or a single 9 meter inner barrels have been pulled out ofthe outer barrel, the inner barrel must be transferred to the processing area. In order to avoid coredamage from bending of the inner barrel it is important to use a CORE�CRADLE�(Figs. 10,11). If availableit should have internal rollers so that after core lay-down, the core can be moved directly onto the sawfor cutting. The core cradle should also have wheels at the ends, which will make the laying down ofthe core easier and more gentle.

The core cradle often supplied by DBS has both these functions, but it has three weak points :

1. The plateau at the end of the cradle for the saw should be broader in order to position the saw in astable position.

2. The plateau should have been at the other end of the cradle. With this core cradle it is necessary tocut from the bottom and upwards. In order to establish the top of the core you therefore have tofirst move the saw to the other end of the cradle for cutting at the top of the core (see point 1. inchapter 5.3 below), or use an other small portable saw for this purpose. Both are possible, but theprocedure is much easier with the sawing-plateau at the other end of the cradle.

3. The height of the internal rollers in the cradle and the roller on the saw are not the same, whichmakes it very difficult or even impossible both to secure the core properly before sawing and to cutthe core in one continuous movement.

21/26

Coring.doc

The core cradle often supplied by BHI also has the internal rollers and the wheels at the ends, but atthe same time it has its weak points :

1. The spacing between the rollers is too large, not giving sufficient support for the core.

2. The rollers on the cradle and the rollers on the saw, which is built into its own box, do not have thesame elevation. This makes the sawing more difficult. It can be solved by putting something underthe cradle, but it should not be necessary to have to improvise when doing this operation, and BHIhave been asked to modify their equipment. (Which we have also requested DBS to do)

If cores longer than 9m are planned, 2 or 3 cradles may be necessary.

There exists different designs depending on both the coring company and the location, but the bestsolution is to pull the inner barrel directly from the outer barrel and into the core cradle, thereby givingimmediate protection. With this design the inner barrel will already be inside the core cradle when thesplitting of the inner barrels is taking place.

With some of the core cradles a frequently occurring problem is that the lifting nipple screwed on topof the inner barrel does not fit through the core cradle. The fitting should be checked by running thenipple through the cradle prior to use.

When the inner barrel is firmly secured within the core cradle, it is time to transfer it to the processingarea. Whether this is done by using the tuggers and/or the crane will vary from one rig to the next, butthe important point is that it must be planned beforehand and that the rig crew is told specifically howto handle the core. This is a critical operation, because the potential shocks during this transfer canbe severe and very damaging to the core. In exceptional situations where cradles are not readilyavailable they can be made at the wellsite using a piece of casing (7") cut in half lengthways.

Note : During the core handling, always keep the core in a « UP » position, to avoid the core fromsliding inside the inner barrel / sleeve.

������#ORE�PROCESSING

Core processing is defined as core marking, cutting, sampling and capping, prior to stabilisation &preservation. This is already described in detail in "Wellsite Geology and Associated Specialities -Exploration Procedures" et " Wellsite Geology - Techniques and Methods". This handbook willtherefore concentrate on techniques and equipment that are new and/or particularly important whentreating poorly consolidated sediments.

��������#ORE�LOGGING

Modern wellsite core logging includes not only classic Gamma Ray measurements but also densityand most recently, NMR. On Girassol 2A in Angola the Schlumberger CMR tool was run both on coreat surface and downhole. Comparison of the two measurements enabled core depth matching andgave us valuable information concerning the representativity of the core petrophysical characteristics.

Core logging is a moderately expensive technique and should only be chosen when operationaldecisions will be based on the data acquired. In most cases, if the core has been correctly stabilisedat the wellsite, measurements carried out in the local core lab or in Pau will be sufficient.

However, if the pursuit of coring depends on a more precise knowledge of the reservoir content, corelogging should be considered and planned well in advance.

The logging process can either be carried out on 9m sections as soon as the core is laid down or on1m sections after stabilisation, depending on the urgency of the data.

����� %XTRACTION�OF�THE�06#�SLEEVE��WITH�(YDROLIFT�SYSTEM�IN����|��HOLE

22/26

Coring.doc

This operation will always be quite time consuming, because of the very tight fit between the PVCsleeve and the inner barrel. The following procedures are believed to represent the quickest andeasiest way of doing the job, and most important, the least disturbing for the core.

It is necessary to keep the PVC sleeve in a fixed position, while pulling away the inner barrel onto asecond core cradle :

• Unscrew the catcher / lower part of the Hydrolift. As mentioned in Ch. 3.4.2, the core inside thislower end of the core is unprotected by the PVC sleeve, and must be extracted from the steelshoe. The core inside the shoe should be measure before emptying it, as doing this afterwards isless accurate or even impossible if it consists of sand.

• After having unscrewed the shoe, there will be about 15cm of PVC sleeve sticking out from thesteel inner barrel. In order to hold the PVC sleeve firmly in place while pulling the steel tube, it isnecessary to put on a conventional core catcher which is then secured with a clamp.

• This clamp is then secured to the lower end of the core cradle by rope or two pieces of wood.

• A tugger line is connected to the top of the steel inner barrel, which is equipped with a proper liftinghandle, and one starts to pull the steel tube onto a second core cradle which is placed perfectly inline with the first core cradle. This operation has to be done very carefully, because the PVC sleeveis easily damaged. If you do not manage to get a good grip around the 15cm sleeve because it hasbeen damaged, you have to cut through the steel inner barrel, and even with a proper saw blade,this will take a long time.

• One continues to gently pull away the inner barrel until the complete PVC sleeve is free, and it isnow ready for washing, marking and cutting.

�������#UTTING�OF�THE�CORE��(Figs. 12,13)

The following points are important when cutting the core.

1. Cutting should start from the top of the core, and the first action is to find and mark the top of thecore. If the core barrel is not full it is necessary to first find the approximate top of core either byknocking gently along the core and hearing where the « hollow sound » stops/starts, or by insertinga measuring rod into the barrel. After cutting slightly above this point it must be visually confirmedthat it really is the top of the core and not some junk or perhaps a lost piece of core from a previouscore run. Both have been experienced before, and as a result the core had to be re-measured.#ONCLUSION��� )T� IS� IMPORTANT�TO�CONFIRM�THE�TOP�OF�CORE�BEFORE�MARKING�AND�STARTING�CUTTINGTHE��M�SECTIONS.

2. Cutting should be performed without lubrication fluids, unless otherwise instructed. The use of awax stick to cool the saw blade is acceptable, and this can also be used when cutting aluminiuminner barrels.

3. It is often recommended to cut the core with a slight angle to facilitate the later matching of the 1msections. It is however doubtful if this method is very useful in unconsolidated sands. Anotheroption is to mark the face of the cut with a paint pen, following the reference line on the tube. Thismay also be difficult due to the friable nature of the core. However, if mousse is used as a corepreservation / stabilisation agent, and rotation is avoided all through the processing of the core, thereference line on the inner barrel / sleeve can be used to re-orientate the 1 meter sections.

4. The saw must be able to cut through the core in one continuous movement. In order to do thisproperly, the core must be well secured in a completely horizontal position, without bending, beforesawing.

5. The annulus at each cut face should be examined and noted, as this is a good indication ofdisturbance/core damage.

6. It is not recommended to saw a « window » along the length of the core sleeve.

23/26

Coring.doc

��� Remember to have spare blades available !

�������'EOLOGICAL�SAMPLING

1. SEALED SAMPLES (SCAL)

Sealed samples could either be taken at the well site or in a laboratory at the base. There areadvantages and disadvantages linked to both approaches.

Taking the sealed samples on the site only has one advantage : That the sample will be in a more« virgin » state when taken immediately after the coring operation, than when taken after a certainperiod of time in the laboratory. The difference this makes depends on the time it takes to bring thecore from the well site to the laboratory, but also on different interpretation approaches.

The advantages of taking the sealed samples in the laboratory in controlled circumstances are many :

1. There is no risk of taking an unnecessary large number of seal peals since the core at this stagehas been logged, and depth matched with the wireline logs, so one can pick with certainty theinteresting zones for taking the sealed samples. One might also have a lithological description ofthe core if this is practically possible before slabbing.

2. The risk of picking a sealed sample over a geological boundary/marker bed always exists on thewell site even when using a core gamma logger. This risk is minimised in the laboratory for thesame reasons as mentioned in point 1.

3. With a X ray CT scanner one can avoid sandstones with clay inclusions etc., which of course is notpossible on the well site.

Which method is the most suitable must be discussed before each well. There is of course also a« third » alternative, namely to use both methods. Take only a few sealed samples at the well site andtake the bulk part of the samples in the laboratory. (This way it may also be possible to quantify whatdifference the time delay makes to the measurements).

A study has been carried out by ARCO on several hundred core samples. Their analyses proved thatfluid saturation in the core measured at the wellsite showed significant differences between the outerpart of the core (invaded zone) and the heart of the core (virgin zone). The same analyses carried outeven a few days only after transport to a laboratory showed no such differences. The fluid saturationshad been homogenised by capillary forces. The core was therefore polluted throughout.

If Swi is a priority then punch plugging should be carried out in the centre of the core at the wellsiteimmediately after sawing into 1m sections. The plugs are transferred to rubber sleeves which willserve as a support for the lab measurements (Fig.14). The sleeves are plugged with aluminium disks,wrapped in cling film, aluminium and solid tape. They should be stored in a refrigerator andtransported in insulated packages to avoid evaporation. Freezing is not a necessity but is analternative wherever it is easy to maintain the frozen state all the way to the lab.

2. CORE CHIPS

After having cut the core, the 1 meter sections must be placed on some sort of « Lay-down rack »simply in order to prevent them from rolling about.

When collecting core-chips the following points should be kept in mind :

• It is not recommended to « slide » unconsolidated or friable core out of the inner barrel to enable amore complete geological description or for any other purposes. Such an action will represent acore disturbance, and it can prove impossible to get the core back into the liner.

24/26

Coring.doc

• The sampling should be done gently, disturbing the core as little as possible. Normally a pocketknife is sufficient to free a small sample.

• The core-chips should not be too large as this can

1. cause difficulties fitting the core back together and drawing the reference line, and

2. create a void which offers the possibility of core collapse in unconsolidated sediments.

�������0LASTIC�CAPS���CLIPS

are necessary for stabilisation and preservation of the core. A pneumatic screwdriver for securing theclips makes the job much easier.

The black rubber caps that BHI so far has provided for the PVC sleeve should be avoided. They aretoo soft for the purpose, and when using mousse preservation, they seem to make the setting of themousse against these end caps more difficult. Elf has urged BHI to always provide the regular yellowplastic end caps commonly used with fiberglass sleeves.

�������#/2%�34!"),)3!4)/.���02%3%26!4)/.

It is extremely important to be able to preserve friable/unconsolidated core samples in a manner whichensures the maximum possible retention of core integrity with minimum contamination of thepetrophysical characteristics. Injection of a preservation material in the annulus between the core andthe inner barrel is often the best form of preservation.

������2ESINATION

A preservation method using resin was developed by Chevron La Habra around 1987. It allowed thecore to be transported, without freezing, in a very stable manner from the well site to the laboratory.However, laboratory tests have showed clearly that the resin, due to its physical properties, quiteeasily invades the pore spaces. Resin may also affect the wettability of the rock. It is therefore NOTRECOMMENDED��

������)NJECTION�OF�FOAM�(Fig. 15)

The use of polyurethane mousse / foam as a preservation material has been used by Elf for severalyears, particularly in Nigeria and is very well suited for the purpose :

• There is practically no invasion into the pores in the sandstone. Laboratory experiments haveshown that for an artificial core, made of river sand with excellent porosity/permeability, theinvasion was less than 2 mm, which is acceptable.

• Does not affect wettability

• Not visible when using X-ray CT scanner, density or gamma-ray measuring equipment.

• Easily assessable.

Note that ELF has regularly used the same contractor, KIRK PETROPHYSICS, for foamconsolidation. The know-how and experience of DONALD KIRK and his associates based in Scotlandare well proven. Given sufficient advance warning they are available worldwide.

25/26

Coring.doc

������)NJECTION�OF�'9035-

The use of gypsum is relatively recent. The method was developed by ResLab in co-operation withNorsk Hydro, Shell and Agip and is now world-patented. Elf has never tried the method. The paperdescribing this new stabilization method (), gives the following characteristics and advantages :

• Only 1-2mm invasion in a synthetic high permeability (40 Darcy) sandstone.

• « The gypsum contains no surface active material and no « side-products » are created duringreaction. It is therefore not expected that the gypsum in any way will influence wettability of thecore samples »

• Gamma ray response is slightly attenuated.

• The method is simple to perform, and easily available throughout most parts of the world.

• The pump system is commercially available, simple and portable.

• The reaction is not temperature dependant in the range of 2 - 60°C.

• Compared to MOUSSE it is easier to use, more efficient (more liquid), and cheaper !

If pore water chemistry is an important factor then gypsum should be avoided as several ionconcentrations may be affected. Industrial plaster is generally far from pure.

For both methods, in addition to actual preservation material and the associated equipment, one mustat the well site have ANGLED�RACKS�(see fig.15). These are used to stabilise and drain the annulus ofthe 1 meter sections prior to the injection.

�������0!#+).'���42!.30/24!4)/.

The packing and transportation of the core is the last part of the core handling that is performed at thewell site, before sending it to the laboratory. Apparently a trivial matter but one that should not beunderestimated. It is not sufficient to treat the core with great care on the well site if it is packedcarelessly and as a result of this can move around in the core-boxes, receive hard impacts, etc.

0ACKING

There are at least five different kinds of core boxes for transportation. Which type to use will probablydepend on availability, but the main point is that the core is completely immobilised duringtransportation in order to avoid physical damage.

1. Standard wooden (or plastic) boxes, one box for each one meter section. These must either beadapted to the sleeve diameter, or the core must be immobilised by using foam or rags, etc.

2. Core boxes made of aluminium, with a fluted top and bottom that fits into each other. They have aspongy internal lining adapted to the sleeve diameter, and the boxes come in a tailored aluminiumbasket that gives full support and can be fork- or crane lifted.

3. Special wooden pallets for sleeved core transport : This system consists of layers of pallets, withina wooden frame, with a « zig-zag » pattern in which the sleeves can lay stable.

4. A plastic box that can contain 15-18 one meter sections of core, combined with mousse, is a verygood way of packing the cores (Fig. 16). The packing is done in the following manner :

• The 1 meter sections are packed in transparent plastic bags.

• The bottom of the core box is covered by mousse, and immediately after, before the mousse hasset, the 1 meter sections are being pushed into the mousse.

26/26

Coring.doc

• Then some more mousse is sprayed on top of the first layer of core sections, and a second layer oftubes is put into the box. This is continues until the box is full.

This way of packing is very good, it stabilises the core 100%, and the mousse will also act as anefficient shock absorber.

5. The last type is the best and the most simple type to use. It was developed by ResLab in Norwayand is patented in Norway only. It consists of a block made of a very firm foam material, into whichthere are pre-drilled holes that fit exactly the size of the inner barrel (but leaves enough space toaccommodate for the plastic caps with clips). This block is protected by a solid aluminium cratewhich has pallet feet and can be fork- or crane lifted.

4RANSPORTATION

The transportation of the cores should be AS� FAST� AND� GENTLE� AS� POSSIBLE�� Fast, because somecritical physical properties (Sw) change with time, and gentle to avoid core damage that could alsochange the physical properties of the rock.

The means of transportation is probably the part in the whole « core handling » that can vary the mostfrom one well location to the other. Here are some general recommendations/keywords :

• The operation geologist together with the personnel responsible for transportation should reviewtransportation options well before spudding of the well. The transportation route should be as fastas possible, secure, minimise handling steps, and if possible avoid extremes of temperature andhumidity.

• The labelling must be complete and clear.

• Direct flights should be used where possible to minimise unsupervised core handling.

• As large temperature and pressure changes can have a negative effect on the core integrity, it isrecommended to use aircrafts with pressurised holds.

• And last but not least, remember to use the « #ORE�&OLLOW UP�3HEET » (see Exploration Rules &Procedures 8.3)!

�������0%23/..%,

It is generally recommended to have two geologists at the well-site, for obvious reasons. Coring inunconsolidated sediments requires extra work and attention from the geologist, and extra supervisionof both contractors and rig crew.