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#/.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
4.
3.4.1
Conventional systems
3.4.2
Full closure systems
3.4.3
Gel coring system
3.4.4
Glider coring system
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
5.
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
CORE HANDLING & PROCESSING 5.1 Inner barrel separation 5.2 Core lay-down 5.3 Core processing
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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
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5.3.5
6.
Plastic caps & clips
CORE STABILISATION & PRESERVATION 6.1 Resination 6.2 Injection of foam (mousse) 6.3 Injection of gypsum
7.
PACKING & TRANSPORTATION
8.
PERSONNEL
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,OW INVASION CORE BIT "()
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3TANDARD COREHEAD $"3
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#ONVENTIONAL FACE DISCHARGE COREHEAD $"3
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#$ COREHEAD LOW INVASION $"3
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&LUTED ALUMINIUM INNER TUBE $"3
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#ORE LAY DOWN CRADLE "()
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#ORE LAY DOWN CRADLES "()
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3AW MODULE ON CORE CRADLE "()
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/BSERVATION OF THE ANNULUS
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4RANSFERRING PUNCH PLUGS TO RUBBER TRANSPORTANALYSIS SLEEVES
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#ORE CONSOLIDATION BY FOAM INJECTION ON ANGLED RACKS
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0REPARATION FOR TRANSPORT PLASTIC BOXES AND FOAM PROTECTION
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The purpose of this handbook is to present a set of guidelines on how to approach coring in unconsolidated sediments. Coring is an expensive operation, mainly in terms of rig-time, but nevertheless 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, temperature etc. However, the core data can vary from being very representative to being almost useless, and this depends 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 unconsolidated reservoirs. It is known that significant structural damage can occur to unconsolidated sediments during coring, trip-out, surface handling and transportation, and that this will lead to unrepresentative core data. Standardised procedures are therefore required in order to provide for high quality, intact core from which accurate and reproducible geological, petrophysical and reservoir engineering measurements can be made. The aim of this handbook is to describe the equipment and procedures which are considered best suited in order to obtain the highest possible quality core sample. Laboratory techniques and procedures 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 subject already 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 equipment described here, and various operational circumstances will in some instances necessitate compromises, but hopefully this handbook can act as a guide to establish the best adapted coring program in each specific case.
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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 that are 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.)
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The team should have a « core project leader », who should co-ordinate the work and also act as a contact 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 will meet all the objectives • A sampling / core processing program that satisfies all the various users. The geologist and the reservoir engineer may not necessarily have the same priorities, and such different interests are best resolved before the beginning of the job. It is also essential to decide exactly in which order the 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 technical solutions to the "coring team" at the well site.
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#ORE BITS The kind of bit that should be used in unconsolidated sediments (or in any other type of sediments) is a LOW INVASION CORE BIT WITH FACE DISCHARGE. Both Baker Hughes Inteq (BHI) and Security DBS produce 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 », but also 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 the core. The anti-whirl design will minimise these vibrations, and thereby decrease the risk of core collapse and jamming. 3. Both the face discharge ports which gives an effective removal of cuttings, and the anti-whirl design 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 spin on the pebbles. In such cases a bit with a parabolic cutting face could be more efficient in cutting the pebbles or more probably pushing them into the borehole walls.
)NNER BARRELS 3.2.1 FIBERGLASS
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This is the best solution for coring unconsolidated sediments, as the fiberglass has the lowest friction coefficient among the materials used for inner barrels. It is also very easy to cut through, a fact that simplifies core processing. The only limitation is when temperatures are too high, > 175 °C (unlikely in this case, most unconsolidated sediments being at shallow depths) 6ENTED FIBERGLASS SLEEVES is an option that is very useful if there is a risk of trapped gas inside the sleeve, for instance if you have a gas bearing sand in between two shale layers that could have a tendency to swell and thereby trap the gas. This is the safety aspect, but there is also a core integrity aspect : When pulling out of hole the expanding reservoir fluids / gas will normally travel along the core towards the bit and the valve at the top of the inner barrel. If this passage is partly blocked, the gas and fluids can escape through the valves instead of forcing its way along the core, which can cause core damage. The only disadvantage with this system is that the valves will be a weak point in the inner barrel. It is not totally uncommon to see that the valves have been washed out, and it has also occurred in some rare 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 very shallow 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 in case of high temperatures, > 175 °C. The aluminium inner-barrel is of course more difficult to cut than the fiberglass, but with the proper saw blade the cutting does not represent a problem. As for fiberglass, it is not necessary to use water as a cooling agent. The aluminium sleeves can, as the fiberglass, be made with valves. The advantages are of course the same, and as the steel valves are set in aluminium instead of fiberglass, they do not in this case represent 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 IN UNCONSOLIDATED SEDIMENTS. The reason being that the unconsolidated sediments will have a tendency to fill the flutes, and thereby increasing instead of decreasing the friction. This phenomena will naturally also be very unfortunate for the core integrity. The fluted aluminium sleeve has an additional disadvantage which is similar for unconsolidated and consolidated 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 or steel inner barrel. See Ch. 3.4.2
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#ORE JAMMING INDICATORS Being able to know whether you are coring or drilling / milling the formation because of jamming or core collapse, is one of the most important aspects of the coring operation. A jamming / core collapse MAY 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 (voir exemple 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 the cuttings 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 blocking between the inner- and outer barrel and/or blocking of the nozzles occurs, there should be an INCREASE in the pump pressure. • The result may also simply be a very ERRATIC pump pressure, which in many cases will mean no change at all. 3. RATE OF PENETRATION : In hard formations the ROP will be strongly reduced, but in unconsolidated 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 coring unconsolidated sediments. -ECHANICAL CORE JAMMING DETECTORS exist or are under development, that should be able to tell when 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 in the outer barrel, being able to move independently of the outer barrel. When a jamming occurs the inner barrel will be lifted up, pushing up a pressure relief plug which will restrict (but not seal) the ports in 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 drop ball. • 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.
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• Requires modification for 6.25” barrel. • This device will only work when the core has entered into the core barrel. That means if the core jams or collapses just inside the bit or the core catcher (which can often be the case), the increase in standpipe pressure will not take place. • False jam indications may happen if the equipment is not set up correctly, and this is quite difficult. « 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 been incorrect, the result can also be that the core has collapsed but without triggering the Core Jam Indicator 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 to many 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 level indicator. They have however not succeeded yet. BHI have recently taken over a coring system which means a completely different approach to the problem of jamming. « Jam buster » consists of a telescopic inner barrel, which allows three jamming incidents to occur before coring must be terminated. When a jamming occurs it will lock the core to the innermost telescopic tube. This tube can then move with very little friction free within the next telescopic tube once a set of shear pins are sheared. The operator can then keep on coring and cope with one more jamming, being obliged to pull out of hole if a third jamming should occur. When the core barrel is full or 3 jams have occurred, the jam detector valve will close and a pressure increase will be noted on the surface. This coring system could be considered for example in a well consolidated, fractured reservoir where the risk of jamming is considered very high, but it is NOT recommended 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/milling through 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 drilling parameters. Even though they are uncertain, they often DO give an indication of when a core jamming/core collapse has taken place, but because the signals are often inconclusive they are much too often ignored. If good core recovery/quality is the aim, this practise must be changed. )F A REASONABLE DOUBT EXISTS TO WHETHER ONE IS CORING OR MILLINGWASHING THE FORMATION CORING SHOULD BE TERMINATED Better safe than sorry, and the coring operator and drilling supervisor must be clearly made 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 spring core 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 may scrape off some of the protecting mudcake, which could thereby lead to further invasion.
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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 a risk, particularly in very unconsolidated sediments, of loosing parts of the core or the complete core when pulling out of hole . The conventional coring system has nevertheless proved to be a very good system in poorly consolidated sediments, for instance in Norway, where the traditional core catcher does not appear to be a weak point. The anti-whirl core bit and proper coring procedures are probably especially important when using a conventional system (see Ch. 3.1 & 4.4). When encountering extremely poorly consolidated or totally unconsolidated sediments however, the above mentioned disadvantages will be even more difficult to overcome, and a Full Closure System may be the best solution. &OR BOTH CONVENTIONAL CORING SYSTEMS AND THE SPECIAL SOLUTIONS HEREUNDER IT HAS BEEN SHOWN STATISTICALLY THAT IN UNCONSOLIDATED FORMATIONS RECOVERY RATES INCREASE WITH CORE DIAMETER )T IS RECOMMENDED WHEREVER POSSIBLE TO CORE IN DIAMETER WITH THE LARGEST POSSIBLE CORE DIAMETER 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 closure catcher 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 that provides the means to close the full closure catcher, and the « Travel release » which locks the slip joint 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 travel joint. By extending the travel joint, the inner tube is lifted in relation to the outer barrel. This exposes the two catchers and closes the clam shells across the bottom of the inner tube, cutting the core. • POSICLOSE LOWER SECTION : An aluminium inner sleeve conceals the core catchers and provides an unrestricted bore for the core into the inner tube. The first 9” of extension on the travel joint raises the inner sleeve which exposes the core catchers. The next 3” of travel joint extension raises the lower part of the shoe assembly, including the clam shells. As they move, the clam shells come into contact with a closure cone which mechanically forces them to shut. If the clam shells are unable to cut through the core, the conventional spring catcher will catch the core in the usual 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 signal on surface to identify the system unlocking prior to catching the core. • Simplicity of the system.
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• 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 in relation to the outer barrel. The operation depends on the outer barrel staying on bottom due to gravity / friction, and this could possibly cause problems in deviated wells. According to DBS the system 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 friction coefficient 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 core barrel 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. The latest 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 is dropped, 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 a smooth unrestricted entry of the core into the inner barrel. The Posiclose and Hydrolift systems are similar in this phase of the operation, but from this point the Hydrolift system is different and slightly more complicated. When coring is complete a 1 1/4” ball is dropped through a wireline access port in the swivel and pumped down hole. The ball seats in the Hydrolift assembly diverting the flow of drilling mud into a smaller chamber at the top of the system. Pressure from the drilling mud provides force to lift the inner barrel several inches. This action pulls the smooth, core-protecting sleeve out of the catcher assembly allowing a heavy spring and cam to close 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 8 1/2" 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 of fiberglass, aluminium or steel.
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• 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 proper functioning 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 the core, 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 the tool 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 system can be triggered unintentionally. That means there is a temperature limit for the use of this combination, which for a 9m barrel is a bottom-hole temperature of 110°C. With higher bottom hole temperatures 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 performed very 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 recommended because 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 is very 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 any drawback, 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.
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• The way the Hydrolift system is constructed, the clam shells close ~30cm above the bottom of the core, thereby loosing this part of the core.
• The lowermost 70-75cm of the recovered core is below the protective PVC sleeve, which means this part of the core must be emptied out of the steel shoe. This does not cause any problems if the core is claystone or consolidated / cemented sandstone, but for poorly consolidated sandstone it is impossible 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 in the 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 the closing of the clam shells. Under certain circumstances the first ball may activate this closing mechanism : 1. If there are lots of cuttings inside the core barrel at the time when the 1" ball arrives at the upper ball seat (for the 1 ¼ " ball), these cuttings might have partly blocked the ball seat enough for the 1" ball to seat in this position, and thereby doing the job of the 1 ¼ " ball and activating the closing of the clam shells. This can be avoided by ALWAYS USING A FLOAT FLAPPER VALVE IN THE STRING, in this way ensuring that 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 exactly according to procedure. If this is not done properly, an accidental closure of the clam shells might be 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 Posiclose system. This does not mean that one system is necessarily better than the other, but it simply reflects the fact that 1) Elf has some more experience with Hydrolift, and 2) that the Hydrolift is a slightly more complex tool than Posiclose. It is not possible to state which of the two coring systems is the superior. The various advantages and disadvantages of the two systems must be seen in relation to the objectives of each specific coring program, and earlier experience with the two coring companies in the specific subsidiary might also have 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 NOT RECOMMENDED in unconsolidated sediments. Throughout the coring process, the core is encapsulated by a pre-loaded, viscous and non-invasive gel material. The technique has proved to be quite effective both in giving good recovery and a high quality core-sample, for example in limestones.
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In very poorly consolidated / unconsolidated sediments however, the forces that the core is exposed to when entering this gel-filled inner barrel, are too great and could by themselves cause core damage or 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 in a virtually solid state at surface and virtually liquid down hole because of the low formation temperature. 3.4.4 GLIDER CORING SYSTEM Statistical analysis shows an increase in coring efficiency of over 150 % when coring in an oil/pseudooil 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 a mixture of environmentally safe oil based fluid and solid lubricant. The inner tube assembly is unsealed downhole at the start of coring. The coring fluid is expelled by the action of the coring assembly moving downward over the core. This motion provides a high lubricity between the core and the 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 specially designed piston. GLIDER CORING FLUID The pre-filled fluid that DBS recommends is a Petrofree (Ester) based drilling fluid formulated to achieve 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 of fluid if preferred. LUBRA BEADS Co-polymer plastic beads have been used in conventional drilling operations for many years to reduce torque 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 high concentration of lubra beads in the pre-filled fluid. The system is being used for the first time in Norway (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 as consolidated cores are concerned, but it is not believed to have the desired effect for unconsolidated material. Note: Neither the Gel coring nor the Glider system has been tested by Elf.
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0RE CORING WELL SITE ACTIVITIES #OMMUNICATION The use of the right equipment is very important, but what is equally significant is that ALL THE PEOPLE involved in the coring, core processing, packing, etc. at the well site, are well informed about the following : • 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 for equipment ! A pre-job meeting should be held at the wellsite prior to each coring run. It is essential that everyone be in agreement with the core termination criteria. Because of the fast penetration rates in this type of formation 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 a decision 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 the mud 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 and minimise interference with normal rig activities. The coring engineer should set up and test all his processing 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 DRILLINGCORING FLUID
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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 be solved between drilling, subsurface and reservoir engineering. In most cases the above mentioned requirements must at the same time, for economical reasons, be compatible with the associated drilling phase and other acquisition processes (wireline logs). In certain cases where residual saturation measurements are of fundamental importance the use of a specific coring mud may be considered. The multidisciplinary team will weigh the evaluation advantages against the significant extra cost involved. In all cases the composition of the drilling fluid should be discussed before and during the pre-spud meeting. Note : For the petrophysical laboratory, remember to collect one mud sample (about 1 litre) from the mud « IN » and one sample from the mud « OUT ». This sampling must be done at the beginning and at the end of the coring job, and also intermediate sampling must be done if considerable changes are done 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 of invasion into the cores, and thanks to tracing techniques this objective can be achieved. This is, in general, 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 development and 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 a weak 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 and kept constant during the coring operation.
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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 can be 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 experience of this kind of tracing (IFE, which is among the leaders in the industry, have not yet tried this method in the field). However, tracers exists that are supposed to be suitable for tracing the oil phase. They are all organic molecules 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 of tracing one intends to perform, is that this kind of operation can not be a last minute decision. One must also be aware of the fact that this is NOT a job that can be done by either the mud-engineer or by the mudlogging company, but ONLY by highly specialised companies, such as IFE in Norway or similar enterprises. It needs thorough planning, and the selected company needs all the specifications concerning the job and 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 of the oil phase, and/or the operation is to be done in some exotic area far away from the base of the tracing 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 while drilling the hole section above the coring point (mostly applicable in appraisal/development wells when 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 should ideally 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 creates vibrations 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. If serious coring problems are expected or have been experienced before, it might even be a good idea to stop coring after approximately 5 meters. If the result is successful using a 9m barrel, the question of running a 18m core barrel will clearly arise. Whether this should be recommended or not will depend 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 core appears to be of good quality (and this is not necessarily easy to determine on the well site after having observed only the cut faces of the core) it does not mean that a 18 meters core barrel can be run without having core compaction or possibly core collapse.
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But then again ; 18m and even 27m cores have been taken in relatively poorly consolidated sandstones with success before, and of course there is always the economics aspect. This decision must therefore be taken if / when the situation arises. As a general rule, the more the formation is consolidated, the longer the core that can be taken. If petrophysics are not a priority and the core is mainly 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 Closure systems 4 3/4”; maximum size available (Hydrolift). The greater the core diameter the less the opportunity for structural damage and invasion. If, in 12 1/4” hole one chooses to take a 4” core instead of 5 1/4” or 4 3/4”, one must be aware of the following extra operations that such a choice implies : 1. Before running the core barrel, it is necessary to drill a few meters of 8 1/2” hole in order to stabilise the coring assembly. This means an extra trip and in most cases it also means drilling of reservoir 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 an accidental 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 and fill is very important in order to obtain the best result possible. At the same time however, AVOID EXCESSIVE CIRCULATION BOTH IN TERMS OF DURATION AND FLOW RATE AS THERE IS ALWAYS THE RISK OF MAKING WASH OUTS IN THE SANDY FORMATIONS. The ball must be pumped down at a moderate flow rate. If high flowrates are used the pressure pulse as the ball seats may cause structural damage to the inner barrel, and it might also result in an unintended 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 that could damage the core. • 2/0 : Should if possible be high. The faster the core can enter into the core barrel, the less it will be exposed to flushing / invasion and erosion. At the same time the ROP should not be too excessive. It is important that a good mudcake can form on the core. This mudcake has two functions : 1) It prevents further invasion and 2) it helps to hold the core together.
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• -UD FLOW RATE : The minimum safe mud flow for the selected core head should be applied in unconsolidated sand, to reduce washing / flushing of the core, but at the same time it must be sufficiently 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 well suited 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 the properties of the formation, strength, grain size, sorting, etc, that will determine which parameters will give 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 fill it completely or even core an extra ½ meter. The intention might be good, but this will cause compression of the core and must be avoided. This technique could also diminish the efficiency of the full closure system. • If a certain length of core is pre-determined, then of course this is also a termination criteria, as long 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 a good idea when coring with a conventional system in formations where one has doubt if the spring catcher will manage to hold the sandstone. The intention of the method is to provoke a jamming, and in 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 the spring 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. The reason is that such a practise increases the risk of washing the core in the catcher causing core slippage 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, while laboratory 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
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PS : The time specified here is only the time when actually pulling the stand, meaning the time from removing the slips, pulling the stand to setting the slips. Care should also be shown when setting the slips, in order to avoid unnecessary vibrations and vertical shocks, which could lead to sand compaction / core damage and possible loss of core. This is increasingly important as the core barrel is approaching the surface.
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When the core reaches the drill floor it is important that everybody knows what to do and how to handle this fragile material. From the moment the core reaches the drill floor until it arrives in the laboratory, a lot of core damage can and often does occur. But most, if not all, can be avoided by following some simple procedures. During the complete core handling process it makes everything much easier if only the people concerned with the coring job and processing are present, meaning the coring engineer, well site geologist, drilling supervisor, mudlogger and of course the drill crew when they are physically handling the corebarrel.
)NNER BARREL SEPARATION If a 18m core barrel is used it is preferable to use a « Shear plate boot » /guillotine when splitting the inner barrel at the drill floor (Fig.9). When the upper sleeve has been unscrewed, a « shear plate boot » is clamped around the connection. This shear plate should preferably be driven through the core by a mechanical (or hydraulic) 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 damages the 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 of the outer barrel, the inner barrel must be transferred to the processing area. In order to avoid core damage from bending of the inner barrel it is important to use a CORE CRADLE (Figs. 10,11). If available it should have internal rollers so that after core lay-down, the core can be moved directly onto the saw for cutting. The core cradle should also have wheels at the ends, which will make the laying down of the 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 a stable position. 2. The plateau should have been at the other end of the cradle. With this core cradle it is necessary to cut from the bottom and upwards. In order to establish the top of the core you therefore have to first move the saw to the other end of the cradle for cutting at the top of the core (see point 1. in chapter 5.3 below), or use an other small portable saw for this purpose. Both are possible, but the procedure 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, which makes it very difficult or even impossible both to secure the core properly before sawing and to cut the core in one continuous movement.
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The core cradle often supplied by BHI also has the internal rollers and the wheels at the ends, but at the 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 the same elevation. This makes the sawing more difficult. It can be solved by putting something under the cradle, but it should not be necessary to have to improvise when doing this operation, and BHI have 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 best solution is to pull the inner barrel directly from the outer barrel and into the core cradle, thereby giving immediate protection. With this design the inner barrel will already be inside the core cradle when the splitting of the inner barrels is taking place. With some of the core cradles a frequently occurring problem is that the lifting nipple screwed on top of the inner barrel does not fit through the core cradle. The fitting should be checked by running the nipple 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 processing area. Whether this is done by using the tuggers and/or the crane will vary from one rig to the next, but the important point is that it must be planned beforehand and that the rig crew is told specifically how to handle the core. This is a critical operation, because the potential shocks during this transfer can be severe and very damaging to the core. In exceptional situations where cradles are not readily available 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 from sliding 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 will therefore concentrate on techniques and equipment that are new and/or particularly important when treating poorly consolidated sediments. #ORE LOGGING Modern wellsite core logging includes not only classic Gamma Ray measurements but also density and most recently, NMR. On Girassol 2A in Angola the Schlumberger CMR tool was run both on core at surface and downhole. Comparison of the two measurements enabled core depth matching and gave us valuable information concerning the representativity of the core petrophysical characteristics. Core logging is a moderately expensive technique and should only be chosen when operational decisions will be based on the data acquired. In most cases, if the core has been correctly stabilised at 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, core logging 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 on 1m sections after stabilisation, depending on the urgency of the data. %XTRACTION OF THE 06# SLEEVE WITH (YDROLIFT SYSTEM IN | HOLE
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This operation will always be quite time consuming, because of the very tight fit between the PVC sleeve and the inner barrel. The following procedures are believed to represent the quickest and easiest 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 a second core cradle : • Unscrew the catcher / lower part of the Hydrolift. As mentioned in Ch. 3.4.2, the core inside this lower end of the core is unprotected by the PVC sleeve, and must be extracted from the steel shoe. The core inside the shoe should be measure before emptying it, as doing this afterwards is less 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 the steel inner barrel. In order to hold the PVC sleeve firmly in place while pulling the steel tube, it is necessary 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 lifting handle, and one starts to pull the steel tube onto a second core cradle which is placed perfectly in line with the first core cradle. This operation has to be done very carefully, because the PVC sleeve is easily damaged. If you do not manage to get a good grip around the 15cm sleeve because it has been 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 is now 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 the core. If the core barrel is not full it is necessary to first find the approximate top of core either by knocking gently along the core and hearing where the « hollow sound » stops/starts, or by inserting a measuring rod into the barrel. After cutting slightly above this point it must be visually confirmed that it really is the top of the core and not some junk or perhaps a lost piece of core from a previous core 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 CUTTING THE M SECTIONS. 2. Cutting should be performed without lubrication fluids, unless otherwise instructed. The use of a wax stick to cool the saw blade is acceptable, and this can also be used when cutting aluminium inner barrels. 3. It is often recommended to cut the core with a slight angle to facilitate the later matching of the 1m sections. It is however doubtful if this method is very useful in unconsolidated sands. Another option is to mark the face of the cut with a paint pen, following the reference line on the tube. This may also be difficult due to the friable nature of the core. However, if mousse is used as a core preservation / stabilisation agent, and rotation is avoided all through the processing of the core, the reference 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 this properly, the core must be well secured in a completely horizontal position, without bending, before sawing. 5. The annulus at each cut face should be examined and noted, as this is a good indication of disturbance/core damage. 6. It is not recommended to saw a « window » along the length of the core sleeve.
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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 are advantages 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 certain period of time in the laboratory. The difference this makes depends on the time it takes to bring the core 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 stage has been logged, and depth matched with the wireline logs, so one can pick with certainty the interesting zones for taking the sealed samples. One might also have a lithological description of the 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 the well site even when using a core gamma logger. This risk is minimised in the laboratory for the same 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 not possible 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 and take the bulk part of the samples in the laboratory. (This way it may also be possible to quantify what difference the time delay makes to the measurements). A study has been carried out by ARCO on several hundred core samples. Their analyses proved that fluid saturation in the core measured at the wellsite showed significant differences between the outer part of the core (invaded zone) and the heart of the core (virgin zone). The same analyses carried out even a few days only after transport to a laboratory showed no such differences. The fluid saturations had 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 wellsite immediately after sawing into 1m sections. The plugs are transferred to rubber sleeves which will serve 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 and transported in insulated packages to avoid evaporation. Freezing is not a necessity but is an alternative 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 a more complete geological description or for any other purposes. Such an action will represent a core disturbance, and it can prove impossible to get the core back into the liner.
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• The sampling should be done gently, disturbing the core as little as possible. Normally a pocket knife 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 the clips makes the job much easier. The black rubber caps that BHI so far has provided for the PVC sleeve should be avoided. They are too soft for the purpose, and when using mousse preservation, they seem to make the setting of the mousse against these end caps more difficult. Elf has urged BHI to always provide the regular yellow plastic end caps commonly used with fiberglass sleeves.
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It is extremely important to be able to preserve friable/unconsolidated core samples in a manner which ensures the maximum possible retention of core integrity with minimum contamination of the petrophysical characteristics. Injection of a preservation material in the annulus between the core and the 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 the core 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, quite easily invades the pore spaces. Resin may also affect the wettability of the rock. It is therefore NOT RECOMMENDED
)NJECTION OF FOAM (Fig. 15) The use of polyurethane mousse / foam as a preservation material has been used by Elf for several years, particularly in Nigeria and is very well suited for the purpose : • There is practically no invasion into the pores in the sandstone. Laboratory experiments have shown that for an artificial core, made of river sand with excellent porosity/permeability, the invasion 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 foam consolidation. The know-how and experience of DONALD KIRK and his associates based in Scotland are well proven. Given sufficient advance warning they are available worldwide.
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)NJECTION OF '9035The use of gypsum is relatively recent. The method was developed by ResLab in co-operation with Norsk Hydro, Shell and Agip and is now world-patented. Elf has never tried the method. The paper describing 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 during reaction. It is therefore not expected that the gypsum in any way will influence wettability of the core 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 ion concentrations may be affected. Industrial plaster is generally far from pure. For both methods, in addition to actual preservation material and the associated equipment, one must at the well site have ANGLED RACKS (see fig.15). These are used to stabilise and drain the annulus of the 1 meter sections prior to the injection.
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The packing and transportation of the core is the last part of the core handling that is performed at the well site, before sending it to the laboratory. Apparently a trivial matter but one that should not be underestimated. It is not sufficient to treat the core with great care on the well site if it is packed carelessly 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 probably depend on availability, but the main point is that the core is completely immobilised during transportation in order to avoid physical damage. 1. Standard wooden (or plastic) boxes, one box for each one meter section. These must either be adapted 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 a spongy internal lining adapted to the sleeve diameter, and the boxes come in a tailored aluminium basket 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, within a 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 very good 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 has set, the 1 meter sections are being pushed into the mousse.
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• Then some more mousse is sprayed on top of the first layer of core sections, and a second layer of tubes 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 an efficient shock absorber. 5. The last type is the best and the most simple type to use. It was developed by ResLab in Norway and is patented in Norway only. It consists of a block made of a very firm foam material, into which there are pre-drilled holes that fit exactly the size of the inner barrel (but leaves enough space to accommodate for the plastic caps with clips). This block is protected by a solid aluminium crate which 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 some critical physical properties (Sw) change with time, and gentle to avoid core damage that could also change the physical properties of the rock. The means of transportation is probably the part in the whole « core handling » that can vary the most from one well location to the other. Here are some general recommendations/keywords : • The operation geologist together with the personnel responsible for transportation should review transportation options well before spudding of the well. The transportation route should be as fast as possible, secure, minimise handling steps, and if possible avoid extremes of temperature and humidity. • 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 is recommended 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)!
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It is generally recommended to have two geologists at the well-site, for obvious reasons. Coring in unconsolidated sediments requires extra work and attention from the geologist, and extra supervision of both contractors and rig crew.
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