Coring operations guidelines. Core samples are taken in order to measure accurately the reservoir parameters in hydrocarbon and water bearing formations, and also for other geological investigation.

1  General Coring Guidelines

• Fibre glass inner core barrel should be used.
• Use a full closure system type core catcher including i.e. a clamshell device that completely closes off the bottom of the inner barrel once the coring is finished.
• Use a PDC core head with a face discharge design. This design will divert a large amount of the fluid away from the core and thus minimise core erosion and filtrate invasion. This is especially important in the unconsolidated Formation sands. A consequence of the diversion of fluids away from the core is a lower cooling effect of the cutters near the core, and a thermally stable construction of this part of the bit is necessary.
• The mud weight should if possible give an overbalance of  20 - 28 bars. This improves recovery whereby loose grains will enter the core barrel without fluidizing.
• Hard lost circulation materials should be avoided due to the risk of jamming.
• The fluid invasion should be between 4 and 9 cm3/30 min (API Standard).
• The fluid viscosity should be low to prevent rotating the inner barrel, core jamming and reduce  swabbing.
• If the section to be cored has the same diameter as the hole drilled to the point where the coring is to start, then the initial coring run can use an 27 m (90 ft) core barrel. If desired the use of top drive and singles makes it possible to core an uninterrupted length of core.
• For 8 1/2" (216 mm) hole, a 6 3/4" (171 mm) x 4" (102 mm) core barrel with an 8 1/2" (215 mm) core head will be used.
• For 12 1/4" (311 mm) hole, either an 8" (203 mm) x 5 1/4" (133 mm) core barrel with a 12 1/4" (310 mm) core head or an 6 3/4" (171 mm) core barrel with an 8 1/2" (215 mm) core head may be used.
• Ensure that the bore of the whole drill string (i.e.: jar, etc.) is of sufficient diameter to allow the ball to pass.
• When spacing out the inner barrel, up to 1/4" (6 mm) too high is acceptable when coring hard formations. However, for soft formations, the spacing out should be as accurate as possible. Also, when spacing out the inner barrel refer to the relevant manufacturer's literature, as additional clearance may be necessary due to the difference in thermal expansion between fibreglass and steel.
• Space out the drill string in order to have a full stand up when starting to core.
• Close to bottom, when running in, the hole should be circulated to remove sloughing and other loose material that can plug the core barrel.
• Always check the on-bottom and off-bottom circulating pressures.
• Use minimum circulating rates while coring.
• If the pump pressure increases, and the torque decreases, then a possible formation change is indicated.
• If the pump pressure, penetration rate and torque, all decrease simultaneously, then it may be an indication that the core has jammed (or that the core barrel is full).
• If the pump pressure and torque both increase simultaneously, then an "o" ring groove may have been developed on the cutting face of the core head.
• Coring the last 30 cm without circulation will cause the core to burn into the core head and help preventing the core from sliding out.
• Avoid reaming with a core barrel assembly. If it has to be done, use the maximum circulating rate and minimum WOB. Do not exceed 30 rpm.
• When pulling out of hole, do not rotate the drill string or the core barrel in the rotary table. Use the pipe spinner to spin out connections. Once the core has been cut, care must be taken while tripping out in order not to mechanically disturb the structure of the core. Slips should be set gently in order to prevent chock loads.
• Pulling out of hole should be done carefully to avoid core damage from too rapid de pressurisation.1.5 min/ stand or slower is recommended. During the last 100 - 200 m the speed should be even lower; 3min/ stand.
• Coring a rathole:

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  • When coring a rathole, the initial run shall use a 9 m (30 ft) core barrel. If the recovery is good, then subsequent runs can use an 27 m (90 ft) core barrel. However, should the recovery be poor, then subsequent runs shall continue to use a 9 m (30 ft) core barrel.
  • When taking the first core from a 12 1/4" (311 mm) hole, install two 12 1/4" (311 mm) stabilisers; one directly above the core barrel, and the other 9 m (30 ft) above it.
  • When taking subsequent cores, run the same stabilisers and also run sufficient 6 1/4" (159 mm) drill collars, stabilised at every 18 m (60 ft), for entering the rat hole.
  • Whenever hole problems develop, enlarge hole to full size after 80 m (266 ft) of rathole has been cored in 12 1/4" hole

2 Coring Parameters

There are three main parameters which have to be considered when coring:

• Rotating speed of the core head.
• Weight on bit.
• Pump discharge.

In general terms, the speed of the core head should be varied to suit the formation, and the pump discharge varied according to the size of the core head. The weight on bit depends both on the core head size, and the type of formation being cored. With higher pressure drops, hydraulic lift should be considered when selecting weight on bit.

To avoid barrel jams or core damage, light bit weights and relatively low rotary speeds should be used to minimise the bending of the core.

3  (Spot) Coring Procedures

The actual procedure for (spot) coring depends on the size of the core required, but generally the steps of the procedure are similar to these given in the example below for spot coring in a 12 1/4" (311 mm) hole.

1. Install a plain 6 1/4" (159 mm) stabiliser body between the bit sub and the 12 1/4" (311 mm) stabiliser.

2. Circulate clean on bottom and work the junk sub with fluctuating pump pressure.

3. Pull out of hole.

4. Make up the core barrel assembly comprising:

Core head - 8 1/2" (215 mm) stab. - core barrel - 8 15/32" (215 mm) stab. -safety joint - cross over - 12 1/4" (311 mm) stab. - rest of 8 1/4" (210 mm) drill collar assembly.

Ensure that all I.D.'s are sufficient to allow the passage of the drop ball, and check that the Totco ring has been removed.

Using a safety clamp under the stabiliser, make up the outer barrel and stabilisers to the required torque with the rotary tongs. Make up the inner barrel with a chain tong. The safety joint and swivel       joint can then be made up. Pick up the barrel and check the spacing with a gauge; adjust the shims. Using the special bit breaker make up the bit. Add the rest of the assembly.

5. Run in slowly, checking whether the string is filling up. Use the drill pipe wiper, and do not rotate in the casing. Start pumping and locate bottom (space out the string so that coring starts with a full stand, but avoid having pup joints in the vicinity of the wellhead/BOP).

Circulate just off bottom for 15 minutes at approximately 400 gpm (1500 l/min) or higher depending on fill. Drop the ball and continue circulating until a pressure increase indicates that the ball has seated.

6. Record the on-bottom and off-bottom circulating pressures, then start coring at 30 rpm with 2000 lbs (0.9 tonnes) WOB. After 150 mm (6") increase the WOB to 10,000 - 15,000 lbs (4.5 - 6.8 tonnes) and the rotary speed to 90 -110 rpm. Maintain circulation at the pre-selected rate:

Soft formation: 160 - 250 gpm ( 605 -  945 l/min)

Hard formation: 280 - 300 gpm (1060 - 1135 l/min)

7. During the coring, maintain a uniform WOB and keep a record of the penetration rate, pump pressure and torque.

4  Fibreglass Inner Core-Barrels

In order to meet the Geologist's demand for undisturbed core samples and safe transportation from the field to the laboratory, the conventional steel inner barrel may be replaced by a fibreglass tube.

The fibreglass inner tubes are produced from vinyl ester resin material reinforced with glass fibre roving, and they have moulded formed pin threads on both the upper and lower ends which will accommodate internal flush connections. The connection between the standard inner tube shoe and the fibreglass inner tubes is made using a special pin and box sub.

Coring with the fibreglass inner tube does not involve any additional difficulties with respect to making up, adjusting nor breaking out the core barrel, and is performed in exactly the same manner as coring with conventional steel inner tubes.

Note:

• Do not install the core-marker when using fibreglass inner barrels.
• With fibreglass inner barrels in hard formations do not rotate the core barrel to "turn  off" the core.

Fibreglass inner tubes are available for the following core barrel sizes:

• 6 1/4" x 3"     (159 mm x  76 mm)
• 6 3/4" x 4"     (171 mm x 102 mm)
• 8" x 5 1/4"     (203 mm x 133 mm)

The low friction rate of the inside of the fibre glass tube allows easy passage of the core along the tube, and core jamming in broken and fractured formations is minimised, resulting in longer cored sections, faster penetration rates and better core recovery.

The middle catcher assembly - designed as either a standard or slip type core catcher - allows breaking out of the 9 m (30 ft) inner tube sections while pulling the inner tube from the outer barrel after the coring job has been finished.

In order to prevent the fibreglass core barrel to bend when laying it down/transporting it to a suitable work site, it may be requested to secure it to some sort of metal cradle or basket. Alternatively it can be lowered into a handling sock, a specially made metal tube, some 7 metres long and of adequate diameter, with welded on plate in one end and lifting lugs. Until the core barrel is secure in this way, it should be kept in a vertical position.

After it has been laid down, the fibre glass inner tube and the core can be cut into sections of desired lengths. In order to stabilise the core inside the core barrel, resin injections may be performed. The resin and hardener used in this process constitutes certain hazards like irritation both on inhalation and contact in addition to being poisonous and carcinogenic. Therefore only people needed for the actual injection process should be near the injection site and chemicals, and the people involved should wear protective clothing and goggles. Resin should only be used in open air. The Well Site Drilling Engineer shall make sure that adequate safety measures are taken.

This technique of core handling keeps the core in as near as possible the original shape and allows optimisation of the evaluation of the core material (see chapter 6.2 "Coring Evaluation").

If there are any indication of the core being pressurised (strong smell of gas, gas bubbling out of the top of the core, hissing noises) the core should be unscrewed into two sections at the rotary table with extreme caution, and all personnel in the area should use eye protection.

The core should be laid down and left for several hours before any attempt is made to cut it into sections. Failure to follow this procedure may result in the core being blown out with considerable force, risking injury to personnel and loosing valuable information.

5  Coring In Deviated Holes

5.1. General

In deviated holes (certainly with angles over 20o), coring is more difficult because of the additional forces caused by WOB, and the weight of the core barrel and of the drill collars at such angles. These gravitational forces bend the drill-collars and the core barrel, and as a result the inner barrel may also rotate and the bit may start to wobble. This can result in poor recovery and unequal wear of the core head. Also the hole angle and direction may be affected.

In order to minimise the bending, additional stabilisation is required. When drilling with an 8 1/2" (216m) core head in a larger hole, e.g. 12 1/4" (311 mm), consideration should be given to the fact that difficulties may arise when trying to re-enter the rathole.

5.2. Stabilisation of Outer Barrel

All coring in deviated holes shall be carried out using a core head equipped with a piggy-back stabiliser whenever possible. This keeps the core head flat on bottom, promotes good cooling and the proper removal of cuttings and results in good core recovery.

The stabilisation of the assembly should be designed such that gravitational forces induces a dogleg of the hole.

5.3. Stabilisation of the Drill Collar Assembly

The design of the string stabilisation will depend to a large extent on the hole conditions, and will not be further specified here, however, it is required that adequately stabilisation is managed.

5.4. Coring Assembly for Deviated Holes

The following is an example of an assembly for "spot coring" short intervals in an 8 1/2" (216 mm) deviated hole:

8 1/2" (215 mm) corehead - 9 m (30 ft) x 6 1/4" (159 mm) x 3" (76 mm) stabilised core barrel - One 6 1/4" (159 mm) DC - Stab - Two 6 1/4" (159 mm) DC's - Stab - One 6 1/4" (159 mm) DC - 6 1/8" (156 mm) drilling jar - HWDP (incl. dart sub)

Note:

• All drill collars should be spiral.
• With this assembly the jar will probably be in compression, (depending upon WOB). Ensure that the jar is not at neutral point.

6  Core Barrel Safety Values

the recommended safety values for steel core barrels should be based upon the ultimate tensile strength in the pin thread area using a safety factor of 3.