Practical guidelines for wellsite coring operations. Three of the key inputs to formation evaluation for effective reservoir development and management are:

• Reservoir geological description
• Log calibration for hydrocarbons in place
• Input to reserves calculation

These inputs all rely on obtaining good quality core.

1 Pre Coring Rigsite Communication and Checks

Actions to be performed by the wellsite geologist and core hand on arrival at the wellsite.

1.1 Communication

• The wellsite geologist and core hand must meet with and brief the company representative on the proposed coring and core handling processes
• The second more detailed meeting should include presentation of objectives with the involved rig personnel (company representative, tool pusher, core hand, mud engineer, wellsite geologist, core stabilisation personnel, crane and forklift operators)
• Safety requirements specific to the rig and core handling process will be reviewed
• Coring related personnel must identify local hazards.   Appropriate working procedures must be agreed.

1.2 Equipment Check Out

• Conduct an inventory of all coring, core handling and preservation equipment

1.3 Selection and Set-up Core Processing Area

• Select a core processing area which will not interfere with other rig activities
• Arrange the equipment in the core processing area for ease of operation and safe laying of compressed air and electrical lines. Core laydown area needs to be flat, well lit and sheltered if possible
• Access for gently laying down cradles and braces must be good
• Supplies required for core processing are:
  • 100 psi air supply via at least ¾ inch hose for air saw
  • 220 volts ac single phase 15Kw supply for Core Preservation Unit
  • Supplies for lighting if rig area lighting is inadequate for safe night time core processing (220v )

2 Pre Coring Borehole Conditioning for Good Core

• The coring BHA will be a stiff packed assembly, it is therefore essential the hole is finished with an equally stiff BHA before running the core barrel
• Reaming with the core barrel will decrease the chance of successfully starting to core.  Excessive reaming often results in the corehead being worn undergauge.  Weight is then held on stabilisers during coring, and core washout and poor recovery may result
• Mud should be in good condition before starting to core.  Excessive solids in the mud can lead to early core jamming as solids pack the annulus between the core and inner barrel

3 Core Point Selection

• Core point will be defined by the wellsite geologist at the rigsite and discussed with the operations geologist.

3.1 Optimum Coring Conditions & Parameters

• Trip In : Reduce instantaneous run in rate by 50% at 5 stands from the bottom to minimise whole mud injection from surge pressure.  Mud should be circulated during run-in below casing shoe to prevent blocking of the inner barrel and face discharge ports on the corehead
• Commencing coring : Circulate bottoms up before starting coring.  The ball must be pumped down at a moderate flow rate for fibretube coring, if high flowrates are used the pressure pulse caused by the ball seating may cause fibretube to implode
• Mud Flow Rate :  The minimum safe mud flow rate for the selected corehead should be applied initially, though impact of flowrate on ROP should be evaluated and optimised to get best ROP.  Beware that high flowrates for extended periods can erode coreheads, pilot shoes and inner tubes (especially around non return valve vents if fitted)
• WOB & ROP :  The driller should try and maintain a constant bit weight suitable for the corehead used. Weight should be applied smoothly, not piled on and allowed to drill off.  Sudden increases in weight on bit are likely to cause core breakage and pack off
• Rotary Speed : Moderate rotary speeds are initially recommended to minimise vibration that could damage the core.  Very low rotary speeds that can lead to stick slip action at the corehead should be avoided.  Rotation must be optimised and balanced to get best ROP performance

3.2 Connections While Coring

• The kelly will limit the maximum length of core that can be cut before making a connection to about 13m.
• If coring appears to be progressing normally from review of surface parameters upto ‘kelly down’, a connection may be attempted.
• Overpull on lifting off bottom should be checked, if overpull is seen, and hole condition is good, then it is likely that the core has been successfully caught.
• The barrel should be lifted off bottom, and then a check for dropped core made by re-tagging bottom.
• If it appears that the core has been broken in solid rock, and that there is no stub left on bottom, the connection should be made and coring re-started (a connection may be attempted if there is a stub on bottom, but chances of a successful re-start are much diminished).
• Do not rotate the string while breaking core, tagging bottom or making connection unless it is absolutely necessary to avoid differential sticking.  Not rotating the string generally increases the chance of a successful re-start, but the company man must be consulted before this route is taken.
• After connection is made, return to bottom and apply full coring weight to try and free the core in the catcher.
• Back off weight to a light weight.
• Coring should be re-started by maintaining light weight on bit, beginning to rotate, and then gently building up to full coring parameters.
• Parameters must be carefully monitored to ensure that coring has been re-established, and that the formation is not being drilled.
• Core already in the barrel will be damaged by centrifugation if the inner barrel begins to spin with the outer.  If coring parameters do not return to ‘normal’ within a short period of time then the core should be pulled.

3.3 Coring Termination Criteria

• Coring should be terminated when barrel is full (i.e. core is within 50cm of top of barrel) or when evidence of jamming has occurred (drop in torque or erratic torque, very low ROP, erratic pump pressure).  Barrel must not be over-filled, core will fail in compression if barrel is overfilled
• If jamming and pack-off is suspected then the core hand should check his parameters against those in the logging unit, and consult with the company representative and geologist before pulling the barre

4 Mud Sampling During Coring

Bulk mud samples will be taken during all coring operations:

• Take one mud sample at the beginning of each continuous cored section
• Take one mud sample every 5m through each cored section
• Samples will be 1.5 litres, sealed in robust polythene bottles
• Sample bottles must be well packaged such that they will survive the journey to Hassi Messaoud intact
• A full listing of samples should be provided to the operations geologist

5 Core Trip Out

Field studies have indicated that reducing the trip-out rate in the latter stages of the trip yields core of improved structural quality.  Reduced trip rates will also minimise gas drive reduction of core liquid saturation. It is also recommended to reduce trip rates close to surface to allow gas to escape from the core barrel for safety

5.1 Rig-floor Precautions

• Braking and slip setting should be performed without jarring the core (especially near the surface).  Sudden vertical shocks to the drill string frequently result in substantial core damage and may result in loss of core
• Minimise rotation of the core barrel when breaking connections.

5.2 Required from Mud Logging Data Engineer

• Continual monitoring of coring parameter trends with feedback to drillfloor to safeguard against drilling formation after core pack-off
• If torque, ROP or stand pipe pressure vary substantially from the baseline, the corehand,  driller, wellsite geologist  and core preservation engineer should be notified and shown plot of parameters
• Coring parameters ROP, WOB, TQA, TQMX, TQMN, MFIA, SPPA, SPPMX, SPM, RPMA ECD, in paper form & electronic / ASCII format to be provided to the wellsite after each core run (depth base data to be presented at 0.2m intervals)
• Trip monitor information (depth of bit vs. time, instantaneous pipe speed) in electronic / ASCII format to be provided to the wellsite geologist at wellsite immediately after each core run

5.3 Required from Corehand after each Core Run

• Details of BHA run for each core
• Coring performance (WOB, ROP, torque, pressure, & comments every 0.5m)

6 Rigfloor Core Handling and Laydown

Once at the surface, the core must be quickly removed from the drill floor to allow further operations to be carried out without costly delay.  Fibreglass inner core barrels are ideal for this purpose.  However, surface handling and processing of fibreglass encased core demands special techniques and equipment, if core damage by inner barrel flexure and impact is to be avoided.  This section outlines key recommendations and equipment requirements.

6.1 General

• Company drilling representative, wellsite geologist & corehand must hold informal pre-core laydown meeting with rig crew & other key personnel to highlight importance of safe effective core handling, and to promote good teamwork
• The rigfloor breakdown of the core barrel, laydown of the core inner barrel, and breaking of the catcher will be led by the corehand

6.2 Inner Barrel Separation

• Inner barrels containing core must be laid out in 9m sections, if a corebarrel longer than 9m is used, the inner barrels must be separated on the rig floor
• Before the connection is unscrewed, a "shear device" is clamped around the connection
• The shear device is tightened onto the core, and breaks the core in a gentle and controlled manner
• The shear device must be driven through the core by a mechanical or hydraulic jack.  It has been shown by visual and X-ray CT examination that the use of a hammer-in shear devices damages core up to a metre from the joint
• The bottom of the upper inner  barrel is then securely capped, and the shear device removed
• A short stub of core will be removed from the lower inner barrel before the lifting eye can be attached.
• The mud logger involved with core preservation should catch this piece, seal it in a ziploc bag (or in cling film if it is too large)  mark it for way up, and with the core number and joint location
• The length of the piece must be included in core measurements.  If possible it should be added to the core in the position from whence it came during core processing

6.3 Securing the fibreglass inner barrel in the core cradle or core brace

After removal from the core barrel, the inner barrel must be transferred to a processing area, which provides a safe environment for the core processing team, and minimises disruption to drilling operations. This must be done without allowing the inner barrel to bend.  Core cradles or core braces are used for this purpose.

• Core cradles are rigid sections of "I" beam equipped with rollers and clamps to support and restrain the inner barrel.   The core cradle is suspended vertically in the derrick alongside the 9m inner barrel section and is secured to the inner barrel with a number of straps
• Core cradles must be rigid and in good condition (free moving rollers for barrel movement, secure lifting lugs, saw mounting points in good condition & alignment, secure means of fixing inner barrel to cradle)
• Core braces are sections of  modified casing that are large enough to accept a 9m inner barrel and end cap or shear device.   The core brace is loaded into the mousehole, and the section of inner barrel lowered into the brace
• When the inner barrel is secured in the cradle or brace, a tugger line is connected to the top, and the air-hoist line removed from the inner barrel pick-up sub
• One cradle or brace is required for each 9m inner barrel, i.e. 6 are required if  54m barrels will be run

6.4 Core Laydown

Various techniques are suitable for the successful laydown of core cradles or braces.

• Usually the best method for cradles is to lower gently down the pipe skid and onto the catwalk.  To this end, core cradles should be equipped with stabilising wheels at the base to ease laydown.
• The best option for laydown seems to be picking the cradle or brace directly from the rigfloor by crane, and then lifting by crane to the processing area

7 Core Processing

This is defined as core mark-up, cutting into manageable lengths, geological sampling, capping, and preservation.  A suitable location must be found for safe and effective processing.  This location must be out of the way of normal drilling operations, and shaded and sheltered to prevent core drying, and hazard to the core processing crew.

The detail of the core processing operation will depend on the exact subsurface study requirements for the core, and on the Sonatrach regulations covering core related activities

The following is an outline of the types of processes used:

• The wellsite geologist will find and mark the core top, and confirm core recovery
• Core mark up will then be led by the wellsite geologist
• Firstly  the inner barrel will be marked with parallel red and black lines (red to right looking up the core).
• Secondly depth markings will be applied in black
• Thirdly sample cut marks will be applied in black
• The core and inner barrel will then be cut
• Primary cuts will always be made at 1m intervals at the whole integer (rather than at fractions of a metre)
• The capping and catching and layout of core pieces on the core racks will be performed by the mudloggers
• Top caps will be briefly removed from the core pieces to allow inspection, but must not be left off such that core begins to dry out
• The wellsite geologist will carefully clean the core at each metre mark (by simple wiping - damping the core with any fluid is not acceptable as samples may go on to sampling for Sw measurements)
• The wellsite geologist will inspect the core at each metre mark, note any sedimentary feature
• The  geologist will take small chip samples from the face of the core samples which are going to routine analysis at each metre mark, if required.
• Samples which are going to be preserved should not be chip sampled
• When inspection and sampling are complete, the core samples will be quickly re-capped
• 20cm lengths of core for preservation will be taken to the core preservation unit

A possible core marking and cutting scheme designed for 20% preservation is as below:

(Depths are for illustration only)

7.1 Core Processing Equipment - Core measuring & marking

• Rags for cleaning fibre glass inner barrel, solvent for erasing marking errors, paint pens that will indelibly mark fibre glass inner barrel under rigsite conditions, good quality steel tape at least 9m long

7.2 Core Processing Equipment - Core Brace Transfer Stands

• Supplied by coring contractor
• If core braces are to be used in core laydown, then a system for transferring the core inner barrel to a core cradle for mark up and cutting must be provided

7.3 Core Processing Equipment - Core Cutting Saw

• Supplied by coring contractor
• Saw must be capable of cutting through inner barrel & core safely in one pass with minimal vibration
• The saw blade must be adequately guarded
• Core saw blades must be intrinsically safe, composite silicon carbide blades are not recommended as these can explode during use
• Spare saw blades must be supplied (at least 2 spare diamond blades for fibretube)
• Core must be cut dry
• Saw blade orientation must be fixed so that cuts normal to the long axis of the core can be made consistently, i.e. all core samples must be right cylinders
• Core support system in the saw and cradle must offer support for cutting short core sections (0.20m lengths for preserved samples), without hazard to the core or  operators
• Saw & cradle rollers should be "height compatible" it must be possible for one person to advance the core in fibre glass barrel along the cradle for sawing

7.4 Core Processing Equipment - Endcaps, clips and tools

• Supplied by coring contractor
• Used to protect core faces
• Secured with hose clips - Supplier to ensure that clips are correct size, in good condition & corrosion free (spray with light oil before dispatch)
• Coring company to supply good quality pneumatic or electric "screwdriver" (+ 1 spare) to secure caps & clips

7.5 Core Processing Equipment - Core Racks

• Recommended that 3 ½” drill pipe be secured together in pairs using cable ties.

7.6 Core Processing Equipment - Core Sampling for Geological Inspection

• Hammer, chisel, pens,  sealable bags - Supplied by wellsite geologist

7.7 Core Preservation By Wax Dipping

Each 20cm length of core that requires full preservation for subsequent special core studies will be carefully examined.

The core sample will be wrapped in an inert plastic film, aluminium foil and sealed by dipping in molten wax.  The plastic film (Saran or Barex) prevents fluids in the core contacting and reacting with the aluminium foil.  The foil is designed to provide the vapour barrier, assisted by the wax which also provides a cushion against external damage.

Because of the high ambient temperatures, core preservation should be carried out immediately that sample selection has been made, to prevent core drying invalidating results.

• Allow approximately 70 kg of wax to fill a bath 60x30x45 cm and 0.75 kg for each sample to be preserved.
• A high quality wax high melting temperature (100 deg. C plus) wax such as Coreseal is recommended.
• Switch on wax bath at least 4 hours before required.  Do not over heat the wax.  The wax temperature must be maintained at the recommended operating temperature during dipping operations.
• After identification and cutting of core piece, remove from inner barrel section and mark the core with way-up orientation lines (red and yellow paintstick).
• Wrap the core in a minimum of 5 layers of Saran or Barex wrap, then a minimum of 3 layers of heavy duty aluminium foil.   Ensure wrapping is tight to core, and ends are crimped neatly together.
• Clearly label the foil with way up marks (red and black felt tip pens, red on the right looking up as always) and a sample identifier number, e.g. C1-B
• The sample should then be sealed in wax.  Copper wire should be wrapped round the sample to form a handle and used to hold the core while dipping.  String should not be used as it can provide a permeable pathway for escaping fluids
• On first dipping, the core should be completely submerged in the wax until major bubbling ceases.  On second and third dips the core should be completely submerged for the minimum of time.   The core should be hung on the draining rack to cool between dips
• When cool after the third dip, check the wax layer for defects (i.e. air bubbles, insufficient wax coverage).  If required the sample should be re-dipped to ensure an effective seal
• Mark sample with paper label that shows well number, sample identification number, top and bottom depth and way up
• One final wax dip should be used to seal the label permanently, and the label checked to ensure legibility.  Illegible samples must be re-labelled
• The copper wire handle must be cut off and the exposed ends sealed with wax by dipping first one, then the other end of the core piece
• Wax-dipped core will be put in coreboxes (3 per box) and stored in a cool place.
• The wax required for preservation at high ambient temperatures will be at about 130 deg. C in the bath.  Appropriate personal protective clothing must be used while working with hot wax, i.e. overalls, gloves, safety glasses
• The condition of the wax must be continually monitored at all times while wax baths are in operation.  If it overheats it will darken and obscure core markings (change it).  In the event that serious overheating occurs it could ignite. The fire blanket should then be used to cover the bath, the power switched off, and the unit vacated until the bath has cooled to a safe temperature (suggest at least 2 hours allowed)

7.8 Core Wax Dip Preservation Equipment

All wax dipping equipment and consumables including will be provided by the mud logging contractors

8 Core Transportation

It is essential that all of the people involved with core handling are briefed as to the fragile nature of the core.  The following are some of general guidelines, though transportation is always area specific and a set of procedures should be developed for Algerian operations.

• All individual core tubes should be securely braced for transportation
• An inventory of the core should accompany all shipments.  A separate copy of this inventory should be sent to recipients via Fax along with an estimated time of arrival.
• All communications regarding the core consignment should be channelled through the wellsite geologist.  All communications should make it very clear that the core is fragile, and request gentle handling.
• The wellsite geologist will brief all of the handling teams at rigsite.  Operations contacts will brief all of the handling teams at points where the core will be handled en-route to final destination, and will also brief the receiving lab.