Coring Operational Planning, Guidelines and Procedures

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The requirements for coring shall be agreed with the Drilling Department. The Programme shall include the follwing:

  • the prognosed coring point (TVD - RKB or MD - RKB for deviated well)
  • the length of core to be cut
  • specifications for the determination of coring
  • specifications for determining when coring is complete
  • any special coring or packing requirements.

Preparation

The following preparation work shall be undertaken prior to coring.

  • Hole shall be conditioned prior to pulling out of the hole.
  • Once pulled out,the bit and BHA shall be carefully checked for broken and lost cutters. Unless there is a severe loss of cutters, a junk run shall not be made but the hole shall be circulated after running to bottom before commencing to cut core.
  • The last bit pulled shall be carefully checked for gauge. If the bit is more than 1.16in under gauge, consideration shall be given to reaming the hole with a full gauge bit and drilling assembly.

Conventional Coring Procedure

The following generic procedures shall be used to verify the detailed procedures proposed by the Coring Contractor.

1. RUN IN the core assembly slowly into the hole. BEWARE of hanging up in open hole.

2. If reaming is necessary PUMP at maximum allowable rate (determined by the core barrel specification and normal drilling engineering considerations ). DO NOT exceed 30 rpm and MAINTAIN minimal WOB. After reaming a section PULL BACK to check trip the reamed section.

3. TAG the bottom gently with high circulating rate without rotation until the mud weight in is similar to mud weight out. Typical circulating rates shall be:

8.1/2in hole     150 gpm

4. DROP the setting ball and when it seats MEASURE slow circulating rates (SCR’s ). START rotating and RECORD the pressures on and off bottom. If back flow is present before dropping the ball PUMP a heavy slug. Prior to the ball seating SLOW the pump.

5. APPLY starting WOB slowly with additional weight and rpm applied smoothly until the coring rate is maximised. WATCH carefully for any indication of torque increase, ROPdecrease, or pump pressure change.

The Driller shall inform the Drilling Supervisor of any change immediately, as changes shall indicate the following:

A pressure increase during coring shall be due to plugging of the barrel, “O” ringing or plugging of the waterways of the corehead or a change in formation.

If the ROP is simultaneously reduced, the corehead is probably ringed or plugged. Continuation in this condition will seriously damage the corehead.

A decrease in pump pressure and ROP, accompanied by erratic torque readings, indicates jamming of the core. The barrel shall be pulled out of the hole.

6. CUT core until the barrel is full or becomes jammed, the end of the programmed coring interval is reached, or cuttings indicate that the required section is cored.

7. CIRCULATE bottoms up, CONDITION the mud and POOH.

Observations While Coring

  • Barrel plugging can be checked by comparing the off-bottom pressure with that recorded prior to coring. If plugging is suspected the barrel shall be pulled out off the hole.
  • When making a connection or pulling off bottom, overpull shall be seen as the core catcher grips the core. Pull to a maximum of 2 200 lbs overpull, after allowing for drag. If the core fails to break, start circulating up to the maximum used while coring and hold the overpull until the core breaks.
  • Extreme care shall be taken when tripping with a core barrel. Flow checks shall be performed as normal when tripping out of hole, and any deviation from expected hole fill-up volume shall be investigated. When pulling out with a core, do not rotate and attempt at all times to minimise jarring or shock loads. The slips shall be set carefully.
  • POOH slowly and watch the well closely as the core barrel is a tight fit in the hole and acts as a piston. Swabbing the well can easily occur.

Oriented Coring

Oriented coring provides the data to determine the amount of dip and direction of tilt of the formations. Due to magnetic interference, orientated coring shall be done more than 65 ft below the shoe and two NMDC’s shall be run above the coring equipment. Additional checks shall be made as follows:

  • Identity that the main knife and centre punch are installed in accordance with manufacturer’s drawings.
  • Check that the electronic multi-shot survey instruments have sufficient battery life and memory for the duration of the coring and surveying.

Coring Unconsolidated Formations

The following guidelines shall be adhered to:

  • In unconsolidated formations, face discharge core heads, fibreglass inner barrels, extended pilot shoes and special core catchers shall be used.
  • Circulating rates shall be the minimum required to keep the hole clean and sufficiently cool the corehead. Typical circulating rates are 150 - 200 gpm for 8.1/2in hole.

Sponge Coring

Same as conventional coring but downhole formation fluids are kept at downhole pressure as far as possible. The difference is in the methods of core recovery and packing.

 

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