|Development Geology Reference Manual|
|Series||Methods in Exploration|
Core handling is a critical phase in the core acquisition process. There are common techniques to handling both “hard” rock and “soft” sediment cores. All handling procedures are aimed at ensuring proper labeling, minimizing damage, and carefully transporting for analysis. In all cases, one should have all equipment prepared before the core is pulled. Every effort should be made to prevent the core from weathering or changing if s fluid contents. Speed is important when handling hard rock to prevent alteration of reservoir and character fluids. In contrast, patience is paramount when handling soft cores to prevent mechanical damage.
Labeling is probably the most important and visible feature in wellsite core handling. Mud should be wiped off the core or inner barrel to get a good marking surface. Do not wash the core. Washing changes the fluid content or wettability and impacts the subsequent analysis.
All cores should have orientation marks. The general convention is to use two different color markers and draw parallel lines on the core or inner barrel. Each operator has preferences for colors and arrangements, such as red and black with red on the right. Up arrows are marked on every segment of the core to ensure proper orientation.
Hard rock core handling
Hard rock cores represent a major workload for the wellsite geologist. The core must be unloaded from the core barrel on the rig floor and taken elsewhere for processing. Safety in the extraction process is paramount. Heed the following rules:
- Hold a safety meeting with all floor personnel and make sure everyone will work at your direction and speed.
- Everyone on the floor should wear eye protection.
- Make sure the driller has a good view of the operation.
- Only one person gives directions to the driller.
- Never put anything (such as hands or feet) under the core.
- Always run the core barrel back to the floor before releasing the core. Stand the barrel on the floor and raise the barrel. This prevents the core from dropping and shattering on the floor. An alternative is to extrude the core on the catwalk.
- Never raise the core barrel more than length::18 in before breaking off.
- Bag all core rubble.
- Support core to keep it from breaking while moving to core boxes.
- Thank all the floor hands for a job well done.
Move core in core boxes marked with a box number and “top” and “bottom” on each box. Guarantee that core is placed in proper orientation in the boxes. Scribes should be used on hard rock cores to both orient the core and help with reassembly (see “Core orientation”). If the catwalk or pipe rack is the location of choice for reassembly, two joints of pipe tied together make an ideal brace for handling.
After the core is laid out, the following should be done:
- Wipe the core clean; do not wash it.
- Use the scribe lines to reassemble the core; put the primary groove up.
- Draw the twin orienting lines on the core; put the primary line in the scribe groove.
- Mark each segment of the core with up arrows.
- Determine where missing footage is located and space it out appropriately. Bag rubble and place it with the core. If the depth interval of missing core is unknown, place it at the bottom of the core.
- Mark the core with depths. Proper marking is critical to tying core footage to wireline measurements. Mark the depth on each segment of the core.
Note that not all core is lost from the catcher at the bottom. Compare the rate of penetration and lithology of the core. Shale sections will often break up when entering the catcher and be lost. Spin marks are generally seen in the shale sections and may represent lost segments. When taking multiple cores, examine the top segment for additional catcher marks that represent recovery of a post from the last core. Be sure to include this when depth marking the core.
Soft sediment handling
Soft sediment core represents an especially difficult handling problem. Soft implies that the sediment is not cemented and therefore has little internal strength. Disturbing the grain orientations will negate the ability to measure porosity, permeability, and other reservoir properties accurately. The sediment can be disturbed in several ways:
- Gas expansion while tripping out of the hole
- Slumping caused by rotating or jarring the core barrel while tripping
- Slumping caused by rough handling of the core barrel at the surface
- Slumping caused by vibration in transport
- Shearing caused by flexing the inner barrel
Soft sediment cannot be removed from a steel barrel without disrupting it. Therefore, a disposable inner barrel made of fiberglass, PVC, or aluminum can be used. The inner barrel is generally mated to a specific core catcher design (see Conventional coring).
Well-designed equipment cannot alleviate the problems caused by poor handling. Since the core is hidden in the inner barrel, these problems are not noticed unless the core is CAT scanned. Scanning will determine the extent of damage and is an excellent aid to picking analysis plug points. Based on experience, the following steps are recommended to prevent damage:
- Use a perforated inner barrel to allow for gas expansion.
- Do not rotate the drill string while tripping out.
- Set the slips softly to prevent jarring of the core.
- Stop length::100 ft below the rotary table to let the core degas.
- Use a brace for the inner barrel to prevent it from flexing while laying it down.
- Do not rotate, bump, or jar the inner barrel while laying down the core or cutting it into sections.
- Freeze the core to prevent disruption during transport.
- Drilling problems
- Mudlogging: drill cuttings analysis
- Introduction to wellsite methods
- Wellbore trajectory
- Conventional coring
- Drilling fluid
- Land rigs
- Rig personnel
- Wellsite math
- Mudlogging: equipment, services, and personnel
- Core alteration and preservation
- Wellsite safety
- Sidewall coring
- Show evaluation
- Rate of penetration
- Mudlogging: the mudlog
- Well planning
- Pressure detection
- Drill stem testing
- Measurement while drilling
- Offshore rigs
- Core orientation
- Mudlogging: gas extraction and monitoring