Cores are taken in order to measure accurately the reservoir parameters in hydrocarbon and water bearing formations, and for geological purposes. Whether a conventional steel/aluminium inner barrel coring or fibreglass inner barrel coring technique will be used for a particular well is specified in the drilling or test programme and is based on the expected formation conditions. In certain circumstances it may be necessary to change the technique, e.g. a pay zone may be found to be much more viable than expected.
1. Coring Point Decision
A number of criteria may be used to determine when to start coring:
1) The prognosed depth of the formation top to be cored in accordance with the Drilling Programme. This method is used in the case where the top of the objective zone has to be cored. The correlation of the seismic data with that of previous wells has to be very refined to be able to tell when to start coring in this case.
Another option is to circulate bottoms up every 2 m (6 ft) from the depth of the prognosed formation top and check for oil/gas, and any indications in the cuttings.
2) Oil/gas-cut mud, indications in cuttings etc.
3) A drilling break when penetrating into an over pressured or more porous formation, e.g. sandstone or limestone after drilling through the cap rock.
When encountering a drilling break or oil/gas is observed in the mud, drilling should be stopped and a flow check should be made, before circulating bottoms up to determine hydrocarbon indications in the mud and cuttings, or to identify a change in the lithology.
If hydrocarbon indications are good, Base should be consulted before pulling out of hole to start coring. If the indications are poor drill a further 2-3 m (6-10 ft) and repeat the observations above.
When coring has started the exact depth of the top of the core shall be determined. A similar determination has to be made when the core is pulled out and from the two set of depths an accurate figure for the cored interval can be determined. As a check on the depths determined above the mud logger's depths may be used.
2. Core Handling
General
Prior to coring the following procedures for recovering of cores should be observed:
1) Hold a meeting of personnel involved to confirm procedures and techniques involved.
2) Ensure that there are available adequate supplies of equipment to handle, label and dispatch the core samples. This equipment includes core boxes and trays with fitting lids, indelible black and red label pens, polythene sheets, rags, suitable labels, hammers and nails, core description sheets and core recovery forms, hydrocarbon solvent for 'cut colour' and 'cut fluorescence' tests. In case a fibreglass core barrel will be used also include a saw, end caps, jubilee clips, sandpaper and two handling socks.
3) Prepare the core boxes and labels as far as possible before the actual recovery of the cores.
4) Arrange adequate space for the cleaning and laying out of the cores and place the core boxes or trays ready to collect the core from the core barrel.
The cores are cut up at the well site and placed in the core boxes. They are accurately labelled for depth. The samples should be sent ashore as soon as possible for analysis & care should be taken to prevent the freezing of cores during storage & transportation. The onshore analyses include standard core analysis such as porosity, vertical permeability (Kv) and horizontal permeability (Kh), other special core analysis (e.g. relative permeability, mechanical properties etc.), & a full lithological description.
Conventional Cores
When the cores have been taken and are brought up in the conventional steel core barrel the following procedures should be observed:
1) Place the boxes or trays in the correct sequence so that the tops are facing the core barrel.
2) Feed the boxes in the correct sequence to receive the core from the core barrel. Box No 1 is the first box to be fed and it receives the bottom part of the core. Ensure that personnel handling the cores do not have their hands at any time between the core and the floor.
3) The bottom of the core barrel should never be more than 1 m (3 ft) above the drill floor, otherwise if part of the core samples fall out the result will be difficulty in identifying the bottom and top of this core. Once the core barrel is empty, the boxes should be taken one by one to the place set aside for cleaning and reboxing. The boxes may be moved by basket but not on a pallet.
When the cores have been moved from the derrick floor for cleaning and reboxing the following procedures should be observed:
1) Rebox the core from the boxes in which they were carried from the derrick floor into the final boxes. While doing so clean the cores with rags only. Do not use damaged boxes. The reboxing shall be done so that the original drill floor box No. 1 (core bottom) becomes the last box number from the final boxes e.g. DF Box No. 1 is now Box No. 20 of the final boxing. Where the core recovery is not 100%, decide which part is missing and state the reason in the report. If this is not possible then it is always assumed that the bottom part is missing. Labels, inserted in small plastic bags, giving the following information shall be securely fixed on to the inside top rim of the core boxes:
Well number; Core number; Depth; Total percentage recovery; Interval.
To identify the top and bottom of the core draw red and black lines parallel to each other on the core so that looking from the bottom of the core upwards the red line is to the right.
2) Make a core description. Take a few evenly spread small samples for the cut colour and cut fluorescence hydrocarbon tests and for a detailed description of the lithology. No other tests or analyses are carried out at the well site.
3) Make out the required core recovery and sampling record in triplicate detailing the well number, core number, box numbers and intervals. One copy accompanies the core inside the first box, one is retained offshore and the other is sent onshore to the Geologist. A copy of the core description sheet shall also accompany the core inside the box.
4) Prior to shipment pack the box with rags to minimise movement of the core in the box and include all the required papers (protected in a plastic cover). Close the core box lids as soon as possible and ensure they are nailed down securely. Label the lids of each box to show the box number and the total number of boxes, e.g. Box No 3 out of 20, and also the core number.
Fibreglass - Sleeve Cores
When the cores have been taken and are brought up in fibreglass inner core barrels the following procedures should be followed:
1) Break the inner core barrel into 9 m (30 ft) sections in the rotary table. Put a clamp on the bottom part of the inner barrel and lift the top part of the inner barrel (using the block or a tugger) into a handling sock. Clamp end caps on each end of the core barrel.
Lift the bottom part of the inner barrel into a handling sock in the same way as above, and clamp on end caps. Lift both socks with the crane and lay them down in a convenient place on deck.
Make sure that handling sock is constructed such that it can take a 5 1/2" core and whatever device attached to the inner barrel during lifting.
Note: Remember to mark which is the lower and upper barrel.
2) Starting with the lower barrel, remove the core from the handling sock without bending and unscrew the metal shoe, catcher (from the bottom core barrel) and connector at both ends. Measure the length of core by pushing it to the bottom in the barrels. A first indication of recovery can be given. Mark the fibreglass clearly with black and red parallel lines (the red line on the right side). Use waterproof pens and ensure the fibreglass is dry and sanded otherwise it will not mark.
3) Measure the core and mark out 1m sections starting at the bottom of the sleeve. Mark the top (T) and the bottom (B) and number each section starting at the top of the core e.g. the top section of core number 5 is numbered 5.1.
4) If there are any indication of the core being pressurised (strong smell of gas, gas bubbling 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 attempts 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.
As an alternative if required, small holes can be drilled in the core barrel to allow any gas to escape before cutting it into sections. Check the identity of any gas with Draeger tubes. (If resin injection is going to be performed, the method of drilling holes in the core barrel is obviously not appropriate).
5) Cut the core into sections with a rotary saw. Calculate the core recovery, reconciling this figure with depth cored.
6) Take samples for lithology/biostratigraphy from the top and bottom of each section and number them from the top sample e.g. the first sample in core number 7 is numbered 7.0.
7) In order to stabilise the core inside the barrel, resin injection should 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 people involved should wear protective clothing and goggles. Resin should only be used in open air. The Well Site Drilling Engineer should make sure that adequate safety measures are taken.
If there is a concern that the mud inbibition may change the wettability of the core, it should be considered to take samples from some ends of the 1-metre lengths and rush these to the laboratory for measurements.
8) The ends of the core will to a certain extent be sealed by resin. In addition both ends of the inner barrel will be capped by clamped on plastic/rubber caps, possibly in combination with a silicon gel and strong adhesive tape. The hole in the cap through which the resin is injected should be sealed with gel and tape.
9) The core boxes must be of an adequate size to fit the approximately 6 1/4" inner core barrel (OD) and the padding which will surround the core. Ensure that the core is padded adequately, using ample rubber foam and/or air bubble film. The core shall be unable to move once the lid of the core box is secured.
Include all the required papers in the boxes and close the boxes securely and label them in the same way as described above.
Padding should be put in between each layer of the core boxes in the container. If times allows, the container should be loaded onto a supply boat in calm sea and the crane operator should be made aware of the contents of the container and be instructed to be very careful.
It should be considered to transport some of the core boxes by helicopter.
Note: Two handling socks (each 9 m (30 ft) long and approx. 8" I.D.) should be available on the installation.
3 Reporting
General
1) All relevant data on which the decisions are based, or are to be based, when to start or finish coring shall be reported on the Daily Drilling Report.
2) A summary of the core, giving recovery figures, a general lithological description including hydrocarbon indications should be reported to Base as soon as possible. Similarly information regarding indications whilst coring such as cutting description, gas readings and penetration rate should be reported. The detailed core description shall be distributed to the same addressees.
3) The mud logging contractor prepares a blank core grapholog just showing the lithological column.
4) The core description should be sent to Base as soon as possible and should therefore be written clearly. The original should be sent ashore and a copy of the original description should be kept on the well site.
Core Description
1) The actual core description is preceded by the following data: Drilling unit, well, date and time, number of core, cored interval, recovery, formation cored, size of core and type of corehead.
2) If only part of the core is recovered state any observations which may indicate which part of the core is missing.
3) The core description should be comprised of the following details in the appropriate columns:
1. Depth. A suitable depth scale may be freely chosen, e.g. 1 div./m, 2 div./m or even a hybrid scale to allow for thin lenses, although scale changes should be kept to a minimum.
2. Indicate with the symbol > where samples were removed from the core. The type of sample should be noted with a P (porosity), S (saturation), W (well site), or Pa (palyeontology).
3. Lithology - as per "tapeworm".
Colour - as per "tapeworm". The colour test for fluorescence is described below.
5. Composition - as per "tapeworm".
Sedimentary and Diagenetic Structures - use words and/or symbols.
7. Cementation and Porosity whenever applicable.
8. Fluorescence. Fluorescence tests are described below. Spots/lenses and zones should be indicated by the symbols > and _ respectively (see "tape worm").
9. Gas %. Spot values or a continuous line may be used.
Note: The 'tapeworm' is a small guide to basic oil/gas geology.
Hydrocarbon Tests
These tests are made to record the fluorescence colour of core samples and the results are reported in the core description. The tests are made as follows:
1) Direct Fluorescence
Fill watch glass with fresh rock and record the sample fluorescence as follows:
Blue/White
White
Yellow
Yellow/Brown
Grey
Report the fluorescence intensity on the following scale:
Zero
Weak
Moderate
Good
2) Solvent Cut
Fill one inch of a test tube with fresh rock. Add hydrocarbon solvent and report the colour of the cut as follows:
Colourless
Light Yellow
Yellow
Yellow/Brown
Dark Brown
Green/Brown
Report the solvent cut types as follows:
Slow
Blooming
Streaming
3) Solvent Cut Fluorescence
Report the solvent cut fluorescence as follows:
Blue/White
White
Milky-White
Green-Brown
Report the solvent cut fluorescence intensity on the following scale:
Zero
Weak
Moderate
Good