Most Read

Latest

Contracting strategy for exploration drilling material

Contracting strategy guidelines for key drilling material to be used for exploration and new venture drilling operations (casing, wellhead, bits, mud cement, liner hanger, fuel)

1 Wellhead

General

Standardisation is very important. Wellheads can be passed from one company to another with relatively minor refurbishing of the larger parts.

Wells with conventional casing schemes should have 10,000 psi wellheads and be H 2S resistant, unless it is known that higher pressures will be encountered. 9 5/8" and 7" casing hangers should be ordered with premium threads so that they are universally applicable by using crossovers.

Onshore operations

For land wells the use of a "Unihead" design should be considered. This has several advantages over previous types which were used:

- There is increased well safety due to uninterrupted BOP protection at each casing point after the 13 3/8".

- Less time spent nippling down/up after the 13 3/8". Less time for inexperienced crew to work in high risk areas.

- Shorter so shallower cellar.

- Less steel, bolts, nuts, gaskets, etc. should be cheaper, lighter and easier to transport.

- Consider using a "reduced bore " design, with 13 3/8" casing as the conductor followed by 9 5/8" and 7". "Uniheads" are also available for use with this casing scheme.

Offshore operations

- Wells drilled from jack-ups should use the same wellhead as land wells.

- For use with floating rigs all major wellhead manufacturers supply weight set systems.

- If the bottom connection on sub-sea BOP stacks is not know yet, the wellheads may have to be ordered with the top connection "to be specified".

- For a single well two complete wellheads should be ordered. The second one should be taken on a "consignment" or buy-back basis.

- A decision has to be made whether or not a Temporary Guide Base (TGB) is required. Points in favour of using them are:

  • - Passive heave compensators only work after landing part of the weight, either on the TGB or on the bottom of the hole. Landing on the TGB allows the casing to hang vertically, whereas landing on the bottom of the hole increases the chance of the top section being inclined. It is essential that the upper section is vertical in order to guarantee BOP integrity.
  • - If the casing is landed on the bottom the length run will be slightly longer than necessary to ensure that the top end is above the mud line, resulting in a section of uncemented casing above the sea-bed and thus decreasing BOP stability.
  • - Without a TGB the BOP leverage increases with time. Its vibration progressively breaks down the cement around the upper section of the 30" casing, increasing the free length of the latter.

Points against their use are:

  • ·If there is little current they will tend to be buried by the cuttings produced while drilling the 36" hole and the cement returns while cementing the 30" foundation pile.
  • ·If the sea-bed is soft they will tend to sink into it.

It is recommended to discuss the requirements for individual cases with Company specialists, taking the results of the sea-bed survey into account.

2 Casing

Because of delivery time, the casing may have to be ordered while a decision has not been made on which prospect to drill. The exploration team should provide some idea on the type of prospect and order of magnitude of depth and formations. The design should then be based on this information, with allowance for more demanding conditions.

A few points to consider:

- An exploration well should always be considered as expendable.

- If the pressure regime is not known the casing should not be designed for the worst imaginable case. A pore pressure equivalent to a gradient of 13 ppg would be adequate in the majority of medium depth exploration wells.

- If the formation breakdown gradient is unknown, setting depths should be based on an average value taken rather than an extreme case. The burst pressure rating of the casing should be sufficient to contain a column of gas from the shoe when the breakdown pressure at the shoe is as high as can reasonably be expected i.e. in case of complete evacuation to gas, it should be ensured that any problem does not manifest itself at the surface

- In an unknown area, it should be assumed that H2S will be present. The wellhead will be H2S resistant and it is reasonable to specify L-80 casing as a standard, rather than N-80.

- Because of potential poor handling in remote locations, if a high strength casing is required it is better to go use a heavier L80 casing rather P110.

- If the casing will have to be carried by a helicopter at any stage it is better to go for lighter, higher grade casing.

- Because of contingency stocks, large amount of surplus casing left will be left over. Even if a light casing is sufficient to meet the casing design criteria, a more common casing should be specified. In the absence of other factors the preferred choice for 9 5/8" casing would be 43.5 or 47 lbs/ft L80, and for 7" would be 29 or 32 lbs/ft L80. Both of these sizes should be ordered with common premium connections (e.g. VAMTOP or BTC).

- If possible, casing should be chosen from the following list:

  • ·4 1/2" 11.5 lbs/ft L80 VAM
  • ·7" 29 or 32 lbs/ft L80 VAM
  • ·9 5/8" 43.5 or 47 lbs/ft L80 VAM
  • ·13 3/8" onshore - 68 lbs/ft K55 BTC
  • ·13 3/8” offshore - 72 lbs/ft N80 BTC
  • ·18 5/8" 87.5 lbs/ft K55 BTC or X52 EW
  • ·20" (for "floaters" only) 133 lbs/ft K55 with quick connectors

3 Mud and Cement

- Service company should be made responsible for the supply of the products required to do the job and all chemicals should be on "consignment" basis.

- For used mud, if possible, include a "buy-back" clause in the contract.

- Since barytes is a standard material, it is possible to consider purchasing barytes from a separate supplier in order to reduce costs.

- Mud and cement products should be charged on a product utilisation basis.

- Land transport should be included in the service contract.

- The effect on the environment of discharges has to be taken into account in the evaluation when comparing different mud systems.

- Required contingency stock for Barytes will depend on the cost of special shipments to replenish the supply, the delay involved, and the probability of requiring such a shipment.

- Packaging should be specified in the invitation to tender.

- Cement and barites should be supplied in "big bags" of 1 M.tonne for convenience in handling. The scope of work of whichever contractor will be doing the bulking should include the handling of these bags. This will be the drilling contractor on a land rig or either the base operator or the mud company itself for an offshore rig.

- If possible a Quality Control check should be made on the mud chemicals before shipment. This is especially to be recommended in the case of barytes which could have been obtained by the vendor from several different mines, and could also be someone else's returned contingency stock.

4 Bits

In a new area it is recommended to utilize specialist services to assist in bits selection and send specialists to the rig to assist the driller in optimising the operations.(ASE - Smith International, Oasis Engineering Services - Baker Hughes)

Arrange to have the bits on a consignment basis.

5 Liner hanger

Different agreements should be considered: call off on 36 hours notice, no fee to be paid until actually mobilised.

Check is that the connections of the sets on stand-by are compatible with the casing ordered.

If a call out option is exercised, and the hanger is not used, care must be taken to return the equipment in optimum condition. Failure to do this will result in the full price being charged for the hanger.

6 Fuel

If possible, contractors should be made responsible for providing his own fuel.

Fuel is the most important consumable in every phase of the operation, and that a guaranteed supply is essential. That applies not only to diesel but also to aircraft fuel.

Typical diesel consumptions are :

  • - Slim-hole land rig 2-3 tonnes/day
  • - Standard land rig 5-10 tonnes/day
  • - Jack-up 10-15 tons/day
  • - Anchored floater 15-20 tons/day
  • - DP floater 30-40 tonnes/day
  • - Supply vessel 5 tonnes/day
  • - Oilfield truck on rig move 250 kg/day/truck

Note that even for land operation it may be more economical (guaranteed supply) to charter a fuel barge and import a bargeload of diesel than to rely on local stockists.