6. Disolids controlharge reduction strategy

The effect of the withdrawal of OBM on drilling operations will require significant rig upgrades, logistics planning and reappraisal of drilling fluid selection. The costs associated with these changes will also be significant and the phase out strategy should be designed in the context of a business case to minize the impact. Given the uncertainty in a number of key considerations, it is important to retain a high degree of flexibility in any planning.

Planning a co-ordinated approach to phasing out the use of OBM involves comparison of the costs associated with the various options applicable to each drilling project on a well section by well section basis. Furthermore, the cost efficiency of the cuttings re-injection option is heavily dependant upon the level of drilling activity of each installation over a number of years i.e., the long term drilling sequence.

There are currently three options for managing the drilling waste from any well section.

1. The simplest is the use of WBM as this allows the unrestricted discharge of drill cuttings. Where the performance of WBM is compatible with the demands of the hole section to be drilled this should be the first choice for environmental and economic perspectives.

2. The second option is to use an OBM drilling mud in conjunction with a total containment system (cuttings re-injection or shipment of waste to shore for treatment and disposal). Where a containment system is planned cheaper mineral OBM is prefered to the more expensive synthetic mud. A containment system will have associated cost penalties but allows the use of high performance drilling muds where required.

3. The third option, rarely available today, is the use of synthetic mud and discharge of cuttings to the sea. Where this option is selected the use of optimum solids control equipment to minimise the amount of synthetic mud discharged will be important. In the short term improved solids control is likely to be an important consideration as it is possible to reduce the oil on cuttings (oil on cuttings (OOC)) level, and hence the volume of oil discharged, by approximately 50% at relatively low expense.

Where WBM is not cost effective for a hole section, as a result of performance penalties compared to OBM, the best alternative must be identified. If the Cuttings Re-Injection option is economically favourable, this will be the preferred option. It should be recognised that Cuttings Re-Injection is only mature in the context of platform drilling. Subsea re-injection is not available as routine technology. At this stage the only containment option considered for mobile rigs is ship to shore.

Once containment costs have been calculated the adjusted cost of using OBM should be compared again with the cost of WBM and associated performance penalties.

The option of cleaning drilling waste at the rig site and then disolids controlharging to sea is not currently available, although there is a considerable effort being made within the industry to develop the necessary technology to achieve this. This and a number of other technologies are reviewed below.