Downsized conventional drilling is the use of a fit-for-purpose rig to drill conventionally a well with a reduced casing scheme ending in 4 3/4" or smaller open hole, possibly with a 3 1/2" production liner. The potential for downsizing oil wells in some fields, with cost savings of up to 35% with no loss of production.

1 Possible applications

1.1 High pressure/high temperature wells

In HP/HT wells the ability to drill and evaluate small diameter holes can lead to a significant reduction in well cost. In some cases, slim hole drilling is the only practical way to reach the objective because additional casing strings are possible.

1.2 Offshore exploration wells

Wells from jackups and floating rigs can be downsized, while being drilled and evaluated conventionally. The reduction in the weight and volume of consumables extends the operational capability of the limited deck loads. Smaller "slim" drilling, riser, and blowout preventer packages could result in additional cost reduction in high water depth or high pressure capability.

1.3 New slim wells

Any well design where the expected production is less than 4000 b/d oil gross, or 50 MMscf/d gas, and where the final hole size is greater than 4 3/4" should be considered for slim hole design. This applies to vertical, deviated and horizontal wells.

1.4 Sidetracks, workovers, multi-laterals and recompletions

The cost and technical difficulties of directionally drilling sidetracks have fallen relative to the costs of workovers and recompletions and the technology of slim holes is fully compatible with extended-reach and horizontal drilling.

1.5 Infill and extended-reach drilling from existing platforms

Many existing platforms, have insufficient room for a full-sized drilling rig. A small rig, coiled tubing unit, or even a snubbing unit, can be used to drill slim sidetracks or new wells.If a platform is accessible by cantilever jackup, slimming down the well design could extend the usable drilling envelope. In some cases, existing platforms could be safely fitted with additional small conductors. It is possible to drill two wells out of a single existing full-sized conductor using a Splitter head.

1.6 Slim subsea producers

The development of smaller Xmas trees, templates and production manifolds could enable existing rigs to integrate the installation of manifolds, templates and (coiled tubing) flowlines with the drilling operation to the point where smaller hydrocarbon accumulations become economic.

1.7 Impairment and bypassed oil and gas

Drilling small holes through the reservoir has the potential to reduce impairment by:

• Using cleaner muds, (a smaller volume, allows more efficient solids control).
• Smaller volumes allow the use of more expensive, low solids drilling fluids.
• Improved kick detection techniques enable a reduction in the overbalance required.
• If a coiled tubing unit is used, the well could be initially perforated underbalanced and only killed on abandonment, since the monobore completion allows practically all well maintenance operations to be performed without killing the well.

2 Downhole data gathering

A comprehensive range of wireline logging and open and cased-hole production testing tools are available for slim holes. For cased-hole testing, the monobore concept allows higher vertical resolution of formation flow potential.

2 Conclusions

Use of slim hole drilling for exploration, appraisal and development is feasible as subsurface equipment suitable for slim hole drilling, evaluation and completion is available.

The downsizing of conventional wells, while maximising the use of existing rigs and service companies, can reduce the costs of a completed well by up to 30%.

In order to minimise costs, slim wells must be drilled with high penetration rates.

It is possible to complete slim wells with 3 1/2" production liners. A wide range of tools and equipment is available which can be adapted to meet the specific needs of various developments.

Use of 3 1/2" monobore completions has enabled slim wells to be produced at rates of up to 4000 b/d (640 m3/d) oil or 50 MM scf/d (1.4 MM m3/d) gas.