This paper covers a five well development program of horizontal and multilateral wells drilled underbalanced on coiled tubing in the UK. This paper discusses the reasons for drilling the wells on coiled tubing, details of the pre-planning, summarises the operational achievements and problems and describes the equipment utilised and the results of the final wells.


In 1994 a CTD system was commissioned on a test well in the UK. The success of the system on this first well was adequate for the field operator to immediately move into a staged development program.

The operator had several marginal fields. Many of the fields had only one or two wells these were drilled conventionally several years ago. Although the wells were producing, the economics of drilling new wells or re-drilling the existing wells often restricted the development of the fields.

Re-entry wells had been drilled with varying and sometimes unpredictable success. Often conventional hole problems, such as differential sticking and excessive losses had prematurely terminated drilling and it was felt that the wells never produced to their full potential due to formation damage and lack of horizontal section.

In 1994, the operator drilled their first well underbalanced to overcome some of the drilling problems encountered earlier. The well drilled further, faster and cheaper than the previous wells.

An immediate commitment was made to underbalanced drilling and many of the marginal prospects became viable.

The first well was drilled with gasified formation brine. However, to drill in other reservoirs where clay and shale layers were present, it was recognised that an oil based fluid was required. The commitment to reduce formation damage, also brought about a commitment to drilling with native formation fluids (brine or crude oil).

The safety aspect of drilling with a conventional rig, underbalanced with crude oil was a major concern of the operator. Drilling with coiled tubing allows the fluid system to be contained and controlled to a much higher degree. Therefore, after proving the coiled tubing drilling system, the operator was eager to move onto the next well.

Pre-job planning

The pre-planning stages required various considerations.

Well planning required CT considerations to be made. The profiles were originally designed to account for conventional formation limitations, namely overpressured or underpressured formations and fluid sensitive formations. CT simulations were then run to ensure the targets were achievable. Several of the wells designed were at the 'simulated limit' of coiled tubing drilling and caution would be required whilst drilling these wells.

The wellplans were established and equipment requirements were considered. The operation was considered as a 'drilling operation using coiled tubing' and the equipment requirements were very similar to those used by the operator for their earlier underbalanced drilling operations, using a rig.

All the coiled tubing equipment was standard offshore equipment, with minor modifications to accommodate drilling. The underbalanced surface system was designed, an hazop undertaken and equipment commissioned. Throughout the program the surface system was modified and improved to increase overall efficiency (see Fig. 1).

The coiled tubing drilling BHA was designed to be short and durable. It utilised all existing, field proven components with the exception of the orienteer.

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