Thru-tubing sidetracking can be the only economically viable way to extend the life of a mature field. In wells using Duplex 25% chrome casing, this option has been unavailable because of the near impossibility of cutting the casing. This paper describes the development of a new system for thru-tubing milling of sidetracks in difficult casing materials and its successful application in a mature North Sea field to produce reserves previously considered out of reach.


After nearly ten years of field production, total platform production rates from Amerada Hess Scott platform in UK North Sea had declined from initial rates of 200,000 bopd to 60,000 bopd, with some early wells producing less than 2,000 bopd.

The operator identified nine potential sidetrack locations from existing platform wells with unswept or attic oil in place. The volume of oil in place that could not be produced from existing wellbores was estimated to be up to 4 MM bbl.

There were two fundamental problems to be overcome in order to conduct cost-effective sidetracking operations. The first and most serious was the presence of Duplex 25% Chrome liner systems in the candidate wells. The second was the requirement that sidetracking had to be a thru-tubing operation to avoid the cost of pulling the completion and recompleting the wells.

With no previous industry experience in thru-tubing windowmilling operations in Duplex 25% Chrome, the project would not be feasible unless this technology could be developed. Cost estimates of up to $14,000,000 for a conventional sidetrack offered an unacceptable return on investment. Coiled tubing would have been less expensive but was ruled out because of the wellbore depth limits of coiled-tubing equipment available at the time.

The operator, with the support and cooperation of Schlumberger, a partner in the project, solicited proposals for a solution and chose a proposal from Baker Oil Tools to jointly develop technology with the operator to meet the challenge. Approximately nine months after acceptance of the proposal, a new thru-tubing rotary drilling (TTRD) system was ready for use. It successfully completed sidetracking operations on the first three candidate wells. Cost savings with TTRD technology are estimated at up to $5,000,000 over conventional sidetracking.

Business Case

The operator was faced with a difficult decision concerning the future of the field. Conventional sidetracking costs were escalating to a point where they could not be justified. Unless the attic oil and smaller compartments could be accessed at a reasonable cost, Scott production would decline, causing loss of revenue and reduced oil recovery from the field. The operator believed that the TTRD approach provided the best means to access the "stranded" oil and a project was embarked upon to modify the platform's hardware and source the necessary technologies for this to take place. It was not well understood by the operator at this stage that thru-tubing 25% chrome casing exits had not been performed in the past.

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