Abstract

This paper covers a one year drilling campaign with a Coiled Tubing Drilling unit in Petroleum Development Oman (PDO). The objective of the campaign was to examine both the technical and the commercial potential of this drilling technique. Fifteen wells were drilled ranging from conventional sidetracks, which formed the main application in PDO, to niche applications such as through- tubing underbalanced multilateral wells. The work scope for the conventional sidetracks included pulling completions, sidetracking from 7" or 9–5/8" casing, drilling and casing build-up sections in 6–1/8" hole and drilling and lining the horizontal section before re-installing the completion. The performance of drilling conventional sidetracks with a coiled tubing unit has been compared with PDO's conventional drilling rigs. The paper concludes that conventional sidetracks do not yet present an attractive application of Coiled Tubing drilling in PDO and highlights the required enhancements of equipment to make coiled tubing units an attractive alternative to rotary drilling rigs. A niche application of drilling through-tubing with Coiled tubing was successfully applied, including drilling underbalanced Moreover, through-tubing multilaterals have been drilled successfully, both overbalanced and underbalanced, in order to access the remaining oil in the mature Yibal field. The system set-up and results will be presented.

Introduction

Within PDO's total producing well population of around 2000, some 800 wells were identified in 1993 as good candidates for re entry and sidetracking utilising Coiled Tubing Drilling (CTD). To establish the merits of CT for this large scope a three well trial programme was initiated to establish the technical feasibility of drilling sidetracks with Coiled Tubing followed by a larger campaign to test the commerciality of the technique. During 1994, three CT Drilling trials were executed which demonstrated the technical feasibility of drilling, evaluating and completing sidetracks with a coiled tubing unit.

Following on from this work a 15 well CTD campaign was executed in 1996, with the objective of demonstrating that the CT Unit could perform workover sidetracks at the cost level of a conventional drilling rig and achieve cost reductions when given a larger scope. Due to the uncertainties in work scope, lumpsums were not found to be workable and one, day rate based, integrated contract was put in place for all services.

The 15 well campaign also covered some non commercial trials with the objective to test the feasibility of short radius and thru-tubing UB applications. The sequence of the wells drilled are shown in Fig. 1 where the commercial and technical trials are shown. Six commercial trial wells have been drilled in the Fahud field which will be reviewed in this paper. The technical thru-tubing trials in the Yibal field will also be reviewed. The other wells drilled in Rima will only briefly be discussed in this paper since the results are similar to the Fahud field results.

The unit consisted of a trailer monted CTU, control cabin, injector head with 80,000 lbs maximum pulling capacity; a CT reel with 2700 m 2–3/8" OD CT and second reel with CT and internal electric line (E-line hepta cable), a substructure with a running and pulling system (integral 200,000 lbs telescopic jacking system and a fairly standard fluids handling system for OB operations.

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