Basis of Design for Coiled Tubing Underbalanced Through-Tubing Drilling in theSajaa Field

摘要

This paper describes the technical basis for the design andimplementation of a Coiled Tubing underbalanced drilling(UBD) campaign in the Sajaa field in Sharjah, UAE. Thecampaign calls for up to three multi-laterals per well to bedrilled underbalanced from existing wells thru-tubing. Thepaper describes the analysis, modeling, result and theirimplication in the selection of the operational approach. Thekey issues associated with the campaign, many of which wereunique and challenging within the context of UBD, and theirconsideration in the basis of design and implementation planare discussed, with specific reference to the Sajaa-3 well,which was the first candidate. These include Temperature modeling and thermal considerationsdue to the high bottom hole temperature, Compositional PVT behavior of the produced fluid, Feasibility, underbalanced operability andcontrollability of CT drilling thru tubing, Risk to underbalance from near-wellbore depletion ofthe reservoir due to long-term production from themotherbore, Estimation and impact of the productivity of themotherbore and open laterals while drillingsubsequent laterals, Risk from reservoir parameter uncertainties, Hole cleaning risks, Designing for zero flare, Consideration of mechanical and hydrauliclimitations to the drillable lengths, and Equipment limitations.The paper details the analysis approach used, assumptions,models used, and results. It should be noted that the Sajaaproduced fluid is a retrograde condensate, and the inflowperformance was non-linear, and therefore consideration ofthese in the basis of design becomes critical.The goal of this work was not only to create the technicalbasis of design for the well Sajaa-3, but also to set theparametric limitations for a wide range of well types that werelikely to be encountered in the continuation of the campaign.Based on this work, an approach to drilling and monitoring thewell so as to minimize the risks was developed and formed thebasis of the drilling operation. The campaign is now on itsseventh well, and the results of the drilling operation of Sajaa-3 and the subsequent wells, and their implications to thedesign basis are also discussed.