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

Abstract

Abstract This paper describes the technical basis for the design and implementation of a Coiled Tubing underbalanced drilling (UBD) campaign in the Sajaa field in Sharjah, UAE. The campaign calls for up to three multi-laterals per well to be drilled underbalanced from existing wells thru-tubing. The paper describes the analysis, modeling, result and their implication in the selection of the operational approach. The key issues associated with the campaign, many of which were unique and challenging within the context of UBD, and their consideration in the basis of design and implementation plan are discussed, with specific reference to the Sajaa-3 well, which was the first candidate. These include ■ Temperature modeling and thermal considerations due to the high bottom hole temperature, ■ Compositional PVT behavior of the produced fluid, ■ Feasibility, underbalanced operability and controllability of CT drilling thru tubing, ■ Risk to underbalance from near-wellbore depletion of the reservoir due to long-term production from the motherbore, ■ Estimation and impact of the productivity of the motherbore and open laterals while drilling subsequent laterals, ■ Risk from reservoir parameter uncertainties, ■ Hole cleaning risks, ■ Designing for zero flare, ■ Consideration of mechanical and hydraulic limitations 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 Sajaa produced fluid is a retrograde condensate, and the inflow performance was non-linear, and therefore consideration of these in the basis of design becomes critical. The goal of this work was not only to create the technical basis of design for the well Sajaa-3, but also to set the parametric limitations for a wide range of well types that were likely to be encountered in the continuation of the campaign. Based on this work, an approach to drilling and monitoring the well so as to minimize the risks was developed and formed the basis of the drilling operation. The campaign is now on its seventh well, and the results of the drilling operation of Sajaa-3 and the subsequent wells, and their implications to the design basis are also discussed.