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

Abstract

Summary This paper describes the technical basis for the design and implementation of a coiled-tubing (CT) underbalanced drilling (UBD) campaign in the Sajaa field in Sharjah, UAE. The campaign calls for up to three multilaterals per well to be drilled underbalanced from the existing wells' through tubing. The paper describes the analysis, modeling, and 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, are discussed, with specific reference to the Sajaa-3 well, which was the first candidate. These include Temperature modeling and thermal considerations because of the high bottomhole temperature (BHT). Compositional pressure/volume/temperature (PVT) behavior of the produced fluid. Feasibility, underbalanced operability, and controllability of through-tubing UBD operations. Risk to underbalance from near-wellbore depletion of the reservoir caused by 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. Equipment limitations. The paper details the analysis approach used, assumptions made, models used, and the results. It should be noted that the Sajaa produced fluid is a retrograde condensate and the inflow performance was nonlinear and, therefore, consideration of these in the basis of design (BOD) becomes critical. The goal of this work was not only to create the technical BOD 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. On the basis of this work, an approach to drilling and monitoring the well to minimize the risks was developed, and it formed the basis of the drilling operation. The ongoing campaign, now well past 30 wells, has been one of the most successful applications of CT UBD. The results of the drilling operation of Sajaa-3 and the subsequent wells and their implications to the design basis are also discussed.