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 includeTemperature 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.

A Methodology for Classifying Operating Regimes in Underbalanced-Drilling Operations

“…In the literature, we find that there can be different mechanisms for severe slugging, for example, terrain slugging (Taitel 1986;Jansen et al 1996), casing heading (Eikrem et al 2006(Eikrem et al , 2008, and density-wave (Hu 2004;Sinegre 2006). However, not one of these mechanisms matches exactly with the boundary conditions of constant liquid rate and drawdown-dependent gas rate, which are explored in this paper.…”

“…This allows the engineer to gauge the WHP and injection rates required to achieve the desired BHCP as well as satisfying holecleaning requirements. Publications referring to the use of such a technique are also numerous; see, for example, Saponja (1998), Guo and Ghalambor (2002), Udegbunam et al (2013), Nguyen et al (2009), Suryanarayana et al (2006), Guo (2002), and Pickles et al (2004).…”

Neutron and density logging responses to gas reservoir for well-balanced and under-balanced logging: Gas reservoirs of sandstone in a western China field

“…Now, there are two protecting methods, one is adding protecting reagent into gas reservoirs, the other is under-balanced drilling. Results indicate that under-balanced drilling can effectively protect gas reservoirs with low porosity and permeability [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] .…”