Flowing bottom-hole pressure prediction in Concentric Casing Gas Injection (CCGI) drilling operation using mechanistic models requires the determination of gas-liquid two-phase flow patterns along the annuli. This paper proposes two flow pattern transition models, namely bubble/dispersed bubble (BL/

Flowing bottom-hole pressure prediction in Concentric Casing Gas Injection (CCGI) drilling operation using mechanistic models requires the determination of gas-liquid two-phase flow patterns along the annuli. This paper proposes two flow pattern transition models, namely bubble/dispersed bubble (BL/DB), and intermittent/annular (IN/AN) in CCGI upward co-current concentric annuli flow of a gas and pseudoplastic non-Newtonian liquid two-phase mixture. The effect of shear-thinning rheological behavior and annular geometry were incorporated into the models ́ development. Sensitivity analysis was performed to investigate the effect of shearthinning fluid behavior on both transition boundaries. The results reveal that the shear-thinning behavior of the liquid phase has a significant effect on the transition boundary of BL/DB, but has a minimal effect on the IN/AN transition boundary. The findings of this study improve not only the hydrodynamic predictions in CCGI, but also the performance of the mechanistic models of gas and non-Newtonian liquid mixtures. Introduction A technical option to drill thru consolidated and highly depleted oil reservoirs is Concentric Casing Gas Injection (CCGI) which could be classified as a Managed Pressure Drilling (MPD) method. It is potentially suitable to maximize productivity and ultimate recovery by reducing formation damage when drilling through pay zones in mature oil fields without lessen heat transfer and directional control capabilities at downhole conditions. The design and operational issues of CCGI have been described by previous authors (Mykytiw et al., 2003; Beltran et al., 2010) as well as some field applications of the method in Mexico (Urbieta et al. 2009; Perez-Tellez et al., 2009; Rodriguez et al., 2013). Practically, it can be assumed that flow is under steady state condition when drilling with a CCGI system and the gas injection point is located in the vertical section as shown in Fig. 1. While using CCGI drilling, a co-current gas-liquidsolid mixture is flowing upwardly inside of the primary annulus above the gas injection point. Unweighted oil-based mud (e.g. invert emulsion) is used as drilling fluid to reduce the bottomhole pressure and hence reduce formation damage and costs associated with drilling fluid losses. In this paper, solid phase is considered part of the liquid phase for modeling flow pattern transitions. Mechanistic modeling of gas-liquid two-phase flow involves flow pattern definition and flow pattern transition criteria. Gas-liquid two-phase models and flow pattern transition criteria were initially developed for a gas and Newtonian liquid mixture in pipe flow. Only a few comprehensive studies concerning to gas-liquid two-phase upward vertical flow in annuli exist in the literature and they are mainly for Newtonian liquid phase. The goal of this work is to develop flow pattern transition criteria which take into account the shear thinning rheological behavior of unweighted oil-based drilling fluids and the nonlinear shear-stress distribution within the annulus. Studies of Non-Newtonian fluid flow in an annulus have received much less attention in comparison to Newtonian fluid flow in pipe. Friction factor is the main parameter of interest in most studies. The flow of non-Newtonian fluids in annuli is quite different from the flow in pipes. For pipe flow, zero shear stress and maximum velocity coincide at the center of the pipe while shear stress varies linearly with the distance from the inner wall. For annular flow, flow is not symmetric; shear stress does not decline linearly with the distance from inner walls. The case of laminar flow in concentric annuli has a theoretical solution for stress distribution (Frederick and Bird, 1958), but transitional and turbulent regimes are still without analytical solution universally accepted. The aim of this paper is to introduce new flow pattern transition criteria that can be applied to improve flowing bottom-hole pressure estimations while drilling with CCGI. Namely, bubble/dispersed bubble (BL/DB), and intermittent/annular (IN/AN) transitions in co-current twophase flow of a gas and pseudo-plastic non-Newtonian liquid mixture in vertical upward concentric annuli. To support the motivation for this work we added some background and fundamentals at Appendix A as a supplement to the topic of incompressible single-phase flow of power-law fluids through annuli. AADE-19-NTCE-SPP-06 Modeling Flow Pattern Transition in Co-current Two-Phase Gas-Non-Newtonian Liquid Flow in Vertical Upward Concentric Annulus Víctor H. Lema and Tan C. Nguyen, New Mexico Institute of Mining and Technology; Eissa Al-Safran, Kuwait University 2 V. H. Lema, T. C. Nguyen and E. Al-Safran AADE-19-NTCE-029 Development of models The bubble/dispersed bubble (BL/DB) transition. In the present work we followed the same phenomenological description suggested by Taitel et al. (1980) and developed for steady upward gas-liquid (Newtonian) two-phase flow in circular tubes. Transition occurs at high liquid flow rates with liquid as the continuous phase and gas as the dispersed phase. At turbulent conditions, shear stresses tend to break up the larger bubbles, inhibiting agglomeration and coalescence of bubbles. Then, the transition is dominated for a force balance between turbulent force of the liquid (disruptive force) and surface tension force of the bubbles (restorative force). The disruptive force acts on the bubble to deform it, whereas the restorative force refrains it from deforming. Dispersed bubble flow pattern is marked by the absence of cap or deformable bubbles, only tiny spherical bubbles remain dispersed in liquid as a homogeneous mixture. At certain level of liquid turbulence there is a maximum stable diameter of the dispersed phase, dmax, which characterizes the dispersion. At this size, bubbles can be deformed, their motion is in a zig-zag path and, if their agglomeration approaches a void fraction value of 0.25, Taylor bubbles can start to appear and transition to slug flow will occur. This size, dmax, is determined by the following expression (Hinze, 1955)