In spite of its complexities, foam underbalanced drilling has unique advantages such as reduced formation damage, improved rate of penetration, higher cutting-transport capacity, and lower circulation losses. Recent experimental studies in a wide range of experimental conditions show that foam flow in pipe can be represented by two different flow regimes: (i) the low-quality regime showing stable plug-flow pattern with fine-textured foams and (ii) the high-quality regime showing unstable slug-flow pattern with alternating free-gas with fine-textured foam segments. Such a new concept was successfully captured by a foam modeling technique with five model parameters, reproducing two distinct pressure contours.This study, for the first time, constructs foam drilling hydraulics model with two foam-flow regimes handling both stable and unstable flow characteristics, and investigates how the model improves current foam drilling hydraulics modeling based only on stable foam flow behavior. The results from three different scenarios (scenario 1: a vertical well, scenario 2: a deviated well with relatively short horizontal section, and scenario 3: a deviated well with relatively long horizontal section) with the transition foam quality between the two flow regimes (f g *) around 75-85% showed that significant differences in drilling hydraulics calculations might occur when the conventional technique was replaced by this new technique. For example, a drilling scenario tested with 10,000 ft deep vertical well (scenario 1) was shown to have as much as 32%, 48%, and 25% deviations in foam density, total velocity, and pressure gradient, respectively, at the bottomhole. Examples from two other deviated well trajectories (scenario 2 and scenario 3) exhibited similar responses, showing even more deviations. These results clearly demonstrate why incorporating two foam flow regimes into the current drilling hydraulics calculations is a crucial step toward evaluating and designing foam drilling practice accurately and reliably.Recent advances in drilling technologies have provided a unique opportunity to explore new horizons for oil and gas industry such as development of deepwater reservoirs, productions from tight formations, and advents of shale gas/oil resources, to name a few. These resulted in increasing drilling activities in the U.S., evidenced by rig count, in the past decade as shown in Fig. 1 -not only horizontal drilling has become popular reaching nearly 61 % in late 2012, but vertical and directional drilling also has become more challenging due to ever increasing well depth and water depth. This trend implies that more and * , m 1 , m 2 , K, and n), if the pressure contours in both high-quality and low-quality regimes can be approximated by a series of straight lines. Fig. 4 shows an example from Edrisi and Kam (2014) where how the model captures experimental data and how such two flow regimes can be reconstructed from the model are presented. Two parameters are employed to delineate the straight pressure contours in bo...
