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Traditionally, several multiphase hydraulic calculations are required to determine the permissible surface-parameter operating envelope in underbalanced drilling (UBD) operations to estimate how bottomhole pressure (BHP) and the desired underbalanced condition can be achieved. Furthermore, the complexity consists of running several combinations of gas- and liquid-injection rates and overlapping these results with operating constraints, including the operating range of equivalent liquid rates for downhole mud motors, minimum annular velocities for effective hole cleaning, desired BHP, and respective surface-equipment limitations, to visualize a comprehensive spectrum of viable surface parameters. This paper explains and illustrates a novel method for obtaining quick and comprehensive operational envelopes for underbalanced drilling operations. The model used for pressure-drop calculations considers the effect of geothermal and string temperatures, detailed drillstring and wellbore geometries (including tool joints), formation fluid influxes at several depth intervals, cutting-slip effects, and surface back pressure. This model also enables the evaluation of the behavior of multiphase drilling fluid using different gas correlations. The system determines bottomhole pressures for several combinations of injection parameters and automatically collects results to generate a permissible operating area (envelope). Measured times to generate the operating envelope graph and a comparison of the results are evaluated for several methods and wellbore hydraulic simulators; a qualitative evaluation and validation against field data is also included. The method outlined in this paper offers an alternative to automatically obtaining a gas- and liquid-injection operating envelope, which results in a time savings of three orders of magnitude, as compared to traditional procedures and computational solutions. One of the limitations of the model is that the operating envelope is for a single wellbore depth and provides a representation of the pressure achieved at a fixed depth. Recommended developments will consist of the ability to visualize BHP values for drilling depths, perhaps through a 3D graph. This paper describes the method used by a multiphase flow simulator to create a graph that aids in determining and visualizing the operational area to achieve a specific BHP, including the hydrostatic and friction-dominant areas of circulation; this development will facilitate the decision-making process while designing the wellbore hydraulic aspects of an underbalanced drilling operation. The benefits of the multiphase flow simulator include a significant time savings to obtain a visual representation of bottomhole conditions and the ability to quickly achieve optimized results by means of a broader sensitivity analysis.

One of the most important factors when designing an aerated-fluid drilling operation is the volume of injected gas and liquid necessary for the downhole mud motor to achieve managed pressure drilling (MPD) conditions. This paper discusses a new method for calculating the equivalent liquid rate (ELR) of the gas/liquid mixture. ELR can be equally treated as the liquid volume to measure mud motor performance and can also be used to refine the permissible MPD operational window. During aerated-fluid drilling, gas/liquid are mixed to form one complex flow regime along the wellbore. To model its flow rheology and mechanisms, both foam and multiphase flow models are used. ELR is calculated by using the surface gas injection rate (GIR)/liquid injection rate (LIR), drillstring configuration, wellbore trajectory, and wellbore geothermal allocation. This calculation also considers tool joint effects and formation influx from downhole and parasite string injection. This paper also describes the impact of ELR to form the MPD operational window by considering GIR/LIR, minimum/maximum ELR, bottomhole pressure (BHP), and annulus flow velocity. Sensitivity analysis is performed for various combinations of GIRs/LIRs. Similar ELR value trending is observed in foam and multiphase flow models for both vertical and deviated wells. Increasing ELR is not always the goal because BHP should be controlled within reasonable ranges during MPD. To ensure operational constraints at the bottom hole, a set of ELRs is calculated from a few combinations of surface GIRs/LIRs. Refined by the target BHP and the desirable hole cleaning capability, the permissible managed drilling operation window can be achieved and visualized. Drilling engineers can calibrate surface GIRs/LIRs to achieve maximum mud motor performance. Conversely, with the fixed GIRs/LIRs, to maintain the desirable BHP and hole cleaning, drilling engineers can select the proper mud motors for use. The calculated ELR in this method is not only variable for evaluating mud motor performance, but it is also novel to combine it with other important operational factors to help manage downhole pressure and meet hole cleaning requirements.