Optimal Equivalent Liquid Rate ELR in Managed Pressure Drilling

Abstract

Abstract 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.