Evaluating the effect of new gas solubility and bubble point models on PVT parameters and optimizing injected gas rate in gas-lift dual gradient drilling
Gas-lift dual gradient drilling (DGD) is a solution for the complex problems caused by the narrow drilling window in deepwater drilling. However, investigations are lacking on using oil-based drilling fluid in DGD, which is the principal novel idea of the present study. Herein, Nitrogen was selected as the injection gas into the riser. This research compares the results obtained from two new models with those obtained from the Standing correlations for solubility and bubble point pressure. Specifically, the study evaluates the PVT behaviors of drilling fluid (oil/water/nitrogen) in gas-lift dual gradient drilling operations in the case of using the new models or the Standing correlations by coding in MATLAB environment. Standing correlation is one of the conventional Black oil models; however, it overestimates the solubility of Nitrogen and underestimates the bubble pressure point in white mineral oil. According to the achieved results, the Standing model has some errors in evaluating the PVT behavior of Nitrogen and oil-based drilling fluids and is not recommended for the mixtures in the gas-lift dual gradient drilling. With regard to optimizing gas flow rate, it was found that the discrepancy between pressures for the new models and the Standing models is higher at both high liquid flow rates and at lower gas flow rates.