Abstract
During drilling of marine natural gas hydrate reservoirs, hydrate cuttings will return upwards along with the drilling mud. As the temperature rises and the pressure drops, hydrate cuttings will decompose at a certain position. At that point a complex multiphase flow will form in the annulus. Moreover, the characteristic parameters of multiphase flow will significantly change in the upward progress of the flow, which is different from that in conventional drilling. Aiming at those situations, a new mathematical method is proposed for sensitivity analysis of multiphase flow in the annulus during drilling of marine natural gas hydrate reservoirs. In this method, temperature and pressure models in the wellbore, a dynamic model of hydrate mass transfer decomposition and complex multiphase flow models are established considering the interactions in the annulus. Then, using a numerical simulation method, models are used to analyze the laws of variation of temperature, pressure, hydrate dynamic decomposition and complex multiphase flow characteristics in the annulus under different operating parameters. After the sensitivity analysis of multiphase flow in annulus during drilling of marine natural gas hydrate reservoirs, it is presented that increasing delivery rate and density of drilling mud, reducing inlet temperature and rate of penetration should be performed to ensure well control safety. The research findings provide important theoretical bases and technical guidance for complex multiphase flow and well control safety analysis during drilling of marine natural gas hydrate reservoirs.
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