Optimal design of the engineering parameters for the first global trial production of marine natural gas hydrates through solid fluidization

More than 90% of natural gas hydrates (hereinafter, hydrate for short) in the South China Sea are non-diagenetic ore bodies, so they cannot be exploited easily by means of the conventional methods. In this paper, the solid fluidization method, as one of the revolutionary technologies in efficient exploitation of non-diagenetic natural gas hydrates, was, for the first time, put forward by Academician Zhou Shouwei. And it is successfully applied in the Shenhu Area of the South China Sea based on the technologies, equipment and processes which rely on domestic independent intellectual property rights. During the production test of fluidization, the ore bodies of hydrates are broken by the jet at the bottom hole into fine particles and carried upward by the drilling fluid. When the phase equilibrium state is reached with the increase of temperature and the decrease of pressure affected by the operation parameters, which is different from conventional phase equilibrium state, the hydrates bearing solid particles are decomposed, and consequently liquid–solid flow in the annulus becomes complex gas–liquid–solid multiphase flow. Therefore, it is necessary to optimize the construction parameters design so as to meet the high-level requirements of well control safety. In this paper, the engineering parameters are optimally designed based on the engineering geological characteristics of the target block, combined with the analysis on complex multiphase flow in the wellbore. Then, a theoretical model and a numerical calculation method for the multiphase flow, temperature and pressure of complex media in wellbores and the phase equilibrium and decomposition of natural gas hydrates were established. And the multiphase flow in the wellbore during the production test of fluidization was analyzed under different operating parameters by means of numerical simulation, software emulation and experimental verification. And thus, the design optimization scheme of on-site engineering parameters of production test of marine natural gas hydrate fluidization was prepared. It is pointed out that the diameter of jet fluidization well section shall not be excessively large; and that it is necessary to increase the flow rate and density of drilling fluid and apply wellhead back pressure to ensure the cutting carrying safety and to mitigate well control risks. The results of this basic theoretical study can provide significant support to field operation and improvement of output in production tests. Keywords: Marine, Natural gas hydrate, Non-diagenetic, Solid fluidization, Production test, Jet breakup, Multiphase flow, Engineering parameter, Numerical simulation, Optimal design, China, Shenhu sea area, South China sea