固态流化采掘海洋天然气水合物藏的多相非平衡管流特征

由密闭管线将破碎的海洋天然气水合物(以下简称水合物)颗粒向上输送至海面平台,是固态流化采掘水合物藏工艺流程的核心环节,但水合物固相颗粒在上升过程中受到温度升高、压力降低的影响,至某一临界位置将会分解产生大量气体,使井筒中的流动变为复杂多相非平衡管流,进一步加剧了井控、固相输送等安全风险。为了研究水合物在上述过程中的动态分解规律,通过建立井筒温度和压力场、水合物相平衡、多相上升管流中的水合物动态分解、耦合水合物动态分解的井筒多相流动数学模型,提出了数值计算方法并予以验证。研究结果表明:(1)应用数值模型分析,得到了不同施工参数条件下的液相排量、固相输送量(日产气量)、井口回压对多相非平衡管流的影响规律;(2)提出了基于多相非平衡管流特征的现场施工措施,适当提高固相输送量可以提高天然气产量,应同时增大液相排量、施加井口回压来保障井控安全。结论认为,该项研究成果为施工参数优化和井控安全提供了技术支撑,也为其他海区水合物藏固态流化采掘多相非平衡管流预测提供了手段。

Non-equilibrium multiphase wellbore flow characteristics in solid fluidization exploitation of marine gas hydrate reservoirs

In the core process of fractured marine gas hydrate (hereinafter referred to as hydrate) particles being transported up to the surface platform by airtight pipeline in the solid fluidization exploitation of marine gas hydrate reservoirs, influenced by the rising temperature and the dropping pressure, the solid hydrates will decompose and produce a large amount of gas at a certain critical point, causing the liquid-solid two-phase flow in the wellbore to change into complicated gas-liquid-solid multiphase non-equilibrium flow, which further aggravate well control, solid phase transportation and other safety risks. In view of this, the dynamic hydrate decomposition law in the above process was studied in this paper by establishing multiphase wellbore flow mathematical models of wellbore temperature and pressure field, hydrate phase equilibrium, hydrate dynamic decomposition in multiphase riser pipe flow, wellbore multiphase flow coupled hydrate dynamic decomposition, and a numerical calculation method was proposed and verified. The following results were obtained. First, by numerical model analysis, the effects of liquid phase displacement, solid throughput (daily gas production rate) and wellhead back pressure under different construction parameters on multi-phase non-equilibrium pipe flow were obtained. In addition, the field construction guidance measures were put forward based on multiphase non-equilibrium pipe flow characteristics as follows: to properly increase the solid throughput so as to increase the natural gas production, to appropriately increase the liquid-phase displacement and the wellhead back pressure so as to ensure well control safety. This study provides not only a theoretical basis for the prediction of multiphase non-equilibrium pipe flow in the solid fluidization exploitation, but a technical support for the field construction parameter optimization and well control safety.