海洋天然气水合物藏钻探环空相态特性

Annular phase behavior analysis during marine natural gas hydrate reservoir drilling

海洋天然气水合物藏钻探过程中,水合物钻屑随钻井液向上返出,随着温度升高、压力降低,水合物钻屑上升至一定位置开始分解,使井筒流动变为环空复杂气液固多相流,这对井下流动安全产生严重威胁。考虑井筒温度、压力与水合物分解的耦合作用与影响,建立了海洋天然气水合物钻井过程中井筒温度模型、井筒压力模型、水合物动态传质分解模型和复杂环空多相流模型。通过模型求解,数值分析了井筒温度、环空压力和水合物分解在不同钻井工况下的变化规律。结果表明:增大钻井液排量,井筒中井底处循环钻井液温度升高,环空中井口处返出钻井液温度降低,分解起始位置下移;增大钻井液密度,环空压力升高,分解起始位置上移;增大钻井液入口温度,井筒温度升高,分解起始位置下移;增大机械钻速,分解起始位置不变。

During the drilling of marine natural gas hydrate reservoirs,hydrate cuttings are transported to platform by drilling mud.With temperature rising and pressure dropping,hydrate cuttings rise to a certain position for decomposition,which changes the wellbore flow to annular complex vapor-liquid-solid multiphase flow,bringing great risks to safe production under downhole flow.Aiming at these situations,wellbore temperature and pressure models,dynamic hydrate mass transfer and decomposition model and complex annular multiphase flow model were established considering the coupling effect and influence of wellbore temperature and pressure with hydrate decomposition.Through model solution,numerical simulation was used to analyze the change laws of wellbore temperature,annular pressure and hydrate decomposition under different drilling conditions.The results show that with the increase in drilling fluid displacement,the temperature of cyclical drilling fluid becomes higher at the bottom of wellbore,and that of drilling fluid returned at annular wellhead will decline,leading to the start position of decomposition downwards.Moreover,in case of increasing the drilling fluid density,the annular pressure will be raised,leading to the start position of decomposition upwards.As the drilling fluid inlet temperature rises,the wellbore temperature will be higher,leading to the start position of decomposition downwards.When the penetration rate is increased,the start position of decomposition keeps unchanged.