南海神狐海域泥底辟/泥火山流体热效应及其对水合物赋存的制约

Mud diapirs/volcanoes, which are often the primary indications for the occurrence of gas hydrate, are widely distributed on the submarine continental slope. Moreover, the reserves for this type of hydrate are huge. However, gas hydrates were not found from drilling in some areas with developed mud diapirs/volcanoes, such as the SH-5 borehole in Shenhu area, South China Sea. Previous studies based on in-situ heat flow measurement, BSR computation, etc., suggested that the thermal anomaly of mud diapir/volcano fluid led to hydrate decomposition. However, most of these studies are not sufficient for quantitative analysis of heat transfer law of fluid flow and its influence on the deposition and distribution of gas hydrate. This study intends to solve the scientific problems on gas hydrate research and drilling in the mud diapir/volcano developed area, based on the thorough investigations on thermodynamics by international cooperation. We plan to carry out numerical simulations of fluid flow and heat transfer in the mud diapir/volcano. Moreover, analyze the characteristics of fluid thermodynamics during the evolution processes of mud diapirs/volcanoes. Afterward, the sophisticated thermal structure of mud diapir/volcano fluid will be revealed. We will then discuss the relationships between the thermal effect of mud diapir/volcano fluid and the deposition and distribution of gas hydrate. In the end, we will finalize a thermodynamics method for evaluation of the potential for gas hydrate resource in the mud diapir/volcano developed area. This work can provide a theoretical basis for the gas hydrate research and exploration in the unique tectonic environments.

泥底辟/泥火山在海底陆坡区广泛分布，是寻找天然气水合物的重要标志，且该类型水合物储量巨大。然而部分泥底辟/泥火山发育区却未钻获水合物样品，如南海神狐海域2007年实施钻探的SH-5井。国内外学者通过实测、BSR计算等手段认为泥底辟/泥火山流体热异常制约了水合物的赋存，但缺少对流体热演化规律及其对水合物赋存的制约的定量研究。本项目拟以国际合作的方式，以热力学研究为主线，针对海底泥底辟/泥火山发育区天然气水合物研究与钻探中存在的相关流体热力学实际问题，以神狐水合物钻探区泥底辟/泥火山为研究对象，开展泥底辟/泥火山流体流动与传热过程数值模拟，分析泥底辟/泥火山演化过程中流体热力学时空规律，揭示泥底辟/泥火山精细的流体热结构，探讨泥底辟/泥火山流体热作用对水合物赋存的制约关系，建立一套泥底辟/泥火山发育区水合物资源评价的热力学指标体系，为流体渗漏构造环境下水合物成藏研究与勘探提供热力学理论依据。

针对海底泥底辟/泥火山发育区天然气水合物研究与钻探中存在的相关流体热力学实际问题，本项目以地热流研究为主线，优化了流体力学与热力学的耦合机制，深入开展了神狐海域泥底辟/泥火山流体流动与传热过程数值模拟，重点分析了流体热力学的时间与三维空间变化规律，揭示了泥底辟/泥火山流体热作用对水合物形成与分布的制约关系，建立了泥底辟/泥火山等流体渗漏环境水合物资源精细评价的热力学指标体系。.建立了海底泥底辟/泥火山地质模型，在传统的砂箱构造模拟设备上加载流体动力系统，开展了不同流体压力、沉积背景、构造环境等条件下泥底辟/泥火山形成演化物理模拟，揭示了泥底辟/泥火山成因机制。通过达西定理和纳维-斯托克斯公式等建立泥浆流体流动的数学模型，并利用Matlab对泥火山/泥底辟流体在管道内向上运移的过程进行数值模拟，分析泥火山/泥底辟形成过程中流体流动速度、密度及压力的变化情况。泥火山/泥底辟的发育和分布与其发育区的沉积厚度、地层超压、构造应力背景及断裂分布特征密切相关，中强地震对泥火山活动具有触发作用。.划分了神狐海域天然气水合物流体输导体系，认为神狐海域天然气水合物稳定带底界与峡谷的侵蚀-沉积-迁移过程存在动态调整的关系。针对海底泥底辟/泥火山发育区天然气水合物研究与钻探中存在的流体热力学问题，开展了泥底辟流体从深层异常流体压力地层向上喷发的动力学过程数值模拟，分析了泥底辟/泥火山流体从深层向浅层运移过程中的热消耗（温度变化），特别是泥底辟流体对水合物赋存层温度场的影响。通过数值模拟建立了SH5泥底辟的热结构特征，根据模拟出的三维热结构分布特征圈定了天然气水合物的赋存范围，为流体渗漏构造环境天然气水合物成藏研究与勘探开采提供了热力学理论依据。同时对神狐海域海底滑坡进行了分析，认为水合物赋存区气体渗漏形成地层超压，引起海底滑坡，导致水合物分解，进而海底滑坡加剧。