地层应力状态下低渗粉砂质水合物沉积物内气水渗流微观机理研究

The gas hydrate-bearing layers in South China Sea are silt-dominated with low permeability, which are difficult to be exploited. Gas-water flow is a key factor affecting the hydrate recovery efficiency and gas recovery rate, whose mechanism is a key scientific problem demanding prompt solution for gas hydrate production in South China Sea at present. Against this background, the project will based on the microfocus X-ray CT hydrate triaxial test system, obtain the pore structure characteristics of gas-water flow channels in hydrate-bearing silty sediments under geostatic stress states, build 3D digital cores; quantitatively describe the gas-water two phase saturation and its distribution in the pore before and during hydrate dissociation (thermal stimulation, depressurization), obtain the absolute permeability, relative permeability curve, threshold pressure gradient etc., and establish a non-Darcy equation of motion considering stress sensitivity; Based on the pore network model, simulating the gas-water flow process under geostatic stress states, indicate the influencing factors of non-Darcy coefficient and the mechanism of non-Darcy flow in low permeability hydrate-bearing silty sediments. The outcomes will provide a theoretical basis and technical support for high efficiency, continuous gas hydrate production and enhancing gas recovery rate of low permeability hydrate-bearing silty layers in South China Sea.

我国南海水合物储层以细粒的粉砂为主，渗透性差，开发难度大。气水渗流过程是影响水合物开采效率和气体采收率的关键因素，揭示其微观机理，是目前我国南海水合物资源开发亟待解决的关键科学问题。本项目以此为研究背景，拟基于微焦点X射线CT水合物三轴实验系统，获取地层应力状态下粉砂质水合物沉积物气水渗流通道孔隙结构特征，建立三维数字岩心；定量描述水合物稳定状态下和注热、降压分解过程中孔隙内气水两相饱和度及其分布状态，获取绝对渗透率、相渗曲线、启动压力梯度等参数并探明其影响因素，建立考虑应力敏感的非达西渗流运动方程；基于孔隙网络模型，模拟地层应力状态下气水两相渗流过程，探明低渗粉砂质水合物沉积物非达西渗流系数影响因素，揭示非达西渗流机理，为我国南海低渗粉砂质储层水合物矿藏高效、持续开采和提高气体采收率提供理论基础和技术支撑。

天然气水合物开采过程水合物分解为气和水，储层胶结结构弱化或消失，引起储层颗粒迁移，孔隙度、孔隙形态及连通性等发生变化，进而改变储层内气水分布状态和渗流特征，影响水合物开采效率和气体采收率。本项目以此为背景，开发了一套微焦点X射线CT水合物三轴实验系统，定量化表征了水合物沉积物气水渗流通道孔隙结构特征，阐明了沉积物内水合物饱和度及其分布状态，建立了高分辨率水合物沉积物三维数字岩心；探明了不同水合物饱和度、有效应力条件下沉积物宏微观渗流参数演化规律，揭示了水合物分解引起的微观结构变化对气水渗流的影响机制，构建了水合物沉积物相对渗透率新模型；基于孔隙网络数值模拟分析，刻画了水合物生成与分解过程沉积物内气水两相渗流特征，揭示了水合物沉积物应力敏感效应和非达西渗流机理。相关理论和模型有望集成到多场耦合水合物开采模拟系统中，进而评价水合物开采气水产出情况，应用于南海水合物试采工程。