粉砂质水合物储层降压开采过程中气-水微观渗流机理研究

The fine-grained hydrate deposits are widely distributed in Shenhu area, South China Sea. There exists a significant difference between production effects in naturally fine- and coarse-grained hydrate deposits. It is crucial to understand the time variability of gas hydrate morphologies in pore space and microscopic gas-water two-phase flow mechanisms during depressurization in order to achieve optimal design of production test scheme and accurate production prediction. It remains a challenge due to poor analysis of hydrate pore morphologies and insufficient understanding of gas-water flow behaviors. For this study, we aim to investigate the microscopic gas-water flow mechanisms of fine-grained hydrate deposits by depressurization using the combined techniques of X-ray CT experiment and pore network simulation. Gas hydrate depressurization-induced decomposition experiment of a fine-grained sandy sediment from Shenhu area is firstly performed, and the X-ray micro-focus CT is employed to acquire high resolution CT images of the test specimen at different experimental stages. To elucidate the time variability of gas hydrate micro-distribution in test specimen, quantitative characterization of gas hydrate pore morphologies need to be achieved on the basis of 2-D segmentation and 3-D reconstruction of CT images. A topologically representative pore network model is further established to determine the relationship between time variability of gas hydrate pore morphologies and characteristic parameters of gas-water fluid flow in porous media. Finally, the microscopic gas-water flow behaviors and their controlling mechanisms will be fully understood, which can provide good insight to efficiently develop the fine-grained hydrate deposits in South China Sea.

我国南海神狐海域主要发育粉砂质水合物储层，试采产气、产水规律与粗粒砂质储层有显著差异。研究水合物微观赋存状态时变演化规律，探讨降压开采过程中气-水微观渗流机理，对水合物试采方案优化设计和产能预测具有重要意义。目前国内外针对水合物微观赋存状态定量描述及气-水渗流特征响应机理的研究还很欠缺。本项目拟采用实验与模拟相结合的手段，进行粉砂质水合物储层降压开采过程中气-水微观渗流机理研究。首先，针对南海粉砂质沉积物样品，开展水合物降压分解X-CT微观实验；对X-CT扫描图像进行预处理、阈值分割、三维重建基础上，实现水合物微观赋存状态的定量统计与表征，阐明水合物微观分布状况及时变演化规律；提取等效的孔隙网络模型，开展水合物降压分解过程的微观渗流模拟研究，认识水合物微观赋存状态时变性与气-水渗流关键参数的响应关系，最终揭示水合物降压分解气-水微观渗流特征及其控制机理，为南海粉砂质水合物开采提供理论支撑。

我国南海神狐海域主要发育粉砂质水合物储层，试采产气、产水规律与粗粒砂质储层有显著差异。研究水合物微观赋存状态时变演化规律，探讨降压开采过程中气-水微观渗流机理，对水合物试采方案优化设计和产能预测十分重要。.为了揭示粉砂质水合物储层降压开采过程中气-水微观渗流机理，采用水合物原位成像CT系统，共完成8组含水合物沉积物分解实验，提出一套考虑水合物分解动力学行为的数字岩心建模、三维孔隙网络提取及微观渗流模拟理论与方法，引入水合物体积、块数、形状因子和欧拉系数等特征参数实现了水合物分解过程中微观赋存状态及拓扑结构的定量化描述，探讨了天然气水合物分解过程中含水合物沉积物渗透率变化及气-水两相渗流特征，揭示了颗粒粒径及初始盐水饱和度对含水合物沉积物气-水微观渗流的影响机理。.结果表明，水合物微观分布非均质特征显著，分解过程开始于气-水合物接触的位置, 分解初期水合物具有孔隙填充、颗粒胶结等多种赋存模式, 而分解后期孔隙中主要赋存颗粒胶结型水合物；随着水合物的逐渐分解，含水合物沉积物平均孔喉半径、孔隙度、绝对渗透率和两相共渗区宽度均不断增大, 配位数、形状因子及束缚水饱和度逐渐减小；水合物分解过程中连续分布的水合物块数不断减少，但孤立水合物块的变化规律存在明显差异，这是水合物分解动力学行为不一致所导致的；相比于完全饱和的情况，部分饱和沉积物孔隙中单个水合物块的拓扑结构复杂性更强，形状因子的变化更加剧烈，孤立水合物块优先分解；当水合物饱和度达到临界阈值后，通过孔喉连通的水合物团簇逐渐分解；对于完全饱和的含水合物沉积物，其水合物降压分解动力学行为截然相反；不论初始状态是否饱和水，颗粒粒径分布越窄，水合物分解过程的渗透率变化越快、微观结构演化越平缓；对于相同粒径分布的沉积物体系，初始盐水饱和度越高，水合物微观结构随分解过程的变化越平缓；水合物分解将导致气相相对渗透率减小，水相相对渗透率增大。