天然气水合物储层泥质细粉砂挡砂介质堵塞规律与微观挡砂机制

中国南海海域部分天然气水合物储层中地层砂为高泥质含量细粉砂,开采防控砂难度较大。针对高泥质细粉砂挡砂机制问题,使用粒度中值为10.13 μm的泥质细粉砂样品,模拟单向气液携砂流动条件,使用绕丝筛板、金属烧结网、金属纤维、预充填陶粒4类挡砂介质在20~80 μm挡砂精度下进行挡砂模拟实验,采用显微成像系统观察挡砂介质内部及表面砂粒沉积与堵塞动态,分析介质流通性能和挡砂性能变化,总结堵塞规律、微观挡砂机制与形态及其控制因素。研究结果表明,不同类型和精度的挡砂介质对泥质细粉砂的堵塞总体呈现堵塞开始、堵塞加剧和堵塞平衡3个阶段。随着驱替进行,挡砂介质渗透率逐渐降低,幅度会高达90%以上;同时过砂速度减缓,最终过砂率为5%~10%。根据堵塞规律和微观图像分析,提出了粗组分分选桥架、局部砂团适度挡砂、整体砂桥阻挡等挡砂介质对泥质细粉砂的3种微观挡砂机制。以粗组分分选桥架挡砂机制为主的挡砂工况下,挡砂介质堵塞渗透率较高,但过砂率超过15%,挡砂效果较差;以整体砂桥挡砂机制为主时,过砂率在10%以下,挡砂性能较好,但各类挡砂介质的堵塞渗透率不足1 D,流通性能较差。局部砂团适度挡砂机制为主时介质挡砂性能及流通性能介于两者之间。挡砂介质对天然气水合物储层泥质细粉砂的微观挡砂机制和形态受挡砂介质类型、精度、地层砂特征以及流动条件等因素控制,其规律对于水合物泥质细粉砂防控砂优化有指导意义。

Plugging law and microscopic sand retention mechanism of sand retaining medium of argillaceous fine silt sand in gas hydrate reservoirs

Fine silt sand with high argillaceous content exists in some gas hydrate reservoirs in the South China Sea, causing the difficulty in sand prevention and control during exploitation. Aiming at the problems of the sand retention mechanism for highly argillaceous fine silt sand, this study takes the argillaceous fine silt sand with the median grain diameter of 10.13 μm as the sample; through simulating the unidirectional flow conditions of the gas-liquid carrying sand, the four types of sand retaining media including wire-wrapped sieve plate, metal sintering filter, metal fiber and pre-filled ceramsite are used to perform the simulation experiment of sand retention with a precision of 20-80 μm. Then the microscopic imaging system is applied to observe the deposition and clogging dynamics of sand in the sand retaining media and on the surface, analyze the changes in the flow performance of the media and the sand retaining performance, and summarize the sand-plugging law, microscopic sand-retaining mechanism and morphology as well as their controlling factors. The results show that different types of sand retaining media with various degrees of precision has experienced three stages while plugging the argillaceous fine silt sand, i.e., plugging beginning, plugging aggravation and plugging balance. As the displacement progresses, the permeability of the sand retaining media will gradually decrease to 90%; meanwhile, the sand passing rate slows down, and the final sand passing rate is about 5%-10%. According to the plugging law and microscopic image analysis, this paper proposes three types of microscopic sand retaining mechanisms for the argillaceous fine silt sand, i.e., separation bridge of coarse components (Type I), moderate sand retention by local sand groups (Type Ⅱ), and overall sand bridge barrier (Type Ⅲ). When mainly applying the Type I mechanism, the sand retaining medium has a high plugging permeability, but the sand passing rate exceeds 15%, so the sand retaining effect is poor. When mainly applying the Type Ⅲ mechanism, the sand passing rate is below 10%, showing good sand retaining performance; however, the plugging permeability of various sand retaining media is less than 1 D, showing poor flow performance. When mainly applying the Type Ⅱ mechanism, the sand retaining performance and flow performance of the media are between those of the above two. The microscopic sand-retaining mechanism and morphology for the argillaceous fine silt sand in gas hydrate reservoirs are controlled by such factors as the type, accuracy, formation sand characteristics and flow conditions of the sand-retaining medium. The sand-plugging laws have certain guiding significance for improving the sand prevention and control of argillaceous fine silt sand in hydrates.