甲烷水合物形成对蒙脱石表面区域电学特性的影响：分子动力学和有限元数值模拟
摘要：摘要含天然气水合物泥质沉积物的电学特性是反演水合物饱和度、渗透率等储层参数的重要物理性质。甲烷水合物形成对黏土矿物颗粒表面电学性质的影响机制尚不明晰,制约了对骨架颗粒表面电导及沉积物电性参数的准确建模和计算。提出分子动力学（MD）与有限元（FE）数值模拟相结合的新方法,基于Gouy-Chapman-Stern（GCS）理论建立无/含水合物-蒙脱石-NaCl溶液体系的MD和FE数值模型,探讨甲烷水合物形成对蒙脱石表面双电层性质和表面区域电势分布的影响规律及机理。研究结果表明：（1）利用MD模型可获得含水合物条件下蒙脱石表面双电层中Stern层和扩散层的厚度以及溶液中离子的扩散系数,为FE模型提供双电层结构参数与离子运动参数,基于FE模型可分析蒙脱石表面区域电势分布特性;（2）蒙脱石表面形成稳定致密的钠离子层与界面水层,甲烷分子无法在蒙脱石表面生成水合物,因此水合物不影响Stern层而仅侵占部分扩散层区域;水合物形成过程中排盐效应使溶液的盐离子浓度增大,对扩散层中Na+ 产生排斥作用,使扩散层中的部分Na+ 进入Stern层,导致扩散层变薄;水合物的生长降低了溶液中离子的扩散系数;（3）水合物对扩散层的侵占使得扩散层部分区域的相对介电常数发生变化,引起该区域中电势的非均匀分布;水合物的存在不影响Stern层内的电势分布,但被水合物侵占的扩散层部分相对介电常数变小,引起扩散层内平均电势的升高;随着水合物与黏土颗粒表面距离的增加,平均电势逐渐降低。研究结果可为认识含水合物泥质沉积物中骨架颗粒的电学特性、量化颗粒表面电性参数提供理论和模型基础。
Abstract:：The electrical properties of clayey sediments containing natural gas hydrates are important to the inversion of reservoir parameters such as hydrate saturation and permeability. The influence mechanisms of methane-hydrate formation on the surface electrical properties of clay particles are still unclear, which restricts the accurate modeling and computation of the surface conductivity of skeleton particles and electrical parameters of sediments. A new method was proposed combining molecular dynamics (MD) and finite element (FE) numerical simulation. Based on the Gouy-Chapman-Stern (GCS) theory, MD and FE numerical models of hydrate-free/hydrate-bearing montmorillonite-NaCl solution systems were established to explore the influences and mechanisms of methane-hydrate formation on the properties of surface electrical double layer (EDL) and surface potential distribution of montmorillonite. It was demonstrated that: (1) the MD model can be used to obtain the thickness of the Stern layer and diffusion layer on the surface of montmorillonite with hydrate, as well as the diffusion coefficient of ions in the solution, providing the FE model with structural parameters of the EDL and ion motion parameters; based on the FE model, the potential distribution characteristics of the surface area of montmorillonite can be analyzed; (2) a stable and dense sodium-ion layer and interfacial water layer are formed on the surface of montmorillonite, and methane hydrates cannot grow onto the surface of montmorillonite; therefore, hydrates do not affect the Stern layer but occupy a portion of the diffusion layer; during the formation of hydrates, the "salt removal effect" results in an increase in the concentration of salt ions in the solution, which repels Na+ in the diffusion layer into the Stern layer, leading to a thinning of the diffusion layer; the growth of hydrates reduces the diffusion coefficient of ions in the solution; (3) the invasion of hydrates into the diffusion layer causes a change in the relative dielectric constant of some areas of the diffusion layer, resulting in a non-uniform distribution of electric potential in those areas; the presence of hydrates does not affect the potential distribution within the Stern layer, but the relative dielectric constant of the areas occupied by hydrates in the diffusion layer decreases, causing an increase in the average potential within the diffusion layer; as the distance between the surfaces of hydrate and clay particle increases, the average potential gradually decreases. The study can provide a theoretical and model basis for understanding the electrical characteristics of skeleton particles in clayey hydrate-bearing sediments and quantifying the surface electrical parameters of particles.
中文标题：甲烷水合物形成对蒙脱石表面区域电学特性的影响：分子动力学和有限元数值模拟
英文标题：Influences of Methane-Hydrate Formation on Electrical Properties of the Surface Area of Montmorillonite: Molecular Dynamics and Finite Element Numerical Simulation
发表时间：2023-11-02
作者：
武晓菲, 邢兰昌, 魏伟, 韩维峰, 吕笑焱, 钟杰
作者简介：
武晓菲（1999-）,女,硕士研究生,主要从事天然气水合物岩石物理数值模拟方法研究。邢兰昌（1983-）,男,博士,副教授,博士生导师,主要从事天然气水合物岩石物理实验及数值模拟、多相孔隙介质电学和声学检测理论与技术、多物理场和跨尺度数值模拟方法等研究。
关键词：甲烷水合物,黏土矿物,双电层,电学特性,分子动力学,有限元数值模拟,
Key words：methane hydrate,clay mineral,electrical double layer,electrical properties,molecular dynamics,finite element numerical simulation,
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