Mechanisms of hydration inhibition on the surface of montmorillonite in deep shale via molecular dynamic simulation
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摘要: 井壁失稳是油气勘探开发过程中最为复杂的技术难题之一，黏土矿物水化膨胀是造成井壁失稳的关键因素，其中表面水化因水化势较大而难以去除。本文通过分子动力学模拟探测了无机盐对蒙脱石表面水化的抑制效果，以及浓度、温度和压强对水化抑制效果的影响，揭示了无机盐CaCl2抑制蒙脱石表面水化的微观机理。研究表明：抑制阳离子通过束缚蒙脱石表面水分子和降低水分子输运传导能力，从而调控水分子侵入蒙脱石表面来实现抑制作用。无机盐抑制表面水化能力依次为CaCl2>NaCl>MgCl2>KCl，钙离子易吸附表面水分子形成稳定的外球络合结构。随着CaCl2浓度增加，钙离子配位数、水化数和水化半径均降低，抑制能力减弱；温度升高和压强降低时，体系中水分子传导输运能力增强、钙离子水化数减小且力学强度降低。Abstract: Borehole instability is one of the most complicated technical problems in oil and gas exploration and development.The hydration and expansion of clay minerals is the critical factor causing wellbore instability in which the surface hydration is difficult to be removed due to the large hydration potential.In this light, through molecular dynamic simulation, this paper probed into the CaCl2 inhibitory effect of concentrations, temperatures and pressures on the surface hydration of montmorillonite which revealed the macroscopic mechanism.Results indicated that inhibition of cations were achieved by binding water molecules on the surface of montmorillonite and decreasing the transport and conductivity of water molecules, thereby regulating the invasion of water molecules into the surface of montmorillonite.The ability of inorganic salts to inhibit surface hydration were CaCl2>NaCl>MgCl2>KCl.The study found that calcium ions easily adsorbed surface water molecules to form stable outer sphere complex.With the increase of CaCl2 concentration, coordination number, hydration number and hydration radius of calcium ion decreased, and the inhibitory effect diminished.When the temperature increased and the pressure decreased, the conductivity and transport capacity of water molecules was enhanced in the system, the coordination number of calcium ion descended, and mechanical strength declined.
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Key words:
- montmorillonite /
- inorganic salts /
- surface hydration /
- inhibition mechanism /
- molecular dynamics
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图 2 蒙脱石表面水化建模过程(单位：nm)
Figure 2. Modeling process of surface hydration of montmorillonite
图 3 4种无机阳离子与水中氧原子的径向分布函数
Figure 3. Radial distribution function of four inorganic cations with oxygen atom in water
图 4 无机阳离子配位数、水化数和水化半径
Figure 4. Coordination number, hydration number and hydration radius of inorganic salt cation
图 5 蒙脱石(0 0 1)面法线方向水分子浓度分布
Figure 5. Concentration profile of H2O in the normal direction of montmorillonite(0 0 1)surface
图 8 不同温压下水分子的扩散系数和钙离子配位数
Figure 8. Diffusion coefficients of water and coordination number of Ca2+ at different temperature and pressure
表 1 脱水蒙脱石弹性常数与实验值和模拟值对比
Table 1. Elastic constants of dehydrated montmorillonite compared to experimentat and simulation values
Cij 弹性常数/GPa 本文值 模拟值[18] 实验值[25] C11 255.181 272.3 181.0±1.2 C22 334.081 323.6 178.4±1.3 C33 50.821 -7.2 58.6±0.6 C44 18.989 -2.3 16.5±0.6 C55 31.793 -6.2 19.5±0.5 C66 69.162 72.6 72.0±0.7 C12 123.128 129.9 48.8±2.5 C13 35.993 30.3 25.6±1.5 C14 -1.162 8.2 — C15 -66.37 -16.2 -14.2±0.8 C16 -1.020 -12.2 — C23 29.983 2.2 21.2±1.8 C24 12.563 4.1 — C25 -26.728 -8.1 1.1±3.7 C26 3.843 -3.2 — C34 0.914 -5.7 — C35 -2.114 11.9 1.0±0.6 C36 -0.225 8.8 — C45 -0.734 2.5 — C46 -14.541 2.1 -5.2±0.9 C56 -0.816 -3.1 — 
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表 2 脱水蒙脱石力学性质的计算结果与实验值和模拟值
Table 2. Computed results of dehydrated montmorillonite mechanical properties compared to experiment and simulation values
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