Molecular Dynamics Simulation of Potassium Formate’s Ability to Inhibit Hydration of Montmorillonite
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摘要: 在盐膏地层或者泥页岩地层钻井过程中，钻遇的地层含有大量的蒙脱石，蒙脱石是典型的水化膨胀性矿物，当钻井液滤液进入蒙脱石含量较高的地层会因水化膨胀作用导致井壁失稳，发生卡钻、井眼缩径、井塌等井下复杂事故。有机盐钻井液的主要成分是含有抑制作用强的甲酸钾，成为目前钻复杂井段的重要水基钻井液体系。为了深入揭示有机盐钻井液中的主要抑制剂甲酸钾抑制蒙脱石水化的机理，采用分子动力学的方法，从分子角度研究其抑制机理。通过建立甲酸钾和蒙脱石的模型，计算两者的相互作用。研究发现在甲酸钾溶液中，电离出的甲酸根主要偏向作用于层间水，且能与水分子形成氢键，电离出的钾离子会吸附在蒙脱石表面，与蒙脱石表面作用，降低蒙脱石的Zeta电位，当质量分数大于一定量时，钾离子对蒙脱石表面作用饱和并开始向层间扩散。通过模拟发现，低浓度的甲酸钾有利于蒙脱石力学性质稳定，随着其浓度继续增加，有利于抑制水分子扩散，而蒙脱石的杨氏模量、剪切模量略微降低，泊松比上升，当浓度大于一定量后，其抑制性趋于稳定，蒙脱石力学性质趋于稳定。模拟认为钻井液中甲酸钾最佳质量分数（相对于水）范围应在32.57%～34.92%。Abstract: Large amount of montmorillonite is generally encountered when drilling in salt/gypsum-containing or shale formations. Montmorillonite, as a typical easily hydrated and swelling clay, when in contact with filtrates from water based drilling fluids, will cause borehole wall instability, stuck pipe, tight hole and borehole collapse etc. Formate water based drilling fluid containing potassium formate, a strongly inhibitive additive, is becoming an important water based mud for drilling complex formations. In laboratory studies, a molecular dynamics method was adopted to further reveal, from the molecular point of view, the mechanisms of potassium formate to inhibit the hydration of montmorillonite. The interaction of potassium formate and montmorillonite was calculated by establishing the models of the two. It was found from the studies that the ionized formate in the potassium formate solution mainly interacts with water molecules existed in between the montmorillonite layers and can form hydrogen bonds with the water molecules. The ionized potassium, on the other hand, is adsorbed on the surfaces of the montmorillonite particles, thereby reducing the zeta-potential of the montmorillonite particles by reacting with the surfaces of the particles. At a concentration that is above a certain level, the number of potassium ions adsorbed onto the surfaces of the montmorillonite particles reaches saturation, and the redundant potassium ions begin to diffuse into the spaces between two montmorillonite layers. Simulation experiments showed that low concentration potassium formate solution is beneficial to stabilizing the mechanical property of montmorillonite; as the concentration of potassium formate increases, it inhibits the diffusion of water molecules, and the mechanical property of the montmorillonite is deteriorated to some extent. When the concentration exceeds a certain level, the inhibitive capacity of potassium formate becomes stabilized, so does the mechanical property of the montmorillonite. The simulation results show that the optimum concentration of potassium formate in water based drilling fluids is 32.57% – 34.92%.
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图 4 动态模拟中蒙脱石体系的构象
注：(a),(b)中标黄的以及(c),(d)中紫色球形原子是钾离子，(a),(b),(d)中层间紫色小点为钠离子红色，表示氧原子，白色表示氢原子
表 1 甲酸钾、甲酸根、钾离子对水和蒙脱石表面非键作用能
体系中甲酸钾个数 水化程度 E（J-M）/
kcal/molE（J-W） /
kcal/molE（JS-M） /
kcal/molE（JS-W） /
kcal/molE（K-M） /
kcal/molE（K-W） /
kcal/mol1 二层饱和水 7.304 149.932 −59.890 73.634 67.195 76.298 三层饱和水 7.581 110.862 −16.608 23.476 24.188 87.385 2 二层饱和水 41.654 395.678 −79.793 112.795 121.447 282.883 三层饱和水 10.255 311.593 −32.674 105.173 42.929 206.420 3 二层饱和水 52.360 424.970 −151.526 118.501 203.886 306.469 三层饱和水 14.143 446.982 −52.942 187.006 67.085 259.976 4 二层饱和水 89.367 470.437 −166.687 125.758 256.054 344.679 三层饱和水 13.852 702.646 −70.085 408.689 83.937 293.957 6 二层饱和水 103.087 485.472 −255.451 186.289 358.538 299.183 三层饱和水 112.112 1092.442 −76.594 481.304 188.706 611.138 8 二层饱和水 109.289 587.242 −262.118 100.818 371.407 486.424 三层饱和水 142.850 1130.380 −28.862 501.557 171.712 628.823 
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