Zhou Zhiyan, Li Junqian, Song Zhaojing, Zhao Rixin
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China.
Shandong Provincial Key Laboratory of Deep Oil & Gas, China University of Petroleum (East China), Qingdao 266580, China.
ACS Omega. 2023 May 16;8(21):18990-19001. doi: 10.1021/acsomega.3c01640. eCollection 2023 May 30.
The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl solution, and a HO/CH mixed phase were created at varied temperatures (80-120 °C) and pressures (70-120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si-O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH-HO mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas-solid interactions and solid-liquid-gas interactions. CH molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.
狭窄孔隙空间中的水的存在会影响甲烷的赋存和流动，进而影响页岩气的产量。因此，研究水的赋存和分布特征对于预测天然气产量具有重要意义。基于分子动力学模拟，本研究原位研究了高岭石纳米孔隙中液态水的赋存特征及影响变量。由于其分布广泛，高岭石是最常见的粘土矿物，具有两个不同特性的表面。在不同温度（80 - 120℃）和压力（70 - 120MPa）下创建了纯水、CaCl溶液和H₂O/CH₄混合相的三个体系。对纳米孔隙中气体和水的存在情况进行了深入研究。结果表明，在原位条件下，水在高岭石的Al - O八面体表面的吸附不受外部条件影响，而水在Si - O四面体表面的吸附随温度升高而降低，但变化较小。当体系中存在离子时，水容量降低。基于上述结果，温度和压力等外部条件不会影响水的基本状态。然而，如果体系中有两种以上流体类型，由于竞争吸附，水在矿物表面的吸附会减少。此外，模拟了CH₄ - H₂O混合体系，其中甲烷分子呈簇状分布。孔隙中有两种吸附类型：气 - 固相互作用和固 - 液 - 气相互作用。由于水中存在强氢键相互作用，CH₄分子被认为聚集在水分子中。
