School of Mining and Petroleum Engineering, Department of Civil and Environmental Engineering , University of Alberta , Edmonton , Alberta T6G 1H9 , Canada.
Department of Mechanical and Industrial Engineering , University of Toronto , 5 King's College Road , Toronto M5S 3G8 , Ontario , Canada.
Langmuir. 2018 Nov 20;34(46):14058-14068. doi: 10.1021/acs.langmuir.8b02789. Epub 2018 Nov 5.
Accurate characterization of the bubble point pressure of hydrocarbon mixtures under nanoconfinement is crucial to the prediction of ultimate oil recovery and well productivity of shale/tight oil reservoirs. Unlike conventional reservoirs, shale has an extensive network of tiny pores in the range of a few nanometers. In nanopores, the properties of hydrocarbon fluids deviate from those in bulk because of significant surface adsorption. Many previous theoretical works use a conventional equation of state model coupled with capillary pressure to study the nanoconfinement effect. Without including the inhomogeneous molecular density distributions in nanoconfinement, these previous approaches predict only slightly reduced bubble points. In this work, we use density functional theory to study the effect of nanoconfinement on the hydrocarbon mixture bubble point pressure by explicitly considering fluid-surface interactions and inhomogeneous density distributions in nanopores. We find that as system pressure decreases, while lighter components are continuously released from the nanopores, heavier components accumulate within. The bubble point pressure of nanoconfined hydrocarbon mixtures is thus significantly suppressed from the bulk bubble point to below the bulk dew point, in line with our previous experiments. When bulk fluids are in a two-phase, the confined hydrocarbon fluids are in a single liquid-like phase. As pore size increases, bubble point pressure of confined fluids increases and hydrocarbon average density in nanopores approaches the liquid-phase density in bulk when bulk is in a two-phase region. For a finite volume bulk bath, we find that because of the competitive adsorption in nanopores, the bulk bubble point pressure increases in line with a previous experimental work. Our work demonstrates how mixture dynamics and nanopore-bulk partitioning influence phase behavior in nanoconfinement and enables the accurate estimation of hydrocarbon mixture bubble point pressure in shale nanopores.
准确描述纳米受限条件下烃混合物的泡点压力对于预测页岩/致密油藏的最终采油量和油井产能至关重要。与常规储层不同，页岩具有纳米级范围内的广泛的微小孔隙网络。在纳米孔隙中，由于显著的表面吸附作用，烃类流体的性质会偏离其在体相中的性质。许多先前的理论工作使用传统的状态方程模型与毛管压力相结合来研究纳米受限效应。由于没有考虑纳米受限中的非均匀分子密度分布，这些先前的方法仅预测了略微降低的泡点。在这项工作中，我们通过明确考虑流体-表面相互作用和纳米孔中的非均匀密度分布，使用密度泛函理论来研究纳米受限对烃混合物泡点压力的影响。我们发现，随着系统压力的降低，虽然较轻的组分不断从纳米孔中释放出来，但较重的组分则在其中积累。因此，纳米受限烃混合物的泡点压力从体相泡点显著降低至低于体相露点，这与我们之前的实验结果一致。当体相流体处于两相时，受限烃流体处于单一液相。随着孔径的增大，受限流体的泡点压力增加，当体相处于两相区时，纳米孔中的烃类平均密度接近体相液相密度。对于有限体积的体相浴，我们发现，由于纳米孔中的竞争吸附，体相泡点压力会按照先前的实验工作中的趋势增加。我们的工作表明了混合物动力学和纳米孔-体相分配如何影响纳米受限中的相行为，并能够准确估计页岩纳米孔中烃混合物的泡点压力。
