Layered Threshold Pressure of Tight Oil in Nanopores: A Molecular Dynamics Simulation Study

Abstract

Nanoscale pores are widely developed in tight oil, and the scale effect significantly affects the dynamic behavior of crude oil molecules at different positions in the nanopores. The molecular dynamics simulation method was used to study the mechanism of threshold pressure of tight oil molecules at different positions in quartz and calcite nanopores and the influence of pore radius on threshold pressure. The results show that the mobilizing of crude oil molecules at different positions in nanopores is mainly affected by liquid−solid interactions and intermolecular friction. The interactions between naphthenic acids and pore walls are the main contributing factor to the threshold pressure of boundary crude oil molecules. Van der Waals interaction and weak hydrogen bonds mainly contribute to the bonding of naphthenic acids with quartz walls, and strong electrostatic interaction and strong hydrogen mainly contribute to that with calcite walls. Except for the boundary layer, the threshold pressure of other layers relates to the interactions between saturated hydrocarbon, aromatic hydrocarbon, and adjacent layers as well as the molecular friction, among which friction between the saturated hydrocarbon and the adjacent molecules is dominant. It is challenging for crude oil to mobilize in quartz pores smaller than 3 nm. However, when the quartz pore width exceeds 7 nm, the mobilizing law of crude oil is scarcely affected by the pore width. The mobilizing laws of crude oil differ in calcite pores smaller than 11 nm, while that stays nearly the same in pores larger than 11 nm. Finally, a prediction model for the threshold pressure of tight oil molecules at different positions in quartz and calcite nanopores is proposed, which provides theoretical guidance for precise control of working fluid properties to improve tight oil recovery.

A Novel Threshold Pressure Gradient Model and Its Influence on Production Simulation for Shale Oil Reservoirs

“…Until now, experiments, numerical methods, and analytical methods have been used to study TPG in low-permeability reservoirs. In general, experimental methods obtain TPG of low permeability porous medium by fitting the experimental results and take TPG as an empirical function of permeability. ,,,,− Although the phenomenon of TPG in porous medium has been measured by many scholars since it was reported in 1963 by Miller and Low, the influence mechanisms of different factors on TPG are difficult to be fully revealed by experiments. ,, In addition, experimental researches require significant instrumentation requirements, consume time and labor, so pore-scale numerical methods (e.g., molecular dynamics models, finite volume methods, and pore-network models) are widely accepted to study TPG-related issues in the development of low permeability reservoirs. , Although pore-scale numerical methods can complement experimental research, their calculation accuracy is affected by algorithm accuracy and discretization error.…”

“…16,28,29 In addition, experimental researches require significant instrumentation requirements, consume time and labor, so pore-scale numerical methods (e.g., molecular dynamics models, finite volume methods, and pore-network models) are widely accepted to study TPGrelated issues in the development of low permeability reservoirs. 30,31 Although pore-scale numerical methods can complement experimental research, their calculation accuracy is affected by algorithm accuracy and discretization error. Compared with experimental and numerical methods, analytical methods can obtain accurate results effectively.…”