Molecular Simulation Study On The Obstructed Behavior Of Oil-water Flow In Nanopores

As an important part of unconventional oil and gas resources,tight oil has abundant reserves.It is another hot spot for exploration after shale gas and has broad development prospects.Due to the widely distributed nano-scale pore throat network system in tight oil reservoirs,the oil and water in the internal migration process are subject to obvious flow resistance,which makes the development of tight oil very difficult and low productivity.Identifying the source of flow resistance in nanopores and quantifying the size of flow resistance are of great significance for regulating flow resistance and improving tight oil recovery.However,traditional observational statistical methods are very difficult to directly observe the obstructive behavior of tight oil-water flow in nanopores,and it is also difficult to investigate the impact of continuous changes in core wettability on the displacement effect in core displacement experiments.Molecular dynamics simulation methods It shows unique advantages in studying the microscopic dynamic characteristics of oil-water migration at the nanometer scale and constructing pores with different wettability.therefore,in this paper,molecular dynamics simulation method is used to study the obstructive behavior of oil-water flow in nanopores,and the microscopic process of oil-water flow is described in detail.the source of flow resistance in the displacement process is analyzed and the fluid mechanics formula is introduced.Quantitative calculations were carried out,and the influence of pore size and wettability on flow resistance was investigated.the main research contents of this paper are as follows:(1)the influence of pore size effect on oil-water flow resistance is studied.Firstly,a model of nanopore water displacement oil was constructed,the process of oil and water obstruction in typical nanopores(10nm)was studied,and the influence of pore size(6,8,10,12,15nm)on flow obstruction behavior was investigated.Studies have found that the oil and water are obviously blocked during the displacement process,but eventually a stable flow will form in the pores.the critical injection pressure required for nanopores with different pore diameters is calculated.It is proposed that the flow resistance is composed of capillary pressure and viscous resistance,and the fluid mechanics formula is introduced to calculate it.the calculation results show that the capillary pressure dominates the flow resistance.the calculated flow resistance is not much different from the adjacent injection pressure,but has obvious size effects.the smaller the pore size,the greater the deviation between the two.This result shows that the fluid mechanics formula has certain applicability at the nanoscale,but it has a significant pore size effect in the nanoscale pore throat network system.(2)the influence of pore wettability on flow resistance was investigated.First,the method of scaling the surface charge of the pores was used to construct a set of pores with different wettability in theory.By investigating the spontaneous imbibition process of tight oil in different pores,different oil-water-pore three-phase contact angles were obtained,and this was verified.the method controls the rationality of the wettability of the pore surface.Subsequently,the process of obstructing the flow of oil and water in pores with different wettability was investigated.the results of the study showed that as the lipophilicity of the pore surface decreases,oil and water can enter the pores more quickly,which is beneficial to improve the efficiency of tight oil recovery.By calculating the flow resistance under different wettability pores,the results show that the weakening of the lipophilicity of the pore surface reduces the capillary pressure during the displacement process,while the viscous resistance remains basically unchanged,which ultimately leads to a decrease in the flow resistance.the study revealed that the wettability of pores affects the three-phase contact angle,thereby changing the capillary pressure,which in turn affects the physical nature of flow resistance.