Numerical Investigation Into Air-water Two-phase Flow And Its Temperature Effects Of Pore Structures Of Rocks

The accurate description and scientific evaluation of multi-phase flow in the reservoir rocks underground have major significance of theory and engineering value for many geological engineering,such as oil and gas exploitation and carbon sequestration.In particular,unconventional oil and gas reservoirs are often buried deep more than 3,000 meters,and carbon dioxide is often captured and stored more than 1,000 meters underground,which often occur in complex rock structures under high pressure and high temperature.It is difficult to accurately detect and analyze the oil and gas migration in complex pore structures of reservoir rocks under high temperature and high pressure.In particular,it is difficult to visually observe and quantitatively analyze the dynamic characteristics and migration paths of multi-phase flow in pore structures of rock mass,which makes it is impossible to accurately evaluate and predict the reconstruction effect of unconventional oil and gas reservoirs and the exploitation efficiency of oil and gas,and carbon sequestration effect.Therefore,it's of great significance to investigate the two-phase flow characteristics and flow paths in pore structures of rock mass under high pressure and high temperature and its influencing factors for understanding and mastering the characteristics of multiphase flow in deep buried reservoirs and accurately evaluating and predicting the exploitation efficiency of oil and gas and carbon sequestration effect.Due to many difficulties and challenges in on-site and laboratory tests,numerical simulation has become an effective mean for quantitative analysis of multi-phase flow processes in complex pore structures of reservoir rocks buried underground.However,how to accurately establish reasonable numerical models and quantitatively analyze the fluid flow inside the reservoir rocks under multi-field coupling conditions has always been a bottleneck problem in academic circles.Many problems have not yet been satisfactory solved,for example:(?)how to accurately and quantitatively characterize the influence of multi-scale heterogeneity of pore structures on multi-phase flow in rock mass,especially its influence on preferential paths.(?)The path selection of multi-phase flow in complex pore structures and influence mechanism are not yet clear.(?)Most researches used 2D models to investigate the two-phase flow in porous structures with assuming the flow channel thickness is much less than the size of pore throats,and less quantitatively explain the effects of channel thickness on flow.Simultaneously,such assumption also makes it difficult to experimentally verify the reliability of numerical results.(?)The existing models less consider the thermo-hydro(T-H)coupling effect on the two-phase flow interface and flow paths.(?)The current numerical methods for simulating and analyzing the two-phase displacement process with the larger viscosity and density ratios,such as air-water flow,can't well deal with the interface instability due to excessive viscosity ratio.(?)The numerical results of the two-phase flow are difficult to be experimentally verified,or there is a large gap with the experimental results.For the above-mentioned problems,the numerical simulations and experimental investigations on the air-water two-phase flow in pore structures of reservoir rocks were carried out in this thesis.Based on the pore structures of natural reservoir rocks,a porous model was established,and 3D porous models with various channel thickness were prepared to analyze and reveal the thickness effect on air-water two-phase flow behavior.On this basis,the phase filed models with larger viscosity ratio was used to simulate the influence of heterogeneous pore structures on the two-phase flow under temperature effect.This paper focuses on the following aspects:(?)Air-water two-phase flow and flow paths in pore structures of reservoir rocksBased on the pore network of natural rocks,a two-phase 2D phase field model for compressible air-water two-phase flow was established.The dynamic characteristics of air-water two-phase interface and preferential paths in the porous structures were simulated,and the influence factors of preferential paths of air-water two-phase flow in porous structures and the path selection mechanism under various wetting conditions were discussed and analyzed.The transparent physical model of complex pore structures was prepared by 3D printing technology.The air-water two-phase displacement experiments by means of dyeing tracer technique were used to visually observe the flow characteristics and path selection behavior of air-water two-phase flow,and the validity of phase-felid model was verified.(?)The effect of channel thickness on air-water two-phase flow were analyzed.On the basis of the 2D phase field model of air-water two-phase flow,in order to facilitate the comparison analysis with the experiment results,a 3D phase field model of air-water two-phase flow was established.The effects of channel thickness and heterogenous pore distribution along thickness direction on the air-water two-phase flow behavior and path selection in the porous structures were simulated and analyzed.The heterogeneity influence of pore structures on air-water two-phase flow was discussed.The validity of numerical analysis was verified by 3D experiments.(?)Two-phase flow and flow paths in pore structures of reservoir rocks under T-H coupling effectsIn order to analyze the influence of T-H coupling on the two-phase flow behavior,based on the MATLAB secondary script provided by COMSOL,a T-H full coupling model of two-phase flow was developed to quantitatively analyze the two-phase interface motion and flow characteristics in the pore structures under various temperatures.The influence of temperature on the preferential path selection of two-phase flow was discussed.This study mainly found that:(?)The flow paths of air-water two-phase fluids in porous structures are determined by driving pressure and pore size in branch channels.(?)A high channel thickness can induce the development of secondary flow paths due to the growth of resistance on flow,leading a different fluid distribution compared with low channel thickness and the final air saturation reduces correspondingly.Therefore,channel thickness effects are not negligible in studies on two-phase flow in porous structures.(?)Under the same displacement conditions,the heterogeneous pore structure in the thickness direction have longer breakthrough time,better displacement effect and less air output than homogeneous ones.For the rock structures with the same porosity,the shape factor of flow channel significantly affects the air-water two-phase flow.The smaller the shape factor,the heterogeneity of pore structures in thickness-direction have a bigger impact on displacement effect and flow paths,(?)Both the reservoir temperature and interface temperature have some effects on the preferential paths of two-phase flow,but the injection temperature can have more obvious effect on the branch flow in some small pore regions.The research has the following innovations:(?)A 2D phase filed model capable of handling two-phase interface kinetics of compressive fluids was built,and the numerical simulation of air-water two-phase flow process in complex pore structures was realized.The analysis reveals the main controlling factors on the preferential path of air-water two-phase flow.The path selection mechanism under various wetting conditions was discussed.(?)The transparent physical model of complex pore structure was constructed by 3D printing technology.The path selection of air-water two-phase flow in complex pore structures was observed visually by dye tracing.The validity and accuracy of 2D phase field model was verified.(?)Extending the 2D phase field model to 3D,the influence of heterogeneous pore distribution on the air-water two-phase flow characteristics and flow path selection was revealed.(?)A numerical simulation algorithm for T-H coupled two-phase flow was developed,and the two-way coupling between thermal and hydraulic fields was realized.The influence mechanism of temperature on two-phase flow behavior and flow paths was revealed.This study provides the research basis and reference for quantitatively and visually analyzing the multi-phase flow and preferential path in the pore structures of reservoir rocks under formation temperature.