| The flow of fluid in tight sandstone reservoir is influenced by its micron structure and surface property.The characters of tight sandstone-low porosity,low permeability,condensation,well-developed secondary porosity,etc.,make it vary from conventional sandstone in percolation mechanism.China has a rich volume of tight sandstone oil and gas,especially tight sandstone gas which has a commercial quantity and attracts increasing attention.This paper regards rocks from tight sandstone gas reservoir as the study object.Seventeen pieces of tight sandstone rocks from one district of west Sichuan Basement were selected and ranged by permeability difference-group 1 has a permeability between 0.01?0.1 ×10-3?m2;group 2 has a permeability between 0.1?0.5×10-3?m2;group 3 has a permeability larger than 0.5×10-3?m.The digital rocks of the 17 rocks are built and quantitative analyses are done to the 17 rocks.After that,one piece of rock was selected from each group for finite simulation.Using the selected rock,studies of the influence of rock micron structure and surface property on gas-water two phase flow were done.According to the studies,several understandings are achieved as followed:(1)Realize the visualization of water driving gas process based on digital rock,as the direct observation and analysis of fluid distribution and flooding frontier are available.Based on this method,the water driving gas simulation is done for studying the effect of pore structure on water and gas distribution.The study shows that flooding frontier turn into arc-shaped from linear with the progress of displacement.The middle flooding frontier has a larger speed compared with the whole frontier,which generates the fengering phenomenon.(2)Analyze the rock pore equivalent diameter frequency distribution chart and cumulative frequency distribution chart.According to the analysis,the rocks,which have a permeability between 0.01?0.1× 10-3?m2,have medium-size tortuosity and low fractal dimension,and the equivalent diameter distribution of which is multimodal with small size.The rocks,which have a permeability between 0.1-0.5×10-?m2,have large-size tortuosity and medium fractal dimension,and the equivalent diameter distribution of which is unimodal with medium size.The rocks,which have a permeability larger than 0.5×10-3?m2,have small-size tortuosity and high fractal dimension,and the equivalent diameter distribution of which is unimodal with large size.(3)Analyze the rock pore structure using fractal method.Based on the results it is found that the bigger the standard deviation of rock pore equivalent diameter,the larger the fractal dimension.As a consequence,the rock will have a rather nonuniform distribution equivalent diameter size and a more complex structure.(4)Study the influence of connection number on percolation in tight sandstone.Using the big ball algorithm,in the case of other conditions stay the same,with the increase of connection number,different permeability saturation curves and capillary pressure curves are received.As the results of the study show,the bigger the connection number,the stronger the rock percolation ability and the lower the residual saturation of wetted phase.According to the capillary pressure curves,the start-up pressure of rocks having a permeability between 0.01?0.1×10-3?m2 is above 1OMPa;the start-up pressure of rocks having a permeability between 0.1?0.5×10-3?m2 is between 1?10MPa;the start-up pressure of rocks having a permeability larger than 0.5 ×10-3?m2 is below 1 MPa.(5)Study the gas-water two phase flow under different wettability.The simulation results show that gas-wetted environment is in favor of stable production and middle-wetted environment is beneficial to high recovery ratio. |
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