Study On Quantitative Evaluation Of Hydrophilic Capillary Water Retention

With the improvement of technological innovation and theoretical innovation,Unconventional oil and gas has gradually become the focus of exploration and exploitation.Among them,tight sandstone gas is an important part of unconventional gas(tight sandstone gas,coal-bed gas,shale gas)and has great development potential.But due to the tight sandstone gas reservoir has a characteristic of poor permeability,strong anisotropy and low initial water saturation,high capillary pressure,and rich in clay minerals,which make tight sandstone reservoir easily suffered serious problems of reservoir damage in the process of drilling,completion and production,especially liquid intrusion damage.With incomplete statistics,the backflow rate of the liquid phase invasion is less than 30%?40%,which undoubtedly bring huge economic losses to the exploration and development of oil and gas reservoirs.In this paper,the microscopic pore structure characteristics of sandstone reservoirs are analyzed by SEM,casting thin section and conventional mercury injection.On the basis of this study,a physical model of micro pore structure,a capillary bundle model is established.Based on this model,the effect of water saturation on gas permeability,the rate of backflow and retention of the liquid phase are further studied.This work provides theoretical guidance and technical support for quantitative evaluation of liquid phase invasion damage and backflow of liquid phase.Through the above research,the main achievements are as follows:(1)The microscopic pore structure characteristics of sandstone reservoir are described by means of the core analysis technology.It is concluded that the high permeability reservoirs has a characteristic of good connectivity and mainly wide-medium pore throat,Low permeability-dense rock sample with narrow throat.(2)Based on the conventional mercury experiment,the capillary tube model is establishecd by using the parameter-tortuosity,which lay the foundation for the quantitative evaluation of water phase invasion damage,backflow and retention.(3)Establishing dif-ferent water saturation from small value to large orderly based capillary bundle model,the average thickness of water film,the effective radius of-gas seepage and the effective permeability of gas are analyzced with the variation of water saturation.The results indicate that the larger the water saturation is,the larger the average water film thickness is,there is a better exponential function relationship between the water saturation and the average thickness of the water film;The larger the water saturation is,the smaller the effective seepage radius of the gas is;the lower the absolute permeability of the rock is,the faster the effective permeability of the gas decrease with the increase of water saturation.(4)Based on the experimental data of different radius'micro-circular tube,the research on the boundary layer thickness and the pressure gradient is carried out by combining the Hagen-Poiseuille equation,and a mathematical flow model is established.It is of great theoretical value to further predict the boundary layer thickness of liquid phase flow in different radius capillary tubes under different pressure gracdient.(5)The backflow and retention of liquid phase are analyzed based on the backflow model of liquid phase.It is indicated that the higher the absolute permeability is,the higher the backflow rate of the liquid phase is,the less the retention is;the lower the absolute permeability of the rock sample is,the lower the backflow rate of the water phase and the more retention.For high permeability reservoir,the problem of working fluid retention in near well zone can be relieved by increasing backflow pressure.In a word,retention of liquid phase is not the most important type of reservoir damage in high permeability reservoir.However,for the low-dense permeability reservoir,it is not enough to reduce or even relieve the damage by means of the self energy of the gas reservoir.It is also more scientific and efficient to remove the invasion by the physical and chemical measures such as injection of surfactant,CO2,acid fracturing.It is of great significance to develop low permeability reservoirs.