| With the severe challenges brought by the use of traditional energy to the environment,the exploration and exploitation of clean energy has become particularly important.Conventional and unconventional natural gas with CH4 as the main component is widely accepted by people in daily life for its cleanliness and low pollution.CH4 breakthrough pressure is a very important parameter in gas reservoir production and caprock evaluation.In the current stage of rapid development of unconventional oil and gas,tight sandstone reservoirs have gradually become the key direction of unconventional natural gas exploration and development due to their abundant resources.Therefore,it is of great significance to carry out breakthrough pressure research on this low-permeability sandstone.In order to reveal the influence of temperature and pressure conditions on the sealability of the caprock under different formation conditions,this paper uses a combination of experiments and numerical simulations to carry out different temperature（20～60℃）and pressures（2～8MPa）,The research on the influence of conditions on CH4 breakthrough pressure,the main conclusions are as follows:（1）The linear functional relationship between temperature&pressure and CH4breakthrough pressure&breakthrough time was determined.Within the temperature and pressure range of this paper,with the increase of temperature or pressure,CH4breakthrough pressure and breakthrough time both showed a linear downward trend.When CH4 broke through,the residual water saturation in the core showed an increasing trend,and the influence of pressure changes on the CH4 breakthrough process was more significant.The experimental and numerical simulation results are consistent with the change of temperature and pressure,and Numerical results agree well with experiment.（2）The influence of temperature and pressure conditions on the breakthrough pressure of CH4 can be attributed to its influence on the interfacial tension of the CH4-H2O system and the contact angle（θ）in the core,thereby controlling the magnitude of the breakthrough pressure of CH4,and the interfacial tension plays a leading role;The breakthrough pressure of CH4 decreases with the increase of the two-phase viscosity ratio,and the higher temperature and pressure,the smaller effect of the viscosity ratio on the breakthrough pressure.（3）With the same temperature（or pressure）,as the pressure（or temperature）increases,the relative permeability and effective permeability of the H2O phase decrease in the same time,and the CH4 phase increases.At this time,it is more conducive to the migration of CH4 in the porous medium,and then shows a smaller breakthrough time and breakthrough pressure.（4）Based on the control variable method,the three key factors of two-phase dynamic viscosity,density and CH4-H2O system interfacial tension were parametrically scanned,and it was found that the three key factors in the CH4displacement process were as follows:two-phase dynamics Viscosity>CH4-H2O system interfacial tension>two-phase density.（5）Through the calculation of the capillary number（Ca）and the two-phase viscosity ratio（M）in this model under different temperature and pressure conditions,and the drawing of the log Ca-log M stability phase diagram,the two phases under the temperature and pressure conditions in this paper were determined.The displacement process is dominated by capillary fingering,and the displacement process is mainly affected by capillary force.Therefore,within the experimental temperature and pressure range,for the unsaturated low-permeability sandstone reservoir,the higher temperature and pressure,the more favorable for gas reservoir development;for the unsaturated low-permeability sandstone caprock,the lower temperature and pressure,the better sealing property of the cap rock and the safer caprock.This paper can provide reference for practical projects such as unconventional natural gas extraction and caprock sealing evaluation. |
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