Influencing Factors Analysis Of The Migration Of CO₂ In Deep Saline Fractured Aquifer

Since the mid-20th century,global warming has increased due to artificial greenhouse gas emissions and activities.In order to control greenhouse gases and protect the environment,carbon dioxide emission reduction has become global research hotspot.The sequestration of CO₂ in deep saline aquifer is widely regarded as one of the most economical and promising emission reduction ways.Due to geological heterogeneity,fractures,faults and other factors,fluid migration in the process of CO₂ storage is complicated.On the one hand,the high permeability fractures in the fractured reservoir will produce the preferred path of CO₂migration;on the other hand,the fractured reservoir can quickly release the bottom pressure of the injection well to avoid excessive pressure around the injection well,resulting in the rupture of the upper caprock leading to CO₂ leakage.In this paper,the geological storage of CO₂ in fractured deep saline reservoirs is taken as the research object,and the influencing factors of CO₂ transport are deeply studied.The main work of the paper is as follows:Firstly,the physical properties of CO₂,brine and rock are selected,and the control equation of CO₂-brine two-phase flow is given base on the embedded discrete fracture model（EDFM）.Considering the influence of reservoir temperature and pressure on the migration of CO₂ in brine,the density and viscosity of CO₂ and brine are calculated according to the range of temperature and pressure in simulated conditions.The intersection types between fractures and matrix,and the calculation methods of the conductivity of non-adjacent connections are introduced.The control equation of CO₂-brine two-phase flow is established based on mass conservation equation,Darcy’s law and non-adjacent connection.Secondly,based on the numerical simulation method,the fractures in the saline aquifer are quantified by EDFM,and the properties of the fractures are clearly characterized.2D reservoir model is constructed to simulate CO₂-brine two-phase flow,to study the effect of fracture properties on the distribution and rate of CO₂ migration,and to study the changes of reservoir pressure caused by CO₂ migration.Results show:The CO₂ plume takes the fracture as the preferred channel and changes the migration direction.The longer the fracture length and wider the fracture aperture,the higher the CO₂migration rate will be.At the same time,due to the increase of the CO₂ migration rate,the CO₂ accumulation near the injection well will decrease and the reservoir pressure will decrease.The direction of CO₂ plume migration is mainly affected by the change of fracture directions.Due to the different angles of plume entering the fracture,the influence on the CO₂ migration rate is also different.When the angle is 45°,the CO₂ migration rate is the fastest,while the rate is the slowest at 135°.Pressure increases in areas where CO₂ accumulates.The difference of CO₂ migration in non-fractured reservoir and complex fractured reservoir is compared.Finally,based on two complex fractured network reservoir models with different fracture apertures,combined with the geothermal properties of the Alberta sedimentary Basin,the influences of reservoir depth,temperature,matrix permeability and matrix porosity on CO₂transport were investigated.It is found that at the underground depth of 1km to 3km,the CO₂migration rate decreases with the increase of underground depth.At the underground depth of1km to 2km,the higher the temperature,the higher the CO₂ migration rate,but with the increase of depth,the migration rate of CO₂ decreases.At the depth of 3km,the migration rate of CO₂decreases in the reservoir with high temperature.The increase of matrix permeability is conducive to the migration of CO₂ in the matrix and the decrease of CO₂ migration in fractures,which reduces the possibility of CO₂ leakage along faults.With the increase of matrix porosity,CO₂ migration in the matrix slows down,and the stability of CO₂ sequestration increases.