A Study On Effect Of CO₂ Geological Storage In Reservoir And Cap Rock

Excessive emissions of CO₂ has brought a serious environmental problems,CCS?CO₂ capture and storage?seems to be the best way to reduce carbon emissions under the condition of existing technology which is gradually aroused people's attention.As a great significance part for the development of CCS,the CO₂ geological storage research means so much for this technology.Resent years,scholars from different country have already carried out a lot of research and achieved fruitful results through indoor experiment and numerical simulation method,and some of the commercialization of CO₂ geological sequestration projects have been launched.In this paper,we carried out the numerical simulation research on the effects of CO₂ on the porosity,permeability,gas saturation and pH of reservoirs and cap rocks at different injection rates.The study is mainly about the effects of CO₂-water-rock reaction and the above parameters on the physical and chemical properties of reservoirs and cap rocks.It has been found that CO₂ is first dissolved in water to form carbonic acid after CO₂ injection into the reservoir,which will cause a dramatic change in the acidity and alkalinity of the reservoir,resulting in a significant reduction in pH.By the effect of H+ in the reservoir,part of the mineral composition?oligoclase as an example?occurred in the dissolution,and some other minerals?calcite as an example?occurred in the precipitation,by the same time the reaction to produce albite,siderite and other secondary mineral precipitation,so that the composition of groundwater solution significantly changed.This CO₂-water-rock reaction rate is very slow,after the injection of CO₂,the various types of mineralization of the localization reaction is still a long time to continue.In addition,continuous injection of CO₂ can cause a significant increase in the local pressure of the reservoir,so that the reservoir to the injection point as the center of the formation of local high pressure area,will damage the original stress balance of the reservoir.Uneven distribution of pressure can lead to shear and stretching of reservoir rocks,which may lead to reservoir rock fractures and fault movement and formation instability.The effect of pressure on the vertical profile of the injection point is particularly pronounced,and if the pressure exceeds the reservoir capacity,it may lead to formation uplift and induced earthquakes.In addition,due to the effect of CO₂-water-rock reaction,reservoir porosity and permeability increase to a certain extent,this situation is conducive to the migration of CO₂ in the reservoir.The CO₂ injected into the reservoir migrates upwards under the action of buoyancy and gathers under the cap rock,so that the original pressure distribution of the cap rock is disrupted and the uneven distribution of local pressure occurs.The change of pressure from the point of inflow point is the inflection point at the junction of the reservoir and the inside of the two layers are linearly changed.With the increase of CO₂ injection,the pressure increment increases in the range of the cover,and the risk of the rupture With the increase.Under the action of injection volume,molecular diffusion,capillary convection and buoyancy,CO₂ will invade the cap rock from the reservoir,but the comprehensive experimental results show that the injection volume is the main driving force of CO₂ intrusion.When the CO₂ injection is stopped,the invasion of CO₂ on the cap is also substantially stopped.The CO₂ that penetrates the cap rock will dissolve some of the minerals in the cap rock,resulting in an increase in the porosity of the cap rock and a weakening of the sealing properties to varying degrees.This effect is related to the depth of CO₂ invasion.During the 200-year simulation period,a total distance of about 10 meters from the cap rock is invaded,but it still affects the safety of the cap rock.