Study On The Influence Of Porosity And Permeability And Mineral Characteristics On CO₂ Geological Storage

Since the Industrial Revolution,the concentration of CO₂ in the atmosphere increases rapidly.The resulting climate change has triggered a series of serious environmental problems,such as sea level rising,frequent extreme weather,etc,which threatens the homeland of humanbeings.However,for a short term,it will still be the fossil fuels?coal,petroleum and natural gas?taking the dominant role in energy consumption structure.Therefore,CO₂ geological sequestration is widely approved by scholars as a way to reduce carbon emissions effectively.The deep saline aquifers are considered to be the most potential storage sites with unique advantages.Different aquifers are of different geological conditions and physicochemical properties,leading to a different hydro-geochemical reaction in reservoirs.In this paper,the upper reservoir of Cretaceous Donggou Formation in the northern part of Fukang,Junggar Basin,Xinjiang is selected as the research area.Based on the analysis of reservoir condition,the geological storage potential of CO₂ in the site is calculated by theoretical formulaand numerical simulation.Then,this paper proposesa method to calculate the effective storage factor,a key parameter of CO₂ geological storage,by comparingthe results of theoretical formula method and numerical simulation method.After that,some factors affecting migration and transformation are discussed,such as,the characteristic of porosity/permeability,mineral composition,and their heterogeneity.The conclusions drawn are as follows:1.According to the richness of geological data,different theoretical formula methods and numerical simulation were employed to calculate the storage perfusion potential in the study area.The effective storage capacity is 13.98 million tons by numerical simulation,and it is 55.82 and 97.38 million tons by“volume analysis”and“dissolution mechanism”two theoretical methods,respectively.2.A method is proposed to calculate the effective storage factor,a key parameter of CO₂ geological storage,by comparingthe calculated results of theoretical formula method and numerical simulation method.The effective sealing factor is the key parameter to calculate the effective storage potential of the reservoir.The method and the specific operation process of determining the effective storage factor of the site are explained in this study.Then,the results of effective storage factor calculated are 0.251 and 0.144 for“volume analysis”and“dissolution mechanism”,respectively.It provides a powerful basis for the rapid calculation of the effective storage capacity of similar sites.3.Determine the mineral which likely to dissolve or to precipitate and the mineral combination that could promote carbon sequestration.The results show that,affected by CO₂,calcite,feldspar,chlorite dissolve,clay minerals,siderite,magnesite and ankerite in this reservoir precipitate.Among them,which can sequestrate CO₂ are siderite and ankerite.4.The evolution of chemical characteristics of reservoir water can indicate the CO₂ migration at different stages.During injection period,supercritical CO₂ dissolves to form HCO₃^-and H⁺,makes the pH of reservoir water lower.Where the CO₂ migrates,a significant change will occur to the cation components and pH,and supercritical CO₂ will arrive in a short term.After injection,minerals dissolve/precipitation cause ion change,so,the appearance of carbon sequestration mineral can directly indicate the hydrogeochemical reaction in CO₂ geological storage process.Ankerite in this reservoir can be regarded as kind of tracer of CO₂.5.Porosity and permeability conditions have a great influence on the CO₂migration,transformation,and the hydro-geochemical reaction time in the reservoir.The content of injected CO₂ is more and the difference in concentration at different locations is greater under relatively high porosity-permeability conditions,which dissipates more easily through the interaction of other forms.Meanwhile,It is mainly influenced by porosity on the extent and distribution of the CO₂/water two-phase zone where water-rock-CO₂ reaction takes place.The time of specific turning point appears later at a given location,where the degree of hydrogeochemical reactions is weaker at the same time,in the higher porosity-permeability condition.In the scope of porosity-permeability condition designed in this paper,the amount of mineral sequestration mainly affected by porosity in a lower porosity-permeability condition,it decreases with the increases of porosity-permeability.And in contrast,mineral sequestrations influenced by permeability,it increases as porosity-perme-ability.More CO₂ is sequestrated in mineral under higher porosity-permeability condition.6.The mineral dissolution pattern has a kind of correspondence to that of precipitation.The cations needed for the precipitated mineral are provided by the corresponding dissolved minerals,while the cations for the carbon sequestration mineral can only be provided by the specific minerals,or,it will never precipitate.Fe²⁺and Mg²⁺in ankerite and magnesite can only be provided by chlorite,as well as Fe²⁺in siderate is mainly comes from chlorite.Ca²⁺of ankerite comes from calcite.And magnesite can only appears in Mg²⁺rich reservoirs after siderite dissolution.Therefore,chlorite has the most significant influence on geological storage.In reservoirs with high salinity and large amount of Ca²⁺,calcite also affects the formation of carbon sequestration minerals,and oligoclase will also indirectly affect siderite and ankerite by affecting calcite dissolution or precipitation.7.Heterogeneity of the reservoir porosity,permeability and mineral composition has significant effects on the evolution of CO₂ transport and hydrogeochemical characteristics.?1?In heterogeneous reservoirs that designed in this paper,at early stage,CO₂migrates along high permeability zone,leading to an irregular distribution of CO₂,instead of the uniform distribution in homogenous reservoirs.When injection stops,the CO₂ accumulated began to dissipate by the preferential flow,more rapidly than that in homogeneous case.?2?There is a large difference of evolution pattern for ion concentration under the condition of mineral composition heterogeneity,which would promote the hydrogeochemical reaction.Hence,it is earlier and more complete for the geochemical reaction in heterogeneous reservoir that designed in this paper.?3?Although,the patterns of dissolution and precipitation presents more irregular under the condition of mineral heterogeneity;there is still a correspondence between dissolution and precipitation in local area.As the minerals dissolving and precipitating,the reservoir is homogenized.