Study On The Sealing Ability Evolution Of Caprock System In The Process Of CO₂ Geological Storage

The industrial revolution opened the door to mass production of machines and increased consumption of natural resources such as coal,oil and natural gas.More and more CO₂ is being released into the atmosphere,causing global warming and other climate problems,such as the loss polar of ice,sea level rising,frequent extreme weather etc.It will threaten the survival of human beings,therefore,CO₂ emission reduction is urgent.CO₂ geological storage?CGS?is a kind of technique that captures CO₂ and injects it into the salt water layer of a certain geological structure.Reservoir and cap are two important geological units of CO₂geological sequestration.The role of reservoir is to seal and let CO₂ stay in the pores of geological unit,while the cap overlying reservoir is to prevent CO₂ from escaping from the pores of reservoir to the layer above the cap,which plays a storage role.The actual geological unit may have discontinuity in some locations?such as fractures and faults?,which will be the preferred path for the migration and diffusion of CO₂.The addition of CO₂ will break the water-rock balance of the original cap,accelerate the dissolution and precipitation of minerals,and then change the porosity and permeability conditions of the cap,resulting in the weakening of the stability and integrity of the cap.In this paper,by means of numerical simulation,in Junggar Basin northern of Xinjiang Fukang,Ziniquanzi formation mudstone as caprock,the underlying Donggou formation as the research reservoir,according to the physical characteristics of the reservoir cap rock and characteristics of mineral composition and systematic studies the CO₂ injection pressure,fracture and the cover layer heterogeneity for CO₂ diffusion penetration distance,cap rock mineral dissolution and precipitation and porosity conditions change,etc.The effect on the stability of the cap.The following major achievements have been made:1.The spatial distribution characteristics of CO₂,the dissolution/precipitation characteristics of minerals and the sealing properties of the homogeneous cap under different pressure differences were systematically analyzed and identified.Different injection pressure conditions affect the distance of gaseous and dissolved CO₂into the cap.With the increase of injection pressure,the vertical distance of gaseous and dissolved CO₂ into the cap increases.When the pressure difference is 1.0 MPa,the vertical penetration distance of gaseous CO₂ into the cap is very short,only 0.5m.When the pressure difference is 2.0 MPa,,3.0 MPa,and 4.0 MPa,,respectively,in the 1000th year,the maximum vertical intrusion distance of gaseous CO₂ is 14 m,32 m and 52 m respectively.The spatial distribution of dissolved CO₂ was not significantly different from that of gaseous CO₂.The maximum intrusion distance of dissolved CO₂ was slightly larger than that of gaseous CO₂,but the increase was very short.Under the four conditions,the maximum intrusion distance of dissolved CO₂ in the year 1000 was 2 m,16 m,32 m and 55 m,respectively.Dissolved CO₂intruded into the caisson,and due to the decomposition of H₂CO₃,the content of H⁺in the caisson also increased,the PH decreased,and the acidity of the water environment in the caisson increased.The spatial distribution of PH in the cap was very similar to that of dissolved CO₂ in the cap.The dissolved minerals in the cap mainly include oligoclase,k-feldspar and chlorite,etc.The dissolution of minerals in the areas of cap PH value smaller,but the strength of the dissolved as caprock buried depth difference is bigger,the intensity of the biggest area in cap rock bottom and reservoir boundary surface of roof,four kinds of solutions,dissolve the regional cap rock bottom respectively more than 2.0 m,14.5 m,31 m and 52 m.Precipitates minerals are mainly quartz,smectite-ca,illite,ankerite,magnesite and siderite,ankerite,magnesite,siderite,etc as secondary minerals,four kinds of solutions,the precipitation of regional and dissolve almost overlap,respectively cover plate more than 2.0 m,14.5 m,31 m and 52 m.The secondary minerals magnesite and siderite were precipitated for the following reasons:due to the dissolution of chlorite and k-feldspar,Fe²⁺and Mg²⁺were provided to promote the precipitation and precipitation of secondary carbonate minerals;The reasons for the precipitation and precipitation of trace secondary mineral iron dolomite are as follows:chlorite dissolution,coupled with calcite dissolution,provides a large amount of Ca²⁺ions,and CO₃^2-provided by the decomposition of H₂CO₃,which provides a material basis for the precipitation and precipitation of ankerite.Dissolved and precipitated minerals include calcite and smectite-ca.Calcite in H⁺enough area is given priority to with dissolved,H⁺is not enough area is given priority to with precipitation,four kinds of solutions,calcite dissolution occurred respectively in the cover plate above 0-1.6 m,0-12 m,0-20 m and 0-30 m,respectively calcite precipitation occurred in the cover plate above 1.6-2.0 m,12-14.5 m,20-31 m and30-52 m.The precipitation region of calcite occurred in the region with a lower PH and rich H⁺,while the dissolution region of calcite occurred in the region with a lower PH and rich H⁺.Different injection pressure,the 1000th year,porosity and permeability of the change in different area,the pressure difference is 1.0 MPa,the porosity and permeability change area for cover plate above 1.4 m,when the pressure difference is 2.0 MPa,3.0 MPa and 4.0 MPa,the permeability change of regional cap rock bottom respectively more than 8.75 m,17.5 m and 25.0 m.With the increase of the injection time,the change of porosity also increases.The maximum change of porosity is about+0.021,an increase of 111%,and the maximum change of permeability in this area is also the largest,with a maximum of 1.1 mD.Compared with the original permeability of 0.1 mD,the change of permeability is+1 mD,an increase of1000%.Compared with the original sealing capacity of the caprock,the sealing capacity of the caprock was weakened when the permeability changed greatly.There are many evaluation indexes for the sealing property of cap,such as the maximum intrusion distance of gaseous CO₂,the maximum intrusion distance of dissolved CO₂ and the porosity change of the cap.According to the index of the maximum intrusion distance of gaseous CO₂,the maximum intrusion distance of gaseous CO₂ was 0.5 m,14 m,32 m and 52m respectively under the millennium scale of the four schemes,and the maximum intrusion distance was 52 m.If the thickness of the cap layer of the numerical model could not be broken through,no gaseous CO₂ would break through the cap layer in 1,000 years.According to the measure of the distance between the biggest intrusion dissolved CO₂ dissolved CO₂ in the biggest intrusion distance to cover slightly greater than that of gaseous CO₂ into distance,but the increase was small,scales,one thousand four plans of dissolved CO₂ maximum intrusion distance of 2 m,16 m,respectively 32 m and 55 m,the maximum intrusion distance is 55 m,not to break through the numerical model of cover layer thickness,there won't be gaseous CO₂ breakthrough cap is 1000 years;According to the changes of the cap rock porosity as evaluation indexes,four scales in one thousand,the increase of porosity and permeability area respectively cover plate above 1.4 m,9.0 m,17.0 m and 27.5 m,increases the maximum distance of hole infiltration conditions cover plate above 27.5 m,four kinds of scheme can guarantee the same one thousand scale caprock sealing effect.Therefore,the sealing performance of Ziniquanzi formation cap is guaranteed in the actual engineering injection situation.2.The spatial distribution characteristics of CO₂,the dissolution/precipitation characteristics of minerals and the sealing properties of the cap with cracks in the cap were defined.When there are cracks in the caisson,CO₂ will preferentially diffuse upward along the cracks and slowly invade into the caisson laterally.Different injection pressure difference not only affects the vertical migration distance of CO₂,but also has a great influence on the invasion in the horizontal direction.Due to the high porosity and permeability at the fissure,CO₂ could penetrate the cap fissure and penetrate into the upper part in a short time?less than2 years?in vertical direction.In the horizontal direction,with the increase of injection pressure,the intrusion distance of CO₂ is larger.Under the millennium scale of the four schemes,the maximum intrusion distance in the direction of gaseous CO₂ level is 0.4 m,0.8 m,18 m and72 m respectively,and the maximum intrusion distance in the direction of dissolved CO₂ level is 1.5 m,4.0 m,20 m and 78 m respectively.The dissolution and precipitation of minerals in the cap layer are symmetrically distributed along the fracture axis.The minerals dissolved all the time mainly include k-feldspar,oligoclase and chlorite,and the maximum dissolution occurs in the fissure region closest to the cap,because the dissolution of potassium feldspar,oligoclase and chlorite requires a large amount of H⁺,and the more abundant H⁺is,the greater the dissolution degree of the above three minerals will be.Remain the state of precipitation of minerals include quartz,smectie-na,illite,ankerite and magnesite,etc.,including magnesite,ankerite and hematite iron belongs to secondary carbonate minerals,ankerite precipitation area and the distribution of CO₂ gas saturation Sg almost unanimously,regional precipitation near the crack generation is the largest,then decreasing,magnesite and hematite precipitation distribution in CO₂ gas saturation of the leading edge of the area.Smectite-ca and calcite are both dissolved and precipitated in the process of CO₂ intrusion,in which calcite is mainly dissolved,and precipitation occurs at the leading edge of the CO₂ gas saturation plume,while smectite-ca is mainly dissolved on both sides of the crack cover,and precipitation mainly occurs at the crack.Under the condition of different injection pressure difference,the porosity of the crack and its adjacent area all showed a tendency to increase,and the maximum value of porosity change at the crack increased with the increase of injection pressure.Since the conditions of porosity and permeability in the fissure are relatively good,both gaseous CO₂ and dissolved CO₂ can intrude into the aquifer above the cap through the fissure in a short time?less than 1day?,so it is not advisable to evaluate the change of sealing performance of the cap with cracks by the index of the maximum intrusion distance between gaseous CO₂ and dissolved CO₂.Therefore,the change of porosity and permeability was selected as the evaluation index.Under the condition of different injection pressure difference,the porosity of the crack and its adjacent area all showed a tendency to increase,and the maximum value of porosity change at the crack increased with the increase of injection pressure.Under the four injection pressures,the maximum value of porosity change at the fracture was+0.0155,+0.02143,+0.02228 and+0.02243,respectively.Compared with the initial value of porosity at the fracture of 0.48,the porosity increased by 3.23%,4.46%,4.64%and 4.67%,respectively.The maximum permeability change was+0.78 mD,+1.133 mD,+1.178 mD and+1.186 mD,and the permeability increased by 0.78%,1.133%,1.178%and 1.186%,respectively,compared with the initial permeability value of 100 mD at the crack.It is obvious that the ability of CO₂ leakage through fractures is further strengthened,and the sealing effect of the cap is obviously weakened compared with the original sealing performance.Therefore,in the actual site selection of CO₂ injection site,full attention should be paid to the risk of cap leakage containing cracks.3.The effects of physical properties and mineral heterogeneity on the spatial distribution characteristics of CO₂,the dissolution/precipitation characteristics of minerals,and the sealing properties of the cap were determined.Cap rock physical properties?porosity,permeability?and the heterogeneity of the spatial distribution of mineral for gas and dissolved CO₂ has a bigger influence vertical maximum intrusion distance,scale in one thousand,the gaseous CO₂ in homogeneous and heterogeneous cover the largest invasion from 44 m and 48 m,respectively dissolved CO₂ in homogeneous and heterogeneous cover the largest invasion from 46 m and 50 m respectively.Due to the heterogeneity of the cap,the migration of gaseous CO₂ in the vertical direction appears non-uniform circumduction phenomenon.Compared with the"horizontal push"migration of gaseous CO₂ in the vertical direction of the cap,the maximum invasion distance of gaseous CO₂ in the heterogeneous cap is increased.However,for the total volume of gaseous CO₂ intruding and diffusing capping,the diffusion degree of gaseous CO₂ in the homogeneous capping is greater than that of the heterogeneous capping in the millennium scale,and the total volume of gaseous CO₂ intruding into the homogeneous and heterogeneous capping is 44%and 30%respectively.In terms of mineral dissolution and precipitation,the minerals that are always in dissolution are mainly oligoclase,k-feldspar and chlorite.Compared with the"banded"distribution of the mineral volume change in the homogeneous cap,the mineral volume change in the heterogeneous cap is inhomogeneous.The minerals dissolved and precipitated mainly include calcite and smectite-ca.Calcite is mainly dissolved in the homogeneous and heterogeneous capping beds,while smectite-ca is mainly precipitated in the homogeneous and heterogeneous capping beds.The dissolution of calcite in the homogeneous cap occurred in the region of 0-35 m above the base plate of the cap.Precipitation occurred in an area of 35-46m above the floor of the cap.Quartz,illite and smectite-na are the minerals that are precipitated all the time,and the secondary carbonate minerals produced mainly include ankerite,siderite and magnesite.Siderite precipitation occurs in a small part of the leading edge of the CO₂ plume,and the precipitation amount is very small,mainly due to the Fe²⁺provided by chlorite.Magnesite precipitation occurs in the internal region of CO₂ plume,and the precipitation amount is also relatively small,mainly due to the dissolution of smectite-ca and chlorite to provide Mg²⁺.According to the maximum intrusion distance of gaseous CO₂ as an index to evaluate the sealing property of the cap,the maximum intrusion distance of gaseous CO₂ in the heterogeneous cap is greater than that of the homogeneous cap under the same number of injection years.With the increase of the number of years of injection,the maximum penetration distance of gaseous CO₂ in both homogeneous and heterogeneous caps increases,but the growth rate of the maximum penetration distance decreases gradually.In the 1000th year,the maximum intrusion distance of gaseous CO₂ in the homogeneous and heterogeneous caps was 43.5 m and 48 m,respectively,and neither of them could break through the 100 m thick caps in the numerical model.According to the actual thickness of the formation and the evaluation index,no gaseous CO₂ leakage could occur in the homogeneous and heterogeneous caps within 1,000 years.According to the dissolved CO₂ into largest distance as evaluation index,the 1000th year,dissolved CO₂ in homogeneous and heterogeneous cover the largest invasion from 45 m and 50 m,respectively,compared with the gaseous CO₂ in homogeneous and heterogeneous cover the largest invasion from 48 m,43.5 m and dissolved CO₂ slightly greater than the maximum distance into gaseous CO₂ dissolved CO₂ has not been able to break through numerical model 100 m thick homogeneous and heterogeneous cover layer,according to the actual thickness of strata,according to the evaluation index,It can ensure that the thickness of the homogeneous cap and the heterogeneous cap will not be breached by dissolved CO₂ for 1000 years.According to the change of the porosity of the cap as the evaluation index,it is concluded that the regions with obvious changes in the porosity of the homogeneous cap and the heterogeneous cap are respectively 25 m and 46 m away from the bottom of the cap at the millennium scale.Both of them can guarantee the security of the cover closure on the millennium scale.In general,through the study of several models of CO₂ intrusion into the cap in this paper,it can be found that dissolved minerals occupy a dominant position in the cap,and the dissolved volume of minerals is greater than the generated volume of precipitated minerals,which reduces the total volume of minerals in the cap and leads to the increased porosity of the cap.The transformation of porosity of heterogeneous cap CO₂ intrusion is more powerful,and the region with increased porosity will become the dominant channel of CO₂,making the region easier to be transformed.In short,in the case of CO₂ intrusion into the cap in this study area,the sealing performance of the cap tends to weaken,and the heterogeneous cap is weakened to a greater extent.Therefore,in the case of actual engineering injection,the sealing ability of Ziniquanzi formation is guaranteed by avoiding the area containing faults or large cracks.