Migration And Transformation Of CO₂ In CO₂ Geological Sequestration Process Of Shiqianfeng Saline Aquifers In Orods Basin

In recent years, the greenhouse effect caused by anthropogenic CO₂ emissions is receiving widespread concern in the world. Governments around the world have begun to recognize the importance of CO₂ emission reduction, and adopted a series of emission reduction policies to reduce CO₂ emission. CO₂ geological sequestration is the direct and effective way of CO₂ reduction that has already got a high degree of attention of Governments and scientists. China is not only the major emitter, but cannot reduce CO₂ emissions from emission sources in the short term due to economic development dependent on fossil fuels highly. Therefore, how to deal with the CO₂ that will be emitted in the future becomes a difficult problem to sustainable economic development. The Ordos Basin is an important energy and chemical base in China, and there are several energy and chemical bases to be built in the northeastern. After the energy bases are put into operation, the problem of CO₂ emissions will become the bottleneck problem that restricts the development of industry in the northeastern of Ordos basin. In order to meet the dual needs of the local industrial development and CO₂ emission reduction, there are many related CO₂ geological sequestration projects have been or are carrying out.The main CO₂ geological sequestration reservoir types include: decline and abandoned oil and gas fields, unminable coal beds, as well as deep saline aquifers. The deep saline aquifers are thought to be ideal reservoirs due to widely distribution and large storage capacity. So far, domestic and foreign researchers have carried out numbers of researches, such as CO₂ geological storage mechanism, the storage capacity and security of and exploration of CO₂ sequestration in deep saline aquifers, to make better understand for CO₂ sequestration processes in deep saline aquifers. However, different deep saline aquifers have different physical characteristics, mineral compositions and water chemistry characteristics in different sedimentary basins so that the migration and transformation characteristics of CO₂ in deep saline aquifers are very complex.There are also a large number of preliminary studies which focused on CO₂ geological sequestration are carried out in China, such as, in large scale, CO₂ storage potential in the basin range, static storage capacity, as well as a CO₂ storage site selection, and in small scale, CO₂ solubility in different type of brine, CO₂ mineral dissolution precipitation reaction in different types of rocks. But the researches of the systematic studies and CO₂ migration and transformation of specific site are rarely. However, the transformation and storage form of CO₂ are the key factors not only whether stable storage and long-term CO₂ trapping, but also whether CO₂ seuqestration is safe or not. Therefore, it is a very important theoretical and practical significance to explore the migration and transformation characteristics and the form of CO₂ storage in geological sequestration process in specific deep saline aquifers. Deep saline aquifers are widely distributed and have big total storage capacity in Ordos Basin. Therefore, it is helpful for solving the CO₂ reducition problem to study the migration and transformation characteristics of CO₂ geological storage process in deep saline aquifers in CO₂ geological storage demonstration sites in the basin.For the above reasons and problems, this study focued on the typical injection layer- deep saline aquifers in Permian system Shiqianfeng Formation of CCS demonstration projects in the northeastern Ordos basin, through the analysis of the migration and transformation of CO₂ in deep saline aquifers, to provide a theoretical basis for the basin deep saline aquifer CO₂ storage capacity assessment and implementation of CO₂ geological storage projects.Shiqianfeng Formation is a thick layer of special low porosity and low permeability reservoir. The pore types are micro-capillary and hair tube pore. From its physical characteristics, the storage capacity of deep saline aquifers in unit thickness is very limited. But it is improved the layer storage capacity that large thickness of the layer and wide extent due to the relatively stable distribution of alluvial plain can be used for CO₂ geological reservoirs. In addition, the major minerals in the sandstone layers are feldspar, quartz, calcite, chlorite, smectite, illite, kaolinite and hematite. The relatively high feldspar content is benefit for CO₂ mineral sequestration. The water chemistry type of saline in the formation is Ca Na-Cl type water, total soluble solids is 31.2g / L, and sulfate and hematite are too saturated, while the carbonate minerals, silicate minerals and clay minerals are in the non-saturated. These characteristics are benefit for CO₂ dissolution sequestration and CO₂ mineral sequestration.Based on the analysis of lithology, mineral composition, and water chemistry characteristics of the deep saline aquifer, TOUGH-MP code was employed as simulation tools to study stratigraphic / hydrodynamic trapping and capillary trapping during CO₂ geological storage process. The results showed that: 1, in the set injection pressure, there were big flow rate in beginning of injection, and then the CO₂ flow rate gradually decreased with the injection time extension, finally reached a stable flow rate. 2, deep saline aquifers for CO₂ geological storage, the proportion of dissolved CO₂ increased over time and it led to storage risk decrease, however supercritical CO₂ storage form was still the main form of storage. Around the injection well supercritical CO₂ density was of more than 700kg/m3. CO₂ injection was mainly gathered around the injection well to form CO₂ plume, and the CO₂ plume gradually expanded from about 300m to 500m over the injection time. The range of CO₂ mass fraction in the salt water increased due to the dissolution of CO₂ increases in CO₂ plume edge. 3, the proportion of residual gas caused by the capillary reached more than 50% in the deep saline aquifers.After the study of stratigraphic trapping / hydrodynamic trapping and capillary trapping during CO₂ geological storage process, this paper employed a combination method of experimental research and numerical simulation to study on the dissolution processes and effect factors of CO₂ in saline. The results showed that: 1, CO₂ solubility in different water chemistry types of water by ascending order: MgCl2-type water <CaCl2-type water <Na2SO4-type water <NaCl-type water <Na2CO3-type water <distilled water. These results were consistent with the calculated results by TOUGHREACT with about 5% error. CO₂ solubility of Shiqianfeng Formation saline was 1.05mol/L. 2, compared with distilled water, the more complex of water chemical composition, and the greater increased in HCO3-concentration. While the water composition was relatively simple, the tested water HCO3-concentrations were in good agreement with the calculated value by TOUGHREACT, and the more complexity of the water composition, the poor agreement was probably due to complex unstable HCO3-complexing matter in aqueous solution system between tested HCO3-concentration and calculated HCO3-concentration. 3, the CO₂ solubility in the saline in the temperature conditions of 55°C and 70°C were 1.21 mol/L and 1.13mol/L, compared with the calculated value 1.20 mol/L and 1.10 mol/L were almost the same with 1 % and 3% error; HCO3- stable concentrations were 402.73 mg/L and 385.65mg/L, while the simulation results were 132.16 mg/L and 128.52 mg/L. From the comparison results between the tested data and the calculated data by TOUGHREACT software, it could be seen that TOUGHRACT could better simulate of the interaction between saline and CO₂, so TOUGHREACT could be used for inter-process prediction of CO₂ long-term geological storage of CO₂. 4, the Ca2+ concentration and SO42-concentration in saline water had less effects on the solubility of CO₂ and HCO3-concentration. In addition, TDS and pH values of saline affected not only the solubility of CO₂, but also the conversion of CO₂ to HCO3-.In addition, this paper also employed a combination method of experimental and numerical simulation to study mineral storage mechanism in the deep saline aquifers during CO₂ geological storage process. The results showed that: 1, during CO₂ geological sequestration in the deep saline aquifer, the dissolved minerals were plagioclase and hematite. The precipitated minerals were quartz, kaolinite and calcite. Illite dissolved on the time scale of the experiment, and with the time scales increased, illite transformed to other minerals, which was dawsonite in 55°C, and kaolinite in 70℃. The precipitation of Calcite did not observe during the experiment. From the simulation results, it was seen that calcite can precipitate in the time scale of thousands of years, and the higher the temperature the occurrence sooner. Mineral storage for thousands of years, the higher the temperature is not only conducive to the earlier occurrence of the mineral storage, and increase the storage of the same mineral composition of the mineral. Taken 10,000-year as standard storage time, mineral storage capacity was 0.786kg/m3 in 55℃,and was 2.18kg/m3 in 70℃. 3, the occurrence of mineral sequestration indirectly increased the solubility of CO₂, and this impact had already appeared early in the reaction, this effect decreased with increasing temperature.Finally, on the basis of the studies before, the 3D CO₂ migration modle and 1D CO₂ transformation model in CO₂ sequestration in the deep saline aquifers were set up to analysis the migration and transformation and related uncertainty. The results showed that: 1, in the set injection pressure, the CO₂ flow rates was from 0.82 kg/s to 0.04kg/s, the average annual flow was 4.8×107kg, include 77% supercritical carbon dioxide and 23% dissolved CO₂. With time increasing, the ratio between supercritical carbon dioxide and dissolved CO₂ changed, the proportion of supercritical CO₂ storage reduced about 2%, the proportion of dissolved CO₂ storage increased about 2%. 2, in the observation point, the CO₂ solubility at 70℃was consistent with Chapter 6 results (almost 1.13mol/L). CO₂ saturation increased in the saline as well as pH decreased rapidly and stabilized at around 4.4. 3, the mineral dissolution / precipitation reactions included: the dissolution of albite and anorthite, illite, kaolinite, montmorillonite, and the precipitation of calcite, dawsonite and quartz reaction. The major CO₂ storage minerals were calcite and dawsonite. 70 years after the stop injection time, mineral storage achieved 1kg/m3 in the bottom of the saline aquifer and CO₂ mineral storage in the top of the salt water layer was only 0.43kg/m3. Compared with the maximum CO₂ mineral storage capacity (2.18kg/m3), it reached 46%, showing that the layer of mineral storage speed was relatively fast. There were different mineral assemblages due to the interaction of CO₂ -saline- sandstone, such as quartz, kaolinite, calcite and dawsonite in the top of the saline aquifers and quartz, kaolinite and calcite in the bottom of the saline aquifers. The mineral precipitation reduced aquifer porosity and permeability. 4, the boundary location and horizontal and vertical permeability ratio were main effect factors to CO₂ migration characteristics and pressure distribution in saline aquifers. The increasing of feldspars proportion could lead to the increase of storage capacity and mineral storage capacity at the same time, also reduced the porosity and permeability of the deep saline aquifer.The above study showed that the interactions among various CO₂ trapping mechanisms during CO₂ geological sequestration in deep saline aquifers affected the migration and transformation of CO₂ in saline aquifers. If this effect was ignored, it would result in poor results of deep saline aquifer CO₂ storage capacity assessment and environmental risk assessment. Therefore, it should be attached importance to consider the interactions among thses CO₂ trapping mechanisms during study CO₂ geological sequestration in deep saline aquifer in related studies.