Evaluation And Simulation Of Dissolved CO₂ Storage In Saline Aquifer

Under the background of"two-carbon"policy,CO₂ geological sequestration,especially in brackish water layer,plays an extremely important role.The traditional CO₂ storage method is to inject CO₂ gas directly into the reservoir,which has many safety risks such as CO₂ leakage.In this thesis,the geological storage method of dissolved CO₂ brackish water layer is explored.Taking Ordos as the study area,the evaluation system and evaluation method of geological storage suitability of dissolved CO₂ brackish water layer are established first,and the geological storage suitability of each brackish water layer in the study area is evaluated.Then,hydrogeochemical analysis software PHREEQC was used to simulate the solubility and hydrochemical change characteristics of CO₂ under different storage conditions,and Shihezi Formation was selected as the optimal reservoir.Finally,the dissolved CO₂ injection process was simulated using TOUGHREACT software,and the characteristics of reservoir pressure field,hydrochemistry,mineral composition and reservoir physical properties were analyzed.The research results of this thesis can effectively guide the geological storage of salt water CO₂ and improve the safety and effectiveness.The specific research results are as follows:（1）Establish an evaluation index system and evaluation method for the suitability of geological storage of dissolved CO₂ saline water layers.The evaluation indicators mainly involve two aspects:geological conditions and storage potential,with 5 primary evaluation indicators and 14 secondary indicators;Using Analytic Hierarchy Process,calculate the weight values of various evaluation indicators through the judgment matrix;Divide each evaluation indicator into 5 levels:suitable,more suitable,average,less suitable,and unsuitable,and provide evaluation criteria for each indicator;Using fuzzy mathematics comprehensive evaluation method and semi trapezoidal distribution function to determine the fuzzy relationship matrix between various evaluation indicators and evaluation levels of the reservoir;The fuzzy comprehensive evaluation vector is obtained by multiplying the weight vector of the relationship matrix and evaluation indicators,and then quantifying the fuzzy vector to obtain the suitability evaluation score.（2）Based on relevant data from the research area,a detailed analysis and assignment of 14 secondary evaluation indicators were conducted;The suitability evaluation method for geological storage of dissolved CO₂ saline water layer was used to evaluate various reservoirs in the study area.The evaluation score of the Liujiagou Formation was 7.0961,the Shiqianfeng Formation was 7.3943,and the Shihezi Formation was 7.4524,all of which were greater than 7.Therefore,the Liujiagou Formation,Shiqianfeng Formation,and Shihezi Formation reservoirs in the study area are more suitable for CO₂ geological storage.（3）The hydrogeochemical simulation software PHREEQC is used to simulate the hydrochemical change characteristics and CO₂ solubility under the conditions of reservoir temperature,pressure and original solution ion concentration.The results show that the p H values of the three reservoirs have significantly decreased,with significant changes in HCO₃^-ions,the largest changes in SO₄^2-ions,and no significant changes in other ions.The CO₂ solubility in descending order is:2.9856mol/L in the Shihezi Formation,2.5617mol/L in the Shiqianfeng Formation,and 1.9658mol/L in the Liujiagou Formation.Taking into account the solubility of CO₂ under various reservoir conditions and the evaluation score of CO₂ geological storage suitability,the Shihezi Formation reservoir is selected as the optimal reservoir.（4）A process evaluation model for simulating the storage of dissolved CO₂ in saline water layers was established by selecting the Shihezi Formation reservoir as the target layer for dissolved CO₂ storage.Based on the geological and hydrogeological conditions of the Shihezi Formation reservoir,a radial vertical two-dimensional model centered on the water injection well was established,and the model boundary was set as an infinite boundary;At the same time,the model parameters such as reservoir mineral composition,fluid chemical properties,and reservoir physical properties were provided in the model;Simulated the changes in reservoir pressure,hydrochemical properties,mineral composition,and physical properties during the injection and post injection stages of dissolved CO₂ into the Shihezi Formation reservoir.（5）After the injection of dissolved CO₂,the reservoir pressure continuously increases,and the influence range of reservoir pressure during the injection period is10000m.After the injection is stopped,the reservoir pressure quickly recovers to the original value,ensuring the feasibility and safety of dissolved CO₂ storage.The p H value is mainly influenced by pressure,water rock reaction,and diffusion;SO₄^2-is mainly affected by pressure and diffusion,and after thousands of years,the concentration of SO₄^2-returns to its initial value,indicating that dissolved CO₂ injection can store in the reservoir for a long time and ensure safety.Illite minerals are dissolved and kaolinite minerals are precipitated,the total dissolved amount of reservoir minerals near the Injection well is greater than the total precipitation amount,which increases the porosity of the reservoir.The porosity of each position of the reservoir will not change after injection.The increase in reservoir porosity increases the permeability of the reservoir,alleviates the increase in reservoir pressure,and ensures the feasibility and safety of dissolved CO₂ storage.