The effects of CO₂-water-rock interactions on the injectivity and safety of CO₂ geological storage have drawn wide attention. The geochemical reaction mechanisms in carbonate formations after CO₂ injection are still controversial. To better understand the transformation of injected CO₂ in carbonates and the involved geochemical reactions, we first conducted autoclave experiments reproducing the in-situ conditions of the Lianglitage Formation, Yingshan Formation, and Qiulitage Formation at the Tazhong Uplift in the Tarim Basin. We then established a batch model using TOUGHREACT-ECO2H, validated with the experimental results, to simulate the long-term CO₂-water-rock interactions. It was found that the initial mineral compositions and water chemistry have important effects on the CO₂-water-rock interactions in carbonate formations. The experiment results show that the dissolution of calcite and dolomite dominates in the early reaction period. However, we still observed some secondary minerals, such as ankerite, montmorillonite, calcite, and dolomite. The CO₂-water-rock reactions can be more dramatic when the contents of calcite and dolomite in carbonates are closer. Moreover, the long-term simulation results show that calcite, magnesite, and hematite are the main formed secondary minerals, whereas dolomite is the major dissolved mineral. This study is helpful for a better understanding of the CO₂ mineral trapping mechanism in carbonate formations.

CO₂ geological storage is considered as an important measure to reduce anthropogenic CO₂ emissions to the atmosphere for addressing climate change. The key prerequisite for long-term CO₂ geological storage is the sealing capacity of caprock. This study investigates the evolution of sealing capacity of caprock induced by geochemical reactions among CO₂, water and caprock using TOUGHREACT code based on the Heshanggou Formation mudstone at the Shenhua Carbon Capture and Storage (CCS) demonstration site of China. The results show that the self-sealing phenomenon occurs in the lower part of the caprock dominated by the precipitation of dawsonite, magnesite, siderite, Ca-smectite and illite. While the self-dissolution occurs in the upper part of caprock mainly due to the dissolution of kaolinite, K-feldspar, chlorite and Ca-smectite. Sensitivity analyses indicate that the precipitation of dawsonite, magnesite, siderite is highly advantageous leading to self-sealing of caprock, with albite and chlorite dissolution providing Na⁺, Mg²⁺ and Fe²⁺. The dissolution of K-feldspar dominates illite precipitation by providing required K⁺, and albite affects Ca-smectite precipitation. The self-sealing and self-dissolution of caprock are enhanced significantly with increasing temperature, while the effect of salinity on caprock sealing capacity is negligible perhaps due to the low salinity level of formation water.

Properly understanding the evolution mechanisms of groundwater storage anomaly (GWSA) is the basis of making effective groundwater management strategies. However, current analysis methods cannot objectively capture the spatiotemporal evolution characteristics of GWSA, which might lead to erroneous inferences of the evolution mechanisms. Here, we developed a new framework to address the challenge of spatiotemporal heterogeneity in the GWSA evolution analysis. It is achieved by integrating the Bayesian Estimator of Abrupt change, Seasonal change, and Trend (BEAST), the Balanced Iterative Reducing and Clustering using Hierarchies (BIRCH), and the Optimal Parameters‐based Geographical Detector (OPGD). In the case study of the North China Plain (NCP), the GWSA time series is divided into four stages by three trend change points in BEAST. An increasing trend of GWSA is observed at Stage IV, and the third trend change point occurs before the third seasonal change point. This distinguishes the positive feedback of anthropogenic interventions and the effects of seasonal precipitations for the first time. Moreover, the spatial distribution of GWSA in the NCP is classified into two clusters by BIRCH in each stage. The differences in GWSA trends and responses to environmental changes between Cluster‐1 and Cluster‐2 are significant. Then the driving effects of 16 factors on the evolution of GWSA are identified using OPGD, in which the contributions of topographic and aquifer characteristics are highlighted by quantitative analysis. This framework provides a novel method for examining the spatiotemporal heterogeneity of GWSA, which can be extended to analyze spatiotemporal trends in GWSA at diverse scales.

The Shenhua CO₂ capture and sequestration (CCS) project has achieved its goal of injecting 100,000 tons/year CO₂ into the saline aquifers of the Ordos Basin. This study analyzes the geochemical interactions between CO₂, formation fluid, and host rock of the major formations in the Ordos Basin, assesses the CO₂ trapping capabilities, and predicts the final mineral forms of injected CO₂. Reactive transport simulations are performed using a 2D radial model, which represents a homogeneous formation. The results show that 80% of injected CO₂ remains as free supercritical gas in each formation after injection, while most of CO₂ is sequestrated in different carbonate mineral assemblages after 10,000 years. The CO₂ mineral trapping capacities of the Shiqianfeng and Shihezi formations are smaller than the Liujiagou formation. Calcite, dawsonite, and siderite are stable CO₂ trapping minerals, while dolomite, ankerite, and magnesite are not. The increase in porosity and permeability of the three formations in the first 100 years agrees with the observation from the Shenhua CCS Project. Also the decrease in porosity and permeability after 100 years shows agreement with other modelling studies using the similar methods. These results are useful for the evaluation of the geochemical process in long-term CO₂ geological storage.