CO2 leakage is a fatal issue for the successful application of CO2 geological storage. In this paper, a method using CO2 triggered gelation is proposed to control the CO2 leakage during CO2 geological storage process. The mechanisms of CO2 triggered gelation was illustrated schematically. The effectiveness of this method for blocking CO2 leakage was evaluated by experimental works using sand-pack model. Reactive flow simulations were conducted to reproduce the experimental results and investigate field-scale feasibility of this method. The results show that this method can control the assumed leakage and the blocking performance was reproduced well by the lab-scale reactive flow simulation model. The results of simulation indicate that gel can be formed and absorbed more easily near the injection point. Feasibility of this method in field-scale were demonstrated with different scenarios of preplaced gel system. Infill drilling and monitoring method with high accuracy are recommended to assist this method.
Natural gas hydrate (NGH) is a kind of potential energy with shallow buried depth, high energy density, huge reserves, and cleanliness. In this study, continuous seawater injection is adopted for NGH production experiments with applying a self-designed reactor which can simulate an NGH-bearing reservoir with a mini well, given that the surface seawater stores tremendous heat. Continuous seawater injection for NGH production can keep the balance between productivity and sand production through controlling production pressure with its thermodynamic and technical feasibilities. Kinetic behaviors of NGH production by continuous seawater injection are investigated using the simulated NGH-bearing reservoir. Meanwhile, the dissociation of NGH and its influence factors are analyzed. The threshold value of temperature for NGH dissociation is also discussed. The experimental results show that the changes in temperature and pressure keep constant at the initial stage of the continuous seawater injection process. However, temperature and pressure show obvious variation with injecting more seawater, which increase first and decrease subsequently. Especially, the methane production rate shows a high level after the temperature exceeded the threshold value for NGH dissociation. But the methane production rate drops quickly after a short period of high level and keeps a low level until the end of the experiment. The maximum value of the methane production rate and cumulative methane production become higher with the presence of a larger overheat and NGH saturation. The existence of a threshold value of the temperature for NGH dissociation is demonstrated by experimental works. Minimum threshold values of the temperature for NGH dissociation vary with the presence of different corresponding reservoir pressures.
In this study, a 3D reactive flow simulation model is built to simulate the leakage processes though assumed leakage channels. The geochemical reactions are coupled with fluid flow simulation in this model with consideration of reservoir minerals calcite, kaolinite, and anorthite. As an essential trigger for geochemical reactions, changes in pH value are investigated during and after the CO2 injection process. By comparing CO2 migration with/without geochemical reactions, the influence of geochemical processes on CO2 leakage is illustrated. The leakage behaviors through leakage channels with different permeabilities are evaluated. Influence of reservoir temperature on CO2 leakage is also exhibited. Furthermore, the effects of the distance between the injection well and leakage zone on the leakage potential are studied. The results indicate that the geochemical reactions have impact on the leakage processes, which can decrease the leakage level with the presence of geochemical reactions. The region of low pH enlarges with continuous injection of CO2. Hence, monitoring changes in pH can reflect the migration of CO2, which can provide an alert for CO2 leakage. The occurrence of the leakage phenomenon is postponed with increasing the distance between the CO2 injection well and the leakage channel. However, the leakage level tends to be consistent with injecting more CO2. The CO2 leakage risk can be reduced through the leakage channels with lower permeability. With the presence of higher reservoir temperatures, the leakage risk can be improved. These results can provide references for the application of monitoring methods and prediction of CO2 front associated with geochemical processes.
Publication Time (P-time for short) of Web pages is often required in many application areas. In this paper, we address the issue of P-time detection and its application for page rank. We first propose an approach to extract P-time for a page with explicit P-time displayed on its body. We then present a method to infer P-time for a page without P-time. We further introduce a temporal sensitive page rank model using P-time. Experiments demonstrate that our methods outperform the baseline methods significantly.
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