Study On Porosity-permeability Characteristics And Damage Mechanism Of High Temperature Sandstone Under Reinjection Cooling Effect

The direct utilization of geothermal resources in China ranks first in the world,mainly through the utilization of hydrothermal geothermal resources.The sandstone reservoirs have rich geothermal water resources,and reinjection of geothermal tail water is an effective measure to maintain sustainable development and solve environmental problems such as wastewater pollution.However,during the reinjection process,the cooling effect caused by the temperature difference between the reinjected geothermal tail water and the sandstone reservoir after surface heat exchange will significantly deteriorate the reservoir properties.At present,the mechanism of the pore-permeability influence of reinjection cooling on reservoirs is unclear.Therefore,this study took the sandstone reservoir of Minghua Town as the research object,carried out indoor displacement cooling experiments under multiple coupled conditions,and established a pore network model before and after the displacement experiment to explore the internal pore-throat changes of the core by combining with the analysis of the water chemical reaction path and the change of ion concentration during each flow stage.The study clarifies the damage to sandstone pore-permeability properties and the rules of water-rock chemical reactions caused by reinjection cooling,and provides theoretical guidance for the scientific regulation of reinjection wastewater temperature.The main conclusions are as follows:During the high-temperature displacement process,the core permeability showed a trend of rapid decline in the early stage,decreasing gradually in the later stage and gradually approaching equilibrium.The cooling effect during reinjection significantly improved the permeability of the sandstone,and the greater the temperature difference caused by the cooling effect,the more significant the improvement in permeability.However,the permeability decline trend was faster during the cooling stage than during the high-temperature displacement process,and the blockage phenomenon was more severe.Using the growth rate of permeability as an indicator,the instant permeability increase caused by different cooling temperature differences in the experiment was sorted as follows:95℃>65℃>80℃>50℃,and the order of the permeability reduction during the cooling stage was 95℃>80℃>65℃>50℃.Based on the analysis of the changes in ion concentration of the injected water and the water-rock interaction pathways at different stages of the experiment,it was found that during the injection process,the ion concentration of Na⁺,K⁺,and H₂Si O₃ increased,while the concentration of Mg²⁺,Ca²⁺,CO₃^2-,and OH^-decreased,mainly due to the hydrolysis of minerals such as albite,montmorillonite,illite,and montmorillonite-illite interlayer.The cooling effect of the injected water increased the shrinkage of the rock skeleton,which enlarged the flow channel and had a certain promoting effect on the permeability.However,it also inhibited the hydrolysis reaction and dissolution rate of minerals.During the experiment,it was found that the inhibitory effect of the cooling on the precipitation of Mg（OH）₂,Ca CO₃,kaolinite,and silica colloids was less than the hydrolysis of minerals,which led to a faster decline in permeability during the cooling and injection stage compared to the high-temperature stage.After comparing and analyzing the pore network models of the rock cores before and after the experiment,it was found that the fractal dimension and tortuosity of the characteristic unit cells of each group decreased after the experiment,indicating that the surface roughness of the mineral in the pore and throat of the rock core decreased and the flow channels became smoother,indicating that obvious dissolution occurred inside the rock core during the flow process.Visual simulation and absolute permeability calculation were performed on the flow paths of the pore network models of the rock cores before and after the experiment.The results showed that the effective flow channels of the rock core after the experiment were significantly reduced,and the absolute permeability value was smaller than that of the initial rock core.Based on the changes in macroscopic permeability of the rock core during the flow cooling process,combined with the changes in ion concentration and microscopic pore structure of the tail water,this study reveals the damage mechanism of the cooling effect on the permeability and porosity of sandstone during geothermal tail water reinjection.The research results can provide theoretical guidance for the temperature control of geothermal tail water reinjection in sandstone thermal storage.