RESEARCH ON OPTIMIZATION OF CO₂ INJECTION SCHEME UNDER THM COUPLINGS IN CO₂ GEOLOGICAL STORAGE

Shenhua multi-layered reservoir is the first GCS demonstration project in China. The gas absorption of its single reservoir increased dramatically. Under the original designed scheme, the cooled CO₂ after compression was injected into the deep high-temperature aquifer, resulting in a sharp change in the fluid pressure, thermal stress and a large number of fractures in the first aquifer. The injectivity of the first layer was increased, but the storage capacity of the whole system was reduced, and it also brought the risk of leakage. A site-scale rock mass cracking module was developed, based on the TOUGH-FLAC, a three-dimensional multiphase multi-component THM coupling numerical simulation program, to study the comprehensive effect of CO₂ injection scheme on the coupling characteristics and cracking characteristics of the target aquifer. Injection schemes including constant rate, first growth rate and then constant rate, intermittent constant rate, intermittent variable rate and secondary variable rate were designed, and the thermodynamic characteristics, multiphase flow characteristics and cracking of reservoir rock mass were calculated and analyzed. The results show that there are many cracks observed in the aquifer under the original designed scheme, and it is the reason of the injectivity increase. Continuous injection of CO₂ causes a significant decrease in the effective stress in the aquifer rock mass and an increase in the permeability. The thermal stress under the constant rate scheme is the smallest. Among the designed injection schemes, the fracture observed in the reservoir under the constant rate injection scheme is the least.