Low reinjectivity of geothermal waste water has become the bottle-neck for sustainable use of low-medium temperature geothermal resources widely found in sedimentary basins of China. Based on conceptual model of CO₂-EATER(CO₂-Enhanced Aquifer Thermal Energy Recovery). Tertiary sandstone reservoir of Guantao formation in Beitang sag of the Middle of Huanghua Depression is taken as a testing example. Water-rock-CO₂ reaction at a temperature of 100℃ and a pressure of 10MPa is carried out for 10d using high temperature & high pressure Parr, to understand the corresponding effects on component changes of formation water and minerals as well as formation porosity and permeability. Results show that after CO₂ injection, pH dropped from original 7.7 to 6.3 and components like HCO-3, Ca, Na and K showed significant rise. The reason is that dissolved CO₂ forming acidic conditions and cations like K, Ca and Na were released into formation water due to dissolution of feldspar and carbonate minerals, which are also confirmed by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) results. Water isotopes results also indicate that artificial CO₂ addition into formation water accelerates water-rock interaction and leads to "Reversed oxygen-shifts" of δ¹⁸O_H2O and δ²H_H2O by 3.5‰ and 3‰, respectively. It can be concluded that CO₂ injection can stimulate the geothermal reservoirs and CO₂-EATER model is feasible for Tertiary Guantao formation of Beitang sag.