Aquifers in the vicinity of a reservoir give rise to water coning and channeling. A high water cut constrains the
effective production of horizontal wells in fault-block reservoirs. Previous field cases demonstrate favorable results of CO2 huff
and puff in reducing water cut and increasing oil production. In this study, a 3D fault-block reservoir model is built similar to the actual reservoir conditions, considering the existence of side water and the reservoir dip. Three experiments are implemented
to study the effect of structural position on the synergistic effects of CO2 huff and puff in horizontal wells. Synergistic results
are observed when gas is injected into one horizontal well, and the other two horizontal wells see a drop in water cut and a rise
of oil production. Gas injected into the low geological position achieves the highest total oil recovery increase (6.6% in the low
position, 5.8% in the middle and 5.0% in the high position) and the most efficient gas utilization (1.73). With the same volume of
gas, the lower the position where the gas is injected, the more significant the water suppression and oil increase. In order to obtain
higher oil stimulus and water drop, the candidate well for CO2 huff and puff should be in the lowest geological position.