Laboratory Investigation on Oil Increment and Water Cut Control of CO₂, N₂, and Gas Mixture Huff-n-Puff in Edge-Water Fault-Block Reservoirs

Excessive water production is a common matter that seriously affects production efficiency during the development of edge-water fault-block reservoirs. Gas huff-n-puff is an effective water shutoff technology that has the characteristics of small injection volume, no interwell connectivity impact, and minor gas channeling. However, gas injection can destroy the stability of the asphaltene to induce asphaltene deposition. In this article, the laboratory experiment had been conducted to investigate the effect of injection ratio and injection sequence on oil increment and water cut control for gas mixture huff-n-puff. Experimental results indicated that the effect of N₂ huff-n-puff on water cut control was the most obvious, while CO₂ huff-n-puff had the best performance on oil increment. Oil increment and water cut control of gas mixture huff-n-puff with CO₂ injected in advance were obviously better than that of N₂ injection preferentially. Subsequently, PVTsim Nova was utilized to investigate whether reducing CO₂ injection volume can inhibit asphaltene deposition and predict the possibility of asphaltene deposition at reservoir conditions. Simulation results demonstrated that the asphaltenes were easily deposited with CO₂ injection while N₂ injection will be unlikely to induce asphaltene deposition. Asphaltene deposition pressure envelope can qualitatively analyze the possibility of asphaltene deposition and provide a reference for screening the appropriate gas injection ratio based on giving full play to the synergistic effect of CO₂ and N₂. In this study, 7:3 is selected as the optimum injection ratio considering the synergistic effect and the possibility of asphaltene deposition.