Gas Production Characteristics and Improved Cylinder Calculation Model of Injection Volume for CO2 Huff and Puff in Heavy Oil with Horizontal Wells

For heavy oil reservoirs, CO₂ huff and puff in horizontal wells can significantly improve the recovery. However, the oil increment mechanism and the difference in IOR potential compared with vertical wells are still unclear. There are many methods for calculating the injection volume in horizontal well, but it is difficult to meet the goal of fast and accurate calculation simultaneously. Firstly, long core experiments with horizontal and vertical placements were carried out to compare the gas production characteristics and oil increment differences between them. Then, orthogonal tests were conducted to determine the main controlling factors and the optimal injection volume for the huff and puff process. Finally, an improved cylindrical model was established based on theoretical derivation. The results show that huff and puff in vertical well includes two breakthroughs of the low gas content front and the “gas barrier”. Compared with vertical wells, horizontal wells can increase the recovery factor by 6.14%. The spatial and temporal differences in the mobilization of oil and gas-oil mass transfer led to the difference in oil increment. The injection volume is the most sensitive factor affecting the huff and puff effect. The improved cylindrical model corrects the three limitations of the conventional cylindrical model. The error between the calculation results and the numerical simulation results is less than 10%, which greatly improves the accuracy. The reliability of the model has been verified through successful field applications. This study provides a fast calculation tool and theoretical guidance for the preparation of similar horizontal well huff and puff schemes.