Derivation and Application of a New Type of Water Flooding Characteristic Curve

The water flooding characteristic curve is a crucial tool in reservoir dynamic analysis, commonly employed to estimate water-driven geological reserves and recoverable reserves. However, due to approximations in theoretical derivations—such as equating average water saturation with outlet saturation or assuming that water cut approaches unity—most conventional curves achieve high accuracy only during the high water-cut stage (>80%). This study eliminates systematic errors and enhances calculation accuracy by establishing an improved water flooding curve equation. Firstly, a theoretical analysis of the error in a WOR (water–oil ratio)-type water flooding characteristic curve is performed. The results demonstrate that as water cut increases, calculated dynamic geological and recoverable reserves gradually rise, approaching actual values only when the water cut exceeds 90%. Secondly, a new type of water flooding characteristic curve is derived by using the Buckley–Leverett water drive oil theory and the Welge equation to modify the saturation approximation. Comparative analysis via reservoir numerical simulation demonstrates that the proposed curve significantly enhances prediction accuracy across all water-cut stages above 50%, outperforming conventional curves. After the water cut reaches 50%, the calculation error of dynamic geological reserves is less than 10%, and the calculation error of recoverable reserves is less than 5%. Field application shows that the new water flooding characteristic curve maintains a stable linear shape under certain development conditions. After the adjustment of development conditions, it jumps to form a new stable straight-line segment, which is conducive to the rapid and accurate evaluation of the adjustment effect.