Types and applicability of waterflooding characteristic curves in fractured-cavity carbonate reservoirs: A case study of Tahe Oilfield

Waterflooding characteristic curves are widely used in analyzing oilfield production dynamics. Most existing waterflooding characteristic curves are derived from statistical analyses of production data from sandstone reservoirs, commonly categorized into four types: Type A, Type B, Type C, and Type D. To assess the applicability of these curve types to fractured-cavity carbonate reservoirs, the Tahe fractured-cavity carbonate reservoir was selected as a case study. By analyzing reservoir fracture-cavity structures and oil-water production data, the Type A waterflooding curve was identified as more suitable for the Tahe reservoir. The study of 255 wells with long-term production data revealed six water cut increase patterns and four waterflooding characteristic curve types. Wells controlled by single cavities exhibited single-straight-line waterflooding characteristic curves and slow water cut increase patterns. Wells controlled by dual cavities displayed double-straight-line waterflooding characteristic curves, with water cut increase patterns categorized as slow rise, rapid rise, or fluctuating. For wells affected by water injection in dual-cavity structures, triple-straight-line waterflooding characteristic curves were observed, with water cut increase patterns featuring rapid rise and catastrophic flooding. Wells located in multi-cavity, complex fracture-cavity structures demonstrated irregular waterflooding characteristic curves, with water cut patterns including slow rise, rapid rise, fluctuating, and catastrophic flooding. A comparison with waterflooding characteristic curves of sandstone reservoirs clarified the applicability conditions for fractured-cavity carbonate reservoirs: adherence to the stable waterflooding principle（i.e., the straight-line principle） and the absence of a fixed water cut threshold. This study provides a foundation for predicting production dynamics in fractured-cavity carbonate reservoirs.