Research advances in water and gas injection development of fractured-vuggy carbonate reservoirs

Fractured-vuggy carbonate reservoirs represent a major domain in global hydrocarbon exploration and development. However, their efficient development is constrained by strong heterogeneity, characterized by spatially discrete distribution of fractures and vugs, complex connectivity, and diverse filling types. This paper systematically reviews research advances in reservoir characteristics, experimental methods, remaining oil distribution, and water/gas injection mechanisms. Based on genesis and controlling factors, the reservoirs are classified into three types: (1) Epikarst: dominated by fractures and dissolution pores (depth < 70 m), with a filling ratio of up to 60%; (2) Subterranean river karst: characterized by large-scale dissolution caves (extending 0.8 ~ 3.5 km), which are further divided into four types based on cave height: hall caves, main channel caves, tributary caves, and terminal caves; (3) Fault-controlled karst: forming vertically connected “fracture-cavity complexes” along fault zones, with full filling ratios ranging from 20% to 38%. Physical experimental models have evolved from conceptual ones (e.g., spliced marble/acrylic plates) toward realistic structures: marble models reveal fundamental flow mechanisms; transparent acrylic models enable visualization of displacement processes; laser-etched models replicate complex fracture-vug networks; full-diameter leached core samples support high-temperature and high-pressure simulations; and 3D printing allows precise control of fracture-vug morphology. However, limitations remain in simulating diverse filling media. After water flooding, remaining oil occurs in various forms, including attic oil, blind-end oil, bottom-water coning oil, oil shielded by high-conductivity channels, and filling-dependent oil. Field analyses, numerical simulations, and physical experiments consistently indicate that attic oil accounts for the largest proportion, making it a key target for enhanced oil recovery. Development strategies have progressed from single-well huff-and-puff to multi-well water/gas flooding, and further to optimized synergistic gas-water injection. Specifically, single-well water flooding mainly reduces water-coning residual oil; gas huff-and-puff mobilizes attic oil via gravity segregation; bottom-water injection suppresses coning and expands sweep efficiency; nitrogen flooding effectively recovers attic oil but requires well pattern optimization to control gas channeling; cyclic water injection enhances water diffusion through pressure fluctuations; flow-reversal injection alters flow paths to enlarge sweep volume; and synergistic gas-water injection couples gravity segregation with mobility control to improve displacement efficiency. Based on these insights, this paper proposes an integrated framework for potential tapping, termed “reservoir-specific adaptation, remaining oil targeting, stage-wise optimization”. This framework entails precisely matching injection-production techniques to reservoir types, implementing targeted controls based on remaining oil patterns, and dynamically optimizing strategies according to development phases. Future efforts should address challenges in three-dimensional modeling of complex fracture-vug systems, suppression of gas channeling, and dynamic prediction of remaining oil. Integrating artificial intelligence and high-precision detection technologies will enable accurate characterization of fracture-vug architectures. Developing novel experimental materials and model fabrication techniques will enhance simulation authenticity. Constructing numerical models that account for multi-physics coupling and optimizing synergistic gas-water flooding strategies are crucial for advancing the development of fractured-vuggy reservoirs from an “experience-driven” to a “precision-controlled” paradigm.