Differentiated Drill-In Fluid Damage Control in Multilayered Fractured Carbonate Reservoirs: Mechanisms, Strategies, and Field Application

Summary Multilayer karst structures in buried-hill fractured carbonate reservoirs frequently cause severe drill-in fluid losses, leading to formation damage and reduced well productivity. To elucidate the damage mechanisms induced by drill-in fluid invasion, we integrate laboratory experiments, microstructural characterization, and field analysis to systematically investigate formation damage within multilayer karst fracture systems and to propose differentiated formation damage control (FDC) strategies. The results indicate that formation fluid sensitivity damage is dominated by velocity and alkali sensitivities, with a maximum permeability damage of 59.18%. Fractures exhibit strong stress sensitivity, with a stress sensitivity coefficient of 0.71. The existing drill-in fluid system fails to effectively seal fractures wider than 200 μm. Well-developed fractures in vertical flow and horizontal underflow zones promote invasion depths of 4.35–6.42 m. By contrast, the deep slow flow zone is dominated by nanopores (<0.1 μm), where filtrate invasion is limited to 0.54 m, yet water phase trapping (WPT) damage reaches up to 85%. Accordingly, a differentiated FDC strategy is proposed by integrating invasion depth prediction, acid-soluble plugging, particle-size optimization, wettability alteration, and acidizing plugging removal. The experimental results demonstrate that acid-soluble plugging agents effectively seal fractures up to 2,000 μm, increasing pressure-bearing capacity by approximately 40% and improving percent permeability recovery by 27.27–44.76%. Surfactant addition further reduces interfacial tension (IFT) and surface tension by 85.71% and 40%, respectively. Field validation shows that, compared with a nontest well, the test wells achieved an average 67.27% reduction in fluid loss, an 80.4% decrease in treatment time, and a 9.67% increase in initial productivity. These results provide practical guidance for FDC in multilayer fractured karst carbonate reservoirs.