Research and Application of a Reservoir Protection Workover Fluid Technology Suitable for Special Lithologic

Biological limestone reservoirs have the characteristics of pore-fracture dual media. The entry of fluids and solid particles or the incompatibility between fluids and formations will lead to a significant decrease in permeability. Therefore, according to the characteristics of biological limestone, it is necessary to prevent the inflow of liquid and solid phases, control the filtration loss of the system, reduce the sensitivity damage, and then improve the flow back capacity of the system and reduce the damage. Based on the solid-free reservoir protection fluid, the shear force is enhanced and the thixotropy is improved through the combination of thickeners and additives. The suspended and easily degradable temporary plugging components are developed to improve the temporary plugging ability. Then the additives such as fluid loss reducer and surfactant are optimized to form a biological limestone reservoir protection workover fluid system. The experimental results show that: (1) The apparent viscosity value of the system is about 24.0 mPa·s, the dynamic shear force is 13.0 Pa, the fluidity is good, the suspension performance is good, and the temporary plugging material can be suspended; (2) The system has good temperature resistance. When the temperature reaches above 100 ℃, the viscosity retention rate is above 40%. (3) The temporary blockage rate of limestone is ≥90%, and the system has good plugging ability; (4) The surface tension of filtrate and gel-breaking liquid is less than 27 mN /m, and the interfacial tension is less than 1 mN/m, which is easier to flow back and reduce the damage to the formation. The field application of 14 wells has achieved good results, which provides a solution to the serious leakage problem of biological limestone reservoirs.