Quantitative Evaluation of Drilling Fluid Damage in Fractured Carbonate Reservoirs

Ultra-deep fractured carbonate reservoirs suffer severe fluid loss and formation damage, requiring accurate characterization of natural fracture parameters and contamination behavior. Current field evaluation methods rely solely on logging data, which cannot obtain parameters such as the number and width of fractures. Taking the Shunbei Block as an example, this study integrates laser particle size analysis, HTHP flooding experiments, and a fracture loss model incorporating dynamic mud cake growth to clarify single-fracture loss behavior for fractures of different widths. Using fracture parameters interpreted from field logging, together with loss data and simulation results, we developed a method to determine fracture parameters and characterize contamination. Experiments reveal that effective plugging becomes difficult when fracture width exceeds 138 μm. Simulations indicate that cumulative loss volume increases with fracture width. The field loss volumes of 40–6600 m3 correspond to single-fracture widths of 0.9–6.2 mm. Based on the simulated width–loss volume relationship, correction coefficients for fracture count and width for two wells were calibrated. The method was applied to 9 wells, yielding an R2 of 0.86 between predicted and measured loss volumes. The method effectively identifies fracture parameters and assesses contamination, supporting lost circulation control in fractured carbonate reservoirs.