Characteristics of mud leakage during workover and its impact on refracturing treatment pressure

Mud leakage during workover can contaminate existing fractures and lead to significant deviations in refracturing treatment-pressure design. This study aims to characterize mud leakage behavior in fractured reservoirs and predict its impact on refracturing treatment pressure.A two-phase Darcy-flow framework was established to simulate mud leakage and to obtain the spatial distribution of mud saturation. Based on the simulated mud-retention geometry in existing fractures, a treatment-pressure prediction model was developed by partitioning the fracture into mud-occupied and un-leaked flow regions and incorporating permeability damage. The workflow was validated using field data from two mud-contaminated refracturing wells in the Tarim Basin, northwest China (Well A and Well B).Field data indicate leaked mud volumes of 13.0 m³ (Well A) and 9.6 m³ (Well B), accompanied by substantial productivity degradation (unrestricted flow rate decreased from 142.3×10⁴/d to 52.2×10⁴/d in Well A and from 38×10⁴/d to 1×10⁴/d in Well B) and residual permeability ratios of 0.366 and 0.026, respectively. A baseline pressure prediction that ignores mud impact underestimates the observed treatment-pressure window (110–120 MPa for Well A; 120–130 MPa for Well B) by 20.3–36.2 MPa and 14.6–42.2 MPa, respectively. After incorporating mud retention and permeability damage, the predicted pressure ranges shift to 109.6–117.4 MPa (Well A) and 122.4–146.9 MPa (Well B), yielding clear overlap with the measured pressure windows and substantially reducing the mismatch.This study provides an additive and practically applicable method for pressure-design correction and risk assessment in mud-contaminated refracturing operations.