In this paper, techniques have been developed to experimentally and mechanistically describe the oil-based cement slurry (OBCS) flow through fractures in carbonate reservoirs when it is co-injected with a pad fluid. Experimentally, a three-dimensional (3D) physical model is used to simulate flow behavior within fractures in carbonate rocks by using the ultra-fine cement and class G cement with or without pad fluids. The injection pressures of an OBCS flow are measured and recorded as a function of time during the experiments at a constant flowrate, while effects of fracture width (i.e., 0.5 mm and 1.0 mm) and cement type (i.e., the class G cement and the ultra-fine cement) on injection pressure are examined and analyzed. Theoretically, the Navier–Stokes (NS) equations are modified and integrated to obtain the explicit velocity equations of visco-plastic materials in a planar fracture, and to further quantify the injection pressure of the slurry flow as a function of viscosity, flowing distance of the injected slurry, fracture width, and flowrate. It is found from the experimental measurements that the fracture width imposes a much larger impact on injection pressure along the fracture than other parameters. Once slurry is made in contact with water, its injection pressure not only increases rapidly with one or two orders of magnitude or even larger but also is changed from its linear to exponential relationship with time after a certain time. During the linear stage, the injection pressure of ultra-fine cement is smaller than that of the class G cement, while an opposite pattern is yielded during the exponential stage, i.e., the exponent of the injection pressure formula pertaining to the ultra-fine cement is found to be about 1.5 times larger than that of the class G cement. By incorporating the experimentally measured patterns of the slurry distribution within the fracture model, the newly developed mechanistic model has been validated by reproducing the experimental pressure measurements, allowing us to perform reliable characterization of the OBCS flow behavior in a fracture and then to efficiently and accurately predict and optimize its water-plugging performance.