Study on drillability evaluation of glutenite reservoir rock using fractal theory and experiments

The strong heterogeneity of glutenite formations poses significant challenges for drilling optimization, including frequent bit failure, low rate of penetration (ROP), and high bit consumption. To address these issues, this study establishes a quantitative evaluation model for glutenite Reservoir Rock drillability by integrating fractal theory with experimental analysis. Based on fractal theory, artificial glutenite samples with different particles fractal dimensions were prepared by adjusting three key parameters: gravel content (20–90%), main particle size (0.25–12.5 mm), and grain size sorting (0.5–0.75). Microdrilling experiments were then conducted using a 32-mm-diameter PDC bit under a weight on bit (WOB) of 500–2000 N and a rotational speed of 55 r/min. Experimental results indicate that gravel content and grain size are the primary factors influencing the drillability of glutenite rock. As the gravel content increases and the main grain size enlarges, the difficulty of drilling significantly increases, resulting in extended drilling time. Notably, there is a strong correlation between particle’s fractal dimension and its drillability. The evaluation model correlating particle’s fractal dimension and drilling time, established through experiments, achieved a coefficient of determination (R²) of 0.9462 when applied in the Bohai Bay Basin. The study results can effectively guide bit selection and drilling parameter optimization for glutenite reservoirs.