Fractured lost circulation management is a critical challenge in drilling engineering, and existing methods often rely on empirical designs with limited effectiveness. This laboratory-scale study, utilizing modified PPT equipment and nut shell-based rigid particles in simplified linear fractures, systematically investigated the bridging and sealing effect of compounded multi-sized rigid lost circulation material (LCM) through physical experiments. It clarified the mechanism by which the proportion of multi-sized plugging particles and the total concentration influence the sealing effect after compounding. A set of quantitative compounding relationships for multi-sized rigid LCM was established based on the particle size-to-fracture width ratio (R = D50/w, where D50 is median particle diameter and w is fracture width). Experimental results show that when using particles with specific R (e.g., R = particle size/fracture width) values, approximately R1 = 0.7, R2 = 0.45~0.3, and R3 = 0.08, in a 1:1:1 ratio, and the total concentration is around 4%, high-efficiency plugging can be achieved. The plugging formulation based on this design principle can effectively seal medium-scale lost circulation fractures, providing a theoretical basis for the scientific design of rigid LCM in fractured formations.

To investigate the mechanism by which the rheological properties of drilling fluids affect the stability of the wellbore in brittle mud shale, this study systematically examines the influence of drilling fluids with different rheological properties on the hydration dispersion and rock strength of brittle mud shale through a series of laboratory experiments, including thermal rolling tests and uniaxial compressive strength tests on core samples. The results reveal that for weakly dispersible brittle mud shale, the rheological properties of drilling fluids have a minor effect on hydration dispersion, with rolling recovery rates consistently above 90%. However, the rheological properties of drilling fluids significantly influence the strength of brittle mud shale, and this effect is coupled with multiple factors, including rock fracture intensity index, soaking time, and confining pressure. Specifically, as the viscosity of the drilling fluid increases, the reduction in rock strength decreases; for instance, at 5 MPa confining pressure with an FII of 0.46, the strength reduction after 144 h was 69.8% in distilled water (from an initial 133.2 MPa to 40.2 MPa) compared to 36.3% with 3# drilling fluid (from 133.2 MPa to 88.7 MPa, with 100 mPa·s apparent viscosity). Both increased soaking time and confining pressure exacerbate the reduction in rock strength; a 5 MPa confining pressure, for example, caused an additional 60.9% strength reduction compared to 0 MPa for highly fractured samples (FII = 0.46) in distilled water after 144 h. Rocks with higher fracture intensity indices are more significantly affected by the rheological properties of drilling fluids. Based on the experimental results, this study proposes a strength attenuation model for brittle mud shale that considers the coupled effects of fracture intensity index, soaking time, and drilling fluid rheological properties. Additionally, the mechanism by which drilling fluid rheological properties influence the strength of brittle mud shale is analyzed, providing a theoretical basis for optimizing drilling fluid rheological parameters and enhancing the stability of wellbores in brittle mud shale formations.