Abstract: Lost circulation is extremely easy to occur during drilling in deep and ultra-deep fractured formations. The failure of plugging zone formed by lost circulation materials (LCMs) may be aggravated under the complex environment of high temperature, high pressure and high in-situ stresses, which results in failure to reach the plugging rate and the fracture plugging effect expectation. Based on the idea of multi-stage and multi-grain bridge plugging, WNDK-1 rigid LCM and high-temperature resistant rubber particles often used in Shuangyushi block located in western Sichuan are selected to conduct LCMs performance evaluation and fracture plugging simulation experiment under high temperature aging environment. The experimental results show that after aging in 150℃ drilling fluid for 24 h, the particle size distribution of WNDK-1 does not change significantly. The maximum friction coefficient and compressive strength of WNDK-1 rigid material decrease by 1.89% and 1.15%. The particle size distribution D₉₀ value of rubber particles increase by 3.55%, the friction coefficient increases by 1.59%, and the compressive strength remains the same. If rigid materials, elastic materials and fibrous materials are mixed with drilling fluid at appropriate concentrations to plug the fractures, the pressure-bearing capacities of the formed plugging zone are generally higher than 13 MPa. Besides, the plugging zone has the characteristics of low porosity and low permeability. The observation of structure and morphology inside fractures after the plugs failure demonstrates that friction/comprehensive failure and shear instability in the plugging zone formed by multi-stage and multi-grain bridge plugging under high temperature aging environment.

Key words: fractured formation, lost circulation material, high temperature aging, fracture plugging zone, pressure bearing and tight plugging