In the process of shale gas well drilling in the southern Sichuan Basin, China, lost circulation of high-density oil-based drilling fluid has become a main challenge. In this study, a novel oil-based crosslinking-curing lost circulation material (OBCC-LCM) system applicable for 100–150 °C shale formation was developed based on polyether, isocyanates and blocking agent. The performance of OBCC-LCM pills was comprehensively evaluated by testing fluidity, setting time, strength, elasticity and fracture sealing capacity. The OBCC-LCM pill setting time could be controlled in 70–120 min. The strength of the pill solid was 5.27–6.51 MPa after 360 min heating, and the volume almost restored to the original states after being compressed by 70%. The pill sealed fractures up to 5 mm wide under a differential pressure of 5.2–6.1 MPa, and a higher maximum fracture sealing capacity when being jointly used with bridging LCMs. The OBCC-LCM clearly demonstrated superior performance of good pumpability at room temperature, stability under high temperature, compatibility with oil-based drilling fluids, adaptability with various fracture widths and sustainability in induced fracture. It is suitable for curing loss of induced fracture in low permeability formation and loss of oil-based drilling fluid, especially for loss of oil-based drilling fluid in shale formations.

Summary: Lost circulation is one type of complex engineering problem in the drilling process. Bridging plugging technology is the most common method of lost circulation control in fractured reservoirs by which loss channels are plugged with lost circulation materials (LCMs). A successful application of LCMs requires selecting suitable materials from hundreds of LCM options, preparing the plugging slurry that matches the loss zone conditions, and adopting the appropriate lost circulation control construction technology. Therefore, through this study, we propose a quantitative evaluation method for LCMs to facilitate the selection of suitable LCMs in field applications. First, we propose characteristic parameters of LCMs based on the entire lifecycle of the fracture plugging zone. Subsequently, we design and conduct experiments to evaluate these parameters based on the temperature and pressure characteristics of the loss zone. Then, we calculate the characteristic parameters’ weight coefficients of the LCMs by the fuzzy analytic hierarchy process (FAHP). Finally, we establish a grading index for characteristic parameters and a quantitative scoring model for LCMs, enabling selection based on the quantitative scores. The reliability of the proposed evaluation method is validated through dynamic fracture plugging experiments and field application results. Research results demonstrate that the plugging zone formed by the plugging slurry, composed of high-scoring LCMs, can withstand a maximum plugging pressure exceeding 15 MPa and maintain structural stability at temperatures up to 150°C. The plugging effect of the slurry composed of high-scoring LCMs is significantly superior to that of the slurry composed of low-scoring LCMs. The high-scoring LCMs were successfully applied in a deep well in the Tarim Basin, effectively controlling the lost circulation and protecting the reservoir. This study enriches the evaluation system for LCMs’ characteristic parameters by integrating multiple parameters comprehensively. It transforms the selection process of optimal LCMs from qualitative analysis to quantitative evaluation and optimization, thereby enhancing on-site evaluation and selection processes for LCMs.