Visualization experiment on dynamic migration and sealing mechanism of irregular materials in rough fractures

Lost circulation is one of the main problems causing low drilling efficiency and high drilling costs in the oil and gas industry. Due to an insufficient understanding of the migration and sealing mechanism of lost circulation materials (LCMs) in rough fractures, the success of fracture sealing is highly unpredictable. This study conducted visualization experiments using four types of LCMs to observe the entire dynamic sealing process of rough fractures. Adaptive fracture sealing experiments were designed to improve the sealing performance of LCMs at low concentrations. The results indicated that the key to sealing rough fractures is forming an effective sealing zone inside the fracture space rather than in the inlet buffer zone. The dynamic sealing process has experienced four typical stages: sporadic bridging, bridging structure expansion, sealing front formation, and sealing layer thickening. The first three typical stages correspond to flow from plate flow to trench flow and seepage. The formation of an effective sealing zone within fractures depends on forming a practical sealing front and subsequent particle accumulation of LCMs that thicken the sealing layer. Compared with regular particles, the sealing front of irregular particles is an area with a specific aperture range. The increase in irregular particle concentration makes the sealing front migrate to the fracture entrance, which is more likely to lead to inlet blockage. The distribution of a sealing zone depends upon the surface morphology of rough fracture. Increasing the particle size distribution improves the favorable bridging area and enhances the sealing performance. The adaptive sealing strategy uses low-concentration granular materials to form multi-point bridging within rough fractures and low-concentration flaky materials to adaptively and quickly extend and connect dispersed bridging structures. Mica flakes are more conducive to extending along the contour of fracture aperture than calcium carbonate particles. Adding mica flakes promotes the expansion of the bridging structures, forming a sealing front and a robust sealing layer in advance. This study delves into irregular materials' migration and sealing mechanisms in rough fractures, enriching theoretical understanding and providing guidance for on-site sealing operations.