LAUSR

Abstract Drilling fluid loss into formation fractures is one of the most common and costly problems encountered during the exploration and development of oil and gas resources. The stability of fracture plugging zone formed by lost circulation materials has a great impact on the lost circulation control effect. Mesoscale structure stability of force chain, which is the way to transferring contact force between particles, determines the strength of the macroscale plugging zone. In the current paper, photoelastic method is introduced and photoelastic experimental system is developed to characterize the mesoscopic structure evolution of plugging zone under shear. The key parameters for the mesoscopic structure characterization of fracture plugging zone are proposed, including the proportion, distribution, elastic energy, and angle of the strong force chain. Experimental results show that the mesoscopic structure of plugging zone experiences continuous formation and destruction during the shear process. Initially, the distribution of the mesoscopic force chain under a vertical load presents a vertical tree branch structure. Then, the proportion of force chains in the horizontal direction increases and form a staggered network structure after a horizontal shear force is continuously applied. During the shear process of plugging zone, the mesoscopic force chain network experiences rearrangement and structural failure. The distribution density of elastic energy in the particles transferring strong force is reduced. Based on the developed photoelastic experimental system, the mesoscopic structure evolution of the plugging zone is accurately described. It can be used to guide the selection of the lost circulation materials and optimization of the loss control formula.