Abstract: The structural stability of fracture plugging zone of is a key factor that determines the lost circulation control effect of fractured reservoirs.Revealing the structural instability mechanism of fracture plugging zone at the mesoscale is crucial for the selection of lost circulation materials (LCMs)and the design of leakproof formula for fractured reservoirs.Based on the theory of mechanics of granular material and the network analysis method of granular system, this paper characterizes the meso-structure force chain network structure of the fracture plugging zone composed of rigid, elastic, and fibrous materials, describes the evolution characteristics of the force chain network during pressure bearing process, and reveals the mechanical mechanism of meso-structure instability of fracture plugging zone.The research results show that the pressure process of fracture plugging zone is accompanied by the formation-failure-reformation of force chains.Under the action of external load, the LCMs contact each other to form a contact force, and the contact force is transmitted in the fracture plugging zone to form a force chain network with certain shear strength, which is the root cause why the fracture plugging zone can withstand the external load.During pressure bearing process, the strong chain network bears the main external load, and the force chains with weak and medium intensity assist in maintaining the stability of the strong force chain network.The failure of the strong force chain network shows the mechanical mechanism of the microstructure instability of fracture plugging zone.The average shear strength and development degree of the strong force chain, and the geometric structure of the force chain network are proposed as the main controlling factors of the pressure stability of the mesoscale structure of fracture plugging zone, and then a new method for evaluating the plugging effect of the plugging formula is formed, which provides a basis for the research and development of new high pressure-containment LCMs.

Key words: lost circulation control, fracture plugging zone, multi-scale structure, meso-force chain network, evolution of meso-structure under pressure, photoelastic experiment

摘要： 裂缝封堵层细观结构稳定性控制着宏观结构承压能力，进而影响工作液漏失控制效果。由于缺少裂缝封堵层动态细观力学参数获取方法与细观结构演化的精细刻画方法，裂缝封堵层结构承压失稳细观力学机制尚不明确。利用自主研发的钻井防漏堵漏裂缝封堵层细观结构表征系统，模拟了承压过程中物理类颗粒材料构成的裂缝封堵层细观结构演化过程，明确了承压过程裂缝封堵层强弱力链演化特征，基于网络科学方法提取了裂缝封堵层细观力链网络，揭示了裂缝封堵层结构失稳细观力学机制。研究结果表明，裂缝封堵层承压过程伴随着细观力链的形成—破坏—再形成。强力链网络构成裂缝封堵层承压的力学基础，强力链网络断裂是裂缝封堵层承压失稳的细观力学机制。裂缝封堵层承压时，颗粒间接触力逐渐增加，强力链占比逐步升高，弱力链占比逐渐降低，链状强力链与三角形、五边形、环状等强力链相连构成了强力链网络。裂缝封堵层承压失稳后，强力链构成的网络形态断裂，演化为交错状、链状力链，强力链占比降低，弱力链占比升高。进一步提出了强力链平均抗剪强度、强力链发育程度和力链网络几何结构3类指标，作为裂缝封堵层细观结构承压稳定性主控因素，进而形成了堵漏配方封堵效果评价新方法，并为新型高封堵强度堵漏材料研发提供依据。

关键词: 防漏堵漏, 裂缝封堵层, 多尺度结构, 细观力链网络, 细观结构失稳, 光弹实验