Formation mechanism and strengthening method of geometric-mechanical structure of plugging zone in deep naturally fractured reservoir

Efficient fracture plugging plays a critical role during drilling and completion of deep naturally fractured reservoirs, minimizing fluid loss, and ensuring reservoir protection. This study investigates the formation process of the geometric-mechanical structure of fracture plugging zones through a series of experiments including microscopic visualization, photoelastic and fracture plugging experiments. By examining these experiments, we reveal the relationship between lost circulation materials (LCMs) at the micro-scale, force chain network structures at the meso-scale, and the pressure stability of plugging zones at the macro-scale. Based on the analytic hierarchy process (AHP), we introduce an evaluation index for assessing the stability of fracture plugging zones and define methods to strengthen their stability. The experimental results show that a synergistic combination of granular and flaky materials, along with fibers, can establish fracture plugging zones with high bearing capacity. Granular materials are distributed throughout the fracture plugging zone structure, flaky materials dominate the front and middle sections, and fibrous materials are prevalent in the middle and end sections. Furthermore, the selection of different types of LCMs and consideration of their characteristic parameters, such as particle size, sphericity and friction coefficient, significantly affect the force chain network structure within the plugging zone. This, in turn, manifests in the pressure stability of the plugging zone. We conduct a comprehensive evaluation based on the multi-scale characteristic parameters of the plugging zone: properties of LCMs, force chain parameters (strong chains ratio and average shear strength), evaluation indexes of bearing stability (plugging pressure-bearing capacity, cumulative loss and pressure-bearing stability time). These assessments yield valuable insights into the pressure-bearing stability of plugging zone and allow for LCM optimization. The field application demonstrates the effectiveness of our optimized plugging formula in enhancing the bearing stability of fractured plugging zones and controlling working fluid loss in deep fractured reservoirs.