Development and application of an experimental device for lost-circulation plugging in stress-sensitive fractures

Stress-sensitive fractures can experience continuous aperture changes under wellbore pressure variations during drilling. However, many existing lost-circulation evaluation methods and material-selection criteria assume a fixed fracture aperture. As a result, the chosen plugging formulations may not match the evolving fracture width, leading to instability of the plugging layer and repeated losses. To address this issue, we developed an experimental device that simulates a coupled wellbore–formation–fracture plugging system.

The device includes a drilling-fluid circulation system, temperature-control system, plugging slurry preparation system, dynamic fracture-aperture regulation system, and data acquisition system. It models lost circulation in stress-sensitive fractures with pressure-driven aperture changes, allowing independent control of fracture closure pressure, fracture inlet pressure, temperature, and different fracture geometries. Pressure, temperature, and fracture deformation are recorded simultaneously, and loss volume can be quantified. After each test, the fracture module is disassembled for direct observation of plugging layer distribution and local bridging morphology. Two plugging systems were compared using the device: a conventional bridging system consisting of rigid bridging and filling materials, and an elastic particle-enhanced system that incorporates recycled tire rubber particles to improve deformation adaptability under dynamic conditions. The effects of fracture closure pressure, total lost circulation material (LCM) concentration, plugging material system, and the performance of composite plugging formulas were systematically evaluated.

Comparative tests show that increasing fracture closure pressure improves plugging stability against dynamic aperture growth; however, the gain is only 7.5% when the closure pressure rises from 1 MPa to 3 MPa, indicating that fracture closure pressure alone has limited capacity to enhance dynamic plugging stability. Total LCM concentration has an effective operational window: at too low a concentration, persistent leakage occurs because a continuous and stable bridging skeleton cannot form; at too high a concentration, premature entrance plugging may happen, increasing the apparent pressure-bearing capacity but risking operational issues during circulation. The effectiveness of elastic particles strongly depends on fracture aperture. In 1 mm fractures, the elastic particle-enhanced system failed to establish a stable pressure-bearing plugging layer, suggesting that small-aperture fractures depend primarily on the rapid formation of a stiff bridging skeleton. In 2 mm fractures, the elastic-particle system reduced fluid loss from 188 mL to 133 mL under similar conditions. This demonstrates that elastic particles are more effective in larger fractures where deformable filling and contact reconfiguration improve loss control. After further optimizing the rubber particle-size combination, fluid loss dropped from 133 mL to 98 mL, while the pressure-bearing capacity remained between 4.5 and 4.8 MPa. This indicates that particle-size recombination mainly enhances loss control and densifies the plugging layer, without significantly increasing ultimate pressure-bearing strength.

This study establishes a dedicated experimental foundation for evaluating pressure-bearing plugging in stress-sensitive fractures. The coupled wellbore–formation–fracture plugging device links macroscopic plugging performance with the evolution of plugging-layer morphology during dynamic fracture deformation. The results provide a reliable basis for dynamic plugging evaluation, mechanism-oriented formulation design, and developing pressure-bearing plugging strategies for fractured formations.