Plugging Mechanism of Rigid and Flexible Composite Plugging Materials for Millimeter-Scale Fractures

Drilling-fluid loss caused by millimeter-scale fractures is a notoriously difficult problem in drilling engineering, and both rigid and flexible plugging materials are commonly used to address this issue. This investigation aims to comprehensively explore the plugging efficacy and underlying mechanisms of rigid, flexible, and fiber materials when used individually and in combination. The findings of our investigations into macroscopic high-temperature and high-pressure plugging experiments divulge a revelation: Under conditions of enhanced concentration, rigid particles evince the remarkable ability to engender a pressure-enduring plugging stratum; in contrast, independent attempts by flexible and fiber materials to yield a stable plugging layer are challenging. In this context, the optimal ratio of rigid, flexible, and fiber materials has been determined through composite plugging experiments. Calcite particles with a concentration of 5–8%, rubber particles with a concentration of 2–3%, and polypropylene fibers with a concentration of 1–2% were compounded for fracture plugging with widths of 1 mm, 3 mm, and 5 mm, respectively. The resulting plugging strengths were 10 MPa, 9 MPa, and 7 MPa. The microscopic visualized plugging experiments showed that the rigid particles form an I-shaped plugging layer with high strength but are difficult to transport to the deep part of the fracture. Flexible particles can be transported into the deep part of the fracture to form a plugging layer, but the “V”-shaped formation is unstable and has low strength. Based on the experimental results of “rigid-flexible synergistic” composite bridging-plugging formulations for different scales of fractured strata, the preferred template for bridging-plugging material formulations in the field is investigated to provide a reference for the bridging-plugging material formulations in the field.