A Multiscale Composite Plugging System for Mitigating Fluid Invasion Into Fractured Coal Rocks

Summary

Coalbed methane (CBM) is an important unconventional gas resource. However, during drilling, drilling fluid invasion and pressure transmission through connected pore-fracture networks frequently cause wellbore instability. In this study, the multiscale pore-fracture structure of coal samples from the Ordos Basin was characterized, and a plugging strategy was developed according to the identified size ranges. Microstructural characterizations show that the coal rock contains large fractures (10–350 μm), mesopores and microfractures (0.1–10 μm), and transition pores (<21 nm). Based on the characterized size ranges and particle-size matching, two multiscale composite plugging systems were formulated by adding to a base slurry with 3 wt% emulsified paraffin, 3 wt% Type III calcium carbonate, and 3 wt% flexible anticollapse powder (FACP) or pressure-bearing plugging agent (PBPA). In the pressure transmission tests, both composite systems maintained pressure transmission efficiencies below 2% after 42 hours. Computed tomography (CT) analysis further showed that the two systems reduced the filtrate saturation within the coal cores to 25 and 24%, respectively. In the American Petroleum Institute (API) filtration tests, the filtrate volume decreased from 10.2 mL for the current drilling fluid to 8.6 and 7.2 mL for Composite Systems 1 and 2, respectively. In the high-temperature and high-pressure (HTHP) filtration tests, it decreased from 15.3 to 10.2 mL and 9.7 mL, respectively. The composite systems also formed thinner and less permeable filter cakes under both conditions. These results indicate that the proposed multiscale plugging systems reduced pressure transmission and drilling fluid invasion, supporting their potential application in maintaining wellbore stability in fractured coal formations.