Permeability Loss and Return Recovery Characterization of Foam Fractured Coal Bodies

Foam fracturing fluid is important for unconventional gas development due to its low water consumption and high flowback efficiency. Using fabricated low-permeability coal samples under confining pressures of 3–6 MPa, this study systematically investigates the performance of guar-based foam fracturing fluids and their impact on coal permeability through an integrated approach combining rheological tests, triaxial seepage measurements, and microfluidic visualization, comparing cationic (CTAB) and anionic (SDS) foam systems. Results show that the CTAB system offers superior foam stability (half-life up to 215 min) but tends to coalesce under high stress, resulting in higher residual saturation (30%). Nevertheless, under low confining pressure (3 MPa), its strong interfacial film stability enables effective mobilization from main flow paths, yielding a superior permeability recovery rate of 83.9%. In contrast, the SDS system exhibits better stress adaptability and flowback efficiency, leaving only about 10% residue. The study further clarifies that excessive polymer concentration severely reduces permeability recovery (e.g., recovery drops from >70% to below 25% at 1.0 wt % HPG), while nanoparticles show an optimal enhancement within 0.1–0.3 wt %. Based on these findings, an optimized formulation (0.5 wt % CTAB, 0.5 wt % HPG, 0.05 wt % nanoparticles) is proposed, demonstrating stable performance under 3–5 MPa confining pressure. This work provides a theoretical basis for tailoring foam fracturing fluids to specific reservoir conditions, supporting more efficient and less damaging stimulation operations.