Long-time seepage evolution in coal fractures during injection of viscoelastic surfactant fracturing fluids

Hydraulic fracturing is widely recognized as a key technology for enhancing coalbed methane production. The fracturing fluid has physicochemical reactions with the coal fractures, along with their duration, critically affecting fracture permeability, thereby determining the effectiveness of the technology. However, the study has not received enough attention. In this study, coal fracture seepage tests were carried out using in situ continuous injection of fracturing fluid. The seepage evolution of viscoelastic surfactant fracturing fluid (VESFF) was investigated at different times (0, 1, 2, 3, 4, and 5 days), and de-ionized water (DW) and potassium chloride solution (KCL) were used for comparison. The results showed that the flow rate increased compared to initial flow rate after VESFF treatment for two to four days, while the flow rate could not be recovered after DW and KCL treatment. The optimal treatment duration for VESFF was two days: marked by a sevenfold increase in the flow rate, an 84% increase in initial hydraulic aperture, and minimal momentum loss. After two more days of VESFF treatment, the pressure gradient and effective confining pressure became greater than 6 MPa/m and 3.5 MPa, respectively, and showed a significant excessive discharge characteristics (β < 0), which resulted from the generation and dilation of microcracks, increasing the number of flow channels due to coupled fluid–mechanical behavior. The degree of flow nonlinearity decreased with increasing VESFF treatment duration and increased with increasing effective confining pressure. These results have profound implications for optimal treatment duration and mechanism of VESFF strengthening coal fracture seepage.