"沼气泄出" can be translated as "methane leakage" or "biogas leakage", depending on the context.
Methane leakage is more commonly used when referring specifically to methane (the primary component of biogas) escaping, especially in scientific, environmental, or industrial contexts.
Biogas leakage is more precise when the focus is on biogas (a mixture of gases, primarily methane and carbon dioxide, produced by anaerobic digestion) escaping.
Choose the term that best fits the context of your usage.