Hydrogen peroxide oxidation and hydraulic conductivity evolution in fractured lignite for subsurface cultivation and gasification

Subsurface cultivation and gasification (SCG) seeks to enhance biogenic methane production from lignite by cultivating methanogens in situ, and its rate-limiting step is the supply of a bioavailable substrate. Hydrogen peroxide (H 2 O 2 ) is injected into a fractured lignite seam to solubilize part of the coal macromolecule into dissolved organic carbon (DOC), which the methanogens then convert to methane. The same oxidation, however, also alters the lignite skeleton, so the hydraulic conductivity that governs oxidant delivery and the subsequent transport of methanogen-bearing groundwater may change during treatment; this response has not previously been resolved. A fractured lignite specimen from the Tempoku coalfield, Hokkaido, Japan, was immersed in 1 wt% H 2 O 2 solution at 22 ± 1 °C and ambient pressure for 169 days. Hydraulic conductivity, H 2 O 2 and DOC concentrations, and specimen height were measured throughout, and structural evolution was followed by sequential photography and X-ray computed tomography (CT). Hydraulic conductivity rose by 64% from its baseline value in water of 1.8 × 10 − 5 m s − 1 to a maximum of 3.0 × 10 − 5 m s − 1 on day 5; it then fell by 87% relative to that maximum to a minimum of 4.0 × 10 − 6 m s − 1 on day 89; and it recovered by 70% relative to that minimum, reaching 7.04 × 10 − 6 m s − 1 on day 169. The H 2 O 2 concentration fell from 1.0% to below 0.03%, while DOC rose to 1756 mg L − 1 , indicating extensive oxidative solubilization of lignite organics. Specimen height decreased by 41% relative to its initial value of 9.9 mm, and the fracture network evolved from dilation through closure to pore coalescence. Quantitative image analysis of the CT mid-sections showed that, between the initial state and day 169, the fracture area fraction fell by 56–60%, the fracture trace intensity by about half, and the mean aperture of the remaining fractures by 14–22%. The hydraulic response follows a chemo–mechanical sequence: oxidation → softening → swelling → compressive stress → fracture closure → partial leaching-driven recovery. A constitutive model coupling oxidation kinetics, sorption-induced swelling and fracture aperture evolution reproduces these trends. For the single specimen and site examined here, the results indicate that oxidant dosing and hydraulic cycling must be managed to sustain injectivity and microbial viability in SCG operations.