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Highlights 11• A novel stimulation technique, namely high-voltage electrohydraulic discharge (EHD) 12 has been developed and validated for coal cracking. 13• Electrohydraulic discharge shows a great potential to create additional flow paths and 14 permeability in coal. 15• Shockwaves generated by EHD increase the accessible porosity of the coal, 16 predominantly in the meso-and micropore size range. 17• The banding in coal has a significant influence on breakage and fractures are more 18 likely to occur in weak plies rather than in the soft dull coal. 19 Abstract 20 Fractures, cleats or cracks provide the main fluid pathways for methane extraction from 21Coalbed Methane (CBM) gas reservoirs. For deep coal seams, where the cleats are often 22 closed by the high overburden pressure, or some shallower but tight coals, fracturing 23 operations to form new pathways or clean out existing but blocked natural cracks are 24 necessary, for CBM wells to produce commercially attractive gas rates. This is usually done 25by injecting fluids under high pressure into the subsurface, an operation that is controversial 26 because of its possible environmental impacts. 27This study focused on developing and validating a novel stimulation technique, namely high-28voltage electrohydraulic discharge (EHD), as an alternative method to effectively crack bulk 29 materials without injecting any new fluids into the well. During the stimulation process, 30 strong pressure shockwaves were generated by underwater electrical discharges, which then 31 served to damage the associated solids, creating fractures and cracks. We used mortar (as a 32 convenient, homogeneous, isotropic materials with consistent properties, and coal (which is 33 naturally fractured and quite heterogeneous), as test materials. EHD significantly enhanced 34 the permeability of mortar core (2.75 to 6.25 times) and coal core (3.6 to 10.8 times). X-ray 35CT scanning showed an obvious increase in fracture density, length and voids in the tested 36 specimens, and mercury porosimetry confirmed that porosity increased at the micro (nm) and 37 macro (µm) scales, in addition to the cracks which were both visible on the samples and at 38 the in the X-ray images. 39The new cracks induced in the coal samples were directional in nature. The fractures tended 40 to more easily occur parallel to the direction of bedding plane (i.e., opening in the weakest 41 direction), or on the bright bands of the coal, which are brittle and fragile compared to the 42 softer and more elastic dull bands. Based on these observations, we are encouraged that EHD 43 has potential as a method for reservoir stimulation and gas development for tight source rocks 44 such as coal or shales. 45 Keywords 46Electrohydraulic discharge; pressure shockwaves; fractures; permeability; induced fracture 47 direction 48 157