Permeability enhancement of coal by chemical-free fracturing using high-voltage electrohydraulic discharge

Abstract

Fractures, cleats or cracks provide the main fluid pathways for methane extraction from Coalbed Methane (CBM) gas reservoirs. For deep coal seams, where the cleats are often closed by the high overburden pressure, or some shallower but tight coals, fracturing operations to form new pathways or clean out existing but blocked natural cracks are necessary for CBM wells to produce commercially attractive gas rates. This is usually done by injecting fluids under high pressure into the subsurface, an operation that is controversial because of its possible environmental impacts.This study focused on developing and validating a novel stimulation technique, namely high-voltage electrohydraulic discharge (EHD), as an alternative method to effectively crack bulk materials without injecting any new fluids into the well. During the stimulation process, strong pressure shockwaves were generated by underwater electrical discharges, which then served to damage the associated solids, creating fractures and cracks. We used mortar (as a convenient, homogeneous, isotropic materials with consistent properties, and coal (which is naturally fractured and quite heterogeneous), as test materials. EHD significantly enhanced the permeability of mortar core (2.75–6.25 times) and coal core (3.6–10.8 times). X-ray CT scanning showed an obvious increase in fracture density, length and voids in the tested specimens, and mercury porosimetry confirmed that porosity increased at the micro (nm) and macro (μm) scales, in addition to the cracks which were both visible on the samples and at the in the X-ray images.The new cracks induced in the coal samples were directional in nature. The fractures tended to more easily occur parallel to the direction of bedding plane (i.e., opening in the weakest direction), or on the bright bands of the coal, which are brittle and fragile compared to the softer and more elastic dull bands. Based on these observations, we are encouraged that EHD has potential as a method for reservoir stimulation and gas development for tight source rocks such as coal or shales.