Fracture stiffness evaluation with waterless cryogenic treatment and its implication in fluid flowability of treated coals

Cryogenic fracturing using liquid nitrogen is a waterless and environmentally friendly formation stimulation method for creating complex fracture networks effectively. This study developed a non-destructive geophysical technique using seismic measurements to probe fluid flow through coal and ascertain the effectiveness of cryogenic fracturing. A theoretical model is established to determine the fracture stiffness of coal inverted from wave velocities, which serves as the nexus for correlating the hydraulic and seismic properties of fractures. In response to thermal shock and frost forces, visible cracks are observed on coal surfaces that deteriorate the mechanical properties of the coal bulk. Consequently, the wave velocity of frozen coal specimens exhibits a general decreasing trend with freezing time under both dry and saturated conditions. For a gas-filled specimen, both the normal and shear fracture stiffness decrease monotonically with freezing time as more cracks are created in the coal bulk. For a water-filled specimen, ice formation due to cryogenic treatment leads to grouting of the coal bulk. Accordingly, the fracture stiffness of wet coal increases initially and then decreases with freezing time. A coalbed with higher water saturation is preferable when applying cryogenic fracturing because fluid-filled cracks can endure larger cryogenic forces before complete failure, and the contained water aggravates coal breaking as ice pressure builds up from the volumetric expansion of the water–ice phase transition and applies additional splitting forces on pre-existing or induced fractures and cleats. Also confirmed is that the stiffness ratio is sensitive to fluid content. The measured stiffness ratio is 0.7–0.9 for dry coal but less than 0.3 for saturated coal. The present findings provide a basis for realistic estimation of coal stiffness ratio for future discrete fracture network modeling.