Multifractal Characterization of Coal Deterioration Induced by Cyclic Hydraulic Pressure Based on the Acoustic Emission Test and X-ray Microscopy

Abstract

Pulsating hydraulic fracturing has been applied in industry to increase gas productivity or clear fluid channel blockage, such as coalbed methane recovery. It produces cyclic hydraulic pressure to generate fatigue damages in coal causing deterioration of the mechanical strength. However, difficulties still exist in the quantification of the coal deterioration in the lab. This work provides a way of measuring the internal events induced by the cyclic hydraulic pressure based on the acoustic emission (AE) test and threedimensional (3D) X-ray microscopy (XRM). It gives a method to quantify the damage in coal with the multifractal dimension and digital image analysis. The results show that the cyclic hydraulic pressure has a great impact on the fracturing behaviors of coal. As the reduction of initial crack pressure is proportional to the pretreated pulse pressure, the AE sensors receive more cracking signals during the whole procedure. For conventional hydraulic fracturing, the AE counts concentrate at the initial crack point with a big pressure drop. However, due to the influence of cyclic hydraulic load, changes exist in the AE count distribution. It represents that the fracture strength has been changed by the cyclic hydraulic loading. The average frequency-rise angle (AF-RA) eigenvalue provides the proportions of tensile and shear failures. It shows that the proportions of tensile and shear failures change from 56.38 and 43.62 to 41.79 and 58.21%, respectively. To observe the internal cracks of coal, the 3D XRM has been applied to directly illustrate the spatial distribution of created cracks. More cracks have been seen in fractured coal samples after higher pulse pressure loading. The multifractal analysis indicates that the AE spectra and computed tomography (CT) images can be characterized by the multifractal dimension. D q monotonically decreases with the increased q value. The relationship of f (α)-α is presented by the inverted U shape. Δf and Δα provide a quantitative analysis of pore structure heterogeneity. It shows that the heterogeneity of pore structure decreases with the increase of pretreated pulse pressures. The change of pore structure heterogeneity has been validated based on the AE localization and the CT images. With the increase of the pulse peak pressure, more microcracks emerge and evenly distribute around the borehole in coal. This work provides an experimental method to quantify the fatigue damage induced by the cyclic hydraulic pressure which is very significant for the investigation of the pulsating hydraulic fracturing.