Fracturing mechanism of coal-like rock specimens under the effect of non-explosive expansion

A non-explosive expansion method (NEM) is proposed to apply in the field of reservoir fracturing for the extraction of coalbed methane. Specimens with mechanical properties similar to primary coal were prepared, and their fracture behavior was investigated under different side stresses. Acoustic emission (AE) and infrared thermal imagery were utilized to record the parameters (energy and amplitude) of the AE events and the surface temperature distributions, respectively. The results showed that specimens were all destroyed using NEM with different crack morphologies under diverse side stresses, and the fracture evolutions were greatly dependent on the structure and side stresses, which caused complex crack numbers and crack distributions. Several periodic energy concentrations and the stair increasing pattern of cumulative energy all indicated that the fracturing process of NEM did not finish instantaneously but continued for a long time. A quantitative analysis of crack number and distribution by box counting showed that the larger the number of fractures, the greater the fractal dimension and the greater the complexity, and the difference in fracture density positively correlated to the fractal dimension was influenced by side stresses. Fractures might be determined by the coupled effect of swelling force and released heat according to the chemical reaction of agents. The relationship between swelling force and tension stress was deduced based on the thick-wall cylinder theory. A semi-submersion test was carried out to evaluate the effect of released heat on crack generation preliminarily in consideration of material volume and influence time. The greater material volume and longer influence time created a number of blocks and produce complex crack faces, largely related to the released heat inducing a complete temperature gradient.