Abstract
A novel method for fracturing coal is presented in this paper. A chemical solution is injected into coal under high pressure, whereby the coal is fractured and subsequently weakened by chemical erosion over time to produce an anti-impact soft structure. In this study, the mechanical properties of coal under chemical erosion were investigated, and the fracturing design parameters were optimized. The uniaxial compression test and the split Hopkinson pressure bar (SHPB) test were used to determine the dynamic and static mechanical properties of coal after 20 days of immersion in different chemical solutions. After chemical solution erosion, the dynamic and static compressive strengths and elastic modulus of the coal decreased according to an exponential power law in the damage variable. The chemical treatment increased the duration of the pore compaction stage and decreased that of the elastic deformation stage, while decreasing the brittleness and increasing the ductility of coal. The acoustic emission (AE) curve of the immersed coal samples consisted of four stages corresponding to those of the stress-strain curve: pore compaction-closure, a slowly rising linear elastic regime, steady-state prepeak crack propagation, and unsteady crack propagation at the peak strain. The increase in the damage variable of the coal sample from chemical erosion led to a lower dissipated energy, a higher fractal dimension, and a more fragmented coal sample. The effect of the investigated chemical solutions on weakening the coal mechanical properties decreased in the following order: alkaline solution > acidic solution > NaCl solution > distilled water. The experimental results provide a reference for weakening fractured coal seams.
Highlights
A novel method for fracturing coal is presented in this paper
For the samples immersed in the NaCl solution, the dynamic compressive strength decreased by 25.9%, the dynamic elastic modulus decreased by 32.3%, and the peak strain increased by 15.6%. us, the effect of the chemical solutions on weakening the dynamic mechanical properties of the coal samples decreased in the following order: alkaline solution > acidic solution > NaCl solution > distilled water. e chemical solutions weakened the sample strength significantly more under dynamic loading than under static loading
Microjoint fissures in the coal propagated, and damage occurred. e static and dynamic mechanical properties of the coal samples decreased according to an exponential power law in the damage variable
Summary
E mechanical properties of coal samples were tested under the action of different acidic and alkaline chemical solutions. E samples were divided into five groups (A, B, C, D, and E), which consisted of samples immersed in the acidic solution, the NaCl solution, the alkaline solution, and distilled water and the original dry rock samples, respectively. (1) Preparation of coal samples: e mass and fissure distribution standard cylindrical samples were compared, and the samples were divided into five groups (A, B, C, D, and E), with six samples per group. (2) Preparation of chemical solutions: An acidic solution at pH 3, an NaCl solution and distilled water at pH 7, and an alkaline solution at pH 11 were prepared. E samples in groups A, B, C, and D were immersed for 20 days in the acidic solution, the NaCl solution, the alkaline solution, and distilled water, respectively, and dried. Test Procedure. e following test procedure was used in the experiment. (1) Preparation of coal samples: e mass and fissure distribution standard cylindrical samples were compared, and the samples were divided into five groups (A, B, C, D, and E), with six samples per group. (2) Preparation of chemical solutions: An acidic solution at pH 3, an NaCl solution and distilled water at pH 7, and an alkaline solution at pH 11 were prepared. e samples in groups A, B, C, and D were immersed for 20 days in the acidic solution, the NaCl solution, the alkaline solution, and distilled water, respectively, and dried. (3) Mechanical properties and AE tests: A static compression test was performed on the samples labelled 1, 2, and 3 in each group, and the split Hopkinson pressure bar (SHPB) test was carried out on the samples labelled 4, 5, and 6 in each group
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