Damage Characterization and Permeability Variation of Acidified Rock: A Theoretical and Experimental Study

Abstract

This study aims to explore the damage and permeability enhancement effect of acidification on rock. First, an X-ray diffraction qualitative analysis was conducted on the roof rock in the coal seam of Jiulishan Coal Mine in Jiaozuo City, Henan Province, China, by an X-ray diffractometer. The optimum acid solution ratio (12% HCl+1% HF) was obtained based on the principle of acidification acting on the mineral composition. Based on previous tests, tests for mechanical properties and gas permeability of rock samples before and after acidification were carried out. Under the condition of different acidification times, the rock damage degree was quantitatively characterized by the changes in the mass loss and mechanical properties of rock, and the relationships of acidification time with the rock damage degree and permeability were explored. The results reveal the following. (1) Macroscopically, acidification damages rock by deteriorating its mechanical properties, and the damage mode transitions from brittle damage to ductile damage. The damage degree is positively correlated with the acidification time; however, when the reaction reaches a certain extent, acidification barely causes mechanical damage to the rock. (2) Microscopically, minerals in the rock mass are dissolved, hence promoting the connectivity between pores and enhancing the permeability. In addition, there is a chain causality for rock permeability enhancement; that is, the acidification time determines the amount of minerals dissolved inside the rock, and the mineral dissolution intensifies rock damage, which then enhances the permeability. The evolution of rock permeability with acidification time can be concluded as follows: the permeability increases rapidly first, then rises slowly, and finally levels off; the optimum acidification time is in the range of 8−24 h with permeability enhanced by 220−260%.