Research on multi-field coupling evolution law of liquid CO₂ filling in difficult water injection seams

In view of the low porosity and permeability of the coal seams that are difficult to inject water, and the difficulty of effectively wetting the coal body with the traditional water injection dust reduction technology, a new idea of improving the water injection effect of the coal seams by injecting liquid CO₂ into the coal seams is proposed. In order to clarify the multi-field coupling evolution law of the liquid CO₂ filling process in the coal seam with difficulty in water injection, this paper selects the Ji-15 coal seam of Minmetals in Pingmei Group as the research object. A multi-field coupling cooperation model of the liquid CO₂ filling in the coal seam with difficulty in water injection was established by combining the Comsol finite element software and the Matlab data processing software. In view of the complex hole-fracture structure of the coal body, the heterogeneous distribution parameters are introduced, and the coal seam is divided into 10 000 areas through the Comsol with Matlab interface, and the Weibull distribution function is called to assign values to the material properties of all areas of the coal seam, so as to achieve the heterogeneous representation of the coal body material. Through the simulation of filling pressure, filling time and other parameter control variables, the multi-field coupling evolution of liquid CO₂ filling process in coal seams with difficulty in water injection is analyzed. The results show that the expansion rate of the seepage channel, the extension range of the cooling zone, the ratio of tensile damage and compressive damage are positively correlated with the filling time and pressure of liquid CO₂, and the evolution rate of seepage, cooling and damage area is more significantly affected by the filling pressure than the filling time; Compared with the filling pressure of 10 MPa, at 20 MPa, the number of branch flow channels with lower flow velocity in the outer region is significantly increased, and the ability to open the coal seam seepage channel is stronger; In the process of loading the filling pressure from 0 MPa to 30 MPa, the proportion of tensile damage and compressive damage of the coal seam finally reached 47.86% and 4.23%, respectively. The main damage of the coal seam is tensile damage, which is consistent with the constitutive relationship that the tensile capacity of the coal body is far lower than the compressive capacity. The research work can provide a theoretical basis for the research and development of coal seam liquid CO₂ fracturing, moisture enhancement and disaster prevention technology.