Analysis of damage characteristics of coal body with supercriticalcarbon dioxide jet impact short time soaking

The mechanical and physical properties of coal are easily deteriorated under the supercritical CO2 atmosphere. Especially under a longtime action,the supercritical CO2 can fully utilize its dissolution and extraction ability to change the physical strength and pore structure of coal. As a highly promising drilling fluid,the supercritical carbon dioxide has a very short contact time with the coal body during the drilling process. Whether the supercritical carbon dioxide can change the physical strength and pore structure of coal within the short contact time is crucial to the efficiency of supercritical carbon dioxide jet impact on coal breaking and drilling efficiency. In order to further study the effect of shorttime immersion of coal body by supercritical CO2 on coal breaking efficiency,this paper carried out a study on the shorttime immersion of coal body by supercritical CO2 on the impact crushing of coal body. It analyzed the impact crushing characteristics of coal body under different immersion conditions and clarified the effect of shorttime immersion on the impact crushing characteristics of coal body by comparing the changes of pore structure and mechanical properties before and after immersion. The results show that the supercritical carbon dioxide jet has an important influence on the impact damage characteristics of the soaked coal body under shorttime soaking conditions. After soaking for only 3 minutes,the supercritical carbon dioxide caused an adsorption and expansion of the coal body,which affected the pore structure and mechanical strength of the coal body. Thus,it reduced the cumulative pore volume of the coal body by 5.54% and the mechanical strength by 13.10%,and increased the impact crushing particle size less than 1 mm coal chips by 5.76%. The degree of coal body impact crushing continued to increase by extending the soaking time. After soaking for 60 minutes,although the supercritical carbon dioxide still mainly adsorbed and swelled the coal body,the coal body showed a mineral dissolution phenomenon. It resulted in the cumulative pore volume of the coal body decreasing by 30.19% and the uniaxial compressive strength decreasing by 33.41%,which led to a significant increase in the degree of coal body fragmentation. Under the same soaking time condition,when the soaking temperature and pressure increased,the adsorption and swelling effect of the coal body was enhanced,the pore volume increased and the compressive strength decreased,which made the degree of crushing increase regularly. However,when the soaking pressure exceeded 16 MPa,the JP2compressive strength decreased by 38.43%,and the impact damage degree was significantly increased. In conclusion,the adsorption and swelling effect of the supercritical carbon dioxide jetassisted drilling process is the main form of influence on the physical and mechanical properties of coal body,which effectively reduces the pore volume and compressive strength of coal body and increases the degree of fragmentation. It is beneficial to the improvement of drilling efficiency.