Study On The Influence Of Coal Bedding Direction On Liquid CO₂ Induced Fracturing

The geological conditions of China’s coal seams are complex,and the characteristics of low permeability and high adsorption generally exist,which have brought great difficulties to gas drainage.Artificial cracking and permeability enhancing technology must be used to improve the permeability of coal seams to promote gas drainage and utilization.The anhydrous fracturing technology has been a research hotspot in recent years.Cyclic cracking of liquid CO₂,that is,the liquid CO₂medium is used as a fracturing fluid,and the coal body is injected into the rock through circulation to improve the permeability of coal seams.The bedding structures have obvious influence on the physical and mechanical properties of coal.In order to further understand the damage evolution characteristics of coal with bedding structures developed during the cyclic injection of liquid CO₂,this paper uses theoretical analysis,experiments and data analysis methods to study the influence of bedding directions on the regular characteristics of coal before and after fracturing to provide theoretical and technical support for the injection of liquid CO₂and enhanced gas drainage in coal seams with bedding structures on site.Firstly,based on the initiation of liquid CO₂ cracking,taking the bedding direction of the coal as the starting point,and combining relevant knowledge of rock mechanics,the damage mechanism of coal cracking caused by bedding direction in liquid CO₂cycle was mainly analyzed based on the thermal stress and pore water action and the advantages of liquid CO₂ cracking were summarized.From a macroscopic perspective,the development of surface cracks of coal during fracturing in different bedding directions was analyzed.The anisotropy characteristics and the acoustic characteristics of liquid CO₂ during cracking were analyzed based on the variation of longitudinal and transverse wave velocity.Two-factor analysis and dynamic parameter calculation were introduced.The experimental results showed that the P-wave velocity of the coal bedding after fracturing was significantly reduced,and the dynamic parameters of the coal were significantly changed,which showed obvious bedding effect.All above showed that liquid CO₂ can significantly improve the internal pores of coal and increase the internal damage of coal,which have a significant relationship with the bedding directions.An experimental study on the mechanical properties of coal with different bedding directions induced by liquid CO₂ cycling was carried out.The uniaxial compression and Brazilian split experiments were carried out to test the changes of various mechanical parameters of the bedding coal before and after fracturing.The experimental results showed that cyclic injection of liquid CO₂ has a significant effect on the mechanical properties of coal.The peak strength,elastic modulus and tensile strength of the coal have significantly decreased,which shows obvious damage variables.This indicats that the strength and rigidity of coal and the ability to resist tensile failure are decreased,which is obviously related to the bedding directions.The influence of bedding direction on the development of internal pores in coal was quantitatively analyzed using NMR from a microscopic perspective.The pore size distribution,porosity,and pore space of coal before and after fracturing were analyzed by the characteristics of T₂ spectrum.The results demonstrated that the internal pore development of the coal was obvious after fracturing from small and medium pore size to large pore size.The total porosity and effective porosity both increased,the pore space was larger,which has a positive correlation with the bedding directions.Tests of permeability of coal in different bedding directions induced by liquid CO₂cycle cracking were carried out,and the variation law of permeability under bedding directions and gas pressure was analyzed.The influencing factors were analyzed through two-factor calculation.The results showed that the permeability increased with the increase of the cycle times and the bedding angle,both having significant changes in the permeability during the cracking process.This paper has 50 pictures,13 tables,and 110 references.