Study on crack propagation law of hydraulic fracturing of coal rock complex based on cohesive element

Abstract

In order to further analyze the propagation path and evolution law of hydraulic fractures in coal rock complex, from the perspective of stress field, coal rock interface strength and coal seam bedding dip angle, a hydraulic fracture model of coal rock complex is established based on cohesion unit method, and the propagation law of hydraulic fractures in coal rock complex under different working conditions is analyzed. It is found that after the hydraulic fracture of coal rock complex cracks in the overlying strata, with the gradual increase of the stress difference between σy and σx, the hydraulic fracture is more and more significantly controlled by the stress field. When the strength of coal rock interface is small, it is difficult for hydraulic cracks to form water pressure accumulation in the interface to reach the cracking conditions of coal seams. When the strength of coal rock interface is large, it can expand a certain distance in the coal rock interface and complete pressure holding. When the pressure reaches the cracking conditions of coal seams, water pressure cracks pass through the coal rock interface and induce the crack initiation and expansion of coal seams. Under the condition of 30° dip angle of coal seam bedding, hydraulic fractures form main fractures along the direction of main stress and secondary branch fractures along the direction of bedding. Under different geological conditions, stress field, coal rock interface and coal bedding have different control effects on hydraulic cracks, so they can be effectively combined to form a complex crack network structure.

Keywords

• coal rock complex
• hydraulic fracturing
• interface strength
• cohesive element
• crack propagation