Study on the Law of True Triaxial Hydraulic Fracturing by Acoustic Emission Monitoring

Abstract

Hydraulic fracturing generates fractures in a coal seam, which has been extensively used in coal mining and disaster management. To study the influences of water injection pressure and time on the propagation of hydraulic fracture, we conducted true triaxial hydraulic fracturing experiments under acoustic emission monitoring. Five briquette specimens were prepared by molding the mixture of coal powder, cement, river sand, and distilled water under 60 MPa to simulate the properties of coal seam. True triaxial loading was used to simulate the in situ stress environment of the coal seam. Hydraulic fracturing experiments on the test specimens were conducted under different water injection pressures and times. The following conclusions have been drawn. At the fixed injection time, increasing water injection pressure promotes the propagation of hydraulic fracture and the migration of water in the coal seam after the fractures are connected. The number of fractures increases from 3 to 8 as the water injected pressure increased from 3 to 9 MPa. Under the constant water injection pressure, the increase in longitudinal wave velocity in the test specimen decreases with the prolongation of water injection time. When the water reaches the action boundary of the water injection pressure, the water stops moving. At this moment, the water injection time has no effect on the longitudinal wave velocity anymore. The fracturing influence range can be increased by increasing the water injection pressure to produce a fracture network of a large radius with many fractures. Appropriately prolonging hydraulic fracturing time can ensure that the fracturing fluid fully moistens the coal seam and thus effectively reduces dust pollution.