Study on the Characteristics of Water Jet Breaking Coal Rock in a Drilling Hole

Water jet technology is an effective measure by which to improve the efficiency of deep coalbed methane mining. Nevertheless, the effect of a water jet impinging on coal rock remains unclear. In this study, numerical simulation is used to analyze a water jet impingement drill test block, after which experimental verification is carried out. Next, on the basis of the experimental verification that the simulation method is feasible, the influence factors of the water jet impingement in the drill hole are analyzed. It is concluded that the phase-field variables based on the fracture change method can effectively characterize the damage and destruction of coal rock. The water jet impact in the borehole has a central damage failure zone and two-side damage failure zone, and the damage failure ratio n is used to characterize the degree of damage to the coal rock. When the jet target distance is 70 mm, the damage ratio n is closest to 1, and the effect of water jet impact on coal-rock is the best. When the wall roughness is less than 10 mm, the blocking effect on the jet is dominant, resulting in a negative correlation between the damage size and the roughness. When the wall roughness exceeds 10 mm, the development of cracks and the connection effect exceed the blocking effect, resulting in larger damage. Therefore, it can be seen that the effect of a water jet impacting on coal rock is positively related to jet pressure. When the jet target distance is 70 mm, the damage range of 30 MPa jet pressure to the center and both sides of the coal rock reaches about 1.1 m, thus signifying a good coal rock breaking effect. The wall roughness has a significant effect on the coal rock breaking effect of the water jet.