Induced drill-spray during hydraulic slotting of a coal seam and its influence on gas extraction

Hydraulic slotting can induce drill spray in a gassy, low permeability coal seam. This then influences subsequent gas extraction. This paper describes the drill spray phenomenon from a mechanical perspective and analyzes the effects of water jet damage during slotting. A simulation of the stresses around the drill hole and slot was prepared using FLAC-3D code. It helps explain the induction of drill spray during hydraulic slotting. The stress concentration around the bore increases as the diameter of the hole increases. As the hole enlarges the variation in stress also increases, which introduces an instability into the coal. This allows easy breaking and removal of the coal. Destruction of the coal structure by the water jet is the major factor causing drill spray. Energy stored as either strain or gas pressure is released by the water jet and this causes the coal to fracture and be expelled from the hole. Field tests showed the effect on gas extraction after slotting with drill spray. The concentration of gas increases after drilling. Compared to conventional techniques, the hydraulic slotted bore gives a gas concentration three times higher and has an effective range twice as far. This makes the gas extraction process more efficient and allows reduced construction effort.