| At present, a large number of hydraulic fracturing experiments to extract methanehave been conducted in Chongqing Songzao district. The results showed that thistechnology can greatly improve the permeability of coal seam and increase coalbedmethane production. But the law of fracture’s initiation and propagation of hydraulicfracturing are not clear, due to the multi-layered, large change of depth and dip angle inthe coal seam. Hence, blindness in fracturing parameter selection and boreholearrangement occurred. Therefore, it is necessary to study the law of hydraulic fracturingcrack initiation in the multi-layered coal seams.Based on the theory of rock elastic mechanics and fracture mechanics, themaximum tensile stress theory of failure criteria was used to study the initiationpressure of hydraulic fracturing under different starting-crack location and direction inthe multi-layered coal seam. Combined with the similar simulation experiment ofhydraulic fracturing in the laboratory, the law of initiation pressure, fracturestarting-crack location and developing direction variation with dip angle and depth ofhydraulic fracturing in the multi-layered coal seam was revealed. The acoustic emissionenergy reflects fracture failure in the coal seam and law of acoustic emission energywith dip angle and depth was studied. The main results are as follows:①The initiation pressure calculation model of multi-layered coal seam hydraulicfracturing was built, which affected by multiple factors including stress, angle, tensilestrength, cohesion of interface and borehole tilt angle：②The law of initiation pressure, starting-crack location and developing directionvariation with dip angle and depth of hydraulic fracturing in the multi-layered coal seamwas revealed. Initiation pressure decreases slightly with the increase of dip angle. It isalso increasing linearly with depth. The coal seam fracturing starting-crack location areall located at soft layer and the crack direction are along with the maximum horizontalprincipal stress direction of coal seam. The cracks are developing like mesh in nearlyhorizontal seam and this phenomena weaken with the dip angle increases, which will form a main fracture. The result is consistent with the theoretical conclusions.③The acoustic emission system was used to reflect fracture failure in the coalseam and law of acoustic emission energy with dip angle and depth was obtained. Thechange of acoustic emission parameter has obvious stage characteristics and themaximum acoustic emission energy appear before and after the peak of injectionpressure. The strength of acoustic emission signals are relative with the stress andrelease of coal skeleton, which reflects the fracture failure of coal seam during hydraulicfracturing. Acoustic emission energy increases with dip angle increases and decreaseswith depth increases. |
