Propagation mechanism of hydraulic fracture in sand coal interbedding

The strata of Linxing Area is composed of many layers of coal seam and tight sandstone.When multi-layers with coal seam and tight sandstone are developed together, it is hard to predict hydraulic fracture morphology because of interfacial property, stress difference, fractured layer, thickness of coal seam, construction parameters and other factors.In order to analyze the hydraulic fracture propagation mechanism in sand coal interbedding, the natural sandstone and coal outcrop rocks were mutually combined to carry out hydraulic fracturing simulation experiments with a true tri-axial pressure machine, and a study was conducted on the natural fracture, different stratum, interface bonding strength, thickness of coal seam and the type of fracturing fluid affected the expansion of hydraulic fracture.The experiment result showed natural fracture and bedding in coal rock would change the propagation direction of hydraulic fracture and cause complex fracture morphology, which was bad for hydraulic fracture to propagate across the interface.However, when the target fractured formation was sandstone, the hydraulic fracture morphology was simple and easier to connect interlayers.When hydraulic fracture approached the weak bonding interface between layers, hydraulic fracture could turn to propagate along the interface.Thick coal seams could easily dissipate hydraulic energy, which made hydraulic fractures difficult to penetrate formation.Although the gel had a better ability to generate fracture than the active water, the gel was damage to reservoir.So it is necessary to consider the selection of fracturing fluid according to the field requirement.