Experimental investigation on non-planar propagation of hydraulic fracture and proppant migration for directional well fracturing in coal seams

摘要

True tri-axial test system was deployed for directional well fracturing simulation experiments of coal outcrops, and an ingenious method of proppant adding during fracturing was performed in laboratory. Subsequently, the influence of in-situ stress, azimuth, cleats and proppant on the non-planar hydraulic fracture propagation rule in natural coal was studied. The results show that the propagation mode of non-planar hydraulic fracture is controlled by the combined effects of different factors: the coal cleats and well deviation determine the local fracture geometry, in-situ stress state dominates the main direction of fracture zone and bedding planes impede fracture height growth. As hydraulic fracture initiated along a direction skewed with wellbore, the extension path of hydraulic fracture near the wellbore would affected by cleats, and more complicated fracture geometry with many fracture branches would formed. Moreover, adding proppant will increase propagation pressure significantly, and the increase of proppant diameter will result in a higher degree of complexity for the proppant migration in flexural fracture and higher fracture propagation pressure. Therefore, the main factor, causing the difficulty of hydraulic fracture propagation, is the complex fracture geometry characterized by the twisting main fracture and the generation of secondary fractures near the wellbore.