Experimental investigation on non-planar fractures mechanisms in hydraulic fracturing

Abstract

The deflection of initial fractures and the formation of new fractures as non-planar fractures were investigated, which both are possible mechanisms for production improvement in refracturing or SRV(Stimulated reservoir volume). Firstly, initial fractures were formed under initial tri-axial stresses loading. Secondly, the formation and development of new fractures from refracturing were stimulated with changing horizontal stresses. The results show that (1) During refracturing new fractures were formed, and the initial fractures were re-opened, initiated and deflected. Whlie most of the deflection was identified as tensile and shear failure. (2) New fractures do not always result from the stress field change. This implied the smaller horizontal stress difference was, the easier the initial fractures were deflected, and the larger horizontal stress difference was, the easier the new single-wing fractures were formed on the walls of the initial fractures; (3) Deflection of initial fractures was more easier than formation of new fractures. The study indicated that, the injection pressure of deflection was lower than the initial breakdown pressure, while the formation of new fractures required higher pressure than initial breakdown pressure. (4) The injection rate contributed to a significant effect on the formation of new fractures. When injection rate was low, the formation of new fractures on the walls of the initial fractures was difficult. But when injection rate was high, new single-wing fractures formation was easily occurred on the walls of the initial fractures.