Fracture propagation in an initially stressed anisotropic reservoir under shear: Reorientation and fluid lag

Abstract

We study the propagation of a fracture in an initially stressed anisotropic reservoir under shear. The focus is made on the deviation of the fracture from the principal direction due to the shearing conditions. An asymptotic model is developed, which yields the solution in terms of the effective K2c and the deviation angle. Then, we analyse formation of the fluid lag during propagation of a hydraulic fracture in the cases of plane and radial fracture. It is shown that there is lag between the fluid and the fract ure tip, which may be as large as half of the fracture at early stages of fracture propagation and its actual length is highly dependent on the compressibility of the reservoir rock, tectonic strength and leak-off conditions at the fracture face. In this case the speed of the fracture tip exceeds that of the fluid and is related to the length of the lag. The influence of the fluid front structure on the law of fracture propagation is studied. It is revealed that the magnitude of pressure gradient and its distribution along the fracture have a significant impact on the lag size.