An inversion for asymmetric hydraulic fracture growth and fracture opening distribution from tilt measurements

Abstract

The eigenstrain tensor is defined in terms of the unit vector normal to the fracture plane and the dislocation vector to characterize the fracture geometry and to determine the fracture induced deformation field. Compared to the conventional nonlinear fracture model, the forward fracture model provides a linear relationship between fracture geometry parameters (eigenstrain) and the induced deformation. Therefore, a linear inverse problem needs to be tackled in mapping the fracture geometry by inversion of tilt observations. The fracture orientation and opening distribution of a tensile-dominated hydraulic fracture can be mapped iteratively in a coupled way by using the forward model defined in terms of the dislocation vector. The proposed analysis method is applied to field tilt data collected for monitoring the growth of a hydraulic fracture placed in the preconditioning of a roof rock over a coal longwall panel at a test site. Inversion of the tilt data produces a low fracture dip angle of 6.8°, which agrees with a nearly horizontal fracture orientation confirmed by intersection data collected in offset monitoring boreholes. Tikhonov regularization technique is applied to stabilize the discrete ill-posed inverse problem. The regularized solution provides information on the fracture opening distribution and asymmetric fracture growth. The inversion shows that the fracture grows uniformly and roughly in a radial pattern at early injection times, but grows asymmetrically, preferentially, and consistently to the northwest during later times. The predicted fracture growth agrees with the temperature logging to measure fracture intersections with the offset monitoring boreholes.