Effect of heterogeneity and injection borehole location on hydraulic fracture initiation and propagation in shale gas reservoirs

Abstract

The breakdown pressure and fracture propagation are essential problems in hydraulic fracturing, but how they are affected by rock heterogeneity and injection borehole location has been unclear. In this study, a hydro-mechanical coupling model in micromechanics was established to investigate the mechanisms of fracture initiation and propagation in shale formations. The dependence of the breakdown pressure and fracture propagation on approach angle of bedding plane, particle heterogeneity and borehole location was numerically studied. First, a hydro-mechanical coupling model was developed with both parallel bond model (PBM) and smooth joint model (SJM) for heterogeneous shale formations. Then, this model was validated by theoretical analysis and experimental observations. Finally, the effects of rock heterogeneity and borehole location on the breakdown pressure, fracture modes, stress distribution were analyzed through numerical simulations. It is found that the approach angle of bedding has significant effects on breakdown pressure and fracture network, but has only limited impact on the stress distribution near borehole. The stress distribution near borehole is heavily affected by particle size distribution. The lowest breakdown pressure and hydraulic injection volume are observed at the borehole located at the bedding plane. These results can deepen our understanding on the micro-mechanisms in hydraulic fracture initiation and propagation.