Application of displacement discontinuity method to hydraulic fracture propagation in heterogeneous rocks

The development of multi-stage hydraulic fracturing technique in horizontal wells enables us to produce oil and gas at economic rate from shale formations, leading to the shale revolution in the United States. Field observations including production history, microseismic mapping, and coring in fractured zones have revealed that the heterogeneity of shale rocks such as natural fractures is likely to have a large impact on oil and gas production from shale reservoirs. In this dissertation, a new hydraulic fracturing model based on the displacement discontinuity method (DDM) was developed. The major achievements in this research include the extension of DDM to multilayered media, the modeling of the interaction with natural fractures in three dimensions, and the development of a DDM-based hydraulic fracturing simulator. The formulation of DDM was revisited, and the equivalence of DDM and BEM was mathematically demonstrated. DDM was extended to multilayered media by using the method of images. The new DDM was applied to a three-layered medium in plain strain containing vertical and horizontal cracks. A sensitivity study suggests that bi-material solutions are sufficient for three-layered media under plain strain conditions. A DDM-based hydraulic fracturing model was developed. The discretized DDM and flow equations were solved in a segregated or fully coupled manner. A new splitting scheme was proposed to improve the convergence speed of the segregated method. The interaction between hydraulic and natural fractures was modeled for both intersecting and remotely interacting cases in our simulator. Poroelastic effects were partially incorporated into DDM by assuming an undrained condition. It was found that poroelastic effects under the undrained condition were limited to the vicinity of hydraulic fractures. Hydraulic fracturing simulations were performed in the presence of synthetic natural fracture networks. Synthetic microseismic events were generated, and inversion analyses of the synthetic microseismic data were performed. It was suggested that the density of microseismic events was affected by both the areal density and length distribution of natural fractures