Influence of preexisting discontinuities on hydraulic fracture complexity in a naturally fractured reservoir

Preexisting discontinuities, such as joints and beddings, are widely distributed in unconventional reservoirs. During fluid injection, hydraulic fractures strongly interact with preexisting discontinuities, leading to great uncertainty in prediction of fracture geometry. To investigate how preexisting discontinuities affect fracture complexity, numerical simulations are carried out using the distinct element method. A discrete fracture network (DFN) model for a shale reservoir in the southeastern Ordos basin, China is built. Four cubic models, including the (1) rock matrix model, (2) bedding model, (3) joint model and (4) bedding and joint model, are extracted from the DFN model and then are used for numerical simulations. The results indicate that preexisting discontinuities strongly influence fracture complexity: they impose a barrier for fracture growth in most cases, and thus result in a smaller fracture area and a higher fluid pressure for fracture extension. However, the roles that joints and beddings play in fracture complexity differ. Compared with the bedding model, the joint model causes a less complex fracture geometry under the same injection condition, suggesting that the joints play a more significant role in reducing fracture complexity.