Adaptive finite element-discrete element method for numerical analysis of the multistage hydrofracturing of horizontal wells in tight reservoirs considering pre-existing fractures, hydromechanical coupling, and leak-off effects

Multistage fracturing is widely used to improve gas production in tight hydrocarbon reservoirs. As a sequential process that interacts with pre-existing fractures, multistage fracturing must be properly addressed in numerical simulations, which are important in evaluating and optimising fracturing technology. The challenges of accurately representing the complex fracture structures and physical properties of naturally fractured reservoirs cause that the interaction between hydraulic and pre-existing fractures has not yet been resolved satisfactorily. In this study, we adopt an adaptive finite element-discrete element method to simulate the multistage fracturing of a naturally fractured reservoir by improving the mesh auto-refinement and identification of multiple fracture propagation. The numerical model considers interactions among hydraulic fractures, pre-existing fractures, and microscale pores, and integrates the nonlinear Carter leak-off criterion to describe fluid leak-off and hydromechanical coupling effects during multistage fracturing. The proppant transport equation for idealised parallel plate flow in fractures is introduced, and Darcy's law is adopted to analyse the seepage flow in the fracture network and determine the gas recovery. The fracture network and consequent fluid flow induced by the hydrofracturing of unfractured and naturally fractured models are compared to assess the influence of pre-existing fractures on multistage fracturing behaviour and gas production.