Hydraulic fracture development in conglomerate reservoirs simulated using combined finite-discrete element method

Hydraulic fracturing is known to be the most effective stimulation to enhance fractured reservoir permeability by creating hydraulic fractures (HFs). The efficiency of a HF fluid injection largely depends on the pre-existing discontinuities or sources of heterogeneities. These features need to be considered in a HF operation design. Amongst these features, conglomerate reservoirs containing blocks in a matrix are largely ignored despite their presence in deep rocks. This study focuses on the HF development in such grounds and simulates their hydro-mechanical behaviour by using the combined finite-discrete element method (FDEM), which is fully capable of modelling HF propagation in heterogeneous rocks. First, the capabilities of the FDEM are verified against existing analytical solutions and experimental fluid injection and then employed to model HF development. In the conglomerate models, three blocks with low to high strength properties are simulated to assess the effect of block strength on the HF development pattern. Various controlling parameters, including in-situ stresses, flow rate, fluid kinematic viscosity, directional HF (DHF), properties of the interface between the blocks and matrix, and their effects on the HF path are investigated. The results show a significant influence of blocks in a matrix on the HF pattern. The HF interaction with the blocks of different shapes and dimensions is shown to be complex, and six interaction types for the blocks are identified based on the close observation of the results. It is also found that to achieve a successful HF injection in such grounds, a low flow rate with a low viscous fluid can be a good strategy. Also, it is shown that the blocks' strength significantly affects the type of failure (tensile or shear) and the breakdown pressure.