Combined finite-discrete element modelling of hydraulic fracturing in deep geologically complex reservoirs

Hydraulic fracturing is known to be one of the most effective stimulation techniques to enhance reservoir permeability. A hydraulic fracture (HF) fluid injection's efficiency largely depends on the pre-existing sources of heterogeneities. Amongst these features, conglomerate reservoirs containing blocks in a matrix and rock masses with filled joints are largely ignored despite their presence in deep rocks. This study focuses on the HF development in such reservoir conditions using the combined finite-discrete element method (FDEM). First, the capabilities of the FDEM are verified against available analytical solutions and experimental data. The influence of a single or multiple types of blocks is studied in the conglomerate models. Various controlling parameters including in-situ stresses, flow rate, fluid viscosity, directional HF (DHF), distance between the injection points, properties of the block and matrix interface are considered, and their effects on the hydraulic fracturing path are investigated. In the second scenario, the effect of the joints' filling material is studied, models containing randomly distributed filled joints are built and several controlling factors including different filling materials, angle of approach, in-situ stresses, flow rate and fluid viscosity are considered. The analysis results show a significant influence of both blocks in the matrix and filled joints on the HF pattern. The HF interaction with these features is shown to be complex, and six interaction types for the blocks and nine interaction types for the filled joints are identified based on the close examination of the results.