Multi-Scale Investigations on the Geometries of Hydraulic Fractures in Conglomerate Reservoirs

Currently, the research on hydraulic fracture geometries is mainly focused on tight sandstone and shale. The investigations on the conglomerate tight reservoirs, e.g., Mahu Oilfield in Junggar Basin, China, is still lacking due to its uniqueness and late discovery time. The strong heterogeneity and the existence of gravels in conglomerate tight reservoirs put great challenges on the study of hydraulic fracture geometries. In this paper, a whole field cohesive zone model in finite element method is used to model the fracture nucleation and propagation in rock matrix (sand) and gravels in lab scale. The numerical model is validated against some published experimental results. Based on the analysis of numerical results, a mathematical model for quantitative characterization of fracture growth speed in conglomerate reservoir is proposed. This model is critical to connect the fracture propagation behaviors in lab-scale with the hundreds of meters field-scale hydraulic fractures. For the field scale fracturing simulations, the UFM (unconventional fracture model), which is based on boundary element method, has been widely used. Considering the similarity of crossing behaviors between hydraulic fracture-gravels and hydraulic fracture -natural fractures in conglomerate and shale respectively, a series of natural fractures are used to equivalent the impact of gravels in lab scale in the field scale simulations. The parameters of the equivalent natural fractures are determined according to the propagation model extracted from the lab-scale numerical simulations. The multi-scale research on fracture geometries and methods for field scale fracturing simulations for Mahu conglomerate reservoir could provide important guidance for the future design and optimizations of hydraulic fracturing.