An analysis of tracer flowback profiles to reduce uncertainty in fracture-network geometries

Hydraulic fracturing is extensively used to increase a wellbore contact area by creating complex fracture networks in unconventional reservoirs. The characterization of in-situ fracture networks is a common challenge, which has great significance to hydrocarbon production prediction and fracturing stimulation optimization. Tracer tests are often used to estimate the transport properties of fracture networks. The understanding of characteristics of fracture networks is explored by tracer numerical experiments. An integrated simulation workflow is developed to simulate the process of tracer injection and flowback in an individual stage of fractured horizontal wells. An individual fracturing stage consists of a single hydraulic (primary) fracture and stochastically generated secondary fractures. Fracture density, length, orientation and conductivity of secondary fractures are systematically altered to investigate the impacts of fracture networks on tracer flowback profiles. The fracture intersections and percolation analysis of fracture networks are correlated with the skewness of tracer flowback profiles. A parameter called a conductivity weighted effective fracture density is proposed to describe the relationship between secondary fracture networks and the shapes of tracer flowback profiles, which is a combination of fracture density, length, orientation and conductivity. The conductivity weighted fracture intersections between hydraulic fractures and secondary fractures are used to account for the impact of a boundary on tracer flowback profiles through which a fluid flows into a well. These relationships are validated by new sets of fracture networks and provide a constraint for fracture network characterization.