Three dimensional finite element simulations of hydraulic fracture height growth in layered formations using a coupled hydro-mechanical model

摘要

In this paper, we treat the important problem of hydraulic fracturing in the presence of elastic modulus and stress contrast in layered rock systems encountered in petroleum resources development using a fully coupled 3D hydro-mechanical model. First, the model is validated by simulating a laboratory hydraulic fracturing experiment dealing with the influence of stress contrast. Good agreements in the distribution of fracture aperture, injection pressure and fracture footprint have been achieved. Then, numerical analyses are performed to investigate the influence of in-situ stress and formation layer properties, such as Young's modulus and fracture energy release rate on the height growth and containment of hydraulic fractures. Comparing the results of simulations using conventional thickness-weighted Young's modulus to those from explicit modeling of layers' Young's moduli, it is found given that for the same amount of injection volume, the thickness-weighted modulus generates a higher injection pressure. Explicit modeling of the layers influences the hydraulic fracture aperture distribution in the pay zone as well as in the surrounding layers. A relatively large fracture aperture is observed in the layer with the lower Young's modulus. Also, the hydraulic fracture tends to propagates mainly in the lower Young's modulus layers which could facilitate containment of the hydraulic fracture by limiting height growth in the stiffer layers. When considering the influence of stress contrast on height growth, the conventional equilibrium height model produces a relatively large aperture and overestimates the fracture height. Simulations using typical injection rate, fluid viscosity, and in-situ stress, show stress contrast larger than a certain value, for example 30% of the in-situ minimum horizontal stress, can effectively inhibit height growth. When the payzone is bounded by ductile layers, the injection pressure is higher and the corresponding aperture at the injection point is larger than those obtained using uniform rock properties.