Oil and gas production from unconventional reservoirs generally requires hydraulic fracturing within layered reservoirs, which are usually stratified with layers of different mechanical properties. Hydraulic fracture height growth is one of the critical factors in the success of hydraulic fracturing treatments. It has been well documented that in-situ stress contrast between adjacent layers and interface properties are the dominating factors in fracture height containment, whereas modulus contrast between adjacent layers is generally considered of secondary importance in terms of direct control of fracture height containment. However, arrested fluid-driven fractures at soft layers are often observed in outcrops and hydraulic fracture diagnostics field tests. The objective of this study is to investigate hydraulic fracture height containment due to modulus contrast between adjacent layers.
In order to illustrate the effect of modulus contrast on fracture height containment, this study proposed a new approach, which is based on the effective modulus of a layered reservoir. In this work, two-dimensional finite element models were utilized to evaluate the effective modulus of a layered reservoir, considering the effect of modulus values, fracture tip location, height percentage of each rock layer, layer location, the number of layers, and the mechanical anisotropy. Then, the effect of modulus contrast on fracture height growth was investigated with an analysis of the stress intensity factor, considering the change of effective modulus as the fracture tip propagates from the stiff layer to the soft layer.
This study showed that the detail of layering did not impact the value of effective modulus and the only important parameters were fracture tip locations, modulus values, and the height percentage of each rock layer. In addition, this study empirically derived two approximations of effective modulus depending on fracture tip location, namely the modified height-weighted mean and the modified height-weighted harmonic average. By combining linear elastic fracture mechanics with the appropriate effective modulus approximations, the results indicate that hydraulic fracture propagation will be inhibited by the soft layer due to a reduced stress intensity factor.
The averaging methods developed in this work can significantly improve material balance in hydraulic fracturing simulation. This study also suggested that soft layers inhibit hydraulic fracture propagation in layered reservoirs. As a result, hydraulic fracture height containment within a stratified rock stack can be better evaluated by comparing the modulus contrast between adjacent layers.
