Viscoelastic approach to capture varying least principal stress magnitude and the effect of observed stress layering on hydraulic fracturing-An example from shale formations of the Perth Basin

Accurate prediction of the least principal stress at depth can improve the efficiency of hydraulic fracturing design and mitigate the risk of unnecessary vertical growth of fractures beyond the reservoir interval. Here, we demonstrate that a viscoelastic stress relaxation model can predict the variation of the least principal stress in a vertical well of the Perth Basin. Creep response of clay- and carbonate-rich shales have been recorded in the laboratory for a duration of ~ 6 hours under simulated in-situ stress conditions. A simple power-law captures the primary creep response, and an empirical relationship is established between the inverted creep parameters. To compute the least principal stress magnitude at depth, we combined gravitational loading, horizontal tectonic stress accumulation at a predefined loading rate via the viscoelastic rheology, and uniformity of the relative stress magnitude along depth, with the necessary wireline log curves (Fig.1). The derived stress magnitude matches well the Instantaneous shut-in pressure (ISIP) stimulation data from hydraulic fracturing in the field. Finally, it is shown by performing a 3-dimensional planar hydraulic fracture simulation through Baker Hughes’s MFrac that stress layering can either act as a fracture barrier or a propagator, depending upon the differential stress contrast between layers.