In situ stress prediction from ductile deformation of unconventional reservoir rocks and its relation to the stress dependence of permeability
Keywords: Geophysics.

The principal focus of this thesis is to investigate viscous stress relaxation as a physical mechanism that could be responsible for variations of least principal stress with depth in unconventional reservoirs. Viscous stress relaxation in unconventional reservoir rocks leads to a more isotropic reservoir stress state. One important result of this effect is that variations of the least principal stress with depth have a strong impact on vertical hydraulic fracture propagation. The linkage between ductility (the degree to which the rocks are non-brittle) and layer-to-layer stress variation, and the effect of this link on vertical hydraulic fracture propagation, have been studied previously (Warpinski and Teufel, 1989; Sone and Zoback, 2014a, b). Laboratory creep experiments and associated empirical power laws are a reliable methodology for characterizing the ductile behavior of unconventional reservoir rocks (Sone and Zoback, 2014a). An extensive series of constitutive parameters measured on core samples from several different shale plays are reviewed in the framework of viscous stress relaxation. To better understand the effect of viscous relaxation on least principal stress magnitude within, above, and below unconventional producing intervals, laboratory creep studies are combined with numerical modeling in an attempt to match a measured profile of least principal stress from a borehole in the eastern United States. The core samples used in laboratory testing are from the borehole in which Diagnostic Fracture Injection Tests (DFITs) were carried out at approximately the same depths as core sample intervals to determine the least principal stress. Laboratory creep experiments, used to characterize the ductile properties of the core samples, are typically run for one day under a confining pressure and axial load similar to in situ conditions. Abaqus is used to simulate the creep of the core samples by using a three-dimensional elastic-power creep constitutive law to obtain the constitutive parameters. The three-dimensional elastic-creep constitutive law in conjunction with a formation-loading model is used to simulate the accumulation of least principal stress in situ. The calculated least principal stresses match the relative changes of the observed least principal stress magnitudes, indicating that variations of least principal stress are controlled, in the first order, by varying degrees of viscous stress relaxation in this case study. Matrix permeabilities are measured on core samples from the same well and similar in depths as the core samples conducted laboratory creep experiments. The stress dependence of matrix permeability is examined using an exponential effective stress law. The permeability modulus in the exponential law is found to have a positive correlation with the power law exponent of the creep of nearby core samples. Microseismic events are examined to validate the vertical propagation of hydraulic fractures in another case study involving stacked Wolfcamp formations of the Permian Basin of West Texas. The company drilled one horizontal well right above another to optimize the production of the stacked pay. If we could better understand vertical hydraulic fractures growth, one horizontal well could potentially be used to create hydraulic fractures penetrating all three stacked zones. Prediction of the least principal stress can be very helpful when designing hydraulic fracturing operations, as it allows operators to ensure that the vertical propagation of hydraulic fractures is contained to a desired production horizon or set of horizons.