Abstract
Abstract. In this study we describe and compare eight different strategies to predict the depth variation of stress within a layered rock formation. This reveals the inherent uncertainties in stress prediction from elastic properties and stress measurements, as well as the geologic implications of the different models. The predictive strategies are based on well log data and in some cases on in situ stress measurements, combined with the weight of the overburden rock, the pore pressure, the depth variation in rock properties, and tectonic effects. We contrast and compare stresses predicted purely using theoretical models with those constrained by in situ measurements. We also explore the role of the applied boundary conditions that mimic two fundamental models of tectonic effects, namely the stress- or strain-driven models. In both models, layer-to-layer tectonic stress variations are added to initial predictions due to vertical variation in rock elasticity, consistent with natural observations, yet describe very different controlling mechanisms. Layer-to-layer stress variations are caused by either local elastic strain accommodation for the strain-driven model, or stress transfers for the stress-driven model. As a consequence, stress predictions can depend strongly on the implemented prediction philosophy and the underlying implicit and explicit assumptions, even for media with identical elastic parameters and stress measurements. This implies that stress predictions have large uncertainties, even if local measurements and boundary conditions are honored.
Highlights
By comparing the stress predictions obtained with these two initial models, we explore the possible range of local stress corrections because the stress predictions obtained assuming an initial lithostatic and uniaxial state of stress are likely the upper and lower limits of the tectonic perturbations, respectively
The increase is lower for the uniaxial stress σu than for the lithostatic stress σl
Different strategies to predict the vertical variations in the in situ stresses lead to different answers
Summary
Knowledge of in situ stress magnitudes and their spatial variability is critical to understand the upper crust stress and strain (Zang and Stephansson, 2010; Zoback, 2010; Schmitt et al, 2012; Reiter et al, 2014; Reiter and Heidbach, 2014), which in turn has strong implications for seismotectonics (e.g., earthquake magnitudes; Davies et al, 2012; Langenbruch and Shapiro, 2014; Busetti and Reches, 2014; Scholz, 2015) and their locations (Sibson, 1982; Zoback and Gorelick, 2012; Zakharova and Goldberg, 2014), structural geology (e.g., fault behavior and slip tendency; Gross et al, 1997; Gudmundsson, 2011), and volcano-tectonics (e.g., prediction of dike paths and eruption forecasting; Gudmundsson, 2006) It is key for the civil engineering, mining, and energy industries, covering topics as diverse as hydraulic fracturing, rock stability, and fluid circulation (Simonson et al, 1978; Van Eekelen, 1982; Warpinski et al, 1985; Hopkins, 1997; King, 2010; Fisher and Warpinski, 2011; Davies et al, 2012). Analytic formulations (McGarr, 1988; Gunzburger and Cornet, 2007; Jaeger et al, 2009), or numerical models (Teufel and Clark, 1984; Gudmundsson, 2006; Roche and Van der Baan, 2015), have to be used to assess the stress variations
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