Novel crustal stress profiling based on the criticality of natural fractures – a KTB example

Determination of in situ stress magnitude and orientation is fundamental for understanding crustal mechanics and facilitating subsurface exploration and development as well as hazard assessment. At present, in situ stress at depth is mainly estimated from borehole observations. Traditional methods, such as hydraulic fracturing tests, are mature and practical, yielding reliable estimates of the least principal stress but usually at a limited number of depths. Estimates of the maximum horizontal stress (S_Hmax) and stress orientation rely on observations of compressive or tensile failure of the borehole but can have considerable uncertainty depending on borehole conditions. Therefore, new approaches to estimate in situ stress magnitudes effectively are desired in stress characterization.
In this study, we extend a novel approach for stress determination that utilizes the natural fractures identified in deep boreholes. Critically-stressed natural fractures exhibit distinct thermal anomaly identifiable on temperature logs, whereas non-critically stressed fractures do not. Given an abundant and diverse set of natural fractures, inversion is feasible to estimate the magnitude of the maximum and minimum horizontal stresses utilizing the knowledge of the vertical stress (estimated from density logs).
We illustrate this novel approach with the KTB borehole data set. The classification facilitated a two-stage stress inversion that efficiently inverts the in situ stress orientation and absolute magnitude. The inverted stress matches well with independent borehole observations. The maximum discrepancy between the inversion results and the S_Hmax derived from wellbore failures is 26.6 MPa at 7 km depth, which is lower than the uncertainty of estimated S_Hmax magnitude (~47 MPa). The inverted S_Hmax orientation is N161.3°E, which is quite consistent with the observed S_Hmax orientation obtained from wellbore failures (~N160°E). To investigate stress heterogeneity over finer scales, the inversion was also applied to selected subsets of fractures along the KTB borehole. We evaluate the limitations and scale-dependence of this approach by considering the fracture distribution and fault perturbations. Our results demonstrate that profiling in situ stress via natural fractures is feasible and complementary to existing approaches, and can offer new insights on the characteristics of crustal stress, its spatial heterogeneity, and its interactions with geological discontinuities.