Estimating perturbed stress from 3-D borehole displacements induced by fluid injection in fractured or faulted shales

Abstract

S U M M A R YHydrofracturing stress measurements in fractured and anisotropic shales are notoriously difficult, because opening of existing geological features tends to prevent the creation of a pure hydraulic fracture perpendicular to the least compressive principal stress. Here we show how adding 3-D borehole-displacement measurements while conducting the hydraulic injection test helps to better constrain the principal stress orientations and magnitudes. We developed a 3-D fully coupled hydromechanical numerical model to analyse the displacement, fluid pressure and injection flow-rate data measured during an injection pressure-step-rate test conducted to activate a faulted borehole interval in the Mont Terri Opalinus Clay (Switzerland). We find that injected fluids can only penetrate the fault when it is at or above the Coulomb failure pressure. Borehole displacement orientations are sensitive to a ∼15 • variation in the stress-tensor orientation and a 1 MPa stress magnitude variation. Although some dispersion occurs while rupture is propagating along the fault plane ∼4 m away from the borehole, the maximum density of displacement orientations consistently informs about the stress orientation. Thus, an extended injection step-rate approach coupled with an accurate in situ measurement of the borehole wall displacements can be used to better constrain the local stress field perturbations in fractured shales and in heterogeneous rock in general.

Modelling the water injection induced fault slip and its application to in-situ stress estimation

“…Table 2. Fault orientation and stress tensor of Mont Terri data [24,25], data used in a recent study [26] and data estimated in this study. Further development is needed to improve the model behavior.…”

From Fluid Flow to Coupled Processes in Fractured Rock: Recent Advances and New Frontiers

“…(2020) and Guglielmi, Nussbaum, Rutqvist, et al. (2020) observed a complex interplay between fault slip and fault opening in low permeability shale formations, with most of the hydraulic conductivity variation in faults caused by opening rather than slip. Moreover, Guglielmi et al.…”

“…Finally, since the 2000s (Murdoch et al., 2003), hydro‐mechanical well testing has been refined, using probes that allow the local downhole direct measurement of fractures opening and shear displacement while pressurizing it between two sealing inflatable packers (Guglielmi et al., 2014; Schweisinger et al., 2009). These techniques provide direct input for geomechanical analyses of fracture properties, full stress tensor, and potential for induced seismicity (Guglielmi, Nussbaum, Rutqvist, et al., 2020).…”

Crystalline Disposal R&D at LBNL: FY20 Progress Report

“…We start by applying the stress field estimated in Section 3.4 to all the models' boundaries. Stress magnitudes are then adjusted to match the magnitude and orientation of the measured displacements following the method developed in Guglielmi et al (2020). A sensitivity study to the different model parameters is conducted but it is not described in this report.…”