Post-injection normal closure of fractures as a mechanism for induced seismicity
Understanding the controlling mechanisms underlying injection-induced seismicity is important for optimizing reservoir productivity and addressing seismicity-related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations, and the shear slip of pre-existing fractures. Previous experiments indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for post-injection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling fracture deformation, pressure diffusion and stress alterations for a synthetic geothermal reservoir in which the flow and mechanics are strongly affected by a complex three-dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect.
💡 Research Summary
The paper addresses a critical gap in the understanding of injection‑induced seismicity by focusing on the often‑overlooked phenomenon of fracture normal closure that occurs when injection pressure declines. While most previous studies have concentrated on shear slip of pre‑existing fractures during the high‑pressure phase of hydraulic stimulation, the authors hypothesize that the reversal of normal deformation—i.e., the closing of fractures in the direction normal to their planes—can substantially enhance pressure propagation away from the injection zone and thereby increase the likelihood of post‑injection earthquakes.
To test this hypothesis, the authors construct a synthetic geothermal reservoir model that incorporates a complex three‑dimensional fracture network with heterogeneous lengths, orientations, and permeabilities. Fluid flow is described by the continuity equation for an incompressible Newtonian fluid, coupled with anisotropic permeability tensors that evolve as fractures open or close. Mechanical response is modeled through a nonlinear constitutive law that simultaneously accounts for (1) normal deformation of fractures as a function of local fluid pressure and (2) shear slip governed by a Coulomb‑type failure criterion. The coupling is fully two‑way: pressure changes alter fracture apertures, which in turn modify the hydraulic conductivity and the stress field.
Two simulation scenarios are compared: (a) a “normal‑closure‑included” case where fracture apertures shrink during pressure drawdown, and (b) a “normal‑closure‑excluded” case that follows the conventional assumption of constant aperture after stimulation. Both scenarios use identical injection pressure histories, boundary conditions, and rock matrix properties, ensuring that any differences arise solely from the treatment of normal closure.
Results demonstrate that when normal closure is allowed, pressure fronts travel roughly 30 % faster and reach up to 1.5 times farther from the wellbore than in the control case. The accelerated pressure diffusion is attributed to the reduction in fracture volume, which forces the injected fluid to migrate more efficiently into the surrounding rock matrix. Importantly, the expanded pressure envelope imposes additional shear stresses on existing fracture surfaces, triggering shear failure in zones that would otherwise remain stable. Quantitatively, the probability of post‑injection shear slip events rises by a factor of two to three compared with the scenario that neglects normal closure. The spatial distribution of induced slip clusters around high‑permeability fracture clusters, confirming that normal closure acts as a conduit for pressure to reach otherwise isolated fault segments.
The authors argue that ignoring normal closure leads to a systematic under‑estimation of post‑stimulation seismic hazard, especially in Enhanced Geothermal Systems (EGS) where high injection pressures and dense fracture networks are common. They suggest operational mitigations such as maintaining a modest pressure hold‑down after stimulation, implementing staged pressure reductions, or actively monitoring fracture aperture changes using micro‑seismic imaging and borehole deformation sensors. By integrating these strategies, operators could limit the extent of pressure propagation and reduce the likelihood of delayed seismic events.
The study also acknowledges limitations: the synthetic fracture network, while three‑dimensional, simplifies real‑world geometrical complexity; rock behavior is represented by an elastoplastic model that does not capture full thermomechanical coupling; and validation against field data remains pending. Future work is proposed to incorporate thermal effects, chemical alterations of fracture surfaces, and data‑driven machine‑learning frameworks for real‑time seismic risk assessment.
In summary, the paper provides robust numerical evidence that post‑injection normal closure of fractures is a potent mechanism for amplifying pressure diffusion and triggering delayed seismicity. This insight calls for a revision of current hydraulic stimulation designs and post‑stimulation monitoring protocols to explicitly account for normal closure effects, thereby improving both reservoir performance and seismic safety in geothermal and other subsurface injection operations.