hide
Free keywords:
OPEN ACCESS. Water reinjection
Stress state
Hydromechanical (HM) coupling
Injection-induced seismicity
Numerical modeling
Abstract:
Water reinjection into the formation is an indispensable operation in many energy engineering practices.
This operation involves a complex hydromechanical (HM) coupling process and sometimes even causes
unpredictable disasters, such as induced seismicity. It is acknowledged that the relative magnitude and
direction of the principal stresses significantly influence the HM behaviors of rocks during injection.
However, due to the limitations of current testing techniques, it is still difficult to comprehensively
conduct laboratory injection tests under various stress conditions, such as in triaxial extension stress
states. To this end, a numerical study of HM changes in rocks during injection under different stress
states is conducted. In this model, the saturated rock is first loaded to the target stress state under
drainage conditions, and then the stress state is maintained and water is injected from the top to
simulate the formation injection operation. Particular attention is given to the difference in HM changes
under triaxial compression and extension stresses. This includes the differences in the pore pressure
propagation, mean effective stress, volumetric strain, and stress-induced permeability. The numerical
results demonstrate that the differential stress will significantly affect the HM behaviors of rocks, but the
degree of influence is different under the two triaxial stress states. The HM changes caused by the triaxial
compression stress states are generally greater than those of extension, but the differences decrease with
increasing differential stress, indicating that the increase in the differential stress will weaken the impact
of the stress state on the HM response. In addition, the shear failure p otential of fracture planes with
various inclination angles is analyzed and summarized under different stress states. It is recommended
that engineers could design suitable injection schemes according to different tectonic stress fields versus
fault occurrence to reduce the risk of injection-induced seismicity.
