FAULT REACTIVATION, SLIP, AND SEISMICITY AS A RESULT OF SUBSURFACE FLUID WITHDRAWAL
Rajesh A. Chanpura and Leonid N. Germanovich
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA
Geological discontinuities such as faults are inherent in most petroleum formations. It is well known that depleting a hydrocarbon reservoir can redistribute in-situ stresses sufficiently to reactivate and induce slip of nearby faults. The objective of this work is to understand the mechanism of fault reactivation as a result of subsurface fluid withdrawal. Fault reactivation may have a wide range of consequences. It could shear the boreholes (drilled through the slipped portions of the fault zone), dynamically release stored elastic energy and induce seismicity, change the formation permeability and the production-depletion strategies, etc. A conceptual model of depleted reservoir and slipping fault is developed. The reservoir is modeled as a thin, soft, smooth poroelastic inclusion in a poroelastic matrix while the fault gouge obeys Mohr-Coulomb failure criterion. A method is developed to calculate the redistributed stresses around a depleted, heterogeneous reservoir with non-uniform pressure distribution. The developed method allows considering the reservoir and the fault to be of arbitrary shapes and material properties while the pressure inside the reservoir does not have to be uniform. Using this method, the instability of nearby faults caused by the reservoir depletion is analyzed. It is shown that different reservoir depletion strategies can affect the fault stability differently. Steep gradients in the reservoir pore pressure have a greater tendency to reactivate and induce slip in the nearby faults than gradual pressure gradients. After the fault reactivates, slip develops in the fault zone. The magnitude of fault slip is computed by modeling the slipping fault as a shear (mode II) fracture. Based on the magnitude of fault slip, seismic moment and earthquake magnitudes generated are calculated. The results show that the magnitude of slip induced may not be of considerable significance for seismic activity but more than sufficient to shear off the boreholes. Because the fault stability is not affected at all if the poroelasticity of the reservoir material is not taken into account, the addressed phenomenon represents a clear and robust demonstration of the importance of poroelastic effect.
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