Abstract

Fault reactivation under a given injection pressure is estimated for faults surrounding a well designed for the purpose of drilling cuttings reinjection in offshore West Africa. The poroelasto-plastic finite-element method (FEM) is used to calculate a set of scenarios of injection pressure values under which fault reactivation could occur. Seismic activity related to this fault activation is calculated using an analytical method with the numerical solution of displacement discrepancy across the fault as input. Maximum magnitude of the seismicity induced by this fault reactivation is calculated. The fault being investigated is in the maximum horizontal-stress direction of the field to the injection well. The following assumptions are adopted in the calculation: the pore-pressure boundary condition at the bottom of the fault is determined by the injection pressure, which is given as input data, and the inclination angle is 70°. The FEM being used is a simplified plane-strain model. Numerical results obtained with the FEM model include the distribution of the equivalent plastic strain within the whole model, distribution of the von Mises equivalent stress, and the value of the displacement discrepancy across the fault under the specified pore-pressure boundary conditions at the bottom of the model. The development of the plastic region indicates the status of fault reactivation under injection resulted pressure.

1. INTRODUCTION

Fault reactivation is an engineering problem that can be initiated by the connection of stimulated factures to the fault. Reactivation of a fault could result in environmental pollution owing to fluid migration, particularly in an offshore field. Another environmental risk of injection-related fault reactivation is possibility of induced seismic behavior. Investigations on these topics using different methods have been reported by various researchers since the last decade. To determine a reasonable and accurate solution of fault reactivation and its environmental impacts with minimum cost, it is necessary to briefly review the studies in existing literature.

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