DISCRETE FRACTURE NETWORK MODELING OF INDUCED SEISMICISTY

Fluid injection and withdrawal frequently produce mico-seismicity, and occasionally produce more significant seismic events. In most of these cases, fluid pressures as well as temperature changes influence the stress state on pre-existing faults, leading to fault slip and consequent seismicity. The frequency and magnitude of this seismicity depends upon a combination of the pre-existing fracture and fault geometry, in situ stresses, and in situ pressures.

The discrete fracture network (DFN) approach is used to develop a 3D model for the faults and fractures affected by fluid injection and withdrawal. The frequency and magnitude of micro-seismicity depends on this geometry, particularly relative to the in situ stress field. The procedure for application of DFN technologies to evaluate and predict induced seismicity is as follows:

1) evaluate available structural data, including lineaments, stratigraphic models, and seismic structures to determine the faults which could be effected directly or indirectly by fluid injection; 2) evaluate the in situ stress field and previous seismic history if any to determine the stress states on potentially active faults; 3) simulate the flow field with injection and withdrawal within the DFN model to determine the change in pore-pressure on potentially active faults; 4) calculate thermo-elastic effects using the solution of Mossop and Segal (2002); 5) utilize empirical relationships to relate the change in shear and normal stress on potentially active faults to determine the potential change in frequency and magnitude of seismic events.

The advantage of this approach is that it directly utilizes the specific local fracture and fault information to analyses site specific induced seismicity potential.

This paper describes the development of the DFN approach described above, utilizing data from geothermal and reservoir sites in the US and Asia.