The application of numerical modelling techniques to the mechanisms of reservoir- and mining-induced seismicity

Abstract

A survey of the 31 case histories of reservoir-induced seismicity has failed to Indicate any relationships between the geology of an area and the induced tremors. The occurrence of induced seismicity Is however related to the water height and, to a lesser extent, the water volume. Reservoir load stresses have been modelled by finite element technique, considering both pre-existing topographic and tectonic stress fields, and been found to result In stabilisation tn all cases. The presence of hard, thin layers of rock has been found, however, to significantly increase the effect of both topographic and reservoir loads, and pre-impoundment stress relief due to fracture and creep can then result in reservoir load stress destabilisation. Consideration of the subsequent pore pressure diffusion away from the reservoir using a finite difference technique can explain the induced rock failure by reduction of effective stress. This is particularly so if allowance for initial pore compression Is made, but the effect of fractures can be to considerably reduce the pore pressure effect.
The use of numerical modelling in practical examples of reservoir- induced seismicity is subject to a variety of limitations. Lack of field information Is the main problem, but lack of computer storage and of an understanding of the mechanism(s) involved also contribute. The stored strain energy due to the water load at Koyna Is found to be sufficient to account for most of the Induced seismic activity, but tectonic stress must be utilised to a large extent. A triggering mechanism is invoked to account for the majority of cases of reservoir-induced seismicity.
Finite element technique has also been applied to the study of stress interactions between adjacent mines, with particular reference to the mining-induced seismicity In N.Staffordshlre. Certain configurations of longwal1 panels have been found to significantly Increase the induced stress field.