ABSTRACT:

In this paper the mechanism of seismic events induced by reservoir stimulation in EGS and Petroleum Engineering are studied and compared. For releasing of deformation energy there are the necessary and sufficient conditions, which must be satisfied. But for a great deformation energy release there must be a big shear failed volume, which appears not at the front of fracture propagation but behind it. Through numerical modeling by the simulator FLAC3D this mechanism is also well verified. At the same time the kinetic energy during the injection is continuously registered. It was also found that the kinetic energy has a good relationship with the injection rate and the stimulated volume in the rock depending strongly on the injection volume.

1 INTRODUCTION

In the Deep Heat Mining (DHM) project Basel, a seismic event with a local magnitude of 3.4 was measured after about 12,000m3 water injection, which caused building damages in the city Basel, suspension of the project and suspicion against deep geothermy. For this reason it is very important to investigate the mechanism and to model seismic events induced by reservoir stimulation of an EGS. Through the numerical modeling, the process of energy release and the influencing factors on the energy release can be better understood, so that the procedure of the stimulation can be optimized and the geo-risks can be minimized.

2 MECHANISM OF INDUCED SEISMIC EVENTS BY RESERVOIR STIMULATION

The induced seismic event by reservoir stimulation in principle belongs to a thermal-dynamic process, in which the energy conservation rule must be obeyed. In other words the energy cannot be lost but transform from one type to another. Seismic wave is strictly speaking a form of kinetic energy. In reservoir stimulation the most kinetic energy comes from the deformation energy, which is released by stimulation induced rock failure.

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