Hydraulic fracturing technology is essential for the development of Enhanced Geothermal Systems (EGS) to increase the permeability of tight rock formations and hence the energy recovery from a petro geothermal reservoir. However, fracturing and the subsequent increase in energy production may pose the risk of induced seismicity. Previous studies established the mechanisms of microseismic events during hydraulic fracturing in the Deep Heat Mining project Basel using the numerical simulator FLAC3Dplus. In this paper, an innovative injection strategy using a linear increasing injection rate to reduce the maximum magnitude of microseismic events Mmax has been proposed. In addition, the new EGS-strategy allows a combination of both the linear increased injection rate and the multiple fracture system to be studied numerically. Results show that the risk of induced seismicity can be considerably minimized by the proposed strategy. The simulation shows that increasing the number of fractures in the tight reservoir decreases Mmax significantly. It is recormnendable that multiple hydraulic fracturing technology be applied in the development of enhanced geothermal systems not only to minimize the risk of induced seismicity but also to increase the surface area for heat exchange and recovery efficiency.

1 INTRODUCTION

The exploitation of renewable energy has become a crucial method for mitigating global energy and climate crises (Gou et al., 2015). Compared with other types of renewable energy, e.g. PV or wind energy, deep geothermal energy offers more advantages including being both sustainable and economical (IEA, 2017). In an enhanced geothermal system, hydraulic fracturing is a key technology to increase formation permeability (Lu et ai., 2015) and improve the energy recovery from reservoir. However, this technology has its own shortcomings. Hydraulic fracturing can trigger seismicity (Hou et al., 2012), which is one of the main obstacles hindering its industrial application in production from enhanced geothermal system.

In the previous study by Hou et al., 2013, the coupled Thermo-Hydro-Mechanica (THM) numerical simulator FLAC3Dplus was developed to investigate hydraulic fracturing in the Deep Heat Mining (DHM) project Basel. Meanwhile, the fundamental mechanisms of microseismic events during hydraulic fracturing have been studied in detail. In this paper, innovative injection strategies in combination with multiple hydraulic fracturing technologies have been proposed to reduce the risks of induced seismicity and to increase the heat exchange area in the reservoir and hence the efficiency of heat recovery. These parameters have been studied and tested based on the history-matched Basel model.

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