Title:

Analysis and Numerical Modelling of Pressure Drops Observed During Hydraulic Stimulation of GRT-1 Geothermal Well (Rittershoffen, France)

Authors:

Gabriel MEYER, Clément BAUJARD, Régis HEHN, Albert GENTER, Mark MCCLURE

Key Words:

EGS, ECOGI, Rittershoffen, hydraulic stimulation, CFRAC,

Conference:

Stanford Geothermal Workshop

Year:

2017

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Meyer

File Size:

1994 KB

View File:

Abstract:

Enhanced Geothermal Systems (EGS) are a promising source of geothermal energy. Nevertheless, induced seismicity associated with EGS projects is a major challenge for the large scale development of the technology, and thus many efforts are made to mitigate it. The accuracy with which the 2013 GRT1 Rittershoffen geothermal well stimulation was monitored offered new perspectives with the recording of a rarely observed phenomenon: injection pressure drops that seemed to be linked with the triggering of induced earthquakes. Here we propose to investigate this phenomenon and to characterize the relationship between pressure drops and seismicity. To do so, we analyzed the drop data recorded during the stimulation and made some early observations. We then used a numerical simulator called CFRAC to test the hypotheses developed from the initial analysis of the data. Firstly, we showed that it is difficult to link an earthquake to a specific pressure drop, thus contradicting a possible causality relationship between the seismicity and the drops. We also showed that the drops share a characteristic shape, which is an abrupt pressure loss followed by a more progressive increase of the pressure to a level lower than the triggering pressure. We noticed that most of the pressure drops are followed (after tens of seconds to a few minutes) by a small burst of seismic activity. These observations led us to a new hypothesis in which the drops are the origin of the seismicity, or at least part of it, and that they are the result of the opening of mode I wing cracks near the tips of the natural fractures in the reservoir. This hypothesis was tested using CFRAC. The simulations suggested that the drop signal is attenuated over very short distances and thus a highly transmissive fracture in the reservoir is necessary to convey the drop signal from the stimulated zone to the well. It also appeared that the drops are caused by the opening of the tensile fractures rather than by the equilibration of dry newly connected fractures, as we first thought. A high (10 MPa) matrix tensile strength is necessary to cause pressure drops during a stimulation. In the end, we cannot confirm that this is the origin for the drops, but our study shed light on phenomena that may be involved in the processes behind induced seismicity.


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