In situ experiments of gas/water fracturing: geophysical monitoring of initiation and propagation of fractures

Predicting occurrence of hydraulically induced damage in geological systems constitutes a major challenge in subsurface engineering. The overall goal to improve our understanding of the hydromechanical processes taking place in geo-engineering applications for a better assessment of the associated risks and to propose non-intrusive methods to assess the spatio-temporal evolution of these processes.

An in situ experiment is being performed at the Tournemire site (IRSN’s URL) to evaluate under which conditions fractures could be initiated/propagated in a saturated indurated clay formation due to an increase of gas or water pressure. This fracturing field test will be conducted using three types of boreholes (each being 20m long) drilled from the URL by IRSN: (i) one injection borehole (P0) with 3 chambers; (ii) eight boreholes dedicated to geophysical monitoring of the “dynamic” fracturing process; (iii) one to two boreholes to record deformation and estimate the fracture location, which will help assess the geophysical survey. Two campaigns of electrically-surveyed hydrofracturing has already been performed in Summer 2022, providing promising first results, yet to be interpreted.

The master will be recruited to participate in performing this experiment with a special focus on geophysical monitoring. The works will have two parts:

1. In situ experiments at the Tournemire site. Hydrofracturing with gas will be performed on the already hydrofractured zones and on a pristine zone. Several campaign might be needed to complete the experimental program.
2. Processing of preexisting electrical campaigns of hydrofracturing. This pre-existing dataset will secure positive outcome of the internship. The processing will be done with several steps:
   1. Processing of electrical waveforms to recompute electrical resistivity and chargeability.
   2. Perform tomographic inversion of the large electrical surveys to identify the prior and posterior states around the injection chambers.
   3. Perform forward modeling of the resistivity changes induced by a propagating fracture, to interpret the time-lapse electrical data. This will allow to interpret hydro-mechanically the electrical changes already observed.

The results will provide new insights on the mechanics of hydrofracturing in tight shale formations.

Student profile: The candidate must be a highly-motivated and self-directed person with a solid knowledge in geophysics. The successful candidate must have a strong background in computing and geophysics.