When working with the subsurface, there is always a level of risk involved, such as induced seismicity. Whilst previous drillings in the area show no induced seismicity at all, 4 monitoring boreholes in the area are supplied with geophones and electromagnetic monitoring tools in order to quickly pick up on any seismic activity and actions can be taken accordingly.
Monitoring within and around the campus geothermal well enables to investigate the performance of various aspects of producing heat from the subsurface and to improve the monitoring techniques themselves. The monitoring programme consists primarily of seismic monitoring with fibre optics and geophone stations surrounding the well and newly developed electromagnetic monitoring tools.
Optical fibres for seismic, temperature, stress and pressure monitoring
Fibre-optic cables will be installed in both wells but the cables serve different purposes and therefore have a different design.
The fibre-optic system for the injector well is primarily for monitoring the fluid flow and cold-front advancement and for detecting micro-cracking in the horizontal plane of the overburden and reservoir. In theory, fluid flow and cold-front advancement (which propagate at different speeds) are well understood but the interaction with the strong geological heterogeneities that are omnipresent in the Dutch subsurface is very uncertain. Geothermal doublets are “designed” on a breakthrough-time of the cold water in about 30 years, though with the current technology, no information is available about the propagation of the cold-water front until actual breakthrough. Geophysical methods to monitor the spatial and temporal propagation of the fluid fronts will be done by measuring the temperature gradient via Distributed Temperature Sensing (DTS), Distributed Acoustic Sensing (DAS) for passive and active-source seismic monitoring, the latter via Vertical Seismic Profiling (VSP) along the injector well.
A DAS optical enhanced-backscatter fibre is cemented tightly to the formation in the reservoir section and is supposed to sense micro-cracking events, which can be detected and its origin located. These events are caused by (i) the modified in-situ stress field due to drilling and injection; (ii) the thermally induced volume changes in the rock (shrinkage mainly in the injection well); (iii) changes in the strength properties of the rock due to flow erosion or geo-chemical activity.
The fibre-optic system in the producer well is primarily for monitoring the pressure, known as Distributed Pressure Sensing (DPS), and it will be sensing over the reservoir interval. The main goal of this sensing is to perform regular Pressure Transient Analysis (PTA) to monitor changes in pressure drop because of temperature changes (moving cold front), possible formation damage (skin) around the well, and possible induced fracturing. The pressure sensing across the reservoir will allow a real-time lifelong Production Logging Tool (PLT) under dynamic and static conditions.
Geophones for seismic monitoring
A geophone monitoring system around Delft is used to monitor seismicity locally. The layout of the network is designed to be able to detect smaller earthquakes than the national KNMI seismic network and to locate them more accurately. One of the local geophone stations is an existing KNMI station (ZH03), while the three other stations are newly installed by the TU Delft. The monitoring station at the Delftse Hout (DAPGEO-02) is different from the other stations since it also has geophones at 440 and 490 m depths, allowing higher sensitivity and lower noise levels than the other geophones. This station is currently set up to become part of the KNMI infrastructure.