This study investigates induced seismicity in deep geothermal reservoir associated with fluid injection and production in doublet and triplet reservoir systems with a hydro-mechanical coupled numerical model. The model is using 2D discrete element model with embedded discrete fracture network. Fluid injection is performed for 6 hours with sequentially increased flow rate in three steps and the resulting induced seismicity is monitored. It is followed by constant rate fluid production at a point (doublet) and at two points (triplet) distances away from the injection point maintained for 20 hours. Spatio-temporal distribution of fluid pressure, injection/production induced seismic events, their magnitudes and source characters are investigated. The simulation shows that production induced seismic events mostly occur along the line at steep fluid pressure gradient with mostly implosion source characteristics. It was simulated that larger magnitude events can be induced in the vicinity of the production well and near pre-existing fractures when combined with abrupt slipping.
Hydraulic stimulation is essential in developing Enhanced Geothermal Systems (EGS) to increase reservoir permeability and maximize heat energy extraction. Byproduct of hydraulic stimulation is induced microseismic events that can trigger some events with higher magnitudes.
As well as fluid injection, production of fluid can also induce seismic events resulting from pore pressure decline and large contraction of the reservoir, in particular increasing horizontal stresses above and below the reservoir that could lead to reverse faulting (Segall 1989). The Lacq gas field in sourth western France offers one of the best-documented cases of seismicity induced by extraction of pore fluids (Grasso & Wittlinger 1990, Segall et al. 1994).
Seismicity induced by fluid withdrawal cannot be explained without taking into account the accompanying stress changes, which are associated with the larger-scale contraction of the reservoir caused by pore pressure reduction. The magnitude of the events can be potentially large.
With this background, this numerical modelling study aims at investigating the following issues:
Can fluid production generate major earthquakes in the vicinity of the production well?
Can fluid production result in the seismic activity further away from the production well?
What are the source characteristics of the fluid production induced seismic events?
Where do they occur?
Can fluid production induce larger magnitude events in the already stimulated reservoir?