Fracture network simulations. The simulations followed the onsite stimulation procedure and indicate a fracture growth and coalescence of pre-existing fractures from the Muschelkalk/PCT upwards to the injection borehole. The fracture growth depends on the pressure conditions within the borehole, i.e., fractures propagate and coalescence in case fluid is injected while the fractures remain stable under in-situ conditions. During the first phase, no additional mechanical load or hydraulic pressure is applied on the borehole walls. During that phase, the whole geomechanical system is stable and the fractures do not propagate, independent of the formation (Figure 24A). Applying a pressure of 46 MPa during the second phase on the borehole wall leads to fracture propagation within the Muschelkalk while cracks within the other formations show no propagation (Figure 24B). Once the fractures in the Muschelkalk have propagated, they become unstable, so that in the third phase, where the pressure in the borehole is reduced to zero, they commence to propagate until they reach the borders to the surrounding layers (Xxxx/Dogger and PCT). The PCT is not penetrated by the fractures, since the crack propagation stops shortly before and the Xxxx Formation/Dogger boundary is penetrated before the propagation ends (Figure 24C). In the fourth phase, the pressure in the borehole is increased again, which leads to a fracture growth into the PCT (Figure 24D). By reaching the PCT the pore pressure that was restricted by the Muschelkalk spreads out into the Xxxx Formation /Dogger along the newly created fracture pathway. In the fifth phase, the pressure in the borehole is decreased to zero, however, the fracture propagate upwards and with it the pore pressure until they reach the borehole (Figure 24E and F). The propagation and coalescence of the fracture lead to a fracture network that connects the over-pressured PCT with the remaining formations. At this point the gas kicks in, visible in a higher pore pressure at the borehole. The outcome of these simulations supports the likelihood of a connection from the PCT to the well, as suggested by Xxxxxxxx et al. (2015). Furthermore, the simulations present a pathway for a fluid/pressure transport from the well to the reservoir and vice versa underlining the high probability of seismicity that was induced by the well control measures instead of a poroelastic stress transfer. <.. image(Ein Bild, das xxxxxxx enthält. Automatisch generierte Beschreibung) removed ..>
