Geomechanical response to N2 injection as a means of pressure maintenance in a sandstone reservoir

摘要

Pressure maintenance by injection of nitrogen into a reservoir is one of the potential options to mitigate induced seismicity during gas production. It is expected that seismicity and the seismic hazard can be reduced by decreasing the stress changes and compaction associated with the declining pore pressure in the reservoir rocks. However, injection-related induced seismicity has been observed in many settings. Hence, the question can be raised whether injection can have an adverse effect on the seismic hazard and can cause an unwanted increase in seismicity. We employed 2-dimensional and 3-dimensional Finite Element Modelling to investigate four mechanisms that can lead to induced seismicity during nitrogen injection in a depleting gas reservoir: (1) pressure diffusion into faults, (2) stress path irreversibility, (3) thermal stresses and (4) differential pressure evolution and stress arching. We show that, depending on the particular setting, mechanisms 1 to 3 can lead to fault destabilization. Using fault slip displacement and total fault slip length as indicative estimates for the induced seismicity potential of faults, it is concluded that these mechanisms can increase the seismicity potential of the faults during injection. Mechanism 4 is shown to be of negligible importance in a typical setting.