Compaction-Induced Permeability Loss’s Effect on Induced Seismicity During Reservoir Depletion

Abstract

Producing fluid out of subsurface reservoirs causes stress changes that can lead to induced seismicity. In the fully coupled theory of poroelasticity, these stress changes are caused by the gradient of pore pressure which acts as an internal force in the momentum balance equation. This implies that, if a larger pore pressure gradient is required to produce the fluid, larger stresses will be induced, likely leading to a higher seismicity rate. Based on Darcy’s Law, the gradient of pore pressure required to achieve a certain fluid production rate is inversely proportional to the permeability. The lower the permeability, the larger the pore pressure gradient required. Consequently, the permeability reductions that are seen due to compaction during fluid production may be significant in terms of induced seismicity because this will increase the pore pressure gradient required to produce a certain amount of fluid. Therefore, using a poroelastic flow model and an existing seismicity model, it is shown that compaction-induced permeability loss has the effect of increasing the stress changes associated with the production of a certain amount of fluid and thereby indirectly increases the seismicity rate, even when the compaction remains in the near-elastic range. In the event that compaction begins occurring inelastically, the permeability decreases and seismicity rate increase are much more drastic. Finally, it is shown that the optimal orientation for a horizontal well, if inelastic compaction near the wellbore is to be avoided, is parallel to $$S_{Hmax}$$ . These results have implications for reservoir management in fluid producing reservoirs.