The effect of caprock permeability on shear stress path at the aquifer-caprock interface during fluid injection

The permeability of the cap rock is one of the most important properties for determining whether an injected fluid can be sequestered as desired. A literature review on many previously conducted investigations shows that the mechanical properties of a clayey cap rock, such as Young's moduli and Poisson's ratios, are relatively constrained to a small range of potential values in comparison to the permeability, which can vary greatly by many orders of magnitude. We found that the shear stress on the interface between the aquifer and cap rock results in very different temporal behaviors in response to different cap rock permeability. The largest shear stress changes concentrate on the near well region and remain stationary over a long period of time if the cap rock has very low permeability. The shear stresses increase greatly in the near well region at early times but the largest increases tend to migrate to the far side for semi-pervious or pervious cap rock conditions. Shear stress near the well may experience a reversion at the interface due to the delayed expansion of the cap rock if the cap rock is semi-pervious and more expansive than the aquifer formation. Because of the difficulty in measuring the stresses at depth, these stress changes may only be inferred from in-direct signals, such as induced seismicity. This poses the possibility that we may be able to inversely evaluate whether a cap rock formation is tight (very impermeable), or semi-pervious or even permeable by detecting the temporal and spatial occurrences of seismic swarms on the rock interface associated with fluid injection.