Biot Theory Yields a Specific Storage Coefficient With Natural Deformation of Rock

The traditional specific storage coefficient (S_s) was defined under two assumptions. One is that aquifer rock deforms only in the vertical direction, and the other is that the average rock stress remains unchanged. Consequently, S_s is irrelevant to the shear modulus of rock (G). In this paper, the Biot theory is used to derive a new specific storage coefficient ($$) with the natural deformation of rock. $$ appears to be relevant to G. Compressed glass beads and Berea sandstone are used for illustration. At frequencies lower than 10 kHz, the equation of groundwater flow with $$ yields the same phase velocity and quality factor as the Biot theory, and therefore, it is capable of accurately predicting fluid pressure diffusion in the low-frequency regime. The results also show that S_s is 16%–17% higher than $$. In conclusion, the latter one is superior to the former in its consistency with the Biot theory and unconstraint by the aforementioned two assumptions.