Negative Unjacketed Pore Modulus in Limestones? Critical Examination of a Peculiar Laboratory Observation

There is recent experimental evidence that the effective pressure coefficient for pore volume exceeds unity in limestones with dual porosity structure. Within the realm of linear poroelastcity, this means that the unjacketed pore modulus is negative. This would be a rather unusual rock behavior since it implies that the pore volume increases for a positive pressure increment during hydrostatic compression. Generally, any deviation of the unjacketed pore modulus from strictly positive mineral bulk modulus means that the deformation is inhomogeneous. We critically examine these observations by attempting to reconcile them with the poroelastic constitutive equations of micro-inhomogeneous rocks that encompass inhomogeneous deformations. Consistency checks among measured poroelastic constants reveal that the measurement accuracy of pore volume changes concurring with the deformation experiments is not as high as desired. However, assigning moderate variations to the measured pore volume changes either due to random or systematic measurement errors, we show that the experimental observations are indeed reconcilable with the constitutive poroelastic equations. Hence, the negative unjacketed pore modulus observed is deemed plausible though overestimated. Within the poroelasticity framework for micro-inhomogeneous rocks, a negative unjacketed pore modulus is the result of a partial localization of elastic strain energy in the fluid space. This in turn could be caused by dissolved calcite particles that block thin pore throats and thus generate temporary fluid pressure compartmentalization. A far-reaching consequence of a negative unjacketed pore modulus is that description of the undrained bulk modulus in rocks with dual porosity structure is more complicated than previously thought.

Automated summary

Recent experimental evidence shows that in limestones with dual porosity structure, the effective pressure coefficient for pore volume exceeds unity, resulting in a negative unjacketed pore modulus and inhomogeneous deformation. While there may be measurement inaccuracies in changes of pore volume, the experimental observations can be reconciled with the constitutive poroelastic equations. A negative unjacketed pore modulus is deemed plausible, indicating a partial localization of elastic strain energy in the fluid space within the poroelasticity framework for micro-inhomogeneous rocks.