Applicability of Fluid Substitution in Carbonates

Summary Applicability of Gassmann fluid substitution in carbonate rocks is often questioned, partly because of fluid pressure compartmentalization in poorly connected pore spaces and finding proper choices of the mineral moduli. We address both issues using a generalized fluid substitution theory for micro-inhomogeneous rocks in which the Biot coefficient and the effective pressure coefficient for the bulk volume can be different. We apply this theory to a carbonate dataset spanning a large range of porosities and for which the mineralogy is approximately known. Fluid pressure compartmentalization is indicated by an elevated effective pressure coefficient for the bulk volume exceeding the Biot coefficient. Since the effective pressure coefficient is strictly bounded, it allows us to gauge and constrain the estimates for the effective mineral moduli.