Fluid Substitution in Laminated Sand-Shale Sequences: An Innovative Approach to Gassmann's Equation

This paper introduces an innovative approach to fluid substitution in laminated sand-shale reservoirs using Gassmann's equation. Erroneous results are often observed from this modelling technique; these are due to the high shale content of these reservoirs violating the assumption of a completely connected pore space with movable fluids. The innovative approach introduced in this paper aims to better constrain the results of Gassmann's equation through addressing this assumption. This is achieved through the incorporation of NMR porosity data; this identifies the movable fluid component of the reservoir porosity, which is used in the computation of "dry-frame" bulk moduli from a re-arranged form of Gassmann's equation. This innovative approach is compared to an accepted approach developed for these reservoirs, which uses effective porosity. This accepted approach violates the assumption of a connected pore space with movable fluids as effective porosity includes potentially significant amounts of immobile capillary bound water. Fluid-substitution applied using the dry frame bulk moduli estimates from both approaches indicates the advantage of the innovative approach, which derives more reasonable bulk moduli estimates when the movable fluid is substituted from in-situ brine to gas.