Over-pressured drilling causes mud-filtrate to replace formation fluids close to the borehole. The invading fluid have a different bulk modulus than the formation fluids and therefore changes the elastic moduli of the formation. This causes the velocities measured by sonic logs to differ from the velocity in virgin formation which is measured by seismic surverys.
In order to tie seismic data to sonic log velocities the sonic log have to be corrected for filtrate invasion. As the invading fluid does not completely displace the formation fluids the invaded zone is saturated with two immiscible fluid phases.
Currently sonic logs are corrected using Gassmann fluid subsitution and an effective average fluid modulus. This approach assumes the entire porosity to have the same distribution of fluids. Laboratory data suggests that depdendent on the wettabillity of the formation the invading filtrate will be concentrated in specific areas of the porosity. This affects the elastic moduli in a way not accounted for by fluid substitution using a single fluid modulus.
In this project we prepare sandstone samples with two fluid phases and measure the elastic moduli. We then tie these moduli to the fluid distribution in order to relate this distribution and the effects on the moduli to lithology characteristics such as porosity, permeability, or clay content.
Supervisor: Prof. Ida Lykke Fabricius, ilfa@byg.dtu.dk