Abstract

Microstructural studies using a dual beam FIB-SEM instrument reveal dimensions of pores within shales, which are consistent with macroscopic-averaged dimensions resolved by nuclear-magnetic resonance and mercury-injection capillary pressure. These dimensions are on the order of nanometers to hundreds of nanometers. We compare observations on a limited number of samples from the Haynesville to observations on the Woodford, Barnett, and Marcellus Shales. The FIB-SEM imaging uniquely resolves where the pores lie, that is, mainly within kerogen in the Woodford and Barnett and between clay platelets for the Haynesville samples. Measurement of velocities as a function of pressure and calculated anisotropies display a pressure dependence that reflects difference in the microstructure studied. The Woodford samples show a weak velocity dependence on pressure whereas the Haynesville samples show a very strong dependence. Coupled with the validity of the effective pressure law, the pressure dependence of anisotropy may prove useful in monitoring pressure depletion and compartmentalization in the Haynesville Shale.