Rock physics estimation of cement type and impact on the permeability for the Sn?hvit Field, the Barents Sea

摘要

Laboratory permeability data from a wet sandstone reservoir (the Tub?en formation, Hammerfest Basin, located in the Barents Sea) subject to CO_2 sequestration indicates an order of magnitude permeability variation at the same porosity. The velocity and density well data from the well, obtained prior to CO_2 injection, show that at the same porosity, the samples with higher permeability have higher elastic moduli (both compressional and shear) as compared to the samples with lower permeability. To understand and quantify this effect, we fit the elastic modulus versus porosity well data with the theoretical constant-cement model theoretical curves. This theoretical rock physics analysis shows that the lower-permeability, softer samples have less contact cement that their high-permeability, stiffer counterparts. One interpretation of this meaning is that in the lower-permeability samples, part of the pore space is filled with fines that do not contribute to the grain-to-grain cementation thus reducing the stiffness (as compared to the well-cemented samples). This means, in turn, that in the softer samples, the fines partly clog the pores thus reducing the permeability. This logic is supported by the geological character of the Tub?en formation where the tidal and marine influence acts to worsen the grain sorting compared to well-sorted distributary channel sediment.