Gassmann's fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies

Abstract

Carbonates have become important targets for rock property research in recent years because they represent many of the major oil and gas reservoirs in the world. Some are undergoing enhanced oil recovery. Most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones, but applying relations developed for sandstones to carbonates is problematic, at best. We measure in the laboratory nine carbonate samples from the same reservoir at seismic (3–3000 Hz) and ultrasonic [Formula: see text] frequencies. Samples are measured dry (humidified) and saturated with liquid butane and brine. Our carbonate samples showed typical changes in moduli as a function of porosity and fluid saturation. However, we explore the applicability of Gassmann’s theory on limestone and dolomite rocks in the context of shear- and bulk-modulus dispersion and Gassmann’s theory assumptions. For our carbonate set at high differential pressures and seismic frequencies, the bulk modulus of rocks with high-aspect-ratio pores and dolomite mineralogy is predicted by Gassmann’s relation. We also explore in detail some of the assumptions of Gassmann’s relation, especially rock-frame sensitivity to fluid saturation. Our carbonate samples show rock shear-modulus change from dry to brine saturation conditions, and we investigate several rock-fluid mechanisms responsible for this change. To our knowledge, these are the first controlled laboratory experiments on carbonates in the seismic frequency range.