Seismic wave attenuation in carbonates

TLDR

Pore fluids significantly influence seismic wave attenuation in carbonate rocks, affecting interpretation of remote sensing data for reservoirs targeted by enhanced oil recovery. Laboratory measurements of elastic moduli and attenuation on five carbonate samples (20–30 % porosity, 0.03–58.1 mdarcy permeability) were modeled with Cole‑Cole relations, confirming bulk losses exceed shear losses. Bulk compressibility losses dominate over shear losses for dry and fully saturated samples, a pattern observed in four of five samples across seismic and ultrasonic frequencies and reservoir pressures; attenuation rises 250 % when brine replaces a light hydrocarbon, and while some samples show frequency dependence, attenuation remains essentially constant between 10–100 Hz.

Abstract

The effect of pore fluids on seismic wave attenuation in carbonate rocks is important for interpreting remote sensing observations of carbonate reservoirs undergoing enhanced oil recovery. Here we measure the elastic moduli and attenuation in the laboratory for five carbonate samples with 20% to 30% porosity and permeability between 0.03 and 58.1 mdarcy. Contrary to most observations in sandstones, bulk compressibility losses dominate over shear wave losses for dry samples and samples fully saturated with either liquid butane or brine. This observation holds for four out of five samples at seismic (10–1000 Hz) and ultrasonic frequencies (0.8 MHz) and reservoir pressures. Attenuation modeled from the modulus data using Cole‐Cole relations agrees in that the bulk losses are greater than the shear losses. On average, attenuation increases by 250% when brine substitutes a light hydrocarbon in these carbonate rocks. For some of our samples, attenuation is frequency‐dependent, but in the typical exploration frequency range (10–100 Hz), attenuation is practically constant for the measured samples.