Rock Physics Diagnostics, Attenuation Measurement and Analyses in Wells

Attenuation is a rock anelastic attribute that is sensitive to the presence of fluids, degree of saturation, porosity, fault/fracture, and pressure. Attenuation measured in recorded well logs can provide useful guide for hydrocarbon exploration and field development. The work presented in this paper shows how the sensitivity of P- and S-wave attenuation and the elastic modulus (shear, bulk and compressional) can be used to detect the presence and type of fluids in rocks penetrated by wells. P-wave and S-wave inverse quality factors () are estimated from the log data recorded in well A-10 of the Gullfaks Field, North Sea, Norway. The analysis indicates that the P-wave inverse quality factor is generally higher in hydrocarbon saturated rocks than in brine (water) saturated rocks, while the S-wave inverse quality factor does not show a dependence on fluid content. The range of values measured in gas, water and oil saturated sands are 0.56 – 0.78, 0.39 – 0.55, and 0.35 – 0.41, respectively. A cross plot of the ratio of P-wave and S-wave inverse quality factors () with the ratio of P-wave and S-wave velocities () clearly distinguishes gas sand from water sand, and water sand from oil sand. Gas sand is characterised by the highest and the lowest ratios; oil sand is characterised by the lowest and the highest; and water sand is characterized by the and values that plotted between those of the gas and oil sands. The signatures of the bulk modulus, Lame’s first parameter, and compressional modulus show sensitivity to both pore fluid and rock mineral matrix. These moduli provided preliminary identification for sand intervals saturated with different pore fluids.