Understanding pore-pressure and saturation changes is important in mature reservoirs. Here we analyze shear-wave splitting in a 3D3C onshore survey from Shengli Oilfield, China, where the thin sand-reservoir has been undergone production through water-flooding which altered the fluid composition and the pore-fluid pressure. Dividing the data into orthogonal azimuthal sectors and processing each sector separately reveals significant shear-wave splitting. The amount of shear-wave splitting can be correlated with the degree of water saturation. Furthermore, the slow shear-wave component shows amplitude dimming in water-flooded areas, whereas the zone of original oil in place shows only weak shear-wave splitting. Rock physics modeling based on the evolution of microcracked rocks and anisotropic fluid substitution incorporating both saturation and pressure changes confirm the observations. The saturation changes have little effect on the P and the fast shear-wave as confirmed by core analysis in the laboratory. However, the substitution of water for oil changes the fluid viscosity that has a strong effect on the slow (quasi) shear-wave. Moreover, the fluid substitution due to water flooding also changes the pore-fluid pressure that modifies the crack aspect ratios, further enhancing shear-wave splitting. These observations reveal the potential of using shear-wave splitting for oil-water discrimination.


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