Seismic rock-physics model for carbonates at low effective stress levels

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Earth Sciences
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When considering the seismic response from the Dammam aquifer, the first required component is a simple model relating porosity and stress state to frame moduli assuming a carbonate matrix. We explored two such relationships, one based on the empirical regression presented by Domenico (1984) and a second using the critical porosity model of Nur et al. (1998) calibrated to the ultrasonic measurements of Nur and Simmons (1969). Once frame properties for a given porosity/pressure state were estimated, the effect of fluid changes were calculated using the low-frequency form of the Biot-Gassmann model, better known as Gassmann fluid substitution (Mavko et al., 1998)Our development of a model mapping porosity/pressure to frame properties is hampered by both the absence of site calibration data and the paucity of experimental measurements on carbonates at low effective stress values (≤ 5 MPa) available within the open literature. This experimental gap is problematic for shallow aquifers, which are well within this pressure regime. Additionally, the pressure/velocity relationships for most rocks often exhibit a high gradient at low stresses due to the presence of open micro-cracks. Investigation of a large set of carbonate pressure versus Vp measurements culled from publicly available datasets confirms hightly variable frame properties due to variations in porosity, porosity type (vuggy vs. micro-porosity), and grain mineralology.