The Impact of Rock Microstructure and Petrophysical Properties on the Velocity-Pressure Relationship of Carbonates

This study investigates the impact of petrophysical rock properties on the velocity-pressure relationship in carbonates. It presents an approach to predict the changes in compressional velocity (Vp) as function of pressure in carbonates. The approach honors the complexity of carbonates by incorporating various petrophysical rock properties including bulk density, porosity, mineralogy and pore stiffness. The data used in this study consists of rock properties (density, porosity, mineralogy) and elastic velocity measured as function of confining pressure for 220 carbonate core plug samples from published literature. Pearson correlation coefficient was calculated to evaluate the significance of each property in predicting velocity-pressure relationship. A simple regression was formulated incorporating all significant input rock properties to predict Vp as function of pressure based on initial measured velocity at a given pressure. The predictions were compared with the measured Vp. The results show that the sensitivity of Vp to changes in pressure increases as the porosity and pore compressiblity increases. On the other hand, samples with higher bulk density and Vp / Vs ratio (at initial lowest pressure) show little Vp variations as function of increasing pressure. High Vp / Vs values are observed in samples that are well cemented and have less clay or silisiclastic fraction. Such characteristics reduce the compressibility of pores leading to non-variable velocity-pressure relationship. Incorporating the rock properties in regression analysis could successfully predict Vp as function of pressure with a correlation coefficient of 0.99 and average absolute error of less than 3%. Since all input parameters (rock properties) can be estimated from well logs, the presented approach can potentially be used to predict in-situ changes in Vp due to pressure changes. This can assist the interpretation of time lapse seismic, and in geomechanics-related applications.