Fluid substitution and elastic characterization in carbonate rocks of the Barreiras/Pirabas Formation: An integrated approach using XRD and Digital Image Analysis

Abstract

This study investigates fluid substitution in mixed carbonate and siliciclastic rocks, with a focus on the Pirabas/Barreiras Formation in northeastern Pará, located on the Equatorial Margin of Brazil. To evaluate the elastic response of these rocks under fluid saturation, we applied Gassmann and Biot models using an integrated data that includes ultrasonic measurements, X-ray diffraction (XRD), and Digital Image Analysis (DIA). Dry and saturated velocities, porosity, and mineral composition were used as input parameters, and tortuosity derived from image analysis was included in the Biot model. Our results show that the Gassmann and Biot models correlate positively with laboratory measurements, indicating their effectiveness in predicting elastic properties, especially P-wave velocity. Gassmann and Biot models were implemented independently using both empirical parameters and image- and mineralogy-based inputs. The results showed that the integration of XRD and digital image data improved the prediction of bulk modulus and P-wave velocity, reducing the absolute error from 4.47% to 0.32% and from 1.36% to 0.51%, respectively, in the case of the Gassmann model. Despite improvements, S-wave velocity remained more sensitive to pore structure and signal attenuation, which limited its predictive accuracy.