Characterization of Irati shale from the Paraná Basin of Brazil through Laboratory Measurement and Rock Physics Modeling

Abstract The Paraná Basin in Brazil contains enormous reserves of unconventional shale oil and gas. However, the rock physical research on these shales remains relatively scarce, constraining the comprehensive understanding of intrinsic elastic properties. We investigated the elastic and anisotropic properties of shale from the Paraná Basin by combining laboratory measurements and rock physical modeling. We collected four shale samples from Irati formation and measured the P- (Vp) and S-wave (Vs) velocities in different directions under changing pressure conditions. The velocities were quantitatively compared and analyzed to reveal the influence of kerogen fraction, crack, and pressure. Afterwards, we performed rock physics modeling to investigate the elastic moduli theoretically. The results showed that the velocities increase with pressure, which could be up to 50% for pressure rising from 500 psi to 2500 psi, while Vp/Vs ratio shows 8% decrease. Under dry conditions, the Vp ranges from 1.5 to 3.75 km/s and Vs ranges from 0.8 to 2.0 km/s, while fluid saturation modifies these bounds to a Vp interval of 2.4–3.8 km/s and Vs interval of 1.2–1.8 km/s. Besides, the kerogen content and cracks have negative influence on velocities. The modeling results with pore aspect ratios of 0.006 and 0.008 exhibited well agreement with the measurements. Overall, this study provides a quantitative rock physical characterization of the Irati shales, highlighting the effects of pressure, kerogen content, and cracks on the elastic properties of the samples, which can facilitate better understanding of the elastic and anisotropic properties of Irati shales.