Rock physics and geomechanical application in the interpretation of rock property trends for overpressure detection
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ORIGINAL PAPER-EXPLORATION GEOLOGY
Rock physics and geomechanical application in the interpretation of rock property trends for overpressure detection Dorcas S. Eyinla1,2 · Michael A. Oladunjoye1 · Abel I. Olayinka1 · Boris B. Bate1 Received: 29 June 2020 / Accepted: 5 November 2020 © The Author(s) 2020
Abstract One of the complexities of geomechanical study is in the classification of rock’s properties and overpressured intervals—a knowledge which is not only essential for well safety and cost-effective drilling, but crucial in evaluating exploration risk factors and ensuring a successful hydraulic fracturing program. In this study, a more robust prediction of reservoir pressure regime is presented, where the geomechanical distributions of the rock give a distinct correlation. Three wells from the Niger Delta Basin were studied using empirical equations to estimate the elastic properties, wave velocities and the rock physics parameters for each well. From the results obtained, the velocities of compressional wave (Vp) and shear wave (Vs) decrease as porosity increases. Also, a linear correlation exists between Poisson’s ratio and Vp/Vs, where both variables showed distinct behavior and similar trend serving as useful tools for lithology identification. Another significant observation is the acoustic impedance of the materials which decreases with increasing porosity. Meanwhile, the depth plot of the impedance showed divergence and scattering away from the supposed linear trend. While inhomogeneity of the rock materials and disequilibrium compaction of sediments may account for this scattering, the variation of geomechanical distributions in this study revealed that pore pressure has a first order effect on the elastic strength of formations, also, under normal pore pressure conditions, acoustic impedance increases linearly with depth. Keywords Lithology identification · Rock physics · Acoustic impedance · Linear divergence · Overpressure Abbreviations 𝜌b Density (g/cm3) Δts Shear interval transit time (µs/ft) S Shear modulus (psi) P Poisson’s ratio B Bulk modulus (psi) Y Young’s modulus (psi) 𝜙D Porosity estimated from Density log 𝜙AC Porosity estimated from Acoustic log PP Predicted pore pressure (psi/ft) IP Compressional wave acoustic impedance g cm3 (ft/s)
* Michael A. Oladunjoye [email protected] 1
Petroleum Geosciences Unit, Department of Geology, Pan African University Life and Earth Sciences Institute, University of Ibadan, Ibadan, Oyo State, Nigeria
Department of Earth Sciences, Adekunle Ajasin University, Akungba‑Akoko, Nigeria
2
Introduction Drilling through overpressure formations poses numerous challenges and risks to operations if the properties of the material are not accurately predicted prior to drilling (Zhang 2019). Adequate knowledge of the geomechanical properties of hydrocarbon reservoirs play an important role in designing recovery well, enhance recovery, and in safe drilling (Abijah and Tse 2016; Saikia et al. 2018). From previous studies (e.g. Streit and Hillis
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