A new method for assessing Young's modulus and Poisson's ratio in tight interbedded clastic reservoirs without a shear wave time difference

Tight clastic reservoirs are an important aspect of hydrocarbon exploration and development worldwide. The Xu5 section of the Upper Triassic Xujiahe Formation in the western Sichuan Basin of central China contains tight terrestrial clastic reservoirs. The depth of these reservoirs is greater than 3000 m, while the thickness is generally greater than 500-580 m; the tight sandstone and shale reservoirs are frequently interbedded. Based on the proportion of sandstone and shale, the reservoirs can be divided into four types: sand-rich, interbedded Type I, interbedded Type II, and shale-rich. Horizontal wells and multistage fracturing are used due to the complexity and significant heterogeneity of the tight clastic reservoirs in the study area. The Young's modulus and Poisson's ratio of the reservoir rocks are the two most significant parameters that must be evaluated when implementing these key techniques. Based on mechanical and acoustic tests, a new assessment system for the Young's modulus and Poisson's ratio based on the Hoek-Brown criterion was proposed. This evaluation system does not require the shear wave time difference; thus, the increase in error caused by the unreliability of the shear wave time difference can be avoided when assessing strata with complex geological structures. This method can be directly applied in single wells using only conventional longitudinal wave time difference log data. After converting the wave velocity of the high experimental frequency into the wave velocity of the logging frequency using the acoustic dispersion technique, the analysis results were applicable to the log evaluations. The relationship between the Hoek-Brown criterion parameters and the mechanical and acoustic parameters of the rock were systematically analyzed. By distinguishing the lithologies, the equations for the Young's moduli of tight sandstones and shales were established based on the Hoek-Brown criterion. The Young's modulus can be expressed using only the longitudinal wave velocity (VP). A significant negative correlation exists between the Poisson's ratio and Young's modulus; therefore, the Poisson's ratio can be evaluated from the Young's modulus. The errors of the Young's modulus and Poisson's ratio predicted by the proposed assessment method are small, which meets engineering requirements and validates the accuracy of the method.