Measurement methods and influencing factors of elastic parameters of shales

Shale oil and gas hold considerable exploration potential as significant unconventional hydrocarbon resources. The fracability of shales plays a vital role in the exploration and exploitation of shale oil and gas reservoirs and is typically measured using elastic parameters. In this study, we comprehensively investigate domestic and international advances in research on the elastic parameters of shales, along with associated issues and challenges. The results indicate numerous measurement methods for the elastic parameters of shales, including experimental methods (e.g., compression, ultrasonic measurement, nanoindentation, and acoustic logging) and theoretical calculation methods (e.g., digital core calculation, equivalent medium theory, and molecular dynamics simulation). Given the advantages, limitations, and application conditions of these methods, it is necessary to select scientific, accurate ones based on specific situations. Despite their relatively high accuracy, laboratory experimental methods are affected by sampling rates and experimental conditions. For instance, acoustic logging provides continuous, dynamic elastic parameters, capable of reflecting the mechanical properties of shales under instantaneous loading. However, these properties somewhat differ from those under long-term static loading in actual strata. Regarding theoretical calculation methods based on physical models, albeit with well-defined physical implications, these models require many input parameters and complex equations, which lead to reduced practicality. Additionally, these models typically neglect or make assumptions on non-primary factors excessively. For example, molecular dynamics simulation can simulate the elastic parameters of composite materials composed of multiple minerals while remaining simple and convenient to use. However, it still differs from actual geological models with complex and highly variable subsurface conditions, leading to discrepancies between simulation results and actual values. The elastic parameters of shales are primarily affected by factors including mineral composition, natural fractures, confining pressure, pore structure, diagenesis, and temperature. Additionally, they are influenced by organic matter characteristics, the properties and temperature of fluids within shales, sample size, bedding, and in-situ stress difference. Future studies should focus on the R&D of advanced technologies in terms of the quantitative relationships, multi-scale characteristics, and complex geologic environments of shales.