Sensitivity parameters of tight sand gas: A case study of Lower Cretaceous Yingcheng Formation of Yingtai gas field in Songliao Basin, NE China

Abstract

In tight sandstone, the gas-bearing sensitivity parameters are studied to improved prediction accuracy of thin gas layer due to the small impedance differences between gas-bearing layers and surrounding rocks. In this paper, we propose a new combined elastic parameter, i.e., the ratio of the first Lame coefficient to S-wave velocity based on elastic parameters sensitivity analysis for tight sandstone gas. By considering different geological conditions, we introduce the extending attribute (the ratio of Russell fluid phase to S-wave velocity), which can reduce to the ratio of the first Lame coefficient to S-wave velocity in specific condition. Both Gassmann equation and Brie empirical relationship are applied to calculate elastic parameters of different gas saturation in fluid replacement process. The results verify the validity of the new combined elastic parameter, which is more sensitive to gas saturation than conventional parameters, such as the product of the first Lame coefficient and density and the ratio of P-wave to S-wave velocity. The pre-stack inversion is applied in the second member of Lower Cretaceous Yingcheng Formation in Yingtai gas field. Compared to the section of the product for the first Lame coefficient and density, the results show the new combined elastic parameter presented improves the accuracy of identifying gas-bearing layers, well conforms to the logging interpretation, and greatly enhances the identification ability and prediction accuracy of gas-bearing layers. Key words: Tight sand gas, Gas-bearing, Elastic parameters, Sensitivity parameters, Pre-stack inversion