Research On The Elastic Impedance Characteristics Of Lithology And Fluid Identification

Based on acoustic impedance (AI) inversion theory, Connolly [1999] proposed Elastic Impedance (EI) inversion theory, which breaked the conventional acoustic inversion theory based on the assumption of a P-wave incident the interfaces of rock at normal incidence. Connolly considered the situation of P-wave incident at different incidence and closely linked the acoustic impedance inversion theory with AVO effects. So elastic impedance provides a new inversion method and concept for better identification of the lithology and fluid. However,EI has the shortages in theory where that the constant Vp and Vs radio is assumed. Furthermore, the EI equation is derived from Aki's approximation on Zoeppritz equation. The approximate Zoeppritz equation does exist errors in the formula. In order to overcome these shortages of EI, several scholars proposed Reflection Impedance (RI) and Zoeppritz Elastic Impedance inversion theory.Based on rock physics theory, I made two-layer model in which fluid saturation, porosity, clay content are considered so that Ican analyze the relationships between rock physics parameters and elastic parameters, and the effect of rock physical parameters on EI,AI, RI and ZEI. Based on model analysis, I made a lithology and fluid identification factor on Elastic Impedance. By using the two-layer model and multi-layer model respectively, I compared the EI, RI and ZEI inversion method from precision, inversion stability and anti-noise capability. I also respectively use AI, EI, RI and ZEI to do inversion at a coal field in Shaanxi Province, and then use the identification factor from the model to identify the lithology of coal bed.My research shows that, (1)porosity is the most sensitive elastic parameters to the elastic impendance of reservoir, clay content is relatively small, saturation is the least in the second case typical sandstone reservoir; (2)In model data, the difference between AI, EI and ZEI is small in small incidence angle situation. In large incidence angle situation, the difference between RI and ZEI is also small but the difference between EI and ZRI is large. In the 30 degree inversion result of real data, the result of RI and ZEI is very close, but the result of El have large deviation with RI and ZEI's result; (3)The crossplot of 0.5*ln(large incidence impedance) with 0.5*ln(AI) in model analysis can discriminate lithology and fluid, so we can establish the lithology and fluid identification factor in this crossplot. The identification factor modified by the log data can well identify lithology of coal bed from real data;(4)Because the elastic impendance inversion method is affected by the noise, the better quality of seismic data will lead to better inversion results and get more accurate lithology and fluid identification; (5)Seismic data processing flow and parameters selection will directly influence the inversion results. It is necessary for us to process seismic data by ourself before doing inversion. Otherwise, the precision and reliability of final inversion result could not be guarteened.