Estimation of sonic velocity in shales in abnormally-pressured formations from resistivity data

• Published in: Journal of petroleum science & engineering Vol. 15; no. 2; pp. 375 - 377
• Main Authors: Kerimov, K.M., Chilingar, G.V., Katz, S.A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.08.1996; Elsevier Science
• Subjects: Applied sciences; Crude oil, natural gas and petroleum products; Energy; Exact sciences and technology; Fuels; Geophysical prospecting. Seismic methods and other methods. Exploration in boreholes. Well logging; Prospecting and exploration; Prospecting and production of crude oil, natural gas, oil shales and tar sands; Correlation; Acoustic logging; Resistivity logging; Wave velocity; Shale
• ISSN: 0920-4105; 1873-4715
• DOI: 10.1016/0920-4105(95)00072-0

The writers proposed a method for estimating (1) the sonic velocity υ a in abnormally-pressured shales using resistivity logs and, thus, (2) shale bulk density. Analysis of the well-log data for productive strata in Azerbaijan showed that there is a poor correlation between the sonic velocities and other well-log data, such as resistivity, SP, neutron and gamma-ray. Introducing normal trend of sonic velocity υ n and resistivity ϱ n allowed us to express sonic velocity (and, therefore, the bulk density of the shale) as a nonlinear function of resistivity with good correlation between the normalized velocity and normalized resistivity. The best-fit regression equation is of the following form: υ a υ n = 0.63 + 0.37( ϱ a ϱ n ) 3.26 (where ϱ a and ϱ n are the resisitvities of abnormally-pressured and normally-compacted shales, respectively). The coefficient of correlation between parameters \ ̃ gn[= υ a υ n ] and \ ̃ g9[= ( ϱ a ϱ n ) 3.26 ] is 0.87 and the mean-squared error for υ a is 190 m/s. The average relative error of velocity estimation is 6% — ranging from zero to a maximum of 14%. Thus, υ a can be estimated from the ϱ a ϱ n ratio and the υ n obtained from the normal compaction trend in the area studied.