Estimation of Groundwater Aquifer Formation-strength Parameters from Geophysical Well Logs: The Southwestern Coastal Area of Yun-Lin, Taiwan

Abstract

The purpose of this study is to estimate groundwater aquifer formation-strength parameters including shear modulus, bulk modulus, Poisson's ratio, and Young's modulus by using geophysical well logs. A new dispersed-shale index equation was developed by using the natural gamma-ray log and the compensated formation density log to solve a confusing problem of the compaction factor setting in the calculation of sonic porosity for an unconsolidated groundwater aquifer. A useful Poisson's ratio estimation method was employed to estimate groundwater aquifer formation-strength parameters when shear-wave transit time data is lacking in groundwater wells. Hydrogeologic parameters are characterized in estimating formation-strength parameters. Five wells in the southwestern coastal area of Yun-Lin, Taiwan, were logged, and four shallow aquifers were identified from log-derived hydrogeologic characteristics less than 200 m in depth. The formation-strength parameters for aquifers between 310 and 500 m in depth were calculated in two wells because complete formation density and compressional-wave transit time data were available. The results of the aquifer's formation-strength parameters demonstrate that both shear modulus, ranging from 0.15 to 0.42 * 10⁶ psi, and Young's modulus, ranging from 0.40 to 1.07 * 10⁶ psi, increase with depth, whereas bulk compressibility, ranging from 1.2 to 2.6 * 10⁻⁶ psi⁻¹, decreases with increasing depth.

Keywords:

• dispersed-shale index
• Poisson's ratio
• Young's modulus

Acknowledgments

Special thanks to Dr. C. Lewis for his discussion and invaluable comments on this study. Thanks also to the China Petroleum Corporation for providing well log data. This work was supported by grants NSC 86-2116-M-006-013 from the National Science Council of Taiwan and 85EC2A045206 from the Water Resources Agency of the Ministry of Economic Affairs of Taiwan.

Notes

^a Geophysical well logs abbreviations: GR: the natural gamma-ray log; RSFL: the spherically focused resistivity log, ohm-m; RILM: the medium dual induction resistivity log; RILD: the deep dual induction resistivity log; BHC: the compensated borehole sonic log; FDC: the compensated formation density log.

^b Hydrogeologic characteristics abbreviations: V _sh : shale volume, φ: porosity, Rt: the true formation resistivity, Rw: the formation water resistivity, K: hydraulic conductivity.

^c Formation strength parameters abbreviations: G: shear modulus, C _b : bulk compressibility, μ: Poisson's ratio, E: Young's modulus.

^a avg = average.

^a avg = average.