Comparative Analysis of Porosity Estimates in a Sandstone Reservoir: The Niger Delta as Case Study

Abstract

Sonic logs and density logs – derived porosities was estimated from two different reservoirs in a single well and comparatively analyzed in this study. The well log is taken from an oil field in the Niger Delta. The study is aimed at identifying a more statistically dependable method, relative to the methods described herewith, in estimating porosity values in the Niger Delta Basin. The well was logged for Transit times, bulk density of the formation and porosity from core analysis as a function of depth. Measures of normalized root mean square error (NRMSE), relative to porosities measured from core analysis at the reservoir intervals, and coefficient of variation were employed in determining which of the estimates were more reliable. For RESERVOIR I, the NRMSE for sonic logs-derived porosites and density logs – derived porosities were 17.96% and 18.68% respectively. For RESERVOIR II, the NRMSE for sonic logs-derived porosites and density logs – derived porosities were 17.35% and 19.29% respectively. Further analysis showed that the coefficient of variation for sonic logs-derived porosites and density logs – derived porosities in the first reservoir were 13.90% and 21.80% respectively, while they were 10.18% and 21.59%, respectively, in the second reservoir. Hence, the sonic log-derived porosities were considered to be a more reliable measure of porosity in the reservoirs of the Niger delta as it had a lower measure of NRMSE and coefficient of variation in the reservoirs of interest. The results obtained therefore, shows that sonic logging tool instead of density logging tool, gives a more reliable porosity value of a formation in the Niger Delta.

Keywords: Porosity, Density logs, Sonic Logs, Core Analysis, Normalized root mean square error, Coefficient of variation