TitleNS15A-0246 - Estimation of Porosity Depth Profile Using a Combination of Electrical Log and Core Data
Session Start2022-12-12 14:45:00 CST
Session End2022-12-12 18:15:00 CST
LocationMcCormick Place, Poster Hall, Hall - A
AuthorMotohiro
Presentation TypeIn Person Poster
AbstractArchie's equation is widely used to estimate porosity from electrical log data. On the other hand, Archie's equation is not valid for rocks containing conductive clay minerals, and the estimation of porosity using the equation may significantly overestimate porosity for such rocks. In this study, we estimated porosity along a scientific drilling located in a volcanic region by combining electrical log and core data considering the effect of excess conductivity. The target borehole was drilled through the Futagawa fault, the source fault of the 2016 Kumamoto earthquake mainshock. Electrical log was conducted in this borehole from a depth range of 302-660 m, excluding a section of 383-399 m where log data is not available. The rocks in this borehole consist of sedimentary and altered volcanic rocks, which are thought to contain conductive clay minerals, and are classified into eight units based on their lithology. To account excess conductivity effects, the apparent formation factor, Fa was converted to the corrected formation factor, Fc. The conversion using the Waxman and Smits (1968) model requires a depth profile of BQv, which represents the excess conductivity term. Since this BQv cannot be obtained from log data, we conducted resistivity measurements of core samples saturated by solutions with different salinities and estimated their BQv based on the relationship between rock electrical conductivity and pore fluid conductivity. One representative core sample was selected in each lithological unit for the multiple conductivity-salinity tests for calculating the BQv, and we assumed that the BQv of the representative sample is applicable throughout each lithological unit. Consequently, the BQv depth profile enabled conversion from Fa to Fc at each depth. Then utilizing Archie's equation showing Fc-porosity relation, porosities were estimated at all depths where electrical log was conducted. The depth profile of estimated porosity generally agreed with the core porosity measured in previous study in this borehole. Estimated porosity from Archie's equation using Fa was not very consistent with the core porosity, our results demonstrate the importance to consider excess conductivity for properly evaluating the formation's porosity. .