Effects of clay on resistivity index thin bed: case from Western Niger Delta.

Abstract

Certain clays produce micro-porosities that retain a relatively thick water layer and enhance the path for conductivity of electric currents through capillary forces. Saturation exponent (n) in clay-rich fields is less than the value of 2, usually assumed in well-logging. Accurately determined clay-corrected saturation exponent (“n”) and resistivity index are needed for better reserve estimation.


A clay inclusion study of saturation-resistivity relationships from thin and laminated beds is carried out in the laboratory. Saturation exponents (n) are measured on a number of core samples from several wells in a Niger Delta field Cation exchange capacity (CEC) technique is used to detect clay in the rocks CEC values are in the ranges of 0.0278 and 0.0078 for the interval 5456 - 5574 ft. and 0.0025 – 0.0166 for 8600 to 8695 ft. A correlation of the resistivity index and saturation indicates deviations, from ‘normal', caused by the effect of clay minerals such as kaolinite, montmorillonite and smectite. Clay presence produces a curve towards low saturation point. The average field “n” is 1.83 and acceptable CEC tolerance in heterogeneous formations is within a depth of 0.2 ft. The larger the CEC, the greater the suppression of downhole resistivity and active clays such as montmorillonites have greater influence on log response than kaolinite.

Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 121-126



KEY WORDS:

Resistivity, saturation, clay.