Formation resistivity response to loading and unloading confining pressure

Abstract

Electrical resistivity measurements of sandstone cores were conducted under both laboratory and overburden conditions. At the conclusion of laboratory condition measurements a cementation factor of 1.619 and a saturation exponent of 1.343 were determined. When the measurements were conducted under overburden conditions, cementation factor increased to 1.851 and the saturation exponent increased to 2.144. Lower cementation factors for well cemented samples at laboratory conditions than values observed for poorly consolidated samples denoted that the Archie cementation factor was not indicative of the degree of cementation for the samples studied.The effect of confining pressure was investigated by measuring resistivities of five fully brine-saturated sandstone samples, while the confining pressure was increased from 250 to 2300 psi in a stepwise manner (loading) and than decreased to 500 psi (unloading). The formation resistivity factor increased as confining pressure increased and decreased as confining pressure decreased, but it did not return to its initial value. In all cases the value of the formation factor at the end of the unloading cycle was higher. This is evidence of hysteresis in formation factor in the loading and unloading cycles.The mean values of the formation and cementation factors at overburden conditions were 45% and 13.9% higher than those obtained at laboratory conditions. Low porosity samples were affected to a greater degree in terms of changes in formation factor and cementation factor when confining pressure was increased. This implied that the laboratory condition electrical parameters would lead to underestimation in water saturation calculations. The error in water saturation calculation is more significant in oil zones as water saturation decreases. The error also increases as porosity decreases.