Formation resistivity response to loading and unloading confining pressure

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.