Dielectric Response Interpretation of Mature Source Rocks

Abstract

Estimating water saturation is a commonly proposed interpretation of results for the dielectric response of shale rocks. The presence of clay minerals often has a significant impact on the dielectric constant. Based on existing dielectric models for shaly sands there are two mechanisms that govern the dielectric response at frequencies between 1 MHz and 300 MHz, the polarization of the double layer and the Maxwell-Wagner effect. Both are a function of pore fluid salinity but have dramatically different salinity dependences. This study documents the influence of clay content and salinity on the dielectric constant of source rocks.The dielectric properties for a set of shale samples from various reservoirs were measured as a function of saturation (vacuum dried and partially brine saturated) and as a function of salinity at frequencies between 1 MHz and 250 MHz. The samples were measured dry and then allowed to imbibe brine until an equilibrium saturation was reached. The salinity was then changed by submerging the samples in brines of increasing salinity. The equilibration times (weeks) were based on a stabilization of the measured dielectric constant. The results indicate that the Maxwell-Wagner effect dominates the clay response. This is consistent with their low measured CEC. The extremely low CEC measured for these samples is confirmed by the frequency scaling with salinity consistent with the Maxwell-Wagner effect. A response equation was developed for the Maxwell-Wagner dominated clay effects. This allows the value of water saturation to be determined from dielectric measurements if the salinity is known. This simple salinity dependence is a departure from the generally accepted model for the dielectric response of source rocks.

Keywords: Maxwell-WagnerCECShaleDielectricWater saturationClay