Dielectric Response Interpretation of Shale Rocks with Low Cation Exchange Capacity
The determination of water saturation is essential and crucial for field development decisions. This calculation is critical for unconventional reservoirs. The presence of organic matter, clay minerals, and complex shale pore systems create difficulties for log-based water saturation interpretation. Many practitioners use conventional methods to estimate water saturation in shale reservoirs, such as resistivity measurements and the use of Archie’s water saturation equation. One way to estimate water saturation is by evaluating the dielectric response of shale rocks. The presence of clay minerals in shale will influence the dielectric constant. However, it is possible to gain an improved understanding of the clay effect through the dielectric response. Based on a previous dielectric model proposed for shaly sands there are two mechanism that govern the dielectric response at frequencies from 1 MHz to 300 MHz: the polarization of the double layer and the Maxwell-Wagner effect. Both are a function of pore fluid salinity. This study documents the influence of clay content and salinity on the dielectric constant of high maturity shales. Dielectric properties for a set of four shale samples from various reservoirs were measured as a function of saturation (vacuum dried, ambient condition equilibrated, and brine saturated at frequencies between 1 MHz and 250 MHz). The dielectric measurements were validated by correlating the measured cation exchange capacity with the dielectric constant of ambient humidity equilibrated measurements differenced with vacuum dried measurements. This difference correlated with Boyles’ law measurements of grain volume measured during drying, which is attributed to loss of bound water. This strong correlation validates the low CEC values measured. The dielectric properties of the samples saturated with three different salinity brine were also measured. The samples were allowed to imbibe brine until an equilibrium saturation was reached. The salinity was then changed by equilibrating 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 in these samples. This is interpreted to be due to their low CEC. A response equation was developed which is dominated by the Maxwell-Wagner effect. This model allows the value of water saturation of these high thermal maturity reservoirs to be determined from dielectric measurements if the salinity is known.