A New Approach for Evaluating Capillary Pressures Using Multifrequency Dielectric Technique

Capillary pressure is a fundamental petrophysical property that governs fluid distribution and movement within the porous media. It plays a critical role in processes such as fluid imbibition, drainage, and displacement efficiency, particularly in reservoir evaluation and development. Traditional methods for measuring capillary pressure are often destructive, time-consuming, or limited in resolution, especially for unconventional formations. This study presents a novel and non-destructive approach for estimating capillary pressure in unconventional formations using a multifrequency dielectric technique. In this study, various rock samples were used, including sandstone, carbonate, and shale, with different total organic carbon (TOC) and petrophysical properties were investigated. The samples exhibited porosities between 3% and 18%, permeabilities ranging from 350 nanoDarcy to 403 mD, and TOC values between 3.4 and 15.6 wt.%. A new experimental setup was developed to monitor the dielectric response as a function of capillary pressure. The permittivity and conductivity profiles were obtained across a frequency range of 4 to 3000 MHz in order to capture different polarization mechanisms. Cleaned and oven-dried samples were used to serve as baseline measurements for the dielectric profiles. Dielectric measurements were performed using an open-ended coaxial probe, while NMR T₂ relaxation profiles were acquired using a GeoSpec spectrometer (Oxford Instruments) operating at 2.2 MHz. Experiments were conducted at ambient temperature under various pressures to allow fluid invasion into the tight and permeable samples. The results demonstrated a clear increase in the dielectric response with capillary pressure. Both relative permittivity and dielectric conductivity increased as a function of pressure. Continuous monitoring of the dielectric properties over time enabled correlating the capillary pressure with fluid saturation derived from dielectric measurements. Higher pressures facilitated the determination of maximum water saturation in the treated samples. The observed conductivity profiles were consistent with trends in dielectric permittivity, further validating the analysis. Additionally, dielectric dispersion, which is defined as the decline in dielectric constant between 25 MHz and 1 GHz, revealed valuable insights into the tortuosity, permeability, and TOC content. Samples with higher TOC exhibited greater dielectric dispersion, likely due to the increased surface area associated with organic matter. The dispersion was reduced with increasing capillary pressure, suggesting greater fluid invasion into tight pores. Moreover, NMR T₂ relaxation profiles supported the dielectric data, offering a means to interpret fluid distribution and estimate water saturation across different pore systems, including micro- and macro-pores. Overall, this study presents, for the first time, the use of multi-frequency dielectric measurements to assess capillary pressure for conventional and unconventional formations. Rock samples with varying permeability, porosity, and TOC content were analyzed. The integration of dielectric measurements, NMR analysis, and capillary pressure provided a comprehensive assessment of fluid behavior in tight formations. The findings highlight the significant potential of this approach to enhance the characterization and development of unconventional hydrocarbon reservoirs.