Determination of Porosity and Hydrocarbon Saturation for Heavy Oil Reservoirs Using Dielectric and NMR Techniques

Oil and gas exploration requires precise evaluation of various parameters, among which porosity and saturation are the most crucial and fundamental properties. Conventional logging methods like neutron, density, and sonic logs used for porosity measurements experience various challenges in heavy oil reservoirs. Thus, this study aims to reduce these challenges and uncertainty in estimating porosity and saturations in heavy oil reservoirs by utilizing specialized techniques including nuclear magnetic resonance (NMR) and multifrequency dielectric dispersion approaches. The study uses carbonate and sandstone rocks having an average permeability of 200 mD and porosity of 15% and 18.5%, respectively. The samples were then saturated with 3wt.% KCl brine followed by NMR, gravimetric, and dielectric measurements for porosity determination. Next, the samples were saturated with heavy oil that has a viscosity of more than 4500 cP using high-pressure and high-temperature (HPHT) flooding experiments. The NMR and dielectric measurements were again taken at different oil saturations, followed by CT scan analysis to visualize the oil and brine distribution inside the samples. Finally, the porosity and hydrocarbon saturations were compared with all the determination techniques. The result shows a significant impact of heavy oil on rock porosity estimation and dielectric measurements. The NMR porosities at fully brine-saturated samples were 16% and 19.3% for carbonate and sandstone, respectively. After heavy oil injection up to irreducible water saturation (S_wirr), the total porosity was decreased to 12.5% and 15.3% for carbonate and sandstone samples, likely due to heavy oil components occupying part of the pore spaces. When the heavy oil saturation was increased by injecting more oil, the measured porosities were around 7.9% and 9.8%, respectively. The reduction in total porosity might be due to the entrapment of heavy oil components in the rock samples. This was also confirmed by NMR T₂ shift and CT scan analysis in which patches of dense components were seen after reducing the heavy oil saturation. Dielectric measurements followed a similar trend. For carbonate, the permittivity value decreased from 10.15 at S_wi (100%) to 6.19 at S_wirr (45%) and slightly increased to 6.30 at S_o of 53% (S_w is 47%). For sandstone, the permittivity reduced from 8.27 at S_wi (100%) to 3.76 at S_wirr (25%) and further decreased to 3.64 at S_o of 34% (Sw is 66%). The conductivity profiles showed a decreasing trend with more oil saturation. These findings highlight the challenges in porosity determination in the presence of heavy oil and propose a framework for achieving more accurate porosity estimation in heavy oil reservoirs. This study uniquely combines NMR and multifrequency dielectric techniques to quantify porosity and hydrocarbon saturation at different saturation stages in heavy-oil reservoirs. The resulting workflow improves rock-fluid characterization in both carbonate and sandstone formations, clarifying how heavy oil distributes within the pore system. These insights enable more precise steam-flood design and other EOR strategies, mitigating production challenges and improving recovery efficiency from heavy oil reservoirs.