Reservoir Evaluation Completion Optimization in Heavy Oil Sands with Dielectric Diffusion Measurements

摘要

Major heavy oil accumulations are found in the tertiary sandstones of the Lagunillas Formation of the Costanero Bolivar Field, located in Lake Maracaibo, Venezuela. The hydrocarbons are found in poorly consolidated shaly-deltaic sands, at depths of around 1200 ft to 2350 ft. The oil ranges from 10 to 18 degrees API, with viscosities ranging from 400 cP to 10000 cP. The formation water salinity is below 5000 ppm and variable within the reservoir, after years of injection of fresh water and steam to increase recovery. Consequently there is today no correlation between water cut and resistivity and the differenciation between oil and water with conventional petrophysical techniques is inaccurate. Conventional log analysis has limited potential since the resistivity shows identical values in both oil and water bearing levels. Deciding on a completion strategy from an inaccurate saturation computation is a major challenge. Additionally, the free water presence reduces the net pay and rapidly increases the probability of water production in this high oil viscosity environment. Therefore an accurate assessment of free water and oil viscosity is a critical factor in the economics of the field. The present work incorporates dielectric and molecular diffusion measurements, showing significant progress in detecting free water from oil and defining the most prospective intervals. Movable oil and fresh water are clearly identified using dielectric polarization at multiple frequencies. The dielectric measurement provides the water-filled porosity, while the magnetic resonance identifies the irreducible versus free water within that volume. This allows predicting the likelihood of producing hydrocarbon or water in areas with high oil saturation. In conclusion, the integration of dielectric polarization and diffusion information at multiple depths into the reservoir enable to distinguish oil from free and bound water and to estimate the oil viscosity, a result impossible to obtain with conventional logs in these environments. This integrated methodology allows accurate reservoir characterization and definition of the production potential of these heavy oil sands, leading to improved completion decisions. The development campaign in Lagunillas sands now has a new workable technique to reduce uncertainties and to optimize heavy oil production.