A Heavy Oil Mobility Quicklook Using Dielectric Measurements at Four Depths of Investigation

Abstract

Abstract Heavy oil sands around the world contain vast quantities of hydrocarbons. Evaluation of heavy oil sands is often complicated by low-salinity formation waters, reducing the contrast between oil- and water-bearing sands. Dielectric permittivity measurements are an excellent solution to this issue because they provide a water-filled porosity to compare with total porosity from density-neutron logs. In water sands, total and water-filled porosities read the same. But in oil zones, a difference is observed which is proportional to the oil saturation in the near-wellbore region. This yields an "oil crossover" indicator at the wellsite, allowing for easy identification of pay versus water sands, regardless of formation water salinity. New dielectric measurements are available that offer an enhancement to heavy oil evaluation. A new dielectric tool has been developed that scans the reservoir with various receiver spacings, frequencies, and polarizations to provide a vast array of measurements. A novel application of a subset of these measurements makes it possible to evaluate heavy oil mobility in a wellsite quicklook presentation. This quicklook uses dielectric porosity measurements at four depths of investigation (DOI) in the region from 1-in. to 4-in. into the formation. If oil has moved owing to invasion, the four curves will show separation indicative of the movement. In a recent case study, the new dielectric tool was run in combination with triple-combo measurements on a number of wells in California, USA, containing heavy oil sands with varying oil mobility. In the immobile oil sands, the expected oil crossover was observed, with the four dielectric curves overlaying each other. In the mobile oil sands, each of the four curves showed variations in the oil crossover. Validation of the oil mobility interpretation was provided by a nuclear magnetic resonance (NMR) log run in one of the wells. The NMR porosity showed the well-known porosity deficit, resulting from the short T2 response of heavy oil, in the immobile oil zones. The deficit diminished where the quicklook showed higher oil mobility, confirming the interpretation. The quicklook allowed well-to-well and sand-to-sand mobility comparisons in the field. Nonlinear gradations were observed within the study area. The dielectric mobility quicklook also provides more effective use of oil saturations obtained from sidewall core samples by identifying flushing seen in high-mobility zones, where core saturations can be affected. This novel wellsite quicklook method for identification of oil mobility allows operators to adapt the completion technique and hardware to their advantage and optimize exploitation of the reservoir.