From Molecular Weight and NMR Relaxation to Viscosity: An Innovative Approach for Heavy Oil Viscosity Estimation for Real-Time Applications

Vertical and lateral viscosity variations in heavy oil reservoirs must be accurately understood for proper reserve assessment, well placement, reservoir simulation and well-test interpretation. Well-logs must be used to determine viscosity trends in the absence of a-priori information or direct viscosity measurements. This paper introduces a novel approach to estimate in-situ heavy oil viscosity using triple-combo and NMR logs. Three components, each with a distinct characteristic NMR relaxation time range and a specific molecular weight, are used to define the heavy oil phase. An empirical transform, built from a database of live oil samples collected in heavy oil reservoirs, is used to compute macroscopic viscosity from the relative volume fractions of the constituents. Case histories presented demonstrate the utility of the methodology in complex carbonate environments. The results clearly show that proper characterization of a heavy oil reservoir requires viscosity information, and interpretations based solely on mobility may have limited success. The case histories also highlight that actual viscosity distributions in heavy oil reservoirs can be quite different from the simple models often used to describe them.