A viable method to produce a deep extra heavy oil reservoir with an in-situ viscosity in excess of 1000 cp is viscosity reduction with dilution, and where steam flooding may be an unlikely option. The objective of this study is to determine viscosity reduction of an extra heavy oil sample as a function of pressure, temperature and composition, where composition refers to addition of different amounts of diluents. Results of this study and modeling of the mixture viscosity can be used to guide future operational decision making to develop large heavy oil resources, where steam flood is not economical.

An extensive experimental test matrix was carried out in this study. A high-pressure capillary tube viscometer was used for viscosity measurements, which is the preferred measurement method for extra heavy oil systems. Potential compatibility issues such as wax and asphaltene formation due to changes in composition and/or temperature were considered during the design and execution of experimental work. In excess of 90 viscosity data points were measured for mixtures of an extra heavy oil sample and two diluents (a diesel sample and a light crude oil sample). The system pressure varied up to 4000 psia, the temperature varied in the range of 80-200 °F, and amount of diluents changed up to 40% by volume. Data QC protocols were implemented and quality of measured viscosities were assessed. Some measurements were repeated to ensure high quality of generated viscosity data.

Three to four orders of magnitude of viscosity reduction were observed with changes in pressure, temperature and composition. On average and for studied fluid systems, every 50 °F change, or 20% increase by volume in diluent concentration, reduced the mixture viscosity by almost an order of magnitude. The impact was less significant at higher temperatures and concentration of diluents. Viscosity data were used to develop specific mixture viscosity correlations for the systems of interest, representing measured viscosities with an average absolute deviation (AAD) of less than 3%. Furthermore, a large number of viscosity mixing rules were evaluated to represent measured viscosity data in this study. Some mixing rules predicted the viscosity data with 15-20 % deviation and an average standard deviation of 14%. A new mixing rule (Modified Ratcliff & Khan) was developed with an AAD of 11% and a standard deviation of 5% (at least 50% improvement in the performance).

Currently, universal viscosity mixing rules are not available, particularly for extra heavy oil and diluent mixtures. The novelty of this study is to provide a high-quality database of heavy oil and diluent viscosity data, together with the development of a new general mixing rule applicable to the type of oils and diluents investigated in this study.

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