Abstract

High viscosity is a major concern in the recovery of heavy oil and bitumen. Viscosity reduction could be achieved by mixing bitumen with solvents. Cragoe(1) and Shu(2) have developed widely used methods for liquid mixture viscosity predictions. However, in these two models, the viscosities or densities of the heavy oil/ bitumen and solvents have to be known at some reference condition.

Low field nuclear magnetic resonance (NMR) relaxometry is an effective, non-destructive alternative for determining the petrophysical properties of oil reservoirs, and has also shown to successfully predict the viscosity of conventional oils, heavy oils and mixtures of oils with solvents. Specially, NMR could be a potential tool for in-situ viscosity measurements, which could be implemented on a logging tool allowing viscosity to be estimated without having to extract oil samples in the lab.

In this paper, a regression model of experimental data, Cragoe, Shu and NMR models are compared with experimental data, which were obtained from four heavy oil/bitumen samples mixed with six solvents in different ratios. NMR based predictions are found to be similar to those of the Shu(2) model and superior to the predictions of the Gragoe(1) model.

Introduction

Viscosity and density reduction could be achieved by mixing with a solvent. The information of viscosity of the heavy oil/bitumen-solvent mixture is vital for designing solvent flooding and as input to reservoir simulators both for recovery processes and reserves assessment. Several correlations have been proposed for estimating the viscosity of a mixture of liquids. Cragoe(1) and Shu(2) have developed two widely used methods for mixture viscosity predictions. In both of the models, viscosities of the heavy oil/ bitumen and solvents have to be known for prediction. Sometimes, it is hard to measure the viscosity accurately when it is too high or too low using conventional viscometers and it is not a convenient method for in-situ measurements.

Low field nuclear magnetic resonance (NMR) relaxometry is an effective, non-destructive alternative for determining the petrophysical properties of an oil reservoir. It was also shown to successfully predict the viscosity of conventional oils(3) and heavy oils(4). The greatest advantage of NMR is its potential to translate these density and viscosity measurements to in-situ measurements, which could be implemented on a logging tool allowing density and viscosity to be estimated without having to extract oil samples in the lab. The NMR viscosity model is especially significant for use in designing solvent injection process for heavy-oil recovery

Experimental Procedure

Four oils were used in the solvent experiments(5). They were from Peace River, Cold Lake, Edam and Atlee Buffalo, and have viscosities of 670,000 mPas, 130,000 mPas, 14,000 mPas and 6,000 mPas respectively, at 25 °. Kerosene, toluene, naphtha, heptane, hexane and pentane were added to the oils in several pre-defined mass fractions: 100% oil, 99%, 96%, 93%, 90%, 85%, 80%, 70%, 50%, 30% and 0% (100% solvent). The samples were slightly heated and mixed by stirring, and the resulting solvent-oil mixtures were cooled. NMR spectra were measured at °25 using an Ecotek FTB bench top relaxometer.

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