Evaluation of Bitumen-Solvent Properties Using Low Field NMR

ABSTRACT

The VAPEX (vapour extraction) process is a new technique for the recovery of highly viscous heavy oil and bitumen. This process involves injection of vapourized hydrocarbon solvent into heavy oil and bitumen reservoirs and production of the resulting solvent-diluted oil that drains by gravity in a horizontal well. Research has shown that this process is highly efficient and that different solvents give different results. In this paper, six different solvents were added to several oils of different viscosities and densities.

The solvents were added in different ratios to each of the oils and NMR spectra were obtained. The mixture of solvent and heavy oil or bitumen produces a spectrum that is distinctly different than that of the solvent or oil alone. From the shape and amplitude of the NMR spectra, one can calculate the amount of solvent present. Furthermore, one can predict the viscosity of the mixture without any additional viscosity measurements.

As asphaltenes precipitate with the addition of solvent(s), one can correlate the amount of asphaltene reduction to changes in the NMR spectra. In this manner, NMR can possibly be used to show the asphaltene precipitation of different oils in the presence of solvent(s). By measuring the amount of asphaltene precipitation, NMR can also provide an indication of in situ upgrading of the oil that occurs with the addition of solvent(s). Using NMR as an analysis tool, the effect of the different solvents on viscosity reduction and asphaltene precipitation is quantified.

Introduction

The VAPEX process was proposed by Butler and Mokrys (1) as an alternative to Steam Assisted Gravity Drainage (SAGD) for thin reservoirs where heat lost in the formation would make the process uneconomic. In the VAPEX process, vapour solvents, instead of steam, are injected in the reservoir. The solvents dissolve into the bitumen and dramatically reduce its viscosity. The diluted bitumen can drain down to the producer by gravity. Since the paper outlining the VAPEX process was originally presented, many valuable experimental studies were published using different systems, including Hele-Shaw cells (2-4), pore network glass micromodels (5), Magnetic Resonance Imaging (MRI) (6), and PVT experiments (7). In this paper, low field Nuclear Magnetic Resonance (NMR) was used to measure the physical properties of heavy oil and bitumen samples with kerosene, hexane, naphtha, haptane, pentane, and toluene at several ratios at room temperature and pressure.

Low field nuclear resonance (NMR) has vast potential as a tool for measuring properties of a reservoir fluid (8). NMR measurements are simple and non-destructive, but capable of yielding an incredible wealth of information about the reservoir fluid under investigation in a particular sample (9-10). The mixture viscosity decreases dramatically as the ratio of the injected solvent to heavy oil or bitumen increases, and different solvents show different dilution capacity. The bulk relaxation time of a hydrocarbon fluid is inversely proportional to its viscosity (10). NMR spectra show changes in the response from the mixture after solvent has been added.

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