Abstract

Low-field nuclear magnetic resonance (NMR), whether implemented in a logging tool, bench top analyser or on-line sensor, cannot detect the complete response of heavy oil or bitumen. Both heavy oil and bitumen relax quickly so the spectra from these samples at room temperature appear mostly at relaxation times less than 10 ms. The contribution of heavy oil to the NMR spectrum is distinct in samples that are free of clays or contain bulk water and it is consequently possible to calculate oil and water content based on NMR spectra. Solids content is then determined by difference. However, samples that contain clays and/or relatively little water produce spectra that are more difficult to interpret because the relaxation times of clay bound water are in the same range as bitumen. Experimental results from mixtures containing a layer of illite, kaolinite or montmorillonite in sand exposed to mild brine shows clay bound water to have a characteristic response. These NMR "signatures" were used to develop predictive nomographs of clay content. A second set of experiments involved adding heavy oil to the same samples and the changes in NMR spectra after exposure to heavy oil were compared to the spectra obtained before oil was added. The differences identified in this work allowed for improvements in calculating water, oil and/or solids content and a preliminary predictive algorithm for clay content determination is presented. The purpose of this work is to allow one to more accurately separate the contributions of heavy oil and clay bound water despite the fact that these will overlap in an NMR spectrum from a sample. Improved characterization of oil sands and recovery from these formations are possible consequences of this work.

Introduction

Nuclear magnetic resonance (NMR) logging tools have been used in numerous applications within the petroleum industry for enhancing recovery. In addition to porosity and permeability determination, NMR has been used to characterize heavy oil and bitumen1,2, to determine the composition of oil/water emulsions3 and to predict heavy oil viscosity4,5 among other things. NMR logging tools obtain information regarding fluids in porous media by using magnetic fields to polarize the protons in the fluid and monitoring the time required for the protons to return to equilibrium. This time is commonly termed the transverse relaxation time (T2). Bulk fluids such as water have a T2 value of approximately two seconds, but the T2 values for heavy oils and bitumen are much faster (e.g., between 1 and 10 ms). The reason for this is that the protons in heavy oil and bitumen are restricted due to the viscous environment6. In fact, present NMR logging tools are incapable of detecting the complete spectrum from heavy oil and bitumen formations because of the high viscosities of these samples. As a result, attempts to characterize heavy oil and bitumen are problematic. Restriction of proton movement can occur also because the fluid has sorbed onto clays or organic matter in the sample7. Consequently, clay bound water has low T2 values (e.g., less than 10 ms) 7–9 compared to water residing in the larger pores of the samples, which has T2 values that may be in excess of 100 ms. The fact that the amplitude peaks for heavy oil and clay bound water appear at similar T2 values makes it difficult to differentiate between the oil and water signals from a sample that contains both fluids10. This makes the T2 cut-off method11, implemented to calculate volumes of free and bound fluid, impossible to use because the assumption that the signal below the T2 cut-off value is due to heavy oil alone is incorrect. Some of the signal below the T2 cut-off is due to water bound to clays and if this distinction is not made, estimates of reserves will be higher than the actual reserves. There have been attempts to separate oil and water signals in NMR spectra11,12 but these methods were not developed for samples containing heavy oil or bitumen with clay bound water.

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