Separating the Signals from Clay Bound Water and Heavy Oil in NMR Spectra of Unconsolidated Samples

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.

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