Clarifying the Contribution of Clay Bound Water and Heavy Oil to 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 majority of these oils’ spectra are detected at relaxation times less than 10 ms at room temperature. In clay free sands the contribution of heavy oil to the NMR spectrum is distinct and, as a result, it is still possible to calculate oil and water content based on NMR spectra. However, in sands that contain clays, the relaxation times of clay bound water are in the same range as bitumen. Experimental results from mixtures containing illite, kaolinite, montmorillonite, sand and mild brine show that clay bound water has 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 mixtures containing clay, sand and brine. 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. This paper present preliminary predictive algorithm for clay content determination and this knowledge will 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 is a possible consequence 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 bitumen 1, 2, composition determination of oil/water emulsions 3 and determination of heavy oil viscosity 4, 5. 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 it takes the protons to return to equilibrium. This time is commonly termed the transverse relaxation time (T2). Protons in 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 environment 6. 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 sample 7. Consequently, clay bound water has low T2 values (e.g., less than 10 ms) compared to water residing in the larger pores of the samples, which has T2 values that are approximately 100 ms.

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