Addressing the Clay/Heavy Oil Interactions when Interpreting Low Field Nuclear Magnetic Resonance Logs

ABSTRACT

Low field nuclear magnetic resonance (NMR) in the form of logging tools, bench top analysers or on-line sensors has shown tremendous promise for the interpretation of logs and subsequent characterization of heavy oil and bitumen reservoirs as well as the evaluation of recovery efficiency. The key for this success is the ability to identify properly the NMR response of the heavy oil and bitumen components of the spectra. Contrary to earlier belief on this topic, NMR can detect at least part of the response of heavy oil and bitumen either down-hole or in the production line. The majority of the spectra for heavy oil and bitumen samples are detected at relaxation times less than 10 ms. In clay free sands this spectrum is quite clear and the response of the NMR interpretation algorithms is accurate. However, the spectra of sand containing clays show clay bound water to be in the same range as bitumen. Not properly accounting for the contribution of the clay bound water in NMR spectra results in the overestimation of oil or bitumen content of a given formation. This paper presents experimental results of heavy oil samples and also the spectra of clay bound water for common clays found in Alberta and Saskatchewan. After the spectra are compared independently, sand packs containing different amounts of clay and bitumen are prepared and their NMR spectra are obtained. Patterns of spectra overlapping are identified and a preliminary clay bound water prediction algorithm is presented. This algorithm allows for the separation of the oil contribution from the total NMR spectrum. Examples from different formations illustrate the process by which NMR can be used to determine the oil saturation in different Alberta formations.

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