Low Field NMR Evidence of Geo-Polymeric Behavior of Organic Matter in Shale and the Implications on Recovery

Abstract

The mechanism through which light oil is stored in organic matter and how it moves has implications on unconventional reservoir production scenarios. The traditional view is that this oil is found in sub-micron sized pores observed the organic matter of numerous shale reservoir rocks where phase behavior and flow properties are defined by established physics of confined spaces. An alternative view is that the organic matter behaves more like a plastic or polymer that can absorb certain solvents. The combination of NMR T₁ and T₂ measurements illustrate that a portion of the light oil in a hydrocarbon bearing reservoir rock is dissolved in the solid organic matter. Differences in the relaxation mechanisms associated with T₁ and T₂ processes are the key in identifying the light oil that is captured in the organic matter. The restricted diffusion component of the T₂ mechanism for this captured oil generates a much faster T₂ relaxation component than observed in a T₁ measurement that lacks a diffusion mechanism. Distinct T₁ and T₂ measurements have greater resolution than the combined 2D T₁-T₂ maps that are often used in recent studies. NMR measurements on organic-rich shale and low organic content shale under dried and oil-saturated states illustrate the fast T₂ – slow T₁ component associated with light oil dissolved in the organic matter, which creates complex geo-polymer behavior. Thermogravimetric analysis of oil saturated shales also show that organic matter has geo-polymer characteristics. Comparisons made between TGA data of other polymers with sorbed solvents and organic matter with sorbed oil show similar trends. For organic rich samples, oil evaporates at higher temperatures (200-350°C) compared with the bulk boiling point (174°C).

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018