Relaxation mechanisms and shales

Abstract

Unconventional petroleum resources present in shales have recently seen increased development due to growing demand for energy worldwide, declining conventional petroleum discoveries, and improved technology for production. Shale reservoirs differ from more conventional reservoirs in both matrix composition and structure, tending to be low in porosity and ultralow in permeability, making traditional laboratory characterization methods difficult to apply. The noninvasiveness of nuclear magnetic resonance (NMR) makes it a favored technique for shale analysis. However, interpretation of NMR relaxometry results for shales is more complex than in traditional reservoirs. Surface relaxivity in conventional reservoirs is predominantly caused by the interaction of fluid molecules with paramagnetic impurities on pore surfaces. However, in shales, small pore sizes coupled with the presence of hydrogen‐rich organic matter can result in more interactions. This article discusses different relaxation mechanisms that may be present in shales and situations where they are relevant. Surface relaxation in organic matter is likely caused by homonuclear dipolar coupling, not paramagnetic impurities, and as such, will have a significant temperature dependence. Due to the small pore sizes in shales, the effect of internal gradients on the signal appears negligible in almost all situations. Analysis of transverse relaxation resulting from organic solids by an inverse Laplace transform may lead to results that are anomalously short and overcall signal intensity. Diffusional coupling between the shale pores confounds interpretation of the NMR response as a pore size distribution. There also appears to be rapid exchange of magnetization between mobile and immobile phases in the samples. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 57–78, 2014.