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.

Direct Correlation between Adsorption Energetics and Nuclear Spin Relaxation in a Liquid‐saturated Catalyst Material

“…Such measurements have been widely applied to the study of fluids confined to heterogeneous porous media, and provide information on the effects of pore structure and surface interactions on molecular dynamics. Representative fields of investigation include rock and shale wettability studies for the hydrocarbon recovery industry, and the characterisation of plaster and cement paste hydration kinetics …”

Contributed Review: Nuclear magnetic resonance core analysis at 0.3 T

“…JMitchell16@slb.com lower gyromagnetic ratios than 1 H are more readily accessed (e.g., 23 Na, 19 F, 31 P, 13 C, 2 H) for studies of molecular structure and chemical reaction monitoring. High-field NMR also offers the advantage of shorter radio frequency (rf) probe recovery times, allowing the detection of short relaxation time components in solids.…”

“…14 The slow recovery of the rf receiver chain after an excitation pulse prevents the detection of very short relaxation time components in shale samples. [15][16][17][18][19] Two-or three-dimensional (3D) imaging remains impractical at 2 MHz. Therefore, any low field core analysis may be complemented with measurements at a higher magnetic field strength to provide additional information on fluid distribution and transport in porous rocks.…”

Digital filters for low‐field NMR

“…There is significant interest in studying solid‐like materials with bench‐top NMR. Recent examples include cement and unconventional reservoir rocks . In such systems, we might reasonably expect to measure signals with .…”

“…Recent examples include cement 25 and unconventional reservoir rocks. 26 In such systems, we might reasonably expect to measure signals with T * 2 ≈ 10 μs. To capture such fast-relaxing components, even when using solid-state techniques such as the solid (or "quadrature") echo, 27 the filter response must be fast (ie, we should aim for T fill ∼ T * 2 ).…”