Considerations for the acquisition and inversion of NMR T₂ data in shales

Low-field Nuclear Magnetic Resonance (NMR) is a non-invasive method widely used in the petroleum industry for the evaluation of reservoirs. Pore structure and fluid properties can be evaluated from transverse relaxation (T₂) distributions, obtained by inverting the raw NMR signal measured at subsurface conditions or in the laboratory. This paper aims to cast some light into the best practices for the T₂ data acquisition and inversion in shales, with a focus on the suitability of different inversion methods. For this purpose, the sensitivity to various signal acquisition parameters was evaluated from T₂ experiments using a real shale core plug. Then, four of the most common inversion methods were tested on synthetic T₂ decays, simulating components often associated with shales, and their performance was evaluated. These inversion algorithms were finally applied to real T₂ data from laboratory NMR measurements in brine-saturated shale samples. Methods using a unique regularization parameter were found to produce solutions with a good balance between the level of misfit and bias, but could not resolve adjacent fast T₂ components. In contrast, methods applying variable regularization – based on the noise level of the data – returned T₂ distributions with better accuracy at short times, in exchange of larger bias in the overall solution. When it comes to reproducing individual T₂ components characteristic of shales, the Butler-Reeds-Dawson (BRD) algorithm was found to have the best performance. In addition, our findings suggest that threshold T₂ cut-offs may be derived analytically, upon visual inspection of the T₂ distributions obtained by two different NMR inversion methods.