Determining the transverse surface relaxivity of reservoir rocks: A critical review and perspective

Abstract There is a growing interest in characterizing the pore structure of reservoir rocks using the low field nuclear magnetic resonance technique. The transverse surface relaxivity is widely appreciated as the most significant parameter to connect the transverse relaxation (T2) time and the pore size by scientists. In this study, we reviewed long-established and recently developed methods to predict the surface relaxivity over the last few decades. Moreover, the advantages, shortcomings, as well as the applications of the methods were discussed. What's more, the potential causes of affecting the surface relaxivity such as mineralogical compositions, reservoir conditions, fluid properties, and magnetic field strengths were investigated to analyze the controlling factors of the surface relaxivity. Based on the review and analysis, we recommended a composited experiment to measure the surface relaxivity and proposed a possible workflow to obtain the variable surface relaxivity. The review section shows that the surface relaxivity is generally calculated based on the specific surface area. There is no universally accepted method since each of the experiments only probes a part of the pore system or resolves pore size at a different length scale, and it is better to characterize the full-scale pore space using multi-scale experiments. In addition, the constant surface relaxivity may not hold true for heterogeneous reservoir rocks with wide pore size distributions and the presence of paramagnetic minerals. It is a trend to use more than one surface relaxivity according to petrophysical facies or pore types. Multiple regressions may also serve as an effective way to get variable surface relaxivities. Further work needs to be done to develop a comprehensive understanding of the surface relaxivity, especially the influence of the spatial distribution of paramagnetic minerals which are extensively distributed in unconventional reservoirs such as shale and coal.