A Review of 60 Years of NMR Wettability

Wettability is a key parameter in the development of an oilfield as it strongly affects oil saturations, capillary pressures, electrical properties, relative permeabilities and oil recovery. Despite attempts made to evaluate wettability downhole, the standard methods to quantify it are still laboratory based; the two most commonly used are Amott-Harvey (AH) and US Bureau of Mines (USBM). These techniques are expensive and very time-consuming, requiring a sample to be retrieved from the well and analyzed in the laboratory. In several cases, the results are obtained late and only after several decisions regarding the reservoirs had to be made, without this important piece of information. It is ubiquitously recognized that nuclear magnetic resonance (NMR) is very sensitive to the strength of the fluid-rock interactions, and therefore has been considered as a good candidate for wettability determination since the 1950s. The NMR signal, however, is also sensitive to several other fluid and rock properties, for example viscosity and pore-size distribution, making the practical extraction of wettability information from NMR data not straightforward. NMR has, however, two considerable advantages compared to AH and USBM: it is much faster, allowing much faster turnaround of laboratory measurements, and can be measured in-situ downhole, with the result of the measurement being available in real time. These extreme advantages fueled the research on the topic of NMR wettability despite the above-mentioned difficulties. There are at least three main NMR parameters measurable downhole: T-1, T-2 and diffusion; with additional information extractable from the correlation between these three. Wettability affects all of these parameters, and the correlation between them. This means that there is not a single way to extract wettability information from NMR data, but there are different options. Here, we review 60 years of literature on the topic of NMR and wettability, from the first experimental observations in the 1950s to the most recent advancements. Also, this work aims at presenting strengths and limitations of the techniques being developed nowadays, to help the audience make the best choice for each specific case. In this paper, we discuss both laboratory- and log-based applications, although we place greater emphasis on laboratory-based applications.