Wettability is a critical reservoir and petrophysical evaluation parameter that is often ignored. Both disciplines often assume the formations are water-wet for simplicity and because wettability measurement on cores often carries a high degree of uncertainty. With the expansion of unconventional carbonate reservoirs development and interest in enhanced oil recovery (EOR), the importance of understanding wettability at the native state and its variability with various injection fluids is becoming critical. For practical purposes, a fast and accurate determination method, ideally at in-situ conditions, is desired.
It is widely recognized that nuclear magnetic resonance (NMR) is very sensitive to the strength of the fluid-rock interactions, and therefore, has been long considered as a good candidate for wettability determination. The NMR methodology was first applied in the laboratory using T2 relaxation measurements. For instance, sample wettability is inferred from a shift of the oil peak to shorter T2 values compared with the bulk T2 response of a live oil in the case of oil-wet system. The main practical limitation to the applicability of the T2 shift-based evaluation of wettability is the usually poor separation of oil and water peaks in the T2 spectrum. Furthermore, the bulk T2 of live oils must be measured and the core sample must be perfectly cleaned to quantify the NMR surface relaxation effect. Recently, a method based on two-dimensional mapping of NMR diffusion versus T2 was developed and validated with Amott-Harvey and USBM lab measurements. This method has two advantages. First, separation between the oil and water signals is greatly improved compared with the one-dimensional T2. Second, key properties such as tortuosity, represented by the electrical cementation factor m, and effective surface relaxivity can be inferred from the two-dimensional NMR maps using the restricted-diffusion model. The wettability index can then be estimated from the effective surface relaxivities.
The laboratory results on cores suggest that it is possible to obtain reservoir wettability using downhole NMR measurements. This requires high-resolution, high signal-to-noise ratio (SNR) data and improved processing techniques to separate oil and water signals. We examined the NMR restricted-diffusion wettability technique utilizing log data collected in an observation well completed with plastic casing. This well is used to monitor oil desaturation during different phases of an EOR pilot consisting of water, alkaline surfactant (ASP), and polymer floods. A downhole NMR tool that simultaneously records T1, T2 and diffusion at multiple depth of investigation (DOI) was used. This device allowed to periodically collect high-quality NMR data with SNR higher than 50. The targeted reservoir is a sandstone containing hydrocarbon with viscosity of 90 cP. The computed wettability consistently showed mildly oil-wet condition at the selected depth and over the analyzed time intervals.