2D maps can be used to give an indication of fluid type and viscosity, to separate oil and water and locate it in the pore structure, and to give an indication of pore connectivity.
It is possible that different fluids, such as heavy oils and partially bound water, may exhibit similar T2 values, which makes it difficult to identify which fluid is which using a standard T2 distribution alone. If two fluids have similar T2 values, they are unlikely to have similar diffusion characteristics, so a measurement that collects both T2 and diffusion data should be able to separate the fluid types. The result is a two-dimensional data map illustrating the relative proportions of the different fluid types in the sample.
For unconventional samples diffusion measurements often do not work, so we use T1 -T2 2D maps, which is measuring NMR relaxation, T2, at different T1 values. The ratio of T1 to T2 changes based on the viscosity. This experiment also helps quantify the type of hydrogen (water, oil, clay or organic) present in the sample. Signals with T1/T2 ratios between 1 and 2 are likely due to low viscosity fluid, like water (bound or free). Higher ratios are from more solid like components such as bitumen. Using standard cut-offs, the map can be divided into bound water, mobile water, bitumen, and mobile oil. We can quantify each population and will give a comparison of fluid contributions to each plug’s overall porosity.
Subtraction of measurements taken in the fresh state and again after forced imbibition of brine will yield the free or mobile fluid. From the subtractions it is easier to see which pore sizes the mobile fluid is contained in.
We can also do these T1 – T2 maps at varying temperatures. The three plots below show results at room temperature, 65C and 110C. Based on what we know of NMR parameters, the lower population is believed to be water and the higher population bitumen. The T1/T2 ratio is unchanged for the lower ratio fluid as the temperature increases, which we would expect because water does not change its viscosity with temperature. The higher ratio fluid does change and moves down closer to the 1:1 line which is consistent with bitumen changing viscosity. This allows us to account for bitumen porosity that we were not able to see at room temperature as it was to solid-like. To read the full paper, click here.