Knowledge about rock wettability is critical for optimizing oil recovery from primary to tertiary development stage. It impacts capillary pressure and relative permeability and hence remaining oil saturation. NMR remains the best option as logging tool to infer several petrophysical properties including potentially wettability, with limitation, as it can work as a cross-link between special core analysis (SCAL) and well logging. However, so far neither NMR nor other techniques are capable to quantitatively evaluate wettability conditions of mixed-wet reservoirs. Furthermore, such conditions require corrections even for standard petrophysical interpretation of NMR responses.
Conventional interpretation of NMR responses from partially saturated rocks relies on strong discrimination of wetting and non-wetting fluids due to their physical contact with the solid surface and hence very different effective surface relaxivities. Mixed wettability conditions complicate NMR interpretation due to diffusional exchange of a signal from an individual fluid phase which is partially in contact with solid phase of the rock and partially isolated by a non-wetting surface or fluid boundary. Several theoretical and experimental studies have been carried out to illustrate this behavior with the attempt to employ NMR as wettability indicator on a porous medium.
This work presents an advanced workflow for the interpretation of NMR responses of mixed-wet micro-porous carbonate rocks by employing micro-CT images and forward simulations of NMR responses, providing spatial distribution and strength of wetting properties at mixed-wet conditions. Simulated NMR relaxation responses at different partial saturations and wetting states are validated by corresponding experimental data. The obtained promising simulation results may help to pave the way for more confident NMR interpretation of a broad range of wettability conditions and for better reservoir characterization and development.
