Minh, Chanh Cao (Schlumberger) | Crary, Steve (Schlumberger) | Singer, Philip M. (Schlumberger) | Valori, Andrea (Schlumberger) | Bachman, Nate (Schlumberger) | Hursan, Gabor (Saudi Aramco) | Ma, Shouxiang (Saudi Aramco) | Belowi, Ali (Saudi Aramco) | Kraishan, Ghazi (Saudi Aramco)
Reservoir wettability is a critical parameter affecting hydrocarbon distribution within and recovery from reservoir rocks. The sensitivity of nuclear magnetic resonance (NMR) responses to rock wettability has been demonstrated in a number of publications. These publications suggest that wettability can be determined in the laboratory from NMR T2 relaxation measurements, obtained in cores after proper cleaning, re-saturation, and aging with reservoir fluids. Wettability changes may be noticed on logging measurements as a downward shift of the oil peak in the T2 spectrum from the bulk T2 response of live oils. The main practical obstacle in the T2 shift-based evaluation of wettability is the poor separation of oil and water peaks in the T2 spectrum. The bulk T2 of live oils must be measured and the core sample must be perfectly cleaned to quantify the NMR surface relaxation effect.
We demonstrate an improved method based on two-dimensional mapping of NMR diffusion vs. T2 with two principal advantages. First, the separation between the oil and water signals is greatly improved compared with the T2-based approach. Second, key properties such as tortuosity (represented by the Archie cementation exponent m) and effective surface relaxivity can be inferred from the two-dimensional NMR maps using restricted diffusion models. The wettability index and the rock relaxivity can then be estimated from the effective surface relaxivities. These results are based on a single-step NMR measurement on fresh-state (or “as received”) plugs cored with water-base muds containing no surfactants and that should be available days after the cores are recovered.
A wettability index using this new NMR method was obtained for carbonate samples from Middle East reservoirs. A strong correlation coefficient of R2 = 0.7 is observed between this new NMR approach and the standard, more time-consuming methods such as the U.S. Bureau of Mines technique. A sensitivity study of the NMR wettability index versus signal-to-noise ratio is performed on the core data, to assess the feasibility of this new technique down hole. The results suggest that it is possible to obtain reservoir wettability using downhole NMR measurements under appropriate conditions and provided sufficient signal-to-noise is obtained.