USING MULTIFREQUENCY NMR FOR PROBING WETTABILITY

摘要

We present a new method for probing wettability of rock/oil/brine systems using multi-frequency NMR,i.e.nuclear magnetic relaxation dispersion(NMRD).Unlike conventional transverse relaxation studies,this approach gives a direct probe of the dynamical surface affinity of fluids,thus allowing the separation of wetting from nonwetting fluids through their typical NMRD features.To quantify these features we introduce a microscopic dynamical surface affinity index which measures the dynamical correlation(i.e.microscopic wettability) between a diffusive fluid and fixed paramagnetic relaxation sources at the pore surfaces. For the first time we apply this technique to carbonate reservoir rocks of bimodal porosity saturated with oil(Sw=0%),brine(Sw=100%) and mixture of oil/brine(S_(wirr)).The measured NMRD features clearly discriminate the wetting behavior of the fluids in the pore system.We have interpreted these different NMRD profiles in a model of dynamical surface relaxation.We consider the two following situations:(i) for an aprotic liquid such as oil(dodecane),the affinity index varies between 1(bulk i.e.non wetting case) and infinity(strong surface affinity i.e.wetting case).This index quantifies the degree of dynamical correlation with the paramagnetic impurities present at the pore surface.(ii) for a protic liquid such as brine,we define a similar index by comparing the time of residence of a proton in the ligand field of the paramagnetic ions to the electronic spinlattice relaxation time(i.e.reflecting the presence of water close to the surface and thus a water-wetting behaviour).The experimental results obtained on carbonate core plugs of bimodal porosity saturated with an oil/brine mixture(S_(wirr)) have been processed using the proposed model and clearly reveal the pore size dependence of wettability.