A MECHANISM FOR THE TEMPERATURE DEPENDENCE OF THE SURFACE RELAXATION RATE IN CARBONATES

摘要

Unexplained temperature dependence in some carbonate reservoir rock was reported recently,in which the mean log T2 for the brine-saturated material increased with temperature [8].For many years significant temperature dependence was not observed and was considered unlikely in rock materials based on a variety of evidence [5].Some of the recently announced temperature dependence was explained.More efficient diffusive coupling between pores,which should occur with increased temperature,can shift the T2 relaxation to shorter times.The examples that were reported to display an increase in relaxation time with increasing temperature were problematic and suggested that the surface relaxation rate weakened with temperature.Last year,new theory and experiments were introduced to explain temperature-dependent T2-increases based on the surface diffusion in the vicinity of paramagnetic relaxation centers [4].Our paper demonstrates that a complementary or possibly alternative mechanism for temperature dependence should be considered,too.Brine in rock relaxes faster than in bulk.This augmented relaxation has been attributed to the presence of paramagnetic impurities in the rock,near the rock-water interface.Prior to the general acceptance of surface relaxation by paramagnetic centers as the dominant NMR relaxation mechanism for fluids in rock,hindered motion on the rock surface that caused enhanced hh(hydrogen-hydrogen)dd(dipole-dipole)relaxation was a competitive theory for the accelerated relaxation.The possibility of hh relaxation,which was provided for in the early theory for the relaxation of rock materials,eventually faded away because it did not seem relevant.Indeed,it was the failure to observe temperaturedependence in rock that led to the conclusion that only paramagnetic relaxation was important and that it was reasonable to exclude consideration of surface hh relaxation and further that only the temperature-independent scalar term for relaxation was important in the electron-proton interaction.In this paper,examination of a single carbonate rock,which has a previously documented temperature-dependence,indicates that the surface hh relaxation rate is comparable to the surface relaxation rate due to paramagnetic centers.Significant surface hh relaxation,relative to the total rate,could account for the observed temperature dependence of relaxation times in carbonates.