Determination of Surface Relaxivity from NMR T2 Measurements

Abstract

Nuclear magnetic resonance (NMR) is a very useful tool to determine rock properties. The NMR respond to the hydrogen contained within rocks can be related in a direct or indirect way to porosity, pore size distribution, rock permeability, capillary pressure, wettability and water saturation. The magnitude of the T2 signal is used to obtain the matrix independent porosity. Bound and moveable water can be estimated using the relation between response and saturation. Empirical relationships can be used to several petrophysical properties, however, more detailed information is needed on surface relaxivity. To determine the effective surface relaxivity and establish a methodology, sandstone ranging from tight gas to poorly lithified sands were analyzed. The tests performed included conventional core analysis (porosity-permeability), back scattered image analysis (BSI), NMR T2 relaxation on both fully saturated and drained conditions. The permeability of the samples ranges from 0.01 to 1000 mD and their porosities between 2 to 15%. The mean T2 of the Brine saturated samples ranged from 0.8 to 400 ms. Arithmetic average of T2 cutoff (calculated as the point where Swi intercepts the T2 distribution) is 39.2 ms however values ranged between 1.45 ms and 242 ms where clay content played a key factor in reducing cutoff time. Back scattered images were used to establish the link between T2 relaxation and pore area, this relation was then used to obtain the surface relaxivity. This paper presents an innovative methodology to calculate the effective surface relaxivity using the signal generated from mean T2 relaxation with the objective of obtaining a better understanding of the NMR capabilities in assessing in situ reservoir properties. The methodology combines pore volume from NMR and BSE image analysis. owever, in the case that image data were not available a correlation has been generated, using a large number of samples, whichcan be used to obtain surface relaxivity only from NMR T2 data. The surface relaxivity and T2 distribution can then be used to determine formation capillarity and in consequence be able to model the saturation height function to provide an input to the geological static model. The advantage of this method comes from and the direct use of actual data, while the number of samples analysed enables the final outcome to be generalised, and therefore suitable to be used as empirical approach when experimental results are not available.