oa Determination of Surface Relaxivity from NMR T2 Measurements

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