ABSTRACT: The Effect of Mineralogy on NMR Relaxation in Water Saturated Reservoir Sandstones

OREN, P. E., H. G. RUESLATTEN, and A. T. BULLER, Statoil Research Centre, Trondheim, Norway

Kenyon et al. demonstrated that NMR longitudinal relaxation (T1) measurements of water in reservoir rocks may be used to characterize pore-size distributions and to predict permeability in conjunction with porosity. To achieve further precision, however, Skjetne et al. (1990) tentatively suggested that the measurements should be interpreted in terms of their complex interdependence on pore architecture and mineralogy (e.g., total iron content, and the presence and quantities of authigenic clays). This paper presents a development of this theme by focusing more closely on the influence of mineralogy on relaxation times for a suite of fully brine-saturated reservoir sandstone samples of different mineral compositions.

Relaxation data were analyzed using both the classical three-exponential model and the recently proposed stretched exponential model. The parameter average surface interaction strength was calculated for each of the samples and was found to vary by almost an order of magnitude. Quartz-rich sandstones have the lowest surface interaction strengths, while kaolinite- and pyrite-rich samples have the highest interaction strengths. Furthermore, surface interaction strength varies in an almost linear fashion with increasing kaolinite.

From this study it is concluded (1) that accurate quantitative information concerning pore structure and conductivity from NMR relaxation measurements cannot be inferred from sandstones with different mineralogies without recourse to this specific parameter; and (2) that kaolinite is the most important mineral in our samples to cause significant enhancement of surface interaction strength.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)