A New Method For Separating Lithologies And Estimating Thickness-Weighted Permeability Using Nmr Logs

Nuclear Magnetic Resonance (NMR) logs provide a summation of the pore volume divided into T2 "bins", from which are extracted information about pore size and fluid type. However, the log does not tell us how the different pore sizes are arranged in space. For example, the T2 response in a homogeneous sandy-siltstone could be identical to the response in an interbedded sandstone/siltstone. This homogenization of the pore system does not affect the estimation of static properties (e.g. calculation of fluid saturation), but is important for estimating dynamic properties (e.g. permeability and moveable water). The new thickness-weighted permeability method attempts to divide the T2 spectrum into component parts relating to specific lithologies, e.g. sands, silts and clays. The bound fluid content, free fluid content, and permeability are obtained for each component. A combination of NMR and conventional logs is then used to determine the typical porosity of each lithology, which in turn is used to estimate the cross-sectional area of each component in the T2 distribution. This information is applied to compute a thickness-weighted average permeability. The results provide a range of NMR permeability values at each sample depth, representing minimum permeability (e.g. siltstones), maximum permeability (e.g. sandstones), and the average permeability. The three NMR permeability curves now form an envelope around core permeability measurements. The maximum NMR permeability matches the high-end core permeability, e.g. where plugs target the better sands. The minimum NMR permeability matches the low-end core permeability, e.g. where plugs target the poor quality silts. The position of the average NMR permeability in relation to the end point values and core data reflects heterogeneity. The terms in the individual NMR permeability equations can be adjusted by calibrating against core data, although in most cases the parameters used in the standard equations are suitable, and the method can be used where no core is available. This thickness-weighted NMR permeability is particularly useful for detecting thief zones, and baffles or barriers. It is also useful for estimating moveable water, since it allows inclusion or exclusion of water associated with different lithologies, depending on their level of permeability.