Spatially Resolved NMR T2 Distributions for Pore Size, Surface Relaxivity and Gas Relative Permeability Mapping of Gas Carbonate Reservoirs

Abstract

Abstract A comprehensive petrophysical evaluation of carbonates requires a well-designed workflow to enable rapid integration of data from cores and logs. We describe a methodology that makes use of spatial NMR T2 distribution measurements performed along the length of a core plug in conjunction with micro-tomographic imaging techniques to obtain: 1) the relationship between pore body and pore throat size, 2) surface relaxivity as a function of pore size and 3) a gas relative permeability predictor for the NMR log. An initially fully water saturated core plug is centrifuged in air. Because of the centrifugation, a saturation profile forms in the plug. The plug is virtually treated as a series of thin slices. Water saturation is assumed to be uniformly distributed in each slice but it will vary from one slice to another. Spatially resolved NMR measurements provide the T2 distribution of each slice. For each slice, water saturation and the capillary pressure applied to it during centrifugation are also known. Let Pc1 and Pc2 be the capillary pressures applied to two adjacent slices and let γ be the water-air interfacial tension. Subtracting the T2 distributions of the two slices gives the pore body size distribution of a narrow region of pores whose pore throats range from 2γ/Pc1 to 2γ/Pc2. Doing this calculation for all couples of adjacent slices provides a relationship between pore body and pore throat sizes that is defined over a wide range of sizes.Surface relaxivity is estimated by matching the NMR-derived pore body size distribution to high resolution images (X-ray Micro-CT and SEM) of the core plug.For the gas relative permeability predictor, a SDR-like permeability is assigned to each slice. The parameters in the SDR equation are then optimized by matching the measured effective air permeability to the harmonic mean of the permeabilities of the individual slices. Since one single centrifugation step is needed, the above data (1, 2 and 3) are obtainable in few days. The methodology was applied to a large gas carbonate reservoir. The body/throat size relationship proved to be useful in rock-typing work and a gas relative permeability profile was defined in one of the wells. This represented a significant contribution to our understanding of the reservoir and to the achievement of the objective of reducing the time-to-market. On average, six data-points for the body/throat size relationship are generated for each analyzed core plug. Standard core analysis methods do not provide such a relationship. Usually, surface relaxivity is an assumed parameter but this methodology allows us to estimate it. Relative permeability cannot be estimated directly from logs and this methodology represents a step towards this important goal.