Summary

Saturated and desaturated NMR response was integrated with air-brine and air-mercury capillary pressure analysis and with lithologic and other petrophysical analyses for cores from a carbonate reservoir in Kansas. This integration provides guidelines for selection of appropriate T2 cutoffs. in these rocks and an understanding of lithologic controls on permeability prediction using NMR response. Three cores from the Mississippian reservoir, Schaben Field, Ness County, Kansas were studied (Figure 1). From these wells, 50 core plugs, representing a wide range in porosity, permeability, and lithology were selected for detailed investigation. Special core-analysis testing was performed on these samples including (for most samples): routine and in situ porosity and pore volume compressibility, routine air and in situ Klinkenberg permeability, air-brine capillary pressure analysis and determination of “irreducible” brine saturation, air-mercury capillary pressure on selected samples, effective and relative gas permeability, determination of the Archie cementation and saturation exponents, and saturated and desaturated NMR analysis for selected samples. Core lithologies were described and thin-sections of representative samples were examined. A portion of this work was funded as part of the DOE and industry supported Class 2 project (Cooperative agreement DE-FC22-93BC14987).

The reservoir is composed primarily of dolomite or lime mudstone-wackestone, sponge spicule-rich wackestone-packstone, and echinoderm-rich wackestone-packstone-grainstone. Porosity within these lithologies is generally intergranular, intercrystalline, or moldic but may also contain a significant portion of vugs. Grain or crystal sizes are fine to micrite size (<100um to <2um) resulting in very fine pores. Brecciation, fracturing, and carbonate replacement with microporous chert are common in all lithologies. Each lithology exhibits a generally unique range of porosity and permeability values which together define a continuous porosity-permeability trend. Where fracturing and vugs are present, permeability is enhanced and the range in permeability for any given porosity is broadened. Mercury capillary pressure analysis shows that pore throat size for all lithologies is the dominant control on permeability and threshold entry pressures.

Oil columns in this region are generally less than 50 feet. Water saturations, corresponding to the capillary pressure generated by this column, correlate well with permeability for rocks with little or no vuggy porosity or microporous chert. Because of this correlation, permeability prediction using both porosity and T2 is improved over prediction using porosity alone. While a causal relationship exists between effective porosity (and pore body size), measured by T2, and permeability, the relative influence of effective porosity and pore body sizes appears to be small compared to the influence of pore throats. Based on the significant difference in the correlation between T2 and permeability for rocks with and without vugs, it is probable that this correlation is partially based on a correlation between pore body and pore throat size which can differ between lithologies exhibiting different pore geometries. If this is correct, accurate permeability prediction using NMR will require “calibration” of the T2-permeability relationship for each lithology exhibiting a unique relationship between pore throats and pore bodies. These correlations and the constant or exponents obtained will therefore be lithology specific. However, relationships using T2 in addition to porosity should also provide a significant improvement over permeability prediction using porosity alone. Where vuggy porosity is present, T2 cutoffs appropriate for intergranular porosity do not provide good permeability prediction. Based on these observations appropriate T2 cutoffs for delineating effective intergranular porosity are range from 10 to 100 and increase with increasing permeability and pore throat and body size.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas