Tight gas sands permeability estimation from mercury injection capillary pressure and nuclear magnetic resonance data

There are several methods to estimate permeability from pore throat sizes and NMR T ₂ relaxation time. Although most of the methods are well-established and work well for conventional reservoirs they fail to estimate permeability for tight gas sands. The aim of this study was to establish relationships between permeability and pore throat sizes, derived from mercury injection analysis, and NMR T ₂ relaxation time. Regression analysis was used to achieve a set of relationships between dry gas permeability, porosity and pore throat sizes for 50 tight gas sand samples. Unlike for normal sandstone, pore throat radii corresponding to a mercury saturation of 10% (r ₁₀) is the best permeability predictor for tight gas sands. For tight gas sands, NMR T ₂ relaxation spectra fall on the shorter values corresponding to the smaller pores. This is because pore spaces are significantly reduced both in size and volume due to extensive compaction and cementation. This study shows that using NMR T _2peak and multi-regression analysis, permeability can be estimated with high accuracy even in rocks with substantially constricted pore throats.