Permeability Modeling from Petrophysical Measurements in Tight Gas Sands: Empirical Relations Including Analyses of Saturation Profiles from Individual Porosity Log Responses

Holmes, Michael¹, Antony Holmes¹, Dominic Holmes¹ (1) Digital Formation, Inc, Denver, CO

Standard petrophysical approaches to estimate permeability involve comparisons of porosity with irreducible water saturation of the form:

Permeability = Constant * Porosity Exponent1 Irreducible Water SaturationExponent2

Usually, single values of exponent 1 and exponent 2 are applied to any one reservoir. For tight gas sands, correlations with core-measured permeability improve if porosity and saturation-dependent values of exponents 1 and 2 are applied, calibrated to the specific reservoir.

A second approach to estimating permeability involves consideration of invasion profiles. We have recently developed fluid substitution techniques whereby gas saturation from individual porosity logs (particularly density and neutron) can be calculated and compared with standard resistivity-based calculations of gas saturation. The comparison shows that in tight gas sands, differences between the calculated gas saturations are a function of permeability. The conclusion is reached that for these very low matrix permeability rocks, there is an increasing degree of mud filtrate invasion as permeability increases. At very low levels of permeability — less than 50 microdarcies — there is essentially no invasion.

Examples from the Rocky Mountains and East Texas are presented, comparing core permeability with petrophysical estimates of permeability.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana