In tight gas-bearing sandstones, the degree of invasion of mud filtrate can be shown to be closely related to formation permeability. Traditional wisdom is that the degree of mud filtrate invasion is related to porosity – as porosity increases, depth of invasion decreases, because only a finite volume of filtrate is lost before mud cake build up eliminates further fluid loss to the formation. This model assumes that the formation has sufficient matrix permeability to allow mud filtrate to enter the pore framework during the (relatively) brief period of time that mud filtrate is forming.

In tight gas-bearing sandstones, matrix permeabilities are extremely low – often in the nanodarcy range. In these cases, it is reasonable to assume that the amount of mud filtrate loss is limited by formation permeability, or perhaps entirely prohibited below a certain permeability level (the permeability “jail.”)

The technique developed here involves the creation of pseudo porosity logs – acoustic compressional, density and neutron – from an initial model of porosity, matrix and shale. From a knowledge of fluid properties and log responses at reservoir pressure, pseudo logs can be constructed for any assumed fluid combination (gas/water mixture). Then, pseudo logs are compared with actual log responses for each individual porosity log. The comparison allows construction of a profile of gas saturation as “seen” by each porosity log. The profile is compared with a standard resistivity saturation analysis profile. If the two profiles are the same, this suggests no invasion. Divergence suggests invasion. The technique can be used for both water base and oil base mud systems.

A comparison of the profile divergence with core permeability shows that, as permeability increases, so does the saturation divergence.

The level of permeability where saturation divergence begins is a direct measure of the permeability “jail.”

This technique allows for distinction between rocks inside and outside the “jail” for intervals where no core data exists.

The example below is from the La Barge area, Green River Basin, Wyoming. Other examples will be presented.

Figure 1. Comparison of saturation differences between resistivity analysis and neutron log analysis, compared with core permeability.

Figure 2. Depth log showing petrophysical reservoir properties, including interpretations involving invasion profiles.

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