Vertical Interference Testing Across a Low-Permeability Zone

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Abstract
Summary: Bremer et al.1 described a mathematical model for analysis of a vertical interference test across a low-permeability zone. The test configuration called for recording wellbore pressure vs. time opposite one permeable layer while fluid is produced from the second permeable layer. The two permeable layers are separated by a tight zone of low vertical permeability. This paper offers a more general interpretation technique that allows for distinct properties in the two permeable layers and includes wellbore-storage and skin effects. The purpose of the vertical interference test across a low-permeability zone is to determine the vertical permeability of the tight zone separating the two permeable strata. The results of such a test may influence well completion decisions for primary, secondary, and enhanced oil recovery. Of particular interest is the estimation of the fluid flow rate into or out of the packed-off layer. Field test data have been analyzed with the new techniques. By including wellbore-storage and skin effects, the model presented in this study provides a much better fit with the data presented by Bremer et al.1 Because this method allows for determination of formation properties in both permeable zones, simultaneous recording of the pressures in both the producing and packed-off layers is recommended. Results indicate that the wellbore- pressure difference between the flowing and packed-off zones becomes constant after sufficient time and depends only on the permeabilities in the three zones. If wellbore-storage effects can be minimized, the behavior of pressure vs. log time may exhibit two distinct slopes: the first providing for calculation of the permeability in the flowing layer, and the second allowing calculation of the permeability in the packed-off layer. Type curves are presented for analysis of both the pressure and pressure-derivative transients. The interpretation method is suitable for broader application than previous methods would allow. An important result is an estimate of the rate of crossflow between layers separated by a low-permeability zone.

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