Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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A New Flow-Based Cut-Off Criterion For Permeability In Dry Gas Reservoirs

Eugene Balbinski (eugene.balbinski@ecl-winfrith.com) of ECL Technology Ltd summarises work undertaken for the DTI SHARP Programme on a flow-based permeability cut-off criterion for dry gas reservoirs.  The results of this study were presented at the SPWLA European Symposium on Reservoir Characterisation, September 2002 (Co-authors: Jeff Masters and John Makin).

The use of a petrophysical cut-off to define net pay and exclude unnecessary data, is a traditional practical device used in the industry for many years. Cut-offs may be employed on one or more of several factors including permeability, porosity, net to gross, shale or clay content and water saturation. Permeability cut-offs are usually chosen by rules of thumb and experience, rather than directly from knowledge of likely flows in the reservoir. However, gas in particular is more mobile than oil, so over the course of a reservoir’s lifetime, gas from quite low permeability regions may be producible. It is possible for significant gas volumes to exist in rock with permeability lower than the typical rule of thumb cut-off value used for gas of 0.1 mD on the UKCS.

So how can we do better to do better than the rule of thumb in estimating permeability cut-offs with a practical method? We have developed a cut-off criterion from an analytic flow solution, that is one which can be expressed as a simple formula. This introduces a host of approximations and assumptions, but these are checked using numerical reservoir simulation. For example, we assume a model in which a uniform low permeability region is connected to a higher permeability region, from which gas is produced. The figure below shows that the analytic solution on which the cut-off criterion is based closely reproduces the actual flow over a range of several orders of magnitude in permeability.

Figure 1: Analytic Solution Closely Reproduces Simulated Flow (click image for larger view)

For the full paper follow this link:

http://www.dti-sharp.co.uk/dissemination/openreports/Netpay_extended_abstract.pdf

A New Flow-Based Cut-Off Criterion For Permeability In Dry Gas Reservoirs, E F Balbinski, J H K Masters, J Makin, Proceedings of the SPWLA Symposium on Reservoir Characterisation, London, September 2002.

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