Summary
Published well-test analyses in gas/condensate reservoirs in which the
pressure has dropped below the dewpoint are usually based on a two- or
three-region radial composite well-test interpretation model to represent
condensate dropout around the wellbore and initial gas in place away from the
well. Gas/condensate-specific results from well-test analysis are the mobility
and storativity ratios between the regions and the condensate-bank radius. For
a given region, however, well-test analysis cannot uncouple the storativity
ratio from the region radius, and the storativity ratio must be estimated
independently to obtain the correct bank radius. In most cases, the storativity
ratio is calculated incorrectly, which explains why condensate bank radii from
well-test analysis often differ greatly from those obtained by numerical
compositional simulation.
In this study, a new method is introduced to estimate the storativity ratios
between the different zones from buildup data when the saturation profile does
not change during the buildup. Application of the method is illustrated with
the analysis of a transient-pressure test in a gas/condensate field in the
North Sea. The analysis uses single-phase pseudopressures and two- and
three-zone radial composite well-test interpretation models to yield the
condensate-bank radius. The calculated condensate-bank radius is validated by
verifying analytical well-test analyses with compositional simulations that
include capillary number and inertia effects.
Introduction and Background
When the bottomhole flowing pressure falls below the dewpoint in a
gas/condensate reservoir, retrograde condensation occurs, and a bank of
condensate builds up around the producing well. This process creates concentric
zones with different liquid saturations around the well (Fevang and Whitson
1996; Kniazeff and Nvaille 1965; Economides et al. 1987). The zone away from
the well, where the reservoir pressure is still above the dewpoint, contains
the original gas. The condensate bank around the wellbore contains two phases,
reservoir gas and liquid condensate, and has a reduced gas mobility, except in
the immediate vicinity of the well at high production rates, where the relative
permeability to gas is greater than in the bank because of capillary number
effects (Danesh et al. 1994; Boom et al. 1995; Henderson et al. 1998; Mott et
al. 1999).
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Revised manuscript received:
7 December 2005
- Manuscript approved:
26 July 2006
- Version of record:
20 October 2006
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