Summary
The flow of a gas toward the wellbore of a production well will result in
the evaporative cleanup of water blocks, if the latter exist. This occurs
primarily due to gas expansion. This paper presents for the first time a model
to calculate the rate at which such water blocks are removed, for either
fractured or unfractured gas wells. The model allows us to compute the impact
of evaporative cleaning on well productivity.
The removal of water first occurs by gas displacement. Evaporative cleanup
is caused by gas expansion. The resulting saturation profile is qualitatively
different for low- or high-permeability rocks. As a consequence, the increase
in gas relative permeability, or the well productivity, with time can vary
depending on the rock permeability and the well drawdown. High-permeability
(e.g. fractured) rocks clean up significantly faster. By contrast,
low-permeability unfractured wells may require a very long time to clean up.
Large pressure drawdowns, as well as the use of more volatile fluids, such as
alcohols, also result in faster cleanup.
A distinctive feature of the work presented is that the model equations are
formulated and solved completely without the assumption of skin factors for the
damage zone. Thus, the prediction of cleanup rates can be made more
accurately.
Introduction
Water blocks in low-permeability rocks clean up much more slowly than those
of higher permeability because of the smaller pore sizes and the consequent
higher capillary entry pressures (Mahadevan et al. 2003). In particular, water
blocks in tight gas sands are not easily cleaned up, especially in cases where
the reservoir pressures are too low to initiate flow.
Past studies (Tannich 1975; Holditch 1979, Parekh and Sharma 2004) have
reported the effect of water displacement by gas in the cleanup of water blocks
in gas wells. They showed that when the drawdown in the gas well is
significantly larger than the capillary pressure, cleanup is faster. However,
in cases where the drawdown becomes comparable to the capillary pressure, as is
the case in depleted tight gas reservoirs, displacement alone is not sufficient
to remove water from the near-wellbore region. Subsequent water removal occurs
by evaporation. The flow of a fully saturated compressible gas through a
water-saturated porous rock induces evaporation. Roughly, this is because the
volume of the gas, and hence its capacity for water content, increases as
pressure declines. In past studies, the impact of evaporation caused by the
flow of gas has been neglected. The focus of this paper is precisely on this
regime in gas wells, in which the drawdown is comparable in magnitude to the
capillary entry pressure, and cleanup of water blocks is by
evaporation.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
24 February 2005
- Revised manuscript received:
11 April 2006
- Manuscript approved:
3 February 2007
- Version of record:
20 June 2007
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