Summary
Production of gas can result in drying of the near-wellbore region caused by
gas expansion, which causes evaporation and hence salt deposition. The
deposition of salt leads to a reduction in porosity and permeability of the
rock in the near-wellbore region, which then leads to reduction in well
productivity. The main objective of this work is to understand the role of
capillarity on salt deposition and its impact on well flow efficiency.
We develop and solve the dimensionless-conservation equations for solid-salt
saturation using numerical methods under radial-flow conditions. The results of
the calculations show that when the capillary effects are strong, the salt
accumulates near the wellbore, which leads to plugging and hence higher skin
values. The salt saturation continues to evolve until a limiting value, in the
rock pores, is reached. Higher values of the limiting saturation lead to a
greater reduction in permeability and therefore result in a larger skin value
for the gas well. The flow efficiency of a gas well, calculated as a
dimensionless ratio of the flow with and without skin, decreases continually
and asymptotically approaches a constant value.
In a simulation for a typical field case, we find that the largest increase
in skin factor and corresponding decrease in flow efficiency occur in
approximately the first 30 days, after which the changes are slower. When
connate-water salt concentration is higher, the skin value is much higher and
thus the well productivity is lower. Using the model developed in this study,
the decline of gas-well productivity caused by salt deposition can be predicted
and hence accurate timing of well operations, such as water wash, can be made.
Also, the modeling study can be used to select remediation strategies such as
wettability alteration to reduce capillarity or application of inhibitors,
which can reduce the limiting salt saturation by preventing growth of salt
crystals within the reservoir rock.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
26 November 2010
- Meeting paper published:
28 May 2010
- Manuscript approved:
16 February 2011
- Published online:
3 November 2011
- Version of record:
23 December 2011
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