Development of natural gas hydrates as an energy resource has gained
significant interest during the past decade. Hydrate reservoirs may be found in
different geologic settings including deep ocean sediments and arctic areas.
Some reservoirs include a free-gas zone beneath the hydrate and such a
situation is referred to as a hydrate-capped gas reservoir. Gas production from
such a reservoir could result in pressure reduction in the hydrate cap and
endothermic decomposition of hydrates.
Well testing in conventional reservoirs is used for estimation of reservoir
and near-wellbore properties. Drawdown testing in a hydrate-capped gas
reservoir needs to account for the effect of gas from decomposing hydrates.
This paper presents a 2D (r,z) mathematical model for a
constant-rate drawdown test performed in a well completed in the free-gas zone
of a hydrate-capped gas reservoir during the earlytime production. Using energy
and material balance equations, the effect of endothermic hydrate decomposition
appears as an increased compressibility in the resulting governing equation.
The solution for the dimensionless wellbore pressure is derived using Laplace
and finite Fourier cosine transforms. The solution to the analytical model was
compared with a numerical hydrate reservoir simulator across some range of
hydrate reservoir parameters.
The use of this solution for determination of reservoir properties is
demonstrated using a synthetic example. Furthermore, the solution may be used
to quantify the contribution of hydrate decomposition on production
In recent years, demands for energy have stimulated the development of
unconventional gas resources, which are available in enormous quantities around
the world. Gas hydrate as an unconventional gas resource may be found in two
geologic settings (Sloan 1991): (1) on land in permafrost regions, and (2) in
the ocean sediments of continental margins. During the last decade, extensive
efforts consisting of detection of the hydrate-bearing areas, drilling,
logging, coring of the intervals, production pilot-testing, and mathematical
modeling of hydrate reservoirs have been pursued to evaluate the potential of
gas production from these gas-hydrate resources.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
23 June 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
25 April 2009
- Manuscript approved:
22 May 2009
- Published online:
20 July 2009
- Version of record:
9 September 2009