Abstract
A tight gas reservoir is commonly defined as a reservoir having less than
0.1 milliDarcies permeability. Because of the very low permeability, hydraulic
fracturing is usually carried out in efforts to establish commercial
production. There are several basic concepts and field cases of different well
tests in tight gas reservoirs in the literature. In this paper, we gather
information and provide a guide to some of the most important tests.
Generally because of low permeability, a well will not flow initially at
measurable rates and conventional well testing cannot be applied. We review
procedures for design of pre- and postfracture tests in single and dual
porosity reservoirs. The prefracture test permits estimating preliminary values
of reservoir permeability and initial pressure. The post-fracture test provides
data for estimating fracture half length and conductivity. We also review the
application of convolution/deconvolution methods to analyze well tests with
significant wellbore storage.
Because of economic and environmental reasons, short duration procedures are
preferred. However, although effective in many instances, these methods also
have their own limitations.
Introduction
Unconventional reservoirs (tight gas, coal bed methane, shales gas and gas
hydrates) will be an important pat of the global energy mix for decades to
come. Large reserves, long-term potential, costs and gas prices and some other
factors account for the great influence of these resources on the future of
energy.
There is no formal definition for “tight gas.” A commonly used definition,
describes tight gas reservoirs as those having permeabilities smaller than 0.1
milliDarcies.
Well testing is generally done as an aid to estimate gas in-place and
recoverable volumes. Initial pressure (pi) is a critical parameter not only for
estimating gas in-place, but also for determining how much field development is
required and whether or not the field is overdeveloped. In addition to pi, well
testing provides an estimate of permeability and skin.
A problem associated with well testing in tight gas sands is that usually
long times are required to reach radial flow, due to their extremely low
permeabilities. Therefore, conventional well tests cannot be applied to these
reservoirs. Because of initial uneconomic rates, fracturing is usually
required. Lee(2) has suggested procedures for pre- and post-fracture tests
design. In order to have measurable gas rates for pre-frac testing, often a
breakdown with acid, KCl water or N2 is necessary.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
27 March 2008
- Meeting paper published:
17 June 2008
- Revised manuscript received:
21 August 2009
- Manuscript approved:
26 August 2009
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