SPE Reservoir Evaluation & Engineering
Volume 15,
Number 4,
August 2012,
pp. 401-409
Summary
Permeability is one of the most fundamental properties of any reservoir rock
required for modeling hydrocarbon production. However, shale permeability has
not yet been understood fully because of the complexities involved in modeling
flow through nanoscale throats. In this paper, we analyze the effects of
adsorbed layers of methane (CH4) and of gas slippage at pore walls
on the flow behavior in individual conduits of simple geometry and in networks
of such conduits. The network is based on the scanning-electron-microscopy
(SEM) image and drainage experiment of shale. To represent the effect of
adsorbed gas, the effective size of each throat in the network depends on the
pressure. The hydraulic conductance of each throat is determined on the basis
of the Knudsen-number (Kn) criterion (Knudson 1909). The
combined effects of adsorption and slip depend strongly on pressure and on
conduit diameter. The results indicate that laboratory measurements made with
N2 at ambient temperature and 5-MPa pressure, which is typical for a
transient pulse-decay (TPD) method, overestimate the gas permeability at early
life of production by a factor of four. This ratio increases if the measurement
is run at ambient condition because the low pressure enhances the slippage and
reduces the thickness of the adsorbed layer. Moreover, the permeability
increases nonlinearly as the in-situ pressure decreases during production. This
effect contributes to mitigating the decline in the production rate of
shale-gas wells. Laboratory data available in the literature for CH4
permeability at pressures below 7 MPa agree with model predictions of the
effect of pressure.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
29 June 2011
- Meeting paper published:
31 October 2011
- Revised manuscript received:
18 January 2012
- Manuscript approved:
30 March 2012
- Published online:
17 July 2012
- Version of record:
7 August 2012
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