Summary
The pore structure of unconventional gas reservoirs, despite having a
significant impact on hydrocarbon storage and transport, has historically been
difficult to characterize because of a wide poresize distribution (PSD), with a
significant pore volume (PV) in the nanopore range. A variety of methods is
typically required to characterize the full pore spectrum, with each individual
technique limited to a certain pore size range. In this work, we investigate
the use of nondestructive, low-pressure adsorption methods, in particular
low-pressure N2 adsorption analysis, to infer pore shape and to
determine PSDs of a tight gas siltstone reservoir in western Canada. Unlike
previous studies, core-plug samples, not crushed samples, are used for isotherm
analysis, allowing an undisturbed pore structure (i.e., uncrushed) to be
analyzed. Furthermore, the core plugs used for isotherm analysis are subsamples
(end pieces) of cores for which mercury-injection capillary pressure (MICP) and
permeability measurements were previously performed, allowing a more direct
comparison with these techniques. PSDs, determined from two isotherm
interpretation methods [Barrett-Joyner-Halenda (BJH) theory and density
functional theory (DFT)], are in reasonable agreement with MICP data for the
portion of the PSD sampled by both. The pore geometry is interpreted as
slot-shaped, as inferred from isotherm hysteresis loop shape, the agreement
between adsorption- and MICP-derived dominant pore sizes,
scanning-electron-microscope (SEM) imaging, and the character of measured
permeability stress dependence. Although correlations between inorganic
composition and total organic carbon (TOC) and between dominant pore-throat
size and permeability are weak, the sample with the lowest illite clay and TOC
content has the largest dominant pore-throat size and highest permeability, as
estimated from MICP. The presence of stress relief-induced microfractures,
however, appears to affect laboratory-derived (pressure-decay and pulse-decay)
estimates of permeability for some samples, even after application of confining
pressure. On the basis of the premise of slot-shaped pore geometry, fractured
rock models (matchstick and cube) were used to predict absolute permeability,
by use of dominant pore-throat size from MICP/adsorption analysis and porosity
measured under confining pressure. The predictions are reasonable, although
permeability is mostly overpredicted for samples that are unaffected by
stressrelease fractures. The conceptual model used to justify the application
of these models is slot pores at grain boundaries or between organic matter and
framework grains.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
16 March 2012
- Meeting paper published:
5 June 2012
- Revised manuscript received:
1 October 2012
- Manuscript approved:
9 October 2012
- Published online:
6 December 2012
- Version of record:
27 December 2012
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