Summary
Drainage/imbibition simulations are traditionally performed on finite
regular lattices. If physically representative networks are used instead, the
spatial correlation of pore space features inherent in granular materials is
automatically accounted for. However, even these networks are obtained from
finite samples, and conditions must be specified on the boundaries. Making the
conditions correspond to physically realistic situations is difficult,
especially for simulations of phase trapping. This paper presents a method of
constructing infinite-acting model rocks, in which a well-defined criterion for
phase trapping is possible that is independent of boundary conditions. The
foundation of the model is computer-generated dense random periodic packings of
spheres. We illustrate the method with simulations of drainage and irreducible
wetting-phase saturations. To eliminate possible confounding effects from
grains being arranged differently, we compare simulations in infinite-acting
networks with simulations in the finite network taken from the unit cell of the
periodic packing. Wetting-phase connectivity is assessed globally and accounts
for all phase morphologies, including pendular rings.
The drainage endpoint for a finite network was highly sensitive to the
number and location of the exit pores; while using the infinite-acting network,
we removed this latter sensitivity. Comparison with experiments showed that a
strict criterion of wetting-phase connectivity (pores connected only by throats
filled with wetting phase) cannot account for typical values of
Sw,irr and that connectivity must persist via pendular
rings.
Typical laboratory experiments and simulations in finite networks yield
drainage curves that approach irreducible wetting phase saturation
Sw,irr gradually, with large changes in capillary
pressure inducing only small changes in saturation. Simulations in
infinite-acting networks do not exhibit this behavior. We argue that these
simulations are more representative of behavior in the field and that typical
laboratory measurements underestimate the value of
Sw,irr.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
25 February 2009
- Manuscript approved:
20 March 2009
- Published online:
16 July 2009
- Version of record:
22 December 2009
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