Summary
In tight-gas sandstone, the productivity of a well is sometimes quite
different from that of a nearby well. Several mechanisms for this observation
have been advanced. Of interest in this paper is the possibility that a small
change in water saturation can change the gas-phase permeability significantly
in rocks with small porosity and very small permeability.
We quantify the effect of small saturations of the wetting phase on
nonwetting-phase relative permeability by modeling the geometry of the wetting
phase. We also show how a porosity-reducing process relevant in tight-gas
sandstones magnifies this effect. The basis for these observations is a model
of the grain-scale geometry of low-porosity sandstones. The model is built from
a dense random packing of spheres modified geometrically to simulate
quartz-overgrowth cementation. To compute phase geometry and permeability, we
use a physically representative network model extracted from the model rock. At
small saturations (at or near the drainage endpoint), the wetting phase exists
largely in the form of pendular rings held at grain contacts. Pore throats
correspond to the constriction between groups of three grains, each pair of
which can be in contact. Thus, the existence of these pendular rings decreases
the void area available for flowing nonwetting phase. Because the hydraulic
conductance of the throat varies
with the square of the void area, the effect on permeability is
disproportionate to the volume occupied by the rings.
Convention holds that connate water has little effect on oil or gas
permeability because it occupies the smaller pores. Comparing predictions for
unconsolidated model rocks with those for cemented model rocks allows one to
reconcile this view with the sensitivity reported in the field and the
laboratory.
Introduction
In tight-gas sandstone, the productivity of a well is sometimes quite
different from that of a nearby well. Wells also can be very sensitive to small
amounts of water, whether from an aquifer associated with the reservoir, from
hydraulic fracturing, or from other completion operations. Although the effect
of water saturation on the effective permeability to gas has been the subject
of numerous experiments (Byrnes et al. 1979; Jones and Owens 1980; Sampth and
Keighin 1982; Walls et al. 1982; Ward and Morrow 1987; Chowdiah 1988), a fully
mechanistic explanation has not yet been offered for why the effect appears
larger in tight-gas reservoirs. In this paper, we explore the possibility that
the grain-scale geometry of tight gas is responsible.
Small wetting saturation is mainly an irreducible wetting phase that exists
in two morphologies (Bryant and Johnson 2003). One is volumes of water held in
the smallest pores. The other is pendular rings held at grain contacts or
liquid bridges held between two grains separated by a gap. The former forces
gas to flow around the filled pores, decreasing the average connectivity of the
gas phase. The latter reduces the area open to gas (the nonwetting phase) as it
passes through a pore throat. It is possible to quantify the effects of these
topological and geometrical changes on gas-phase permeability with the methods
described in the next section. The important feature of the method is that its
input is based on a geologic description (sorting, type, and extent of
cementation). Thus, the insights gained can be useful in explaining regional
variations in well performance, if regional trends in diagenetic alteration are
known.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 February 2007
- Meeting paper published:
16 April 2007
- Revised manuscript received:
3 November 2008
- Manuscript approved:
2 January 2009
- Published online:
1 June 2009
- Version of record:
1 June 2009
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