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.
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.