Quantitative Mechanism for Permeability Reduction by Small Water Saturation in Tight Gas Sandstones

摘要

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.