710 Subsurface Geologic Methods 



surface and with the fluid saturation of the rock. The conventional cap- 

 illary-pressure curve (figs. 384 and 385) depicts the relationship between 

 saturation and the capillary pressure above the free-water level and 

 illustrates the fact that the equilibrium between capillary pressure and 

 gravitational forces required by equation 17 results in a vertical satura- 

 tion gradient in a petroleum reservoir, which is known as the transition 

 zone. 



The concept of capillary pressure demands that, at equilibrium, the 

 capillary pressure be everywhere the same at the same horizontal level. 

 Inasmuch as the geometry of a homogeneous rock and, hence, the radius 

 of curvature of fluid interfaces are constant, the fluid saturation must be 

 constant at equilibrium in accordance with equation (17). Consequently, 

 the amount of connate water in a rock is that which can be held in the 

 rock by capillary forces. Experimental capillary-pressure data provide 

 a means of determining the connate water saturation, this being the so- 

 called irreducible water saturation on the curve obtained when saturation 

 is plotted against capillary pressure (figs. 384 and 385). If the geometry 

 and mineral composition of a rock were similar and uniform in all re- 

 spects and in all directions, the permeability and connate water satura- 

 tion should be constant and uniform throughout the rock. The connate 

 water saturation of actual sedimentary rocks, formed by the deposition 

 of particles graded to size or by some other natural process, would be 

 expected to vary with permeability, increasing as permeability decreases. 

 Differences in mineral composition resulting in different degrees of wetta- 

 bility likewise result in diff"erences in connate-water saturation. The 

 Woodbine sand represented by samples 4, 5, and 6 (fig. 383) is an even- 

 grained, friable, almost pure quartz sandstone containing a small amount 

 of argillaceous cementing material, according to Puls.^^ The porosity 

 and permeability of these samples are of the same order of magnitude: 

 namely, 21.1 percent and 401 millidarcys, 26.6 percent and 113 milli- 

 darcys, and 27.1 percent and 641 milldiarcys respectively. The assort- 

 ment of grains of different sizes is that to be expected of a well sorted 

 sandstone; and, hence, the capillary pressure-saturation curve is regular 

 and smooth. Nevertheless, the minimum connate-water saturation of the 

 samples is 9, 26, and 10 percent respectively. The high connate-water 

 saturation of sample 5 is due to a greater amount of cementing material, 

 as well as, perhaps, to a streak of lignitic and feruginous material in the 

 sample. The Grayburg formation (samples 1, 2, and 3, figure 385) is a 

 uniformly fine-grained, gray dolomite — in some places sandy and in 

 others oolitic — the porosity of which is chiefly intergranular, although 

 samples exhibit scattered, very small solution channels. Consequently, 

 both the porosity and permeability are low, being 8.5 percent and 8.2 

 millidarcys, 9.8 percent and 1.4 millidarcys, and 7.9 percent and 1.3 

 millidarcys respectively. The great difference between the average dimen- 



^* Puis, W. L., Thesis for M.S., Degree in Petroleum Engineering, Unpublished, 1950. 



