608 russell-dickey. POROSITY AND PERMEABILITY [Ch. 32 



present. During compaction the interstitial clay is protected from 

 pressure by the bridging and arching of the sand grains. Consequently 

 it may have a porosity in excess of 50 percent. A small amount of 

 clay may, therefore, have only a small effect on the porosity. 



The effect of clay on permeability, on the other hand, is large. The 

 fine clay particles surround the sand grains and reduce the effective 

 diameter of the pores. The presence of a comparatively small percent 

 of clay by weight might have little effect in reducing porosity but a 

 large effect in reducing permeability. 



Because of the smallness and platy character of the clay particles, 

 their surface area is large and pore diameter small. It takes, therefore, 

 a very high capillary pressure to displace water from the clay in a 

 sand. Consequently the connate water content of shaly sands is high, 

 often in excess of 50 percent. Some such sands may contain more 

 than 50 percent water and yet produce clean oil, because the water is 

 retained by the clays. This water was probably never displaced during 

 the original accumulation of oil in the reservoir. Figure 6 shows capil- 

 lary-pressure curves characteristic of low and high clay content. 



Most interstitial clay appears to belong to the group of clay minerals 

 called illite or hydromica. The clay particles have a great affinity for 

 water, and they swell considerably when wet. Montmorillonite, which 

 also occurs, has still greater swelling properties. Clays are also sen- 

 sitive to the salt content and hydrogen-ion concentration of the water 

 in contact with them. Thus if clay is suspended in fresh water of 

 pH 8 or 9, the addition of acid or strong salt water will cause a floccu- 

 lation and precipitation of agglomerates of particles (Baver, 1948). 

 The introduction of fresh water into a sand in which the clay minerals 

 had been in contact with salt water causes them to swell and reduce 

 the permeability of the sand (Johnston et ah, 1945; Breston and John- 

 son, 1945). The introduction of fresh water into oil sands occurs 

 frequently on a small scale during the drilling and completion of a 

 well. There is some evidence that the productivity of the well may 

 be adversely affected by the plugging effect of the water that filters 

 from the drilling mud. Drilling with mud containing oil as a fluid base 

 is commonly practiced to avoid this effect. It has also been shown that 

 salt water is preferable to fresh water for large-scale injection into 

 sands to displace the oil. 



The presence of sufficient clay in sandstones to reduce their perme- 

 ability to very low values is, perhaps, more common than has generally 

 been realized. By no means are all sandstones sufficiently permeable 

 to produce fluids, even though they may contain them under pressure. 

 A comparatively small percent of clay by weight can raise the capillary 



