Miscellaneous Subsurface Methods 751 



The character, continuity, thickness, and extent of porous and per- 

 meable intervals within a carbonate section are of primary concern to the 

 operator. Rock characteristics may change drastically both vertically and 

 horizontally. 



Two types of porosity are recognized in carbonate rocks: (1) primary 

 porosity — ^the percentage of pore space present at or just subsequent to 

 deposition, and (2) secondary porosity — the pore space developed subse- 

 quent to or during lithification. In the latter case, the original openings 

 have been enlarged or new pore space developed by ground-water percola- 

 tion. Development of secondary porosity and permeability gives rise to 

 several types of solution patterns.^^ (1) equi-solution type, which involves 

 a carbonate rock of uniform composition; (2) channel pattern, developed 

 in an alternating limestone and dolomite section where differential solution 

 is evident or in fractured carbonate rocks; and (3) cellular type which is 

 typified by shallow vugs formed by solution on exposed surfaces. 



Carbonate rocks possessing a vuggular pattern do not necessarily in- 

 dicate continuous porosity. However, this type of solution pattern coupled 

 with the equi-solution and channel types tend to develop favorable perme- 

 ability. Acid treatment of the rock fosters interconnection of these solution 

 patterns and thus improves the flow of fluids and gases. 



Sandstone 



Acidization of sandstone reservoirs has in some cases resulted in in- 

 creased production; however, a large number of such treatments have been 

 failures. ^^ Acid testing of core samples prior to formation treatment serves 

 to determine whether increased production may be expected. Fitzgerald, 

 James, and Austin carried on a core-acidizing study in which 332 cores 

 from 30 different producing formations were involved. Of this total 271 

 (81 percent) showed an increase in permeability after acidizing. The aver- 

 age permeability increase was 337 percent. Several examples of increased 

 porosity and permeability in sandstones are given below. 



Formation Average 



Bartlesville (Oklahoma) 



Porosity, % (before) 18.4 



Porosity, % (after) : 19.2 



Permeability, M'd (before) 70.0 



Permeability, M'd (after) 81.0 



"Wilcox" (Oklahoma) 



Porosity, % (before) 12.6 



Porosity, % (after) __ _ 12.4 



Permeability, M'd (before) 83.3 



Permeability, M'd (after) 479.2 



' Howarf], W. V., and David, M. W., Development oj Porosity in Limestones : Am. Assoc. Petroleum 

 Geologists Bull., vol. 20, no. II, p. 1402, 1936. 



'' Fitzgerald, P. E., James, J. R., and Austin, R. L., op. cit., p. 851. 



