Miscellaneous Subsurface Methods 753 



Structural, Stratigraphic, and Lithologic Influence on Acidizing 



To perform properly an acid treatment of a well, the acid engineer 

 should have a concept of the rock structure and the type and relationships 

 of the rocks involved. The subsurface geologist should be consulted on the 

 problem and should assist in the treatment procedure. Before acidizing the 

 attitude of the beds should be known in order to predict the effect of acidi- 

 zation along bedding surfaces. The type and amount of fracturing should 

 be evaluated in order to avoid water entry or increased gas-oil ratios fol- 

 lowing the treatment. The opening of a fracture by acidization, a fracture 

 in which water is confined under high pressure, may influence serious pro- 

 duction difficulties. 



The stratigraphy of a well section should be properly determined be- 

 fore the initial acid treatment. In some cases several porous zones may be 

 present; one may be more permeable than another; consequently, it will 

 absorb the greater portion of the acid, and leave the less-permeable interval 

 unacidized. A permeable sandstone bed in a carbonate section may give 

 similar results. Sections of this type require stage acidation. 



Rarely are carbonate-producing sections lithologically homogeneous. 

 Limestones and dolomites commonly contain impurities which affect acidi- 

 zation treatment. Love and Fitzgerald ^^ comment that sandy limestones 

 are always treacherous to acidize and on acidizing may occasionally result 

 in production decrease. This decrease is attributed to partial plugging of 

 the pores by sand grains freed by the reaction of the solvent. Many car- 

 bonate rocks contain variable amounts of the clay minerals. Upon acid- 

 ization the clay fraction is released and transported into the minute pores, 

 where it restricts the flow of the acid and retards the outflow of the con- 

 tained oil and gas. Iron and aluminum compounds occur in carbonate de- 

 posits. The reaction between these compounds and the injected acid tend 

 sometimes to create colloidal precipitates which act as plugging agents. 



Examples of Acidization 



The results of acidization depend upon the character (composition, 

 texture, and structure) of the rock and the applied method of treatment. 

 Several examples of acidization are given. 



(1) In the Buckeye field, Gladwin County, Michigan,"**^ production is 

 obtained from along modified bedding surfaces and fractures in the Rogers 

 City limestone and from lenticular porous zones near the top of the Dundee 

 formation (Devonian). Acidization is initiated immediately after comple- 

 tion of the well. It is customary to inject from 2,000 to 3,000 gallons op- 

 posite the producing intervals. Occasionally as many as 10,000 gallons 

 have been used. 



(2) In the Cunningham field, Kingman and Pratt Counties, Kansas,^-*^ 



'" Love, W. W., and Fitzgerald, P. E., op. cU., pp. 625-626. 



^^ Addison, C. Ct, Buckeye Oil Field, Gladwin County, Michigan: Am. Assoc. Petroleum Geologists 

 Bull., vol. 24, no. 11, p. 1981, 1940. 



" Rutledge, R. B., and Bryant, H. Sw, Cunningham Field, Kingman and Pratt Counties, Kansas: 

 Am. Assoc. Petroleum Geologists Bull., vol. 21, no. 4, p. 518, 1937. 



