750 Subsurface Geologic Methods 



through perforations. It is also frequently applied to cut off excess gas 

 issuing from the gas-cap section of a producing zone. 



It is always important when attempting the exclusion of water or gas 

 that the squeeze job or plug-back cover the offending section and overlap 

 the contact point. When reperforation is initiated, the new holes should 

 not be allowed to enter the sections which were cemented to correct pro- 

 duction. 



The brief and elementary details which have been given are intended 

 merely to bring attention to certain items which should be kept in mind 

 by the subsurface geologist during oil-well cementing practices. 



WELL ACIDIZATION 

 L. W. LeROY 



Well acidizing is defined "as the process of introducing inhibited 

 hydrochloric acid into predominantly limestone formations to enlarge the 

 pores by removing obstacles and constrictions from them and extending 

 them into new drainage areas, thus lowering the resistance offered to the 

 flow of oil and gas through the oil-bearing formations. "^^ Acidizing of a 

 well results in the modification of the physical aspects of the reservoir 

 strata so that pressure differentials across a portion of it are reduced, 

 thereby permitting a more efficient utilization of the available energy. ^*' 



As a result of well acidization, new oil reserves have been discovered, 

 oil-field developments have been improved, and production has been in- 

 creased. Acidizing has become an adopted procedure in completing and 

 reworking of wells which produce from limestone and/or dolomite rocks. 

 In certain cases, acidization of sandstone-producing intervals has promoted 

 greater and more efficient recovery. 



Limestone and Dolomite 



The carbonate rocks are represented by limestone and dolomite and 

 mixtures of both types. Some limestones are true elastics, whereas others 

 are chemical or biochemical precipitates. Diagenetic changes frequently 

 produce extreme modification of the original deposit. Limestones and dolo- 

 mites vary widely in composition and texture. They include such foreign 

 constituents as quartz grains, clay, anhydrite, gypsum, iron oxides, and 

 chert. These impurities frequently play an important role in acidizing pro- 

 grams. The texture of limestones and dolomites may range from very fine 

 to coarse crystalline. Dolomites more frequently exhibit coarser crystallin- 

 ity than limestones. The chemical composition of the rocks is perhaps the 

 most basic of all geologic factors which control the results of acidizing. 

 However, the texture and structural fabric of the rock are also important 

 factors. 



^ Love, W. W., and Fitzgerald, P. E., Importance oj Geological Data in Acidizing of WeUs: Am. 

 Assoc, of i'etroleum Geologists Bull., vol. 21, no. 5, p. 616, 1937. 



" Fitzgerald, P. E., James, J. R., and Austin, R. Lv, Laboratory and Field Observations of Effect of 

 Acidizing Oil Reservoirs Composed of Sands: Am. Assoc. Petroleum Geologists Bull., vol. 25, no. 5, 

 p. 850, 1941. 



