INTRODUCTION

The practice of introducing acid into wells to increase oil and gas production has been in use since 1894, when it was first used in the Pennsylvania fields. The earliest acid treatment consisted of dumping raw hydrochloric acid down the casing without subsequent application of pressure . It was not until 1930 (see fig. 1 ) that better treating materials and improved techniques in directing the acid into the desired pay sections proved successful in increasing production.fig.1.- Acidizing operations, 1938

Another acidizing method include Acid Washing. This operation is designed to remove acid soluble scales or to perforations. It involves spotting the acid at the desired position in the wellbore and then either allowing it to soak or circulate it back and forth across the exposed perforations or formation face.

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Fig. 2.-Heterogeneous core shows a etching pattern

The main purpose of carbonate acidizing is not to remove the damage to the formation near the wellbore, but to create wormholes through which oil and gas will flow after stimulation. Figure 3 shows wormholes created by

acid dissolution of limestone in a laboratory. Carbonate acidizing is a more difficult process to predict than sandstone acidizing because the physics in much more complex. Because the surface reaction rates are very high and mass transfer often plays the role of limiting step locally, highly nonuniform dissolution patterns are usually created. The structure of the wormholes depends on many factors including flow geometry, injection rate, reaction kinetics, and mass transfer rates.

Fig .3.- Wormholes created by acid dissolution of limestone