A.W. Coulter, Dowell Division of The Dow Chemical Company
Although not recognized as a definite acidizing technique, fracture acidizing has probably occurred in a majority of acidizing treatments throughout the history of the process. Injection rates of most early acidizing treatments were high enough to cause fracturing, and it was common practice to "breakdown" the formation at the beginning of the treatment. Since the introduction of hydraulic fracturing, fracture acidizing has been recognized as a fracturing process, and it is now commonly used in an effort to increase live acid penetration into the reservoir rock. Extensive studies have been made to evaluate the effect of many variables (temperature, pressure, concentration, etc.) on acid penetration. Data from these studies have been incorporated into acid treatment design programs so that the operator will have a tool to use in planning more effective and economical acid treatments. The industry has long recognized, however, that actual results obtained by acidizing carbonate formations were not equal to increases predicted by the design programs. During the past decade, considerable research has been directed toward shortening the gap between actual and predicted results. Some of the approaches taken have been: 1. Acid retardation 2. Increased acid concentration 3. Increased fracture width to decrease the area volume ratio 4. Improved matrix leakoff control 5. Improved computer calculations. All these approaches had one basic purpose to increase live acid penetration of the reservoir rock. Each improvement did provide better response, either separately or when used in various combinations. Too large a gap still remained, however, between the predicted and actual results. The problem with these improvements was that each of them was based on the assumption of flow conditions with matrix leakoff: It is now recognized that such conditions do not exist due to natural hairline fractures that exist in most carbonate formations. In an undisturbed state, these fractures exert little influence on overall permeability, but leakoff of acid during an acid fracturing treatment not only occurs into the matrix but also into these hairline fractures. Calculations can be made to show that very small volumes of acid can enlarge these hairline fractures so that they can increase the average permeability from less than 1 md to more than 900 md. Large volumes of following acid can then leak off into these fractures, and penetration will then be much less than predicted by design programs assuming only matrix leakoff. A new acid fracturing technique using alternating stages of pad fluid and acid has been proven highly successful in achieving results that approximate predicted results of acid fracturing treatments. This success is believed due to the fact that the technique reduces leakoff into the hairline fractures as well as the matrix. A description of fluid flow behavior during acid fracturing will show how the new technique provides better penetration and better results from such treatments.