Carbonate Matrix Acidizing Fluids at High Temperatures: Acetic Acid, Chelating Agents or Long-Chained Carboxylic Acids?

摘要

Matrix acidizing of carbonate formations has been carried outrnfor many years using HCl acid in various strengths. However,rnin some high temperature applications, HCl does not producernacceptable stimulation results due to lack of penetration orrnsurface reactions. Organic acids, like formic acid and aceticrnacid, were introduced to offer a slower reacting a thus deeperrnstimulating acid. These “retarded” acids also had shortcomingsrndue to solubility limitations of acetate or formaternsalts. In recent years, several alternatives have beenrndeveloped, including aminocarboxylic acids and long-chainedrncarboxylic acids. These long-chained carboxylic acids offerrnlow corrosion rates, good dissolving power at highrntemperature, high biodegradability, and easier and saferrnto handle.rnMany experimental and theoretical studies inrncarbonate acidizing have confirmed the existence of anrnoptimal acid injection rate at which major wormholes arernformed, and the benefit from stimulation is maximized. Thisrnoptimal rate depends on reservoir conditions, rock propertiesrnand chemical reaction rate of the acid being used. In ourrnprevious study, a theoretical model showed that under thernsame conditions, the optimal injection rate for weaker acids isrnlower than that for stronger acids. This paper presents arncomparison of the efficiency of stimulation in carbonaternacidizing of three different kinds of high temperaturernstimulation fluids. A chelating agent, EDTA, acetic acid, and arnmixture of long-chained carboxylic acids were used to acidizerncarbonate cores at high temperatures. The effectiveness of thernprocess and the optimal injection rate were studied byrnmeasuring the acid volume needed to propagate wormholesrnthrough 4-inch cores. The dendritic nature of the acidrnpenetration was also determined by making castings of thernwormhole structures after acidizing. The experimental resultsrnfrom this study showed that the optimal injection rate of longchainedrncarboxylic acids is lower than that for acetic acid andrnthe EDTA. This increase in efficiency then determines that arndeeper and more efficient stimulation per gallon of acidrnmixture used is obtained with the long-chainedrncarboxylic acids.