Higher-pH Acid Stimulation Systems

Abstract

Summary Two types of high-pH (4 to 6), mild-acting acidizing systems have been developed for in-depth rock matrix stimulation of both sandstone and carbonate reservoirs:self-generating (SG),** which utilizes the hydrolysis of an ester (or other acid producers) to generate low concentrations of H+, andbuffer-regulated (BR), which utilizes an organic acid buffered with its ammonium salt. For sandstone stimulations, the HF acid generation is provided by the addition of ammonium fluoride. In-depth stimulation can be achieved with these systems from ambient temperature to about 280 degrees F [138 degrees C]. Near-wellbore damage removal can be obtained by using BR systems to at least 380 degrees F [193 degrees C]. Maximum application temperature is being determined now. Three SG systems have been developed for different operating temperature ranges: methyl formate (MF), for 130 to 180 degrees F [54 to 82 degrees C]; the ammonium salt of monochloroacetic acid (CA), for 180 to 215 degrees F [82 to 102 degrees C]; and methyl acetate (MA), for 190 to 280 degrees F [88 to 130 degrees C]. Similarly, BR systems have been developed for different pH ranges: formic acid (F) for pH 3.5 to 4.0, and acetic acid (A) and citric acid (C) for pH 4.5 to 5.0. Because of their moderate pH levels and low corrosiveness, BR systems also have potential application at temperatures where excessive corrosion precludes the use of conventional acids. Laboratory corrosion studies on BR systems show that corrosion rates, without inhibitors, are within acceptable limits to about 380 degrees F [193 degrees C]. Corrosion inhibitors should allow applicability to higher temperatures. This paper reviews the theory and experiments used in the development of these SG and BR systems. Field test results with these systems are presented in Refs. 1 through 4. Introduction Well stimulation acids have been designed traditionally on the basis of dissolving capacity, Therefore, concentrated acids such as 15% and 28% HCl and 12% HCl/3% HF have been used widely. These acid systems have pH values less than zero and are classified as strong acids. Both field experience and laboratory flow test results suggest that the amount of material causing impairment is probably small. However, because of its location in the flow channels in the rock, it can result in significant impairment. Further, the industry has been concerned for a long time with damage that may result from the release of fines and with the rapid reaction and depletion characteristics of low-ph acid stimulations. To meet the need for milder, slower-acting acids, we undertook a laboratory study that led to the development of the SG and BR systems described. Our field experiences with sandstone acidization have shown that mild acids and modified field procedures result in acid stimulation jobs with increased, stabilized oil production. JPT P. 2175^