Acid Stimulation of Power Water Injectors and Saltwater Disposal Wells in a Carbonate Reservoir in Saudi Arabia: Laboratory Testing and Field Results

Abstract

Abstract Seawater power injectors and saltwater disposal wells are used to maintain reservoir pressure in a carbonate reservoir in Saudi Arabia. Various forms of hydrochloric acid were used to stimulate damaged wells and restore the injectivity of these wells. The forms of the acid used were regular, emulsified, and in-situ gelled acids. Acid diversion was achieved by using a coiled tubing unit, nitrogen foam, in-situ gelation, emulsified acid or combinations of these methods. A comprehensive investigation was undertaken to assess the effectiveness of acid treatments performed in the field and improve the outcome of the acid jobs. Laboratory and field investigations were conducted simultaneously to address formation damage and associated problems. These investigations included coreflood tests to screen various acid formulae, and analysis of field samples to identify the damaging mechanism. More than 80 acid jobs were analyzed in this study. A new empirical coefficient was identified and used to evaluate these jobs. The coefficient was based on: 1) the ratio between the pre and post acid injectivity indices, 2) the rate of decline of normalized well injectivity index, and 3) the incremental cumulative injection from the Hall plot following the acid job and before the first change in the slope. In addition, chemical analysis of acid returns (spent acid) was used to assess the damaging mechanisms and determine the type and concentration of various acid additives. Improper use of emulsified acid and in-situ gelled acid was found to result in poor field results. The outcome of acid jobs was found to be a function of the volume ratio of regular to in-situ gelled acid. Good field results were obtained when the volume of in-situ gelled acid was 15-30% of that of the regular acid (15 wt% HCl). Furthermore, the study confirmed that acid injection using coiled tubing enhanced the efficiency of the acid treatment. This paper highlights the importance of conducting laboratory and field evaluation of acidizing treatments. The following new findings were obtained: A new job index was developed and used to assess acid treatments in the field. Severe formation damage can result from the improper use of in-situ gelled acids. Analysis of spent acid was effectively used to optimize the concentration and type of various acid additives.