A simple and practical process has been developed to heat hydrochloric acid to improve its performance as an oil well stimulating agent. Heated acid is used in cold formations, such as cold dolomites, to increase the rate at which the formation is dissolved, thus improving flow capacities. The new technique heats the acid by causing an exothermic chemical reaction that uses a portion of the HCl as one of the reactants.
Heated acid reacts at a faster rate and aids in removal of acid-retarding materials such as oil, asphaltene and paraffins from the formation to give a better acid-to-formation contact. Some other benefits of heating acid are:
Helps prevent precipitation of organic materials from oils that have been cooled by acids.
Limits the amount of tubing contraction due to temperature change.
Problems presented by the slow reaction time Problems presented by the slow reaction time HCl in cold formations and the special problems of conducting winter time acid treatments are largely solved by heating HCl up to bottom hole temperature or more prior to pumping. This paper presents laboratory and field results to illustrate the effectiveness of the heated acid method of stimulation and compares the various methods of bringing the acid to its ideal temperature.
Stimulation of formations with cold treating fluids causes a variety of problems to the oil industry.
1. Cold treating fluids are not reactive enough, especially when the formations are themselves cool.
2. Tubular goods will contract excessively when the formation is hot and the treating fluid is cool. Contraction can cause strain on the tubular goods and unseating of packers.
Historically, in acidizing emphasis has been on the use of retarded acids or retarded systems that place live acid deeper into the formation; place live acid deeper into the formation; however, studies have shown that in many ways accelerated acid systems are necessary because of cold formations or short acid pumping times. Although surfactants can aid to some degree, the most practical means to accelerate the acid reaction has practical means to accelerate the acid reaction has been found to be heating the acid.
Hot oilers and steam have been used to heat acid, but their application with many acid systems was limited or they were impractical for the type of treatment to be run. A system to chemically heat acid has been developed and tested in the laboratory and field, showing excellent results.
Heating acid by chemical means can be accomplished by either batch or continuous methods. The batch heating process consists of flowing ammonia into the container below the surface of the acid by means of a perforated lance. Ammonia is added until the desired temperature or acid concentration is reached. Figure 1 shows the heat derived from three different acid solutions.
Large volumes of acid can also be heated simply by the continuous method. Ammonia is metered into the acid while the acid is being pumped into the well. Heat is developed rapidly and there is little heat loss on the surface since there is no exposure or delay.
Heat used to warm acid is derived from (1) the equation of ammonia and (2) neutralization of the aquated ammonia by hydrochloric acid to make ammonium chloride. Table 1 lists some other compounds capable of heating acid.
Generally, acid is not run through hot oiler systems because of a potential for corrosion.