Lessons Learned from Acid Pickle Treatments of Deep/Sour Gas Wells

Abstract

Pickle treatments are commonly conducted to remove various contaminants from the wellbore prior to performing chemical treatments in various wells. Introduction of these contaminants into the formation during the treatment can cause loss of well performance, especially in tight formations. These treatments should be designed to remove various contaminants from well tubulars in an efficient and cost effective way. A pickle treatment was designed to clean well tubulars in several wells prior to performing acid fracturing in the carbonate formation. The pickle treatment consisted of several fluids such that each fluid removed one type of contaminants. This paper examines the effects of fluid composition, volume, and placement (bullheading versus coiled tubing) on the efficiency of pickling treatments. More than twenty pickle treatments performed on deep, sour, hot gas wells were examined. To assess pickle treatments in the field, spent acid was analyzed and the concentrations of key ions were measured. Sufficient measurements were made to obtain concentration profiles of chemical species in the spent pickle acid. These profiles were numerically integrated and the data were used to assess the outcome of each pickle treatment. In addition, particulate solids present in the spent acid were separated and analyzed using XRD and XRF techniques. Pickle treatments were conducted on more than twenty gas wells. Some of these wells were perforated, while others were not perforated. Un-perforated wells were pickled using 1.75" high-pressure coiled tubing. On the other hand, perforated wells were pickled using bullheading. The pickle treatments consisted of slugs of surfactants, organic solvents, hydrochloric acid and gelled fluids. Samples of all pumped fluids were collected and analyzed. In addition, well flow back samples were collected every two minutes and were also analyzed to determine their composition. Analysis of spent acids indicated that the major tubular contaminants are mill scale and pipe dope. Spent acid samples collected from deep sour gas wells contained up to nearly 105,500 mg/L of total iron. The efficiency of removing pipe dope was followed by tracing the concentration of zinc, the main acid-soluble constituent in the pipe dope, in the spent acid. The concentrations of acid and iron were used to determine acid reaction with the corrosion products present in the tubing. The concentration of the chloride ions was used to determine the dispersion and mixing behaviour of the acid slug. This paper presents the results obtained from analyzing more than 2000 samples obtained from twenty treatments. The results of this work have been used to enhance the efficiency of the pickle treatments and to significantly reduce the treatment cost. Introduction Formation damage occurs during acidizing or chemical treatments due to the presence of contaminants in the tubing. Several researchers investigated the type and concentration of these contaminants.1–4 The main contaminants present in a typical well tubing are mill scale, iron sulfides, pipe dope, sand, and other fine particles that were picked up during storage and transportation of the tubing string. Oil or condensate may also be present in old producers. One of the contaminants in the wellbore is pipe dope. Pipe dope is used to seal tubing joints, minimize thread erosion and enhance the galling resistance of the threads.5,6 Many authors recommended not to use excessive amounts of pipe dope, which may end in the target zone and cause severe formation damage.1–4

New Insights Into the Viscosity of Polymer-Based In-Situ-Gelled Acids

“…Ferric ions have three positive charges and can crosslink the polymer that was used to prepare gelled and in-situ-gelled acids. Acids are usually contaminated with ferric ions because of dissolution of rust from well tubulars and mixing tanks (Nasr-El-Din et al 2002b). Because of surface contamination, the concentration of ferric ions in live acids can reach up to 10,000 ppm in some cases (Al-Nakhli et al 2008).…”

Modeling of Tubing Pickling in Conjunction With Acidizing Treatments

“…This validates our assumption of having an effective corrosion inhibitor. Details of these calculations were given in Nasr- El-Din et al (2002b). Table 3 lists values of dimensionless variables for the pickling treatment that was conducted on Well A.…”

“…Fig. 2 shows the total iron concentration (main reaction product) in the acid front, mainly in the region occupied by the spent acid (Nasr-El-Din et al 2002b). Because the chloride ion is not consumed in the reaction, it can be used as a tracer.…”

“…4, a microscopic visualization demonstrates the proposed mechanism of the heterogeneous reaction between the acid and the reactants on the inner surface of mill scale. Two main processes control the reaction (Williams et al 1979;Loewen et al 1990;Nasr-El-Din et al 2002b):…”

“…Acid additives included a corrosion inhibitor and a water-wetting agent. More details on the acid formulation were given by Nasr- El-Din et al (2002b). It is important to note that the gases produced from these wells contained hydrogen sulfide.…”