Water treatment and injection well stimulation are two important contributions to oil lifting cost at Barrancas oil field. As most of mature fields, waterfloding represents a hugh amount of oil production. Although average water quality is quite good some injection wells require regular hydrochloric acid stimulation in order to restore injectivity, impaired by different solids, mainly calcium carbonate and sulfides. A new stimulation system was designed. The system combines the conventional acid system with an aqueous chlorine dioxide solution. In this approach chlorine dioxide activates at the bottom of the injection well. The oxidating power of chlorine dioxide removes completely all solids that are not sensitive to acid treatments. The synergistic effect of the proposed treatment was determined in two different flow core tests on reservoir samples. Due to successful results at the lab, a decision was made to perform field tests in four injection wells. A dramatic and continuous decrease in wellhead pressure were the results obtained after several weeks of treatment. In order to analyze and prevent corrosion effects a parallel study was designed and conducted on tubing samples at reservoir conditions. Introduction Injectivity losses are a usual problem in several Repsol YPF waterflooded fields. Barrancas, a mature oil field, has started waterflooding in 1967. For several years fresh water was injected but in the 90 s and aggressive program lead to the gradual replacement of fresh water by prodution water. More than one hundred wells currently inject 15,000 m3/day. Injection pressures are quite high (20,000 KPa), leading to a high energy consumption. Table 1 shows an average of Barrancas treated injection water composition at the plant s outlet. The importance of water quality to minimize formation damage has been stated by several researchers. Reference 1 is an excellent summary of guides to follow. Although it can be seen that the water treatment system has a fair good performance, some injection wells need a periodic acid stimulation in order to restore injectivity and decrease wellhead pressure. Historically, 10 % HCl or 10% HCl plus 1 % HF are used for stimulations. In both cases acetic acid and additives are also added. About 100 acid treatments are annually performed. In some cases hydrochloric acid stimulation frequency is less than two months. A typical injectivity decline curve of one of this problematic wells is shown in figure 1. Statistics shows that plugging frequency decreases as time passes. On the other hand acid treatments must be handled with care for preventing injection facilities failures. The effect of this situation on economics is clear: sweep efficiency is reduced, water treatment cost increases and as a consecuence the overall lifting cost of the field also increases. As the oil production has been declining and fields profitability is very senstitive to oil prices the injectivity of problematic wells needs to be restored or improved without affecting the field economics. Previous approaches Many references discussed the use of different oxidizers to remove solids that are not attacked by hydrochloric acid. Many of the papers analyze the possible use of chlorine or chlorine dioxide. Probably the most interesting one is a paper by Mc Cafferty et al2. The authors discussed in detail the use of chlorine dioxide as a stimulation fluid. They stated that more than 1,000 injection and production wells had been successfully treated with a combination of hydrochloric acid and chlorine dioxide. Chlorine dioxide was generated on site using a venturi and mixing three different components, HCl, sodium hypochlorite and sodium chlorite. A detailed description of the generation system is given in their paper.

No abstract