|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Journal Paper|
Field Performance in the Practical Application of Chlorine Dioxide as a Stimulation Enhancement Fluid
McCafferty, J.F., Conoco Inc.; Tate, E.W., Williams, D.A., Exxon Chemical Co.
|Journal||SPE Production & Facilities|
|Volume||Volume 8, Number 1||Pages||9-14|
1993. Society of Petroleum Engineers
New, innovative, and successful application technology has evolved in the practical use of chlorine dioxide for well stimulations. The safe generation and use of chlorine dioxide is paramount in well-stimulation procedures. More than 1,000 well paramount in well-stimulation procedures. More than 1,000 well stimulations have been completed safely with the described method of chlorine dioxide generation. Guidelines and operational procedures have been established for the safe handling of chlorine procedures have been established for the safe handling of chlorine dioxide in well stimulations. Chlorine dioxide is a powerful oxidant that has been coupled safely with conventional acidization to stimulate producing wells, waterflood injection wells, and saltwater disposal wells. Field applications of chlorine dioxide and acid indicate a synergistic relationship that increases the longevity of the stimulation by removal of formation skin damaging agents. Field results correlate well with laboratory core tests for remediation of formation skin damage. This technology is being applied successfully with consistent results in the U.S., although safety has been a primary issue. This paper provides safe guidelines for the process.
Chlorine dioxide has been used for 50 years in Europe to oxidize organic contaminants in potable water and municipal sewage. In the U.S., chlorine dioxide technology spread to the canning industry to control salmonella and clostridium bacteria and eventually to the pulp and paper delignification industry. pulp and paper delignification industry. Adaptation of chlorine dioxide technology to the petroleum industry led to a novel technology in which chlorine dioxide is use to eliminate oxidizable plugging agents in well stimulations. Know oxidizable materials that can hinder formation performance are iron sulfide, bio-organics, and polyacrylamide-based polymers. Further complications arise when these contaminants exist in the presence of calcium carbonate scale and heavy hydrocarbon sludge. Over time, most wells experience a decline in productivity o injectivity. The decline is caused, in part, by acid-soluble plugging agents, such as calcium carbonate scale and iron sulfide. plugging agents, such as calcium carbonate scale and iron sulfide. Acidization cannot affect other plugging agents, such as organic biomasses and polymeric residues. The oxidative power of chlorine dioxide used with conventional acids will remove these plugging agents. Conventional acids will solubilize calcium carbonate scale and iron sulfide. As the pH of the formation fluids rises, however, the solubilized iron sulfide will reprecipitate and again plug pore spaces. The addition of chlorine dioxide in acid stimulations will oxidize the preferentially oil-wet iron sulfide to a water-wet hydrous ferric oxide, which will be chelated by additives in the acid. Chlorine dioxide also will oxidize available H2S created by acid in the presence of iron sulfide. Biomass residues and any polyacrylamide material present also will be oxidized. A mutual polyacrylamide material present also will be oxidized. A mutual solvent should be used to remove heavy hydrocarbons that coat the formation rock and act as a bonding agent for contaminants. This paper describes the theory, safe well-treating methods, an case histories correlated to laboratory core studies.
Any procedure involving the use of aqueous chlorine dioxide must comply with strict safety regulations (see the Appendix). The solubility of chlorine dioxide in fresh water at 77F is about 6,000 ppm (Fig. 1). 3 Chlorine dioxide is an effective oxidant at low concentrations. When used as an oxidant in an aqueous solution of be tween 1,500 and 3,000 ppm, chlorine dioxide is concentrated enough to be an effective oxidant but dilute enough to be handled safely. The method of aqueous chlorine dioxide generation used he involves a water-driven eductor venturi. The suction created as water flows through the venturi pulls three precursor material together into a reactor and eventually into diluent water that drive the venturi. In the first of two reactions, a 15% solution of HCl is mixed with a 10% solution of sodium hypochlorite, through respective flow-control rotameters, to produce a small amount of hydrated chlorine. The reaction is
In the second reaction, the generated hydrated chlorine is reacted with sodium chlorite, which is controlled by a third flow-control rotameter, to produce aqueous chlorine dioxide:
The aqueous chlorine dioxide produced in the second reaction is drawn from the reactor into the diluent water that drives the venturi. The chlorine dioxide dissolves in the diluent water and does not react with water, as in the case of chlorine gas. This enhances safety because no chlorine dioxide can be generated when pumping stops and the concentration of chlorine dioxide generated pumping stops and the concentration of chlorine dioxide generated is below the solubility limit in water at standard temperature and pressure. pressure. Field Experience
The first field trials of chlorine dioxide as a stimulation fluid were completed in late 1987 specifically for crosslinked polyacrylamide damage removal. These treatments were successful polyacrylamide damage removal. These treatments were successful because the polyacrylamide damage required an oxidizing process rather than an acid hydrolysis process. This led to further development of chlorine dioxide as an oxidizer for wellbore stimulations. Acid treatments historically have been used to remove calcium carbonate scale damage from the wellbore. However, acid has proved ineffective for removal of polymer damage, iron sulfide proved ineffective for removal of polymer damage, iron sulfide damage, and/or biomass plugging. The combination of chlorine dioxide and acid provides a synergistic relationship for removal of these damaging mechanisms. Results from stimulations with chlorine dioxide and acid indicate marked improvement in production or injection volumes and longevity of the stimulation. The synergistic relationship between chlorine dioxide and acid has been proved in the field and laboratory to be an effective well-stimulation enhancement process. Improved results were observed in field trials when chlorine dioxide was place between two stages of acid and a micellar solvent was added to the acid. The first acid stage (1) solubilizes iron sulfide and calcium carbonate scale, (2) carries a micellar solvent for heavy hydrocarbon dissolution, (3) carries an iron chelant, and (4) lowers the pH of the downhole environment to activate the chlorine dioxide. The chlorine dioxide stage oxidizes any polymer, iron sulfide, bacteria, and generated H2S gas. The second acid stage then "mops up" the remaining carbonates and can carry a micellar solvent into the formation for increased water wettability. The merit of the sandwiching effect observed in field practice has been proved by laboratory core-analysis data. practice has been proved by laboratory core-analysis data. SPEPF
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