Improving Wellbore And Formation Cleaning Efficiencies With Environmental Solvents And Pickling Solutions
Abstract: TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThe purpose of a chemical stimulation treatment, be it based on acid, solvent, or other systems, is to remove formation damage and increase productivity/injectivity, by restoring or improving the reservoir permeability in the near-wellbore region.
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Traditional "Best Practices" completion procedures recommend an acid pickle treatment to clean-out all tubulars in a wellbore prior to pumping an acid treatment or a "solids-free" completion brine system. The potential contaminants (iron oxidation, hydrocarbon deposits and pipe dope) can create formation damage if pumped into the formation during the main treatment. In addition, contamination of "solids-free" completion brine can lead to increased location diatomaceous earth (D.E.) filtration time. Tubing clean-out treatments normally consist of a slug of acid (10 to 15wt% hydrochloric) being pumped inside the tubulars and then reverse circulated back to surface without fluids being lost to the formation. A solvent is typically added to assist in the removal of pipe dope and organic deposits. Inhibitor and a surfactant, to facilitate wetting, are the normal fluid additives. Handling and disposal of this fluid after clean up of the tubulars can be dangerous and very costly due to its corrosive nature (pH <0). A slightly acidic (pH of 5.8–6.3) fluid has been evaluated in the laboratory for use as a pickling fluid to replace the traditional acids. This fluid's capability to remove iron scales on coiled tubing, prevent rust from reforming, clean up commercial pipe dopes and reduce corrosion rates, was compared to traditional hydrochloric acid. This fluid performed as efficiently as 15wt% hydrochloric acid in removing all potential contaminants associated with a workstring, while producing one fifth the corrosion rate. Furthermore, a work string treated with this fluid prior to being laid down on a pipe rack retained a protective coating that resisted re-oxidization for up to 30 days. Introduction Completion workstrings endure extremely harsh corrosive and erosive conditions during all phases of oilfield operations. As a result of these harsh environmental conditions, significant amounts of rust and iron oxidation are commonly present on the surfaces of the completion workstring, downhole pumps, and flowlines.1,2,3,5–8 Moreover, the surfaces of new low-carbon steel pipes and coiled tubing metals such as L-80 or N-80 are commonly covered with mill scale or oxidization after manufacturing and warehouse storage before even being subjected to well conditions. The dissolution of iron oxidation or mill scale solubilized in HCl acid during an acidizing treatment will form both ferric and ferrous ions in acid.9 Formation damage occurs when these iron compounds enter the perforations, sandpacks and /or the formation. Ferric ions precipitate as ferric solids once the pH value increases above 1–2.2,5–8 Another common completion contaminant in casing and tubulars is excess pipe dope. Pipe dope is used between drill pipe sections during preparation of the drill string to seal tubular joints and reduce thread wear. These materials are usually composed of very high viscosity grease with various fine particles, friction reducers and plating materials. The possibility of formation permeability damage is very high if the pipe dope is introduced into the producing zone. Pipe dope compounds as well as iron scales and rust may seal the formation, making acid treatment extremely difficult and inefficient. Introducing these contaminants into the formation can result in loss of well productivity if they enter the targeted oil-producing zone. Therefore, efforts to remove these contaminants before any chemical treatments begin are very advantageous to future well productivity. "Best practices" completion procedures recommend a pickle treatment to remove rust, iron oxidation and other possible contaminants from the completion workstring before certain treatments of completion stages are implemented. Pickle treatments are usually conducted before an acid matrix/acid fracturing procedure, gravel packing job or before introducing completion brine into the wellbore to prevent completion workstring contaminants from entering into the formation, perforations, or into the "solids-free" completion brine system.10 Introduction of contaminants into "solids-free" completion brines can also result in formation damage as well as increased rigsite D.E. (diatomaceous earth) filtration time. Also, contaminated completion brine can increase reclamation cost to the operator, when the brine is refurbished to new fluid specifications.
Traditional "Best Practices" completion procedures recommend an acid pickle treatment to clean-out all tubulars in a wellbore prior to pumping an acid treatment or a "solids-free" completion brine system. The potential contaminants (iron oxidation, hydrocarbon deposits and pipe dope) can create formation damage if pumped into the formation during the main treatment. In addition, contamination of "solids-free" completion brine can lead to increased location diatomaceous earth (D.E.) filtration time. Tubing clean-out treatments normally consist of a slug of acid (10 to 15wt% hydrochloric) being pumped inside the tubulars and then reverse circulated back to surface without fluids being lost to the formation. A solvent is typically added to assist in the removal of pipe dope and organic deposits. Inhibitor and a surfactant, to facilitate wetting, are the normal fluid additives. Handling and disposal of this fluid after clean up of the tubulars can be dangerous and very costly due to its corrosive nature (pH <0). A slightly acidic (pH of 5.8–6.3) fluid has been evaluated in the laboratory for use as a pickling fluid to replace the traditional acids. This fluid's capability to remove iron scales on coiled tubing, prevent rust from reforming, clean up commercial pipe dopes and reduce corrosion rates, was compared to traditional hydrochloric acid. This fluid performed as efficiently as 15wt% hydrochloric acid in removing all potential contaminants associated with a workstring, while producing one fifth the corrosion rate. Furthermore, a work string treated with this fluid prior to being laid down on a pipe rack retained a protective coating that resisted re-oxidization for up to 30 days. Introduction Completion workstrings endure extremely harsh corrosive and erosive conditions during all phases of oilfield operations. As a result of these harsh environmental conditions, significant amounts of rust and iron oxidation are commonly present on the surfaces of the completion workstring, downhole pumps, and flowlines.1,2,3,5–8 Moreover, the surfaces of new low-carbon steel pipes and coiled tubing metals such as L-80 or N-80 are commonly covered with mill scale or oxidization after manufacturing and warehouse storage before even being subjected to well conditions. The dissolution of iron oxidation or mill scale solubilized in HCl acid during an acidizing treatment will form both ferric and ferrous ions in acid.9 Formation damage occurs when these iron compounds enter the perforations, sandpacks and /or the formation. Ferric ions precipitate as ferric solids once the pH value increases above 1–2.2,5–8 Another common completion contaminant in casing and tubulars is excess pipe dope. Pipe dope is used between drill pipe sections during preparation of the drill string to seal tubular joints and reduce thread wear. These materials are usually composed of very high viscosity grease with various fine particles, friction reducers and plating materials. The possibility of formation permeability damage is very high if the pipe dope is introduced into the producing zone. Pipe dope compounds as well as iron scales and rust may seal the formation, making acid treatment extremely difficult and inefficient. Introducing these contaminants into the formation can result in loss of well productivity if they enter the targeted oil-producing zone. Therefore, efforts to remove these contaminants before any chemical treatments begin are very advantageous to future well productivity. "Best practices" completion procedures recommend a pickle treatment to remove rust, iron oxidation and other possible contaminants from the completion workstring before certain treatments of completion stages are implemented. Pickle treatments are usually conducted before an acid matrix/acid fracturing procedure, gravel packing job or before introducing completion brine into the wellbore to prevent completion workstring contaminants from entering into the formation, perforations, or into the "solids-free" completion brine system.10 Introduction of contaminants into "solids-free" completion brines can also result in formation damage as well as increased rigsite D.E. (diatomaceous earth) filtration time. Also, contaminated completion brine can increase reclamation cost to the operator, when the brine is refurbished to new fluid specifications.
Deposition of iron oxidation products, pipe dope and hydrocarbons on the metallic surfaces associated with wells can become significant. Whether the iron compounds come from the producing environment, from mill scale on a new tubing string, or from exposure to the elements during storage of a work string, their presence creates the potential for formation damage if they are dislodged and carried into the reservoir with treating fluids. Acid stimulation treatments, frac packs and gravel packs are reservoir interventions where this transportation of deposits could occur. Traditionally, an acid pickle treatment is performed prior to such treatments to remove the potential contaminants. Acid pickling, while being effective, presents significant problems with safety, the environment and disposal of acidic fluids that return to the surface only partially spent. An alternative solution has been found using a near-neutral pH fluid to remove iron oxidation products and provide passivation of treated metallic surfaces. The safer acid level of this fluid provides a solution to both the downhole problems and the surface health, safety and environmental issues. Extensive laboratory testing has established the effectiveness of the fluid to removes not only iron oxidation products but also pipe dope and hydrocarbon deposits when used in conjunction with an environmentally friendly solvent. Performance is equal to traditional acidic fluids. Testing also demonstrated that metallic surfaces retained passivity. This new fluid has now been used in several hundred wells in both the United States and South America. This paper will outline the laboratory results defining its capabilities compared to traditional hydrochloric acid systems. Case histories of usage in the United States and South America for the pre-treatment of downhole tubulars to remove contaminants are presented. The special case history of application to remove oxidation scale in a subsea fuel gas line system off the coast of Brazil is also discussed. Introduction Acid stimulation treatments, frac packs and gravel packs are reservoir interventions where transportation of potentially damaging deposits and contaminants can be carried into the reservoir with treating fluids. API standards recommend conducting an acid pickle treatment prior to these reservoir interventions to remove any potentially formation-damaging contaminants. These contaminants may consist of iron oxidation products, mill scale, pipe dope and hydrocarbon deposits, which can be present in surface equipment, flow lines, downhole tubulars, coiled tubing string, and gravel pack screens. A pickle acid treatment is usually initiated prior to all reservoir interventions to prevent these potential contaminants from entering the formation. Benefits of conducting pickling treatments are iron control, sludge prevention (between the treating fluid and the produced hydrocarbons), elimination of spent acid entering the formation, improvement in stimulation results and reduction in overall well treatment cost.1,2 Traditional Pickle Formulations Traditional pickle acid systems are formulated with a 10-to-15wt% hydrochloric acid and additives such as corrosion inhibitors and surfactants to facilitate water-wet surfaces during the treatment. A solvent is typically added to aid in the removal of pipe dope and hydrocarbons. After uniform mixing, the pickling treatment is pumped down inside the tubulars and reverse circulated back to the surface without fluids being lost to the formation. These treatments are conducted to ensure the most soluble and potentially formation-damaging contaminants are removed from the wellbore and will not affect future well productivity. Challenges with Traditional Pickle Systems Pickle acid systems are classified as hazardous materials before pickling and upon return to the surface after treatment with a pH range of less than 0. Upon return to the surface after pickling, fluid requires neutralization with sodium bicarbonate or hydroxide before disposal. In highly sensitive areas, these properties pose safety, environmental, and liability risks associated with handling both before and after pickling.
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