Standard: NACE 1D191
A REVIEW OF SCREENING TESTS FOR GYPSUM SCALE REMOVAL CHEMICALS
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Scale deposition has been occurring in oil and gas production and injection wells and surface equipment for many years. The scale deposits can be located both downhole and in surface equipment. Minerals carried in oilfield-produced brine are the source of these deposits. Although there are many factors that affect scale precipitation, the primary causes are temperature changes, a reduction of pressure on the produced brine, or the mixing of waters having incompatible ions in solution.
The composition of mineral scales differs with the environment. The most common oilfield scale deposits are calcium carbonate, calcium sulfate, barium sulfate, and strontium sulfate. Various iron compounds, such as iron carbonate, iron sulfide, and iron oxide may also form as the corrosion products generated by carbon dioxide, hydrogen sulfide, or oxygen dissolved in the water.
As various solids precipitate from solution and deposit onto the surfaces of tubulars—within the perforations or on the reservoir face—the flow of fluids is reduced. The resulting loss in gas and/or oil production has a negative economic impact. Therefore, an effective scale inhibition program is usually maintained, because preventing scale from forming is usually more economical than removing it after formation and deposition.
If scale deposits do develop, they usually are removed either chemically or mechanically. A summary of the various methods and equipment used for removing scale mechanically has been presented, and there continues to be further development in this area.Further discussion is beyond the scope of this report.
Chemical scale removal can include the use of mineral acids to remove acid-soluble scales such as calcite (calcium carbonate), perhaps the predominant form of scale encountered in oilfield production systems, and various iron-based scales. Sequestrants or chelating agents are sometimes used alone or in combination with acid. The most commonly used chelating compounds, salts of ethylene-diaminetetraacetic acid (EDTA), have been used to remove calcite scale deposits in several oil production systems.4-7 Excellent summaries of developments in this area have been presented.
Given the serious impact that gypsum scale formation can have on hydrocarbon production, it is not surprising that producers and service companies devote considerable effort to developing and marketing effective treating chemicals. The performance of these products can be verified most effectively after an actual field trial. However, field testing can be very difficult and time consuming, especially when evaluating many chemicals. As a result, most testing of the effectiveness and performance of gypsum removers takes place initially in the laboratory. Although most laboratory tests cannot exactly duplicate field conditions, the advantage of such tests is to provide the user with a comparison of one product's performance against that of another under standard laboratory conditions.
The topic that is specifically addressed in this report is the use of screening tests to evaluate chemical scale-removal agents—most notably alkaline solutions and/or various organic acids and sequestrants—to remove acid-insoluble scales, specifically gypsum (calcium sulfate dihydrate). Common scale-removal chemicals include sodium hydroxide, sodium carbonate/bicarbonate, ammonium carbonate/bicarbonate, EDTA salts, nitrilotriacetic acid (NTA) salts, hydroxyacetic acid salts, and gluconic acid salts.
Additives such as surfactants are sometimes added to these chemicals to improve the penetration rate of the chemical treating solution into the scale deposits. Numerous proprietary products have been formulated specifically for the removal of gypsum deposits.
Case histories involving the field application of gypsum scale removers have been reported in the literature. The development of new chemicals and processes specifically designed for removing gypsum scale deposits is described in some recent patents.
|Document Number:||nace 1d191|
|Most Recent Revision:||YES|
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